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Invited review: Current enteric methane mitigation options

Open AccessPublished:October 18, 2022DOI:https://doi.org/10.3168/jds.2022-22091

      ABSTRACT

      Ruminant livestock are an important source of anthropogenic methane (CH4). Decreasing the emissions of enteric CH4 from ruminant production is strategic to limit the global temperature increase to 1.5°C by 2050. Research in the area of enteric CH4 mitigation has grown exponentially in the last 2 decades, with various strategies for enteric CH4 abatement being investigated: production intensification, dietary manipulation (including supplementation and processing of concentrates and lipids, and management of forage and pastures), rumen manipulation (supplementation of ionophores, 3-nitrooxypropanol, macroalgae, alternative electron acceptors, and phytochemicals), and selection of low-CH4-producing animals. Other enteric CH4 mitigation strategies are at earlier stages of research but rapidly developing. Herein, we discuss and analyze the current status of available enteric CH4 mitigation strategies with an emphasis on opportunities and barriers to their implementation in confined and partial grazing production systems, and in extensive and fully grazing production systems. For each enteric CH4 mitigation strategy, we discuss its effectiveness to decrease total CH4 emissions and emissions on a per animal product basis, safety issues, impacts on the emissions of other greenhouse gases, as well as other economic, regulatory, and societal aspects that are key to implementation. Most research has been conducted with confined animals, and considerably more research is needed to develop, adapt, and evaluate antimethanogenic strategies for grazing systems. In general, few options are currently available for extensive production systems without feed supplementation. Continuous research and development are needed to develop enteric CH4 mitigation strategies that are locally applicable. Information is needed to calculate carbon footprints of interventions on a regional basis to evaluate the impact of mitigation strategies on net greenhouse gas emissions. Economically affordable enteric CH4 mitigation solutions are urgently needed. Successful implementation of safe and effective antimethanogenic strategies will also require delivery mechanisms and adequate technical support for producers, as well as consumer involvement and acceptance. The most appropriate metrics should be used in quantifying the overall climate outcomes associated with mitigation of enteric CH4 emissions. A holistic approach is required, and buy-in is needed at all levels of the supply chain.

      Key words

      INTRODUCTION

      Over 110 countries and supporters have signed the Global Methane Pledge (www.globalmethanepledge.org) to decrease methane (CH4) emissions collectively by 30% from 2020 levels by 2030. Due to the relatively short life of CH4 in the atmosphere and its high global warming potential, reducing CH4 emissions is seen as a rapid way to help limit global warming to 1.5°C above preindustrial levels. Given that enteric CH4 from ruminant livestock accounts for 30% of global anthropogenic CH4 emissions (
      • United Nations Environment Programme and Climate and Clean Air Coalition
      Global methane assessment: benefits and costs of mitigating methane emissions.
      ), there is increasing interest in its mitigation.
      In recent years, tremendous advances have been made in understanding factors that affect CH4 production in the rumen and development of mitigation practices. Detailed descriptions of the biochemistry (
      • Ungerfeld E.M.
      Metabolic hydrogen flows in rumen fermentation: Principles and possibilities of intervention.
      ) and microbiology (
      • Morgavi D.P.
      • Forano E.
      • Martin C.
      • Newbold C.J.
      Microbial ecosystem and methanogenesis in ruminants.
      ;
      • Huws S.A.
      • Creevey C.J.
      • Oyama L.B.
      • Mizrahi I.
      • Denman S.E.
      • Popova M.
      • Muñoz-Tamayo R.
      • Forano E.
      • Waters S.M.
      • Hess M.
      • Tapio I.
      • Smidt H.
      • Krizsan S.J.
      • Yáñez-Ruiz D.R.
      • Belanche A.
      • Guan L.
      • Gruninger R.J.
      • McAllister T.A.
      • Newbold C.J.
      • Roehe R.
      • Dewhurst R.J.
      • Snelling T.J.
      • Watson M.
      • Suen G.
      • Hart E.H.
      • Kingston-Smith A.H.
      • Scollan N.D.
      • do Prado R.M.
      • Pilau E.J.
      • Mantovani H.C.
      • Attwood G.T.
      • Edwards J.E.
      • McEwan N.R.
      • Morrisson S.
      • Mayorga O.L.
      • Elliott C.
      • Morgavi D.P.
      Addressing global ruminant agricultural challenges through understanding the rumen microbiome: Past, present, and future.
      ) of fermentation and CH4 production in ruminants have been published, along with thorough reviews of CH4 mitigation (
      • Beauchemin K.
      • Kreuzer M.O.
      • O'Mara F.
      • Mcallister T.
      Nutritional management for enteric methane abatement: A review.
      ,
      • Beauchemin K.A.
      • Ungerfeld E.M.
      • Eckard R.J.
      • Wang M.
      Fifty years of research on rumen methanogenesis: Lessons learned and future challenges for mitigation.
      ;
      • Martin C.
      • Morgavi D.P.
      • Doreau M.
      Methane mitigation in ruminants: From microbe to the farm scale.
      ;
      • Hristov A.N.
      • Oh J.
      • Firkins J.L.
      • Dijkstra J.
      • Kebreab E.
      • Waghorn G.
      • Makkar H.P.
      • Adesogan A.T.
      • Yang W.
      • Lee C.
      • Gerber P.J.
      • Henderson B.
      • Tricarico J.M.
      Mitigation of methane and nitrous oxide emissions from animal operations: I. A review of enteric methane mitigation options.
      ,
      • Hristov A.N.
      • Ott T.
      • Tricarico J.
      • Rotz A.
      • Waghorn G.
      • Adesogan A.
      • Dijkstra J.
      • Montes F.
      • Oh J.
      • Kebreab E.
      • Oosting S.J.
      • Gerber P.J.
      • Henderson B.
      • Makkar H.P.S.
      • Firkins J.L.
      Special Topics—Mitigation of methane and nitrous oxide emissions from animal operations: III. A review of animal management mitigation options.
      ;
      • Knapp J.R.
      • Laur G.L.
      • Vadas P.A.
      • Weiss W.P.
      • Tricarico J.M.
      Invited review: Enteric methane in dairy cattle production: Quantifying the opportunities and impact of reducing emissions.
      ;
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      ). This area of science is evolving at a rapid pace and, with increased pressure on ruminant sectors to decrease CH4 emissions, there is a need for continual review to guide research, policy, and adoption.
      This review stems from a comprehensive technical guidance document for the Food and Agriculture Organization of the United Nations (FAO) under the Livestock Environmental Assessment and Performance Partnership (FAO LEAP Partnership) program developed by an international group of scientists working on solutions for enteric CH4 mitigation. The main intents of this review are to analyze the current possibilities for implementing strategies for mitigating enteric CH4 emissions, establish research priorities for different production systems, and discuss biological, economical, regulatory, and societal barriers for adoption of each antimethanogenic strategy. We also discuss other antimethanogenic strategies being investigated that may have implications for future adoption. Whenever possible, we build on previous reviews and meta-analyses. Our review takes the approach of systematically discussing each mitigation option in terms of its mode of action, efficacy, potential for combining with other strategies, effects on other emissions of greenhouse gases (GHG), impact on animal productivity, safety, adoption potential, and further research needs.

      METHANE MITIGATION METRICS

      Several metrics must be considered when addressing the efficacy of a particular enteric CH4 mitigation strategy. Antimethanogenic strategies may decrease total CH4 production (absolute emissions, g/d), CH4 yield (g/kg of DMI), or CH4 intensity (g/kg of meat, milk, or wool produced). Methane mitigation can also be evaluated in terms of CH4 energy loss as a proportion of gross energy intake (GEI), a variable known as Ym, and as grams of CH4 produced per kilogram of digested OM. Methane yield, CH4 produced per kilogram of digested OM, and Ym are variables important in research for helping to understand how emissions are mitigated by a certain strategy, and potential effects on the animal's energy utilization efficiency. Expressing CH4 relative to DMI reveals how efficacious a mitigation strategy may be, independently of possible changes in feed intake, given that feed intake is the main factor driving CH4 production. Methane production per kilogram of digested OM further adjusts CH4 yield for the proportion of ingested feed actually digested and can reflect changes in the rumen fermentation profile. In turn, Ym provides a measure of how much extra ingested energy is potentially available for increasing animal production when methanogenesis is inhibited.
      Importantly, in some cases, CH4 may decrease when expressed as one metric but increase when expressed as another. Increasing feed intake by increasing forage digestibility or supplementing concentrates can decrease CH4 yield, but absolute CH4 emissions could remain unchanged or even increase if the animal's DMI increases. Strategies that improve animal performance and efficiency of production tend to decrease CH4 intensity because they dilute the feed energy associated with the individual animal or herd maintenance, and thus represent a desirable improvement in the efficiency of GHG emissions relative to food supply. However, a decrease in CH4 intensity may not decrease absolute emissions from a farm, sector, or area if individual feed intake or the number of animals, or both, increase to compensate for the decrease in CH4 intensity.

      INCREASED ANIMAL PRODUCTIVITY

      Intensification through improved feeding practices (quantity and quality of feed), animal management, improved animal health, breeding for greater productivity, and better reproductive performance results in greater individual animal production (
      • Capper J.L.
      • Cady R.A.
      • Bauman D.E.
      The environmental impact of dairy production: 1944 compared with 2007.
      ;
      • Beauchemin K.A.
      • Ungerfeld E.M.
      • Eckard R.J.
      • Wang M.
      Fifty years of research on rumen methanogenesis: Lessons learned and future challenges for mitigation.
      ). Animals that produce more generally eat more, and digest and ferment more feed in their rumens, thus producing more CH4. However, as the production of the individual animal increases, the emission of CH4 on an animal product basis (i.e., CH4 emissions intensity), decreases (
      • Gerber P.J.
      • Hristov A.N.
      • Henderson B.
      • Makkar H.
      • Oh J.
      • Lee C.
      • Meinen R.
      • Montes F.
      • Ott T.
      • Firkins J.
      • Rotz A.
      • Dell C.
      • Adesogan A.T.
      • Yang W.Z.
      • Tricarico J.M.
      • Kebreab E.
      • Waghorn G.
      • Dijkstra J.
      • Oosting S.
      Technical options for the mitigation of direct methane and nitrous oxide emissions from livestock: A review.
      ). This phenomenon largely occurs through the dilution of maintenance effect; as nutrient intake increases, the proportion of nutrients ingested used for maintenance functions decreases, leaving a greater proportion of ingested nutrients for animal production (
      • Capper J.L.
      • Cady R.A.
      • Bauman D.E.
      The environmental impact of dairy production: 1944 compared with 2007.
      ). Mitigation potential through intensification of animal production is greater in low-producing than in high-producing animal systems (
      • Gerber P.J.
      • Hristov A.N.
      • Henderson B.
      • Makkar H.
      • Oh J.
      • Lee C.
      • Meinen R.
      • Montes F.
      • Ott T.
      • Firkins J.
      • Rotz A.
      • Dell C.
      • Adesogan A.T.
      • Yang W.Z.
      • Tricarico J.M.
      • Kebreab E.
      • Waghorn G.
      • Dijkstra J.
      • Oosting S.
      Technical options for the mitigation of direct methane and nitrous oxide emissions from livestock: A review.
      ) and is attractive in low-income countries that need to produce greater quantities of nutritious foods (
      • Tricarico J.M.
      • Kebreab E.
      • Wattiaux M.A.
      MILK Symposium review: Sustainability of dairy production and consumption in low-income countries with emphasis on productivity and environmental impact.
      ).
      Importantly, intensification usually increases upstream emissions of carbon dioxide (CO2) and nitrous oxide (N2O) resulting from the production of animal feed or even from pasture management, and also increases manure emissions. Therefore, changes in the emissions of all GHG must be taken into account in a life cycle assessment (LCA) approach (
      • Beauchemin K.A.
      • Ungerfeld E.M.
      • Eckard R.J.
      • Wang M.
      Fifty years of research on rumen methanogenesis: Lessons learned and future challenges for mitigation.
      ).
      Intensification of animal production has resulted in large decreases in GHG emission intensity or carbon footprint [measured as kg of CO2 equivalents (CO2e) per kg of product] of animal products. For example, in the United States, milk production increased by 59% between 1944 and 2007 through improved animal productivity, leading to a decrease in milk carbon footprint by almost two-thirds (
      • Capper J.L.
      • Cady R.A.
      • Bauman D.E.
      The environmental impact of dairy production: 1944 compared with 2007.
      ).
      The relationship between carbon footprint and total GHG emissions is, however, variable. The change in total GHG emissions from ruminant production over time in a certain region, country, or globally depends on how the rate of decrease in carbon footprint relates to changes in individual animal production and numbers.
      • Ungerfeld E.M.
      • Beauchemin K.A.
      • Muñoz C.
      Current perspectives on achieving pronounced enteric methane mitigation from ruminant production.
      examined the evolution of GHG emissions, expressed as CO2e, of the dairy, beef, and lamb production industries in 9 countries or regions, contrasting yearly rates of change of CO2e per animal product with yearly rates of change of total emissions CO2e for each industry in each country or region. Examination of that historical evidence showed that intensification and lowering the carbon footprint decreased total emissions of GHG in 4 of those case studies but increased GHG emissions in 5 of them. Although intensification and increases in animal productivity ameliorate total GHG emissions relative to a nonintensification scenario, intensification alone is insufficient to mitigate total emissions of GHG to the levels required to maintain global temperature increase within 1.5°C by 2050, unless total emissions are capped (
      • Ungerfeld E.M.
      • Beauchemin K.A.
      • Muñoz C.
      Current perspectives on achieving pronounced enteric methane mitigation from ruminant production.
      ).
      Animal productivity can be increased without an increase in total GHG emissions if GHG intensity is reduced sufficiently to offset the increase in total animal product output. Additionally, selective breeding for improved feed efficiency without a decline in performance could result in animals utilizing less feed to produce the same amount of product (
      • Løvendahl P.
      • Difford G.F.
      • Li B.
      • Chagunda M.G.G.
      • Huhtanen P.
      • Lidauer M.H.
      • Lassen J.
      • Lund P.
      Review: Selecting for improved feed efficiency and reduced methane emissions in dairy cattle.
      ). Precision feeding, targeting nutrient requirements of each individual animal and thus decreasing feed input per unit of product, can be another tool to decrease CH4 intensity, particularly in less-efficient animals (
      • Fischer A.
      • Edouard N.
      • Faverdin P.
      Precision feed restriction improves feed and milk efficiencies and reduces methane emissions of less efficient lactating Holstein cows without impairing their performance.
      ). It is important to continue investigating the improvement of feed efficiency through genetic selection and precision feeding, because greater production efficiency can help decrease total emissions of GHG with equal or greater animal production (
      • Waghorn G.C.
      • Hegarty R.S.
      Lowering ruminant methane emissions through improved feed conversion efficiency.
      ;
      • Tricarico J.M.
      • de Haas Y.
      • Hristov A.N.
      • Kebreab E.
      • Kurt T.
      • Mitloehner F.
      • Pitta D.
      Symposium review: Development of a funding program to support research on enteric methane mitigation from ruminants.
      ). The balance in total emissions of GHG ultimately depends on the relative proportional changes in feed efficiency and total animal production.
      Improved animal productivity has been regarded as an attractive proposition for producers because it has the potential to increase economic margins of production (
      • Gerber P.J.
      • Hristov A.N.
      • Henderson B.
      • Makkar H.
      • Oh J.
      • Lee C.
      • Meinen R.
      • Montes F.
      • Ott T.
      • Firkins J.
      • Rotz A.
      • Dell C.
      • Adesogan A.T.
      • Yang W.Z.
      • Tricarico J.M.
      • Kebreab E.
      • Waghorn G.
      • Dijkstra J.
      • Oosting S.
      Technical options for the mitigation of direct methane and nitrous oxide emissions from livestock: A review.
      ). However, the appeal to producers and success of intensification as a means of improving animal production profitability are highly dependent on the ratios of product prices to cost of production, risk aversion, access to high-producing breeds, access to credit, education and entrepreneurship, size of the production unit, and availability of technology applicable to local conditions, among other factors.

      SELECTION OF LOW-METHANE-PRODUCING ANIMALS

      Individual differences in CH4 production exist among animals within the same herd and with the same feeding management (
      • de Haas Y.
      • Pszczola M.
      • Soyeurt H.
      • Wall E.
      • Lassen J.
      Invited review: Phenotypes to genetically reduce greenhouse gas emissions in dairying.
      ). Heritabilities of absolute CH4 production in cattle and sheep were moderate, and were higher than the heritability of CH4 yield in sheep (
      • Rowe S.J.
      • Hickey S.M.
      • Jonker A.
      • Hess M.K.
      • Janssen P.
      • Johnson T.
      • Bryson B.
      • Knowler K.
      • Pinares-Patino C.
      • Bain W.
      • Elmes S.
      • Young E.
      • Wing J.
      • Waller E.
      • Pickering N.
      • McEwan J.C.
      Selection for divergent methane yield in New Zealand sheep – a ten-year perspective.
      ) but lower in dairy cows (
      • Manzanilla-Pech C.I.V.
      • Løvendahl P.
      • Mansan Gordo D.
      • Gifford G.F.
      • Pryce G.E.
      • Schenkel F.
      • Wegmann S.
      • Miglior F.
      • Chud T.C.
      • Moate P.J.
      • Williams S.R.O.
      • Richardson C.M.
      • Stothard P.
      • Lassen J.
      Breeding for reduced methane emission and fee-efficient Holstein cows: An international response.
      ). As with any selected trait, gains in lowering CH4 production that are associated with host genetics are permanent and cumulative (
      • de Haas Y.
      • Veerkamp R.F.
      • de Jong G.
      • Aldridge M.N.
      Selective breeding as a mitigation tool for methane emissions from dairy cattle.
      ;
      • Manzanilla-Pech C.I.V.
      • Løvendahl P.
      • Mansan Gordo D.
      • Gifford G.F.
      • Pryce G.E.
      • Schenkel F.
      • Wegmann S.
      • Miglior F.
      • Chud T.C.
      • Moate P.J.
      • Williams S.R.O.
      • Richardson C.M.
      • Stothard P.
      • Lassen J.
      Breeding for reduced methane emission and fee-efficient Holstein cows: An international response.
      ).
      Possible associations with other traits of interest need to be considered. Selecting against total CH4 production selects against DMI and production traits (
      • Lassen J.
      • Løvendahl P.
      Heritability estimates for enteric methane emissions from Holstein cattle measured using noninvasive methods.
      ;
      • de Haas Y.
      • Pszczola M.
      • Soyeurt H.
      • Wall E.
      • Lassen J.
      Invited review: Phenotypes to genetically reduce greenhouse gas emissions in dairying.
      ;
      • Breider I.S.
      • Wall E.
      • Garnsworthy P.C.
      Short communication: Heritability of methane production and genetic correlations with milk yield and body weight in Holstein-Friesian dairy cows.
      ;
      • Manzanilla-Pech C.I.V.
      • Løvendahl P.
      • Mansan Gordo D.
      • Gifford G.F.
      • Pryce G.E.
      • Schenkel F.
      • Wegmann S.
      • Miglior F.
      • Chud T.C.
      • Moate P.J.
      • Williams S.R.O.
      • Richardson C.M.
      • Stothard P.
      • Lassen J.
      Breeding for reduced methane emission and fee-efficient Holstein cows: An international response.
      ). Problems caused by using ratios of correlated variables as selection criteria (i.e., CH4 yield or intensity) as a means of overcoming positive correlations between CH4 production and performance have been discussed (
      • de Haas Y.
      • Pszczola M.
      • Soyeurt H.
      • Wall E.
      • Lassen J.
      Invited review: Phenotypes to genetically reduce greenhouse gas emissions in dairying.
      ;
      • Løvendahl P.
      • Difford G.F.
      • Li B.
      • Chagunda M.G.G.
      • Huhtanen P.
      • Lidauer M.H.
      • Lassen J.
      • Lund P.
      Review: Selecting for improved feed efficiency and reduced methane emissions in dairy cattle.
      ;
      • Breider I.S.
      • Wall E.
      • Garnsworthy P.C.
      Short communication: Heritability of methane production and genetic correlations with milk yield and body weight in Holstein-Friesian dairy cows.
      ;
      • Manzanilla-Pech C.I.V.
      • Løvendahl P.
      • Mansan Gordo D.
      • Gifford G.F.
      • Pryce G.E.
      • Schenkel F.
      • Wegmann S.
      • Miglior F.
      • Chud T.C.
      • Moate P.J.
      • Williams S.R.O.
      • Richardson C.M.
      • Stothard P.
      • Lassen J.
      Breeding for reduced methane emission and fee-efficient Holstein cows: An international response.
      ). However, a program selecting for ewes divergent in CH4 yield, obtaining a 12% difference between divergent lines after 10 yr of selection, was also successful in selecting more productive animals: ewes with low CH4 yield weaned heavier and leaner lambs that produced more wool (
      • Rowe S.J.
      • Hickey S.M.
      • Jonker A.
      • Hess M.K.
      • Janssen P.
      • Johnson T.
      • Bryson B.
      • Knowler K.
      • Pinares-Patino C.
      • Bain W.
      • Elmes S.
      • Young E.
      • Wing J.
      • Waller E.
      • Pickering N.
      • McEwan J.C.
      Selection for divergent methane yield in New Zealand sheep – a ten-year perspective.
      ).
      Another option for selection is residual CH4 production, calculated as the residual of the regression of observed CH4 production against variables such as DMI, daily body mass gain (ADG), milk production, and body mass. Working with 15,320 dairy cow records of CH4 production and performance from 2,990 dairy cows from Canada, Australia, Switzerland, and Denmark,
      • Manzanilla-Pech C.I.V.
      • Løvendahl P.
      • Mansan Gordo D.
      • Gifford G.F.
      • Pryce G.E.
      • Schenkel F.
      • Wegmann S.
      • Miglior F.
      • Chud T.C.
      • Moate P.J.
      • Williams S.R.O.
      • Richardson C.M.
      • Stothard P.
      • Lassen J.
      Breeding for reduced methane emission and fee-efficient Holstein cows: An international response.
      found that residual CH4 metrics were more suited for inclusion in selection indices than CH4 production metrics. Inclusion of CH4 production in a selection index will likely require establishment of a price on CH4. If the price of CH4 is too low, the CH4 production weight in the index will be equally low and it might not be worth implementing (
      • Beauchemin K.A.
      • Ungerfeld E.M.
      • Eckard R.J.
      • Wang M.
      Fifty years of research on rumen methanogenesis: Lessons learned and future challenges for mitigation.
      ;
      • de Haas Y.
      • Veerkamp R.F.
      • de Jong G.
      • Aldridge M.N.
      Selective breeding as a mitigation tool for methane emissions from dairy cattle.
      ).
      The association between enteric CH4 emissions and feed efficiency needs to be clearly established. On the one hand, low-CH4-producing animals should theoretically have a better conversion of digestible to metabolizable energy; however, their lower rumen retention times may result in lower digestibility (
      • Løvendahl P.
      • Difford G.F.
      • Li B.
      • Chagunda M.G.G.
      • Huhtanen P.
      • Lidauer M.H.
      • Lassen J.
      • Lund P.
      Review: Selecting for improved feed efficiency and reduced methane emissions in dairy cattle.
      ). Studies comparing animals selected for low and high production efficiency have yielded differing results with respect to CH4 production.
      • Fitzsimons C.
      • Kenny D.A.
      • Deighton M.H.
      • Fahey A.G.
      • McGee M.
      Methane emissions, body composition and rumen fermentation traits of beef heifers differing in residual feed intake.
      observed that heifers with low residual feed intake (i.e., more efficient animals) emitted less total CH4 and CH4 per kilogram of metabolic body mass than those with high residual feed intake (i.e., less efficient animals).
      • Freetly H.C.
      • Brown-Brandl T.M.
      Enteric methane production from beef cattle that vary in feed efficiency.
      reported that feed efficiency expressed either as the ADG:DMI ratio or residual feed intake was unrelated to CH4 production in steers, whereas the ADG:DMI ratio was loosely but positively related to CH4 production in heifers.
      • Arndt C.
      • Powell J.M.
      • Aguerre M.J.
      • Crump P.M.
      • Wattiaux M.A.
      Feed conversion efficiency in dairy cows: Repeatability, variation in digestion and metabolism of energy and nitrogen, and ruminal methanogens.
      observed that highly efficient dairy cows lost less energy as CH4 as a proportion of GEI compared with their low-efficiency counterparts.
      • McDonnell R.P.
      • Hart K.J.
      • Boland T.M.
      • Kelly A.K.
      • McGee M.
      • Kenny D.A.
      Effect of divergence in phenotypical residual feed intake on methane emissions, ruminal fermentation, and apparent whole-tract digestibility of beef heifers across three contrasting diets.
      did not find associations between residual feed intake and total CH4 production in heifers, but more efficient animals lost a greater proportion of GEI as CH4.
      • Velazco J.I.
      • Herd R.M.
      • Cottle D.J.
      • Hegarty R.S.
      Daily methane emissions and emissions intensity of grazing beef cattle genetically divergent for residual feed intake.
      reported a negative association between residual feed intake and CH4 production.
      • Olijhoek D.W.
      • Løvendahl P.
      • Lassen J.
      • Hellwing A.L.F.
      • Höglund J.K.
      • Weisbjerg M.R.
      • Noel S.J.
      • McLean F.
      • Højberg O.
      • Lund P.
      Methane production, rumen fermentation, and diet digestibility of Holstein and Jersey dairy cows being divergent in residual feed intake and fed at 2 forage-to-concentrate ratios.
      ,
      • Flay H.E.
      • Kuhn-Sherlock B.
      • Macdonald K.A.
      • Camara M.
      • Lopez-Villalobos N.
      • Donaghy D.J.
      • Roche J.R.
      Hot topic: Selecting cattle for residual feed intake did not affect daily methane production but increased methane yield.
      , and
      • Renand G.
      • Vinet A.
      • Decruyenaere V.
      • Maupetit D.
      • Dozias D.
      Methane and carbon dioxide emission of beef heifers in relation with growth and feed efficiency.
      did not find associations between residual feed intake and total CH4 production or intensity, CH4 production per kilogram of body mass, or CH4 as a proportion of GEI. Importantly, low residual feed intake animals have consistently demonstrated greater CH4 yield because of their decreased DMI (
      • Fitzsimons C.
      • Kenny D.A.
      • Deighton M.H.
      • Fahey A.G.
      • McGee M.
      Methane emissions, body composition and rumen fermentation traits of beef heifers differing in residual feed intake.
      ;
      • McDonnell R.P.
      • Hart K.J.
      • Boland T.M.
      • Kelly A.K.
      • McGee M.
      • Kenny D.A.
      Effect of divergence in phenotypical residual feed intake on methane emissions, ruminal fermentation, and apparent whole-tract digestibility of beef heifers across three contrasting diets.
      ;
      • Olijhoek D.W.
      • Løvendahl P.
      • Lassen J.
      • Hellwing A.L.F.
      • Höglund J.K.
      • Weisbjerg M.R.
      • Noel S.J.
      • McLean F.
      • Højberg O.
      • Lund P.
      Methane production, rumen fermentation, and diet digestibility of Holstein and Jersey dairy cows being divergent in residual feed intake and fed at 2 forage-to-concentrate ratios.
      ;
      • Flay H.E.
      • Kuhn-Sherlock B.
      • Macdonald K.A.
      • Camara M.
      • Lopez-Villalobos N.
      • Donaghy D.J.
      • Roche J.R.
      Hot topic: Selecting cattle for residual feed intake did not affect daily methane production but increased methane yield.
      ), which indicates that residual feed intake is more influenced by DMI than by CH4 production. The latter aspect is the result of feed efficiency being a complex trait influenced by numerous aspects of digestive and metabolic efficiency apart from energy losses as CH4 (
      • Herd R.M.
      • Oddy V.H.
      • Richardson E.C.
      Biological basis for variation in residual feed intake in beef cattle. 1. Review of potential mechanisms.
      ;
      • Richardson E.C.
      • Herd R.M.
      Biological basis for variation in residual feed intake in cattle. 2. Synthesis of results following divergent selection.
      ).
      It should be noted that in the studies discussed above, animals were selected based on feed efficiency or residual feed intake, and CH4 production was compared in high- and low-efficiency animals; more studies are needed comparing feed efficiency and animal performance of animal lines selected for high and low CH4 production.
      • Manzanilla-Pech C.I.V.
      • Løvendahl P.
      • Mansan Gordo D.
      • Gifford G.F.
      • Pryce G.E.
      • Schenkel F.
      • Wegmann S.
      • Miglior F.
      • Chud T.C.
      • Moate P.J.
      • Williams S.R.O.
      • Richardson C.M.
      • Stothard P.
      • Lassen J.
      Breeding for reduced methane emission and fee-efficient Holstein cows: An international response.
      found positive correlations between residual CH4 and residual feed intake in a multi-country data set of records from cows that had not been selected by either trait, implying that those animals that produced less CH4 converted feed to milk more efficiently. As for any other trait subjected to genetic selection, the possible existence of genotype by environment interactions should be considered when anticipating how a genotype selected for low CH4 production would perform in other countries or regions, or in a different production system, or with different diets and rumen microbiota.
      One of the main challenges in selecting animals with low CH4 production is measuring CH4 of a large number of animals in commercial farms, which is not within the reach of most commercial breeders (
      • de Haas Y.
      • Veerkamp R.F.
      • de Jong G.
      • Aldridge M.N.
      Selective breeding as a mitigation tool for methane emissions from dairy cattle.
      ). Sniffers to measure CH4 concentration in the exhaled air at a feeder or during milking have been used with some success (
      • Difford G.F.
      • Olijhoek D.W.
      • Hellwing A.L.F.
      • Lund P.
      • Bjerring M.A.
      • de Haas Y.
      • Lassen J.
      • Løvendahl P.
      Ranking cows' methane emissions under commercial conditions with sniffers versus respiration chambers.
      ). Methane production needs to be measured for weeks at a time, and a genetic selection program requires thousands of measurements (
      • de Haas Y.
      • Pszczola M.
      • Soyeurt H.
      • Wall E.
      • Lassen J.
      Invited review: Phenotypes to genetically reduce greenhouse gas emissions in dairying.
      ;
      • Løvendahl P.
      • Difford G.F.
      • Li B.
      • Chagunda M.G.G.
      • Huhtanen P.
      • Lidauer M.H.
      • Lassen J.
      • Lund P.
      Review: Selecting for improved feed efficiency and reduced methane emissions in dairy cattle.
      ), although measurements of CH4 production in sires can potentially accelerate the spread of genetic progress. Proxies of CH4 production, such as feed intake and feeding behavior, rumen VFA concentration, composition of the microbial community, and membrane lipids of methanogens in feces, have all been investigated as alternatives to direct measurement (
      • Beauchemin K.A.
      • Ungerfeld E.M.
      • Eckard R.J.
      • Wang M.
      Fifty years of research on rumen methanogenesis: Lessons learned and future challenges for mitigation.
      ). Milk fatty acid composition estimated through mid-infrared spectroscopy initially showed good results at an experimental scale, but to a lesser extent under commercial conditions with more animals (
      • Løvendahl P.
      • Difford G.F.
      • Li B.
      • Chagunda M.G.G.
      • Huhtanen P.
      • Lidauer M.H.
      • Lassen J.
      • Lund P.
      Review: Selecting for improved feed efficiency and reduced methane emissions in dairy cattle.
      ). Development of biomarkers to estimate CH4 production reliably and that are sufficiently practical to implement at the farm scale is an area of considerable interest (
      • Beauchemin K.A.
      • Ungerfeld E.M.
      • Eckard R.J.
      • Wang M.
      Fifty years of research on rumen methanogenesis: Lessons learned and future challenges for mitigation.
      ). The use of genomic selection toward low CH4 production has also been proposed (
      • de Haas Y.
      • Pszczola M.
      • Soyeurt H.
      • Wall E.
      • Lassen J.
      Invited review: Phenotypes to genetically reduce greenhouse gas emissions in dairying.
      ).
      Animal breeding is one of the few antimethanogenic strategies that can be applied to extensive production systems where animals are not supplemented. An additional advantage of this approach is that no major effects on other upstream or downstream emissions of GHG are expected. The greatest challenges to the selection of low-CH4-producing animals are the possible existence of undesirable associations between CH4 production and animal productivity and developing reliable and practical proxies for predicting CH4 production applicable to large numbers of animals.

      DIET REFORMULATION

      Levels of Feed and Concentrate Intake, Source, and Processing

      This section discusses dietary strategies based on their direct effects on CH4 production through rumen digestion and fermentation; effects occurring through an increase in productivity were discussed in a previous section (“Increased Animal Productivity”). Increasing feed intake of ruminants decreases retention time of feed in the rumen due to faster passage rates. Shorter retention time limits microbial access to OM, thus reducing the extent of rumen fermentation and leading to a decline in CH4 yield and Ym, although total CH4 emissions increase as more feed is ingested and digested (
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      ). Increasing the proportion of concentrate in the diet increases dietary energy density, decreases the proportion of structural carbohydrates, increases rumen outflow rate, and lowers rumen pH, decreasing CH4 production per unit DMI and of feed fermented (
      • Janssen P.H.
      Influence of hydrogen on rumen methane formation and fermentation balances through microbial growth kinetics and fermentation thermodynamics.
      ). Processing of grains and feeding concentrates with rapidly fermentable starch promotes starch fermentation in the rumen and increases propionate production, which serves as a sink of metabolic hydrogen alternative to methanogenesis (
      • Janssen P.H.
      Influence of hydrogen on rumen methane formation and fermentation balances through microbial growth kinetics and fermentation thermodynamics.
      ;
      • Ungerfeld E.M.
      Shifts in metabolic hydrogen sinks in the methanogenesis-inhibited ruminal fermentation: Ameta-analysis.
      ). Rapid fermentation rate of grains also lowers rumen pH and inhibits the growth of protozoa (
      • Janssen P.H.
      Influence of hydrogen on rumen methane formation and fermentation balances through microbial growth kinetics and fermentation thermodynamics.
      ), thereby reducing the role of protozoa in protecting methanogens from oxygen toxicity and decreasing the supply of H2 for methanogenesis (
      • Newbold C.J.
      • De La Fuente G.
      • Belanche A.
      • Ramos-Morales E.
      • McEwan N.R.
      The role of ciliate protozoa in the rumen.
      ).
      The efficacy of increasing levels of concentrate is variable. Based on an intercontinental database for beef cattle,
      • van Lingen H.J.
      • Niu M.
      • Kebreab E.
      • Valadares Filho S.C.
      • Rooke J.A.
      • Duthie C.-A.
      • Schwarm A.
      • Kreuzer M.
      • Hynd P.I.
      • Caetano M.
      • Eugène M.
      • Martin C.
      • McGee M.
      • O'Kiely P.
      • Hünerberg M.
      • McAllister T.A.
      • Berchielli T.T.
      • Messana J.D.
      • Peiren N.
      • Chaves A.V.
      • Charmley E.
      • Cole N.A.
      • Hales K.E.
      • Lee S.-S.
      • Berndt A.
      • Reynolds C.K.
      • Crompton L.A.
      • Bayat A.-R.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      • Bannink A.
      • Dijkstra J.
      • Casper D.P.
      • Hristov A.N.
      Prediction of enteric methane production, yield and intensity of beef cattle using an intercontinental database.
      reported a CH4 yield of 20.7 g/kg of DM (range: 6.29 to 35.1 g/kg of DM) and a Ym of 6.3% (1.9 to 10.4%) for high-forage (≥25%) diets compared with a CH4 yield of 15.2 g/kg of DM (7.50 to 30.9 g/kg of DM) and Ym of 4.5% (2.3 to 8.7%) for low-forage (≤18%) diets. The meta-analysis by
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      reported a decrease in total CH4 as well as CH4 yield and intensity, without an increase in total CH4 production through decreasing the forage-to-concentrate ratio. In terms of grain sources, absolute CH4 production and CH4 yield appear to follow the order wheat and steam-flaked corn < corn < barley, with the ranking highly dependent on the composition and extent of processing of the grain (
      • Beauchemin K.A.
      • McGinn S.M.
      Methane emissions from feedlot cattle fed barley or corn diets.
      ;
      • Moate P.
      • Williams S.
      • Jacobs J.
      • Hannah M.
      • Beauchemin K.
      • Eckard R.
      • Wales W.
      Wheat is more potent than corn or barley for dietary mitigation of enteric methane emissions from dairy cows.
      ,
      • Moate P.
      • Williams S.
      • Deighton M.
      • Hannah M.
      • Ribaux B.
      • Morris G.
      • Jacobs J.
      • Hill J.
      • Wales W.
      Effects of feeding wheat or corn and of rumen fistulation on milk production and methane emissions of dairy cows.
      ). In their meta-analysis,
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      found greater decreases in total CH4 with barley than with corn, with differing results for CH4 intensity of growth and milk production. Grain processing methods (application of various combinations of heat, moisture, time, and mechanical actions) can modify the fermentation of starch and rumen pH. Compared with a dry-rolled corn-based diet, feeding a steam-flaked corn-based diet to steers reduced CH4 yield by 17% (
      • Hales K.E.
      • Cole N.
      • MacDonald J.
      Effects of corn processing method and dietary inclusion of wet distillers grains with solubles on energy metabolism, carbon−nitrogen balance, and methane emissions of cattle.
      ).
      Some experiments evaluating concentrate supplementation of grazing animals have shown a decrease in CH4 yield and intensity (
      • Jiao H.P.
      • Dale A.J.
      • Carson A.F.
      • Murray S.
      • Gordon A.W.
      • Ferris C.P.
      Effect of concentrate feed level on methane emissions from grazing dairy cows.
      ), although most reported no change in any CH4 production metric (
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      ;
      • Vargas J.
      • Ungerfeld E.
      • Muñoz C.
      • DiLorenzo N.
      Feeding strategies to mitigate enteric methane emission from ruminants in grassland systems.
      ). The discrepancies for pasture studies may be attributed to the substitution rate (concentrate vs. pasture), pasture quality, or differences in methodology to estimate DMI. Increasing concentrate intake can be easily combined with other mitigation strategies. Several studies have shown additive effects of concentrate and oil inclusion on mitigating total CH4 emissions and emission intensity (e.g.,
      • Bayat A.R.
      • Ventto L.
      • Kairenius P.
      • Stefański T.
      • Leskinen H.
      • Tapio I.
      • Negussie E.
      • Vilkki J.
      • Shingfield K.J.
      Dietary forage to concentrate ratio and sunflower oil supplement alter rumen fermentation, ruminal methane emissions, and nutrient utilization in lactating cows.
      ). Methanogenesis inhibitors such as 3-nitrooxypropanol (3-NOP) show synergy with concentrates, whereby the mitigation potential of inhibitors is increased in high-concentrate diets (
      • Schilde M.
      • von Soosten D.
      • Hüther L.
      • Meyer U.
      • Zeyner A.
      • Dänicke S.
      Effects of 3-nitrooxypropanol and varying concentrate feed proportions in the ration on methane emission, rumen fermentation and performance of periparturient dairy cows.
      ).
      Increased feed intake and use of grain and grain processing are accompanied by increased emissions of CO2 and N2O from the use of fossil fuels and nitrogen (N) fertilizers used during feed production and manufacture (
      • Beauchemin K.A.
      • McGinn S.
      • Benchaar C.
      • Holtshausen L.
      Crushed sunflower, flax, or canola seeds in lactating dairy cow diets: Effects on methane production, rumen fermentation, and milk production.
      ). Furthermore, conversion of pastureland to cropland results in the loss of soil carbon. Changes in digestibility of nutrients can alter manure amount and composition (
      • Beauchemin K.A.
      • McGinn S.M.
      Methane emissions from feedlot cattle fed barley or corn diets.
      ;
      • Hristov A.N.
      • Oh J.
      • Firkins J.L.
      • Dijkstra J.
      • Kebreab E.
      • Waghorn G.
      • Makkar H.P.
      • Adesogan A.T.
      • Yang W.
      • Lee C.
      • Gerber P.J.
      • Henderson B.
      • Tricarico J.M.
      Mitigation of methane and nitrous oxide emissions from animal operations: I. A review of enteric methane mitigation options.
      ,
      • Hristov A.N.
      • Ott T.
      • Tricarico J.
      • Rotz A.
      • Waghorn G.
      • Adesogan A.
      • Dijkstra J.
      • Montes F.
      • Oh J.
      • Kebreab E.
      • Oosting S.J.
      • Gerber P.J.
      • Henderson B.
      • Makkar H.P.S.
      • Firkins J.L.
      Special Topics—Mitigation of methane and nitrous oxide emissions from animal operations: III. A review of animal management mitigation options.
      ) and CH4, ammonia, and N2O emissions from manure.
      Greater concentrate intake increases feed costs and can cause clinical and subclinical acidosis (
      • Hristov A.N.
      • Melgar A.
      • Wasson D.
      • Arndt C.
      Symposium review: Effective nutritional strategies to mitigate enteric methane in dairy cattle.
      ). Milk protein and fat concentrations might also decrease, particularly when feeding wheat- or oat-based diets compared with corn- or barley-based diets (
      • Moate P.
      • Williams S.
      • Deighton M.
      • Hannah M.
      • Ribaux B.
      • Morris G.
      • Jacobs J.
      • Hill J.
      • Wales W.
      Effects of feeding wheat or corn and of rumen fistulation on milk production and methane emissions of dairy cows.
      ). A decrease in milk components could reduce the profitability for dairy producers. Increased feeding of concentrates is easily adoptable in production systems in which intensification is possible. However, substantial increases in concentrate use would be difficult, or even impossible, to implement in many areas of the world where cereal crops cannot be grown or are too expensive (
      • Beauchemin K.A.
      • McGinn S.
      • Benchaar C.
      • Holtshausen L.
      Crushed sunflower, flax, or canola seeds in lactating dairy cow diets: Effects on methane production, rumen fermentation, and milk production.
      ). Such production practices would also be associated with substantial land use change. Furthermore, feeding ruminants grains that could be directly consumed by humans can be regarded as an inefficient practice that does not take advantage of the ability of ruminants to convert fibrous feeds unsuitable for humans into useful products (
      • Beauchemin K.A.
      • Ungerfeld E.M.
      • Eckard R.J.
      • Wang M.
      Fifty years of research on rumen methanogenesis: Lessons learned and future challenges for mitigation.
      ). More research is required to characterize how grain source and processing method influence enteric CH4 emissions, and to identify the appropriate ration formulations with cereal-based diets that counteract the negative effect on milk fat while retaining a CH4 mitigation effect. Further research should also focus on evaluating total GHG emissions using an LCA for individual farms and geographical regions (
      • Beauchemin K.
      • Kreuzer M.O.
      • O'Mara F.
      • Mcallister T.
      Nutritional management for enteric methane abatement: A review.
      ).

      Lipid Supplementation

      Dietary lipids elicit their CH4-mitigating effect through several mechanisms, including toxicity against methanogens and protozoa; biohydrogenation of UFA serving as a minor sink of metabolic hydrogen; and shifting rumen fermentation to promote the production of propionate, resulting in lower CH4 production (
      • Newbold C.J.
      • De La Fuente G.
      • Belanche A.
      • Ramos-Morales E.
      • McEwan N.R.
      The role of ciliate protozoa in the rumen.
      ). Also, as they are largely unfermentable (except for the glycerol moiety), the replacement of carbohydrates with lipids contributes to decreased enteric CH4 emissions.
      Supplementation of dietary lipids is an effective CH4 mitigation strategy, although efficacy depends on the form, source, and amount of supplemental fat; degree of saturation; fatty acid carbon chain lengths; and nutrient and fatty acid composition of the basal diet (
      • Patra A.K.
      The effect of dietary fats on methane emissions, and its other effects on digestibility, rumen fermentation and lactation performance in cattle: A meta-analysis.
      ). Various meta-analyses have been conducted to elucidate the CH4-mitigating effect of dietary lipids in ruminants (
      • Beauchemin K.
      • Kreuzer M.O.
      • O'Mara F.
      • Mcallister T.
      Nutritional management for enteric methane abatement: A review.
      ;
      • Patra A.K.
      The effect of dietary fats on methane emissions, and its other effects on digestibility, rumen fermentation and lactation performance in cattle: A meta-analysis.
      ;
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      ). The antimethanogenic effects of dietary lipids vary considerably over a broad range of conditions. The effects vary from a decrease in CH4 yield of 5.6% (
      • Beauchemin K.
      • Kreuzer M.O.
      • O'Mara F.
      • Mcallister T.
      Nutritional management for enteric methane abatement: A review.
      ) to between 3.8 and 4.3% per 10 g/kg (DM) of supplemental fat depending on source (
      • Patra A.K.
      The effect of dietary fats on methane emissions, and its other effects on digestibility, rumen fermentation and lactation performance in cattle: A meta-analysis.
      ,
      • Patra A.K.
      A meta-analysis of the effect of dietary fat on enteric methane production, digestibility and rumen fermentation in sheep, and a comparison of these responses between cattle and sheep.
      ). Medium-chain fatty acids such as myristic acid and PUFA in fish, sunflower, linseed, and canola oils are the most effective fatty acids for reducing CH4 emissions (e.g.,
      • Grainger C.
      • Williams R.
      • Clarke T.
      • Wright A.-D.
      • Eckard R.
      Supplementation with whole cottonseed causes long-term reduction of methane emissions from lactating dairy cows offered a forage and cereal grain diet.
      ). Supplementation with lipids was more effective, although effectiveness was more variable, for sheep than for cattle in the meta-analysis by
      • Grainger C.
      • Beauchemin K.A.
      Can enteric methane emissions from ruminants be lowered without lowering their production?.
      . However, it should be noted that different lipid sources were used in the sheep and cattle studies, which could have influenced the responses of each species to lipid supplementation.
      Breaking the hull of oilseeds through grinding, crushing, or rolling them before feeding ensures availability of the lipids in the rumen. Oils are typically more effective than crushed oilseeds (
      • Beauchemin K.
      • Kreuzer M.O.
      • O'Mara F.
      • Mcallister T.
      Nutritional management for enteric methane abatement: A review.
      ), although this comparison depends on the extent of processing of the oilseeds. In a meta-analysis,
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      showed that feeding oils or fats versus oilseeds had comparable mitigation effects on total CH4 production (−20 and −20%), CH4 yield (−15 and −14%), and CH4 intensity for milk production (−12 and −12%). However, feeding oilseeds had no effect on CH4 intensity for ADG, whereas supplemental oils and fats reduced CH4 intensity of ADG by 22% (
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      ). Few studies have examined the long-term effects of dietary lipids on CH4 emissions. Indications are that they have persistent antimethanogenic effects (
      • Jordan E.
      • Lovett D.
      • Monahan F.
      • Callan J.
      • Flynn B.
      • O'Mara F.
      Effect of refined coconut oil or copra meal on methane output and on intake and performance of beef heifers.
      ;
      • Grainger C.
      • Williams R.
      • Clarke T.
      • Wright A.-D.
      • Eckard R.
      Supplementation with whole cottonseed causes long-term reduction of methane emissions from lactating dairy cows offered a forage and cereal grain diet.
      ), although a recent study with grazing dairy cows found transient effects of oilseed supplementation (
      • Muñoz C.
      • Villalobos R.
      • Peralta A.M.T.
      • Morales M.
      • Urrutia N.L.
      • Ungerfeld E.M.
      Long-term and carryover effects of whole oilseeds on methane emission, milk production, and milk fatty acid profile of grazing dairy cows.
      ). The inhibitory effect of dietary lipids on CH4 emissions is greater with concentrate-based diets than with forage-based diets (
      • Patra A.K.
      The effect of dietary fats on methane emissions, and its other effects on digestibility, rumen fermentation and lactation performance in cattle: A meta-analysis.
      ), possibly because of the lower rumen pH associated with concentrate-based diets, which enhances the inhibitory effect of fatty acids on methanogens (
      • Zhou X.
      • Zeitz J.
      • Meile L.
      • Kreuzer M.
      • Schwarm A.
      Influence of pH and the degree of protonation on the inhibitory effect of fatty acids in the ruminal methanogen Methanobrevibacter ruminantium strain M1.
      ).
      Combinations of dietary lipids with other mitigation strategies have been investigated. An additive effect of dietary lipids on CH4 abatement was confirmed when canola oil was combined with 3-NOP (
      • Zhang X.M.
      • Smith M.L.
      • Gruninger R.J.
      • Kung Jr., L.
      • Vyas D.
      • McGinn S.M.
      • Kindermann M.
      • Wang M.
      • Tan Z.L.
      • Beauchemin K.A.
      Combined effects of 3-nitrooxypropanol and canola oil supplementation on methane emissions, rumen fermentation and biohydrogenation, and total tract digestibility in beef cattle.
      ) and when linseed oil was combined with nitrate (
      • Guyader J.
      • Eugène M.
      • Meunier B.
      • Doreau M.
      • Morgavi D.P.
      • Silberberg M.
      • Rochette Y.
      • Gerard C.
      • Loncke C.
      • Martin C.
      Additive methane-mitigating effect between linseed oil and nitrate fed to cattle.
      ). However, there was no additive effect when soybean oil was combined with extracts rich in tannins (
      • Lima P.R.
      • Apdini T.
      • Freire A.S.
      • Santana A.S.
      • Moura L.M.L.
      • Nascimento J.C.S.
      • Rodrigues R.T.S.
      • Dijkstra J.
      • Garcez Neto A.F.
      • Queiroz M.A.Á.
      • Menezes D.R.
      Dietary supplementation with tannin and soybean oil on intake, digestibility, feeding behavior, ruminal protozoa and methane emission in sheep.
      ) or saponins (
      • Mao H.L.
      • Wang J.K.
      • Zhou Y.Y.
      • Liu J.X.
      Effects of addition of tea saponins and soybean oil on methane production, fermentation and microbial population in the rumen of growing lambs.
      ).
      Feeding a high concentration of lipids can decrease feed and fiber digestibility (
      • Patra A.K.
      The effect of dietary fats on methane emissions, and its other effects on digestibility, rumen fermentation and lactation performance in cattle: A meta-analysis.
      ,
      • Patra A.K.
      A meta-analysis of the effect of dietary fat on enteric methane production, digestibility and rumen fermentation in sheep, and a comparison of these responses between cattle and sheep.
      ;
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      ), which might increase the excretion of OM and nutrients and emissions of CH4 from manure (
      • Hassanat F.
      • Benchaar C.
      Methane emissions of manure from dairy cows fed red clover-or corn silage-based diets supplemented with linseed oil.
      ), although this may not occur with levels of total dietary lipid supplementation <6% DM. Supplementing fats leads to an increase in feed emissions associated with the cultivation and transportation of refined oils or of whole or crushed oilseeds. Supplementing fats at 4 to 6% of dietary DM (total dietary fat of 6 to 8% maximum) can improve milk production but feeding higher concentrations of fats can have detrimental effects on rumen fermentation, feed digestion, and animal performance (
      • Patra A.K.
      The effect of dietary fats on methane emissions, and its other effects on digestibility, rumen fermentation and lactation performance in cattle: A meta-analysis.
      ,
      • Patra A.K.
      A meta-analysis of the effect of dietary fat on enteric methane production, digestibility and rumen fermentation in sheep, and a comparison of these responses between cattle and sheep.
      ). The meta-analysis conducted by
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      showed that feeding oils and fats decreased DMI (by 6%) and fiber digestibility (by 4%) but had no effect on milk production or ADG. However, feeding oilseeds did not affect DMI but decreased digestibility (by 8%) and ADG (by 13%), with no effect on milk production (
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      ). Supplementing dietary lipids rich in long-chain UFA can improve the nutritional quality of meat or milk by increasing the content of healthful fatty acids including PUFA, CLA, and vaccenic acid (
      • Bayat A.R.
      • Kairenius P.
      • Stefański T.
      • Leskinen H.
      • Comtet-Marre S.
      • Forano E.
      • Chaucheyras-Durand F.
      • Shingfield K.
      Effect of camelina oil or live yeasts (Saccharomyces cerevisiae) on ruminal methane production, rumen fermentation, and milk fatty acid composition in lactating cows fed grass silage diets.
      ). However, high dietary UFA may decrease milk fat production, especially when diets contain high concentrate or rumen pH is low (
      • Bougouin A.
      • Martin C.
      • Doreau M.
      • Ferlay A.
      Effects of starch-rich or lipid-supplemented diets that induce milk fat depression on rumen biohydrogenation of fatty acids and methanogenesis in lactating dairy cows.
      ;
      • Sun Y.
      • Allen M.S.
      • Lock A.L.
      Culture pH interacts with corn oil concentration to affect biohydrogenation of unsaturated fatty acids and disappearance of neutral detergent fiber in batch culture.
      ).
      Lipid supplementation is not known to pose a risk to the safety of animals and humans; it is readily available and can be easily implemented in intensive or confined feeding systems. Feeding refined oils can be costly, and they often do not fit into least-cost ration formulations. Alternatively, processed oilseeds can be less expensive and might stimulate the adoption of supplementing dietary lipids. Although limited options exist to apply this strategy in grazing systems, there have been promising efforts to breed grasses with high levels of fats rich in PUFA (
      • Winichayakul S.
      • Cookson R.
      • Scott R.
      • Zhou J.
      • Zou X.
      • Roldan M.
      • Richardson K.
      • Roberts N.
      Delivery of grasses with high levels of unsaturated, protected fatty acids.
      ) or providing supplemental fat through drinking water (
      • Osborne V.R.
      • Radhakrishnan S.
      • Odongo N.
      • Hill A.
      • McBride B.
      Effects of supplementing fish oil in the drinking water of dairy cows on production performance and milk fatty acid composition.
      ).
      Further research is needed to identify cost-effective and sustainable fat sources and their respective supplemental level that would reduce CH4 emissions without impairing feed digestibility and animal production. Studies are also required to ascertain the long-term effect of supplemental fats in suppressing CH4 emission. Considering the potential impact on feed emissions and nutrient excretion, the effectiveness of this mitigation strategy needs to be addressed using LCA.

      FORAGES

      Pastures and forage crops comprise 26% of the land and 70% of agricultural area globally (
      • FAO
      FAOSTAT Database.
      ) and are the main component of ruminant livestock diets. The unique digestive system of ruminants allows them to produce high-quality protein in the form of meat and milk from forages, avoiding direct competition for grain that can be used as human food. However, intake of cellulosic material augments enteric CH4 emissions, with substantial variation due to forage source, chemical composition, digestibility, forage preservation, grazing management, and other factors. This variation creates opportunities for CH4 mitigation through forage management. Forage production systems are highly variable and dependent upon farm site conditions (e.g., soil type and fertility, water, climate) and management practices. These factors affect forage yield and nutritive value, carbon storage in soils, animal performance, manure excretion and, ultimately, GHG emissions. Therefore, in all cases, a change in forage management to decrease enteric CH4 emissions needs to be assessed using regionally specific farm-level LCA that account for changes in forage and animal productivity, as well as emissions and sinks from all components of the farming system, including soil carbon.

      Digestibility

      Increasing forage digestibility usually increases DMI and improves animal performance, which decreases CH4 yield and intensity. Digestibility of forages conserved as hay or silage can be maximized by harvesting at a vegetative stage; in pastoral systems, digestibility can be enhanced by optimizing grazing management to decrease forage maturity (e.g., adjusting stocking rates, ensuring pregrazing herbal mass is not excessive;
      • Vargas J.
      • Ungerfeld E.
      • Muñoz C.
      • DiLorenzo N.
      Feeding strategies to mitigate enteric methane emission from ruminants in grassland systems.
      ). Although CH4 intensity decreases, absolute CH4 production due to increased forage digestibility usually remains constant or increases due to greater DMI and increased OM fermentation in the rumen (
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      ). For example, cows fed fresh herbage cut after a shorter regrowth period produced more milk and the same amount of CH4, thus CH4 intensity was 12% less with a shorter grass regrowth period (
      • Warner D.
      • Podesta S.C.
      • Hatew B.
      • Klop G.
      • van Laar H.
      • Bannink A.
      • Dijkstra J.
      Effect of nitrogen fertilization rate and regrowth interval of grass herbage on methane emission of zero-grazing lactating dairy cows.
      ).
      • Warner D.
      • Hatew B.
      • Podesta S.C.
      • Klop G.
      • Van Gastelen S.
      • Van Laar H.
      • Dijkstra J.
      • Bannink A.
      Effects of nitrogen fertilisation rate and maturity of grass silage on methane emission by lactating dairy cows.
      compared grass ensiled at 3 stages of maturity and reported that ensiling less-mature grass resulted in greater DMI, digestibility, and milk and CH4 production, with CH4 intensity being 24% less for the least compared with the most mature silage. On the other hand, total CH4 production was 6% greater compared with that of the most mature silage.
      • Macome F.M.
      • Pellikaan W.F.
      • Hendriks W.H.
      • Warner D.
      • Schonewille J.T.
      • Cone J.W.
      In vitro gas and methane production in rumen fluid from dairy cows fed grass silages differing in plant maturity, compared to in vivo data.
      evaluated grass ensiled at 4 stages of maturity and reported that CH4 yield and intensity of dairy cows was 16 and 21% less, respectively, for the least compared with most mature grass. Total CH4 production was not reported in that study.
      Improved forage digestibility is easy to combine with other CH4 mitigation strategies at the farm level. Forage management to enhance digestibility affects many other aspects of the farming system, highlighting the need to consider impacts on net GHG emissions. The other aspects of farming that need consideration include animal productivity, amount and composition of manure, forage biomass yields, carbon sequestration during forage growth, and forage crop inputs. Immature forages have greater N content, which can increase N voided to the environment if not balanced. Implementation of increased forage digestibility at the farm can be hindered by lack of agronomic information and technical support, as well as additional costs. Furthermore, some ruminant production systems (e.g., nonlactating beef cows, animals at maintenance energy intake) fill the unique niche of consuming high-fiber, low-digestible feeds and crop residues and co-products not suitable for highly productive animals.

      Perennial Legumes

      At the same physiological stage of maturity, legume forages contain less NDF than grasses. Although fiber in legumes is more lignified, the decline in fiber digestibility with advancing maturity is much greater for grasses than for legumes, especially in tropical locations. In addition, some legumes can contain secondary compounds that decrease CH4 production (refer to section “Tannins and Saponins”). Rate of passage from the rumen, and consequently DMI, can be greater for legumes than grasses, which should theoretically decrease CH4 yield. Animal performance is often increased with inclusion of legumes in ruminant diets, which decreases CH4 intensity. For example,
      • Johansen M.
      • Lund P.
      • Weisbjerg M.R.
      Feed intake and milk production in dairy cows fed different grass and legume species: A meta-analysis.
      conducted a meta-analysis of temperate forages in dairy cow diets and concluded that, overall, legume-based diets resulted in higher DMI and milk yield than grass-based diets, although not all legumes were equally effective.
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      and
      • Vargas J.
      • Ungerfeld E.
      • Muñoz C.
      • DiLorenzo N.
      Feeding strategies to mitigate enteric methane emission from ruminants in grassland systems.
      , on the other hand, indicated variable and mostly no effects, respectively, of the inclusion of non-tannin-containing legumes in pastures, on different CH4 production metrics.
      It is difficult to quantify the mitigation effect due to dietary inclusion of legumes because it depends on the quality of the forages being compared, as differences in feed intake and digestibility due to phenological stages can confound results. For temperate forages, a meta-analysis showed no difference in CH4 yield between legumes and C3 grasses, whereas in warmer environments, legumes produced 19% less CH4 than C3 or C4 grasses (
      • Archimède H.
      • Eugène M.
      • Magdeleine C.M.
      • Boval M.
      • Martin C.
      • Morgavi D.P.
      • Lecomte P.
      • Doreau M.
      Comparison of methane production between C3 and C4 grasses and legumes.
      ). Similarly, working with sheep,
      • Hammond K.J.
      • Pacheco D.
      • Burke J.L.
      • Koolaard J.P.
      • Muetzel S.
      • Waghorn G.C.
      The effects of fresh forages and feed intake level on digesta kinetics and enteric methane emissions from sheep.
      did not find differences between ryegrass and white clover in DMI, digestibility, or any CH4 production metric; in their experiment, rumen liquid and solid outflow rates were greater or tended to be greater for ryegrass than for white clover. Inconsistent results for CH4 production in experiments comparing ryegrass with white clover were reported in other studies (
      • Hammond K.J.
      • Hoskin S.O.
      • Burke J.L.
      • Waghorn G.C.
      • Koolaard J.P.
      • Muetzel S.
      Effects of feeding fresh white clover (Trifolium repens) or perennial ryegrass (Lolium perenne) on enteric methane emissions from sheep.
      ,
      • Hammond K.J.
      • Burke J.L.
      • Koolaard J.P.
      • Muetzel S.
      • Pinares-Patiño C.S.
      • Waghorn G.C.
      Effects of feed intake on enteric methane of sheep fed fresh white clover (Trifolium repens) and perennial ryegrass (Lolium perenne) forages.
      ).
      • Kennedy P.M.
      • Charmley E.
      Methane yields from Brahman cattle fed tropical grasses and legumes.
      also reported similar Ym values for cattle fed tropical grasses (5.4–7.2%) compared with tropical grass-legume mixtures (5.4–6.5%). However, there was one exception; the legume Leucaena leucocephala decreased Ym by 11% when its inclusion rate was doubled (
      • Kennedy P.M.
      • Charmley E.
      Methane yields from Brahman cattle fed tropical grasses and legumes.
      ). Although increased use of legumes may not consistently decrease CH4 production or yield, CH4 intensity can decrease if the nutritive value of the diet is improved with increased animal performance.
      Perennial legume forages biologically fix N, which reduces the amount of N fertilizer used and associated emissions (
      • Schultze-Kraft R.
      • Rao I.
      • Peters M.
      • Clements R.
      • Bai C.
      • Liu G.
      Tropical forage legumes for environmental benefits: An overview.
      ). The N fixed by legume forages is still subject to losses and thus contributes to N2O emissions when their residues decay (
      • Guyader J.
      • Janzen H.H.
      • Kroebel R.
      • Beauchemin K.A.
      Forage use to improve environmental sustainability of ruminant production.
      ). Perennial legumes can increase soil carbon storage (
      • Little S.M.
      • Benchaar C.
      • Janzen H.H.
      • Kröbel R.
      • McGeough E.J.
      • Beauchemin K.A.
      Demonstrating the effect of forage source on the carbon footprint of a Canadian dairy farm using whole-systems analysis and the Holos model: Alfalfa silage vs. corn silage.
      ) and help prevent erosion and rehabilitate degraded soils, especially in tropical areas (
      • Schultze-Kraft R.
      • Rao I.
      • Peters M.
      • Clements R.
      • Bai C.
      • Liu G.
      Tropical forage legumes for environmental benefits: An overview.
      ). Methane emissions from dairy manure slurry were less for alfalfa than for corn silage (
      • Massé D.I.
      • Jarret G.
      • Hassanat F.
      • Benchaar C.
      • Cata Saady N.M.
      Effect of increasing levels of corn silage in an alfalfa-based dairy cow diet and of manure management practices on manure fugitive methane emissions.
      ), although ammonia and N2O emissions can be greater if CP intake of animals is increased by feeding legumes (
      • Rotz C.A.
      • Montes F.
      • Chianese D.S.
      The carbon footprint of dairy production systems through partial life cycle assessment.
      ). The high CP concentration of legumes can decrease the use of purchased supplements and associated emissions (
      • Schultze-Kraft R.
      • Rao I.
      • Peters M.
      • Clements R.
      • Bai C.
      • Liu G.
      Tropical forage legumes for environmental benefits: An overview.
      ). Therefore, the net effect of increased use of perennial legumes is complex and regionally specific, and LCA need to be conducted before recommendations can be made. Further research is needed to assess impacts on animal and forage productivity under different management systems to identify optimum legume inclusion levels that minimize emission intensity in different regions.

      High-Starch Forages

      Use of high-starch forages such as corn silage and small-grain cereals (e.g., barley, oat, triticale, and wheat in temperate locations; sorghum in semi-arid, warmer climates) can increase starch and decrease fiber concentration of diets. The resulting rumen fermentation promotes propionate production, which competes with methanogenesis for metabolic hydrogen and can also lower rumen pH and inhibit methanogens. With some diets, incorporating high-starch forages increases digestible energy intake of animals and enhances animal performance, thereby decreasing CH4 intensity. A meta-analysis for corn silage diets indicated that milk yield per tonne of DM was positively correlated with starch concentration (r = 0.65) and NDF digestibility (r = 0.49) and negatively correlated with NDF concentration (r = −0.72;
      • García-Chávez I.
      • Meraz-Romero E.
      • Castelán-Ortega O.
      • Zaragoza-Esparza J.
      • Osorio-Avalos J.
      • Robles-Jiménez L.E.
      • González-Ronquillo M.
      Corn silage, meta-analysis of the quality and yield of different regions in the world.
      ). In addition to decreased CH4 intensity, CH4 yield decreased up to 15% for diets containing corn silage compared with other forages (
      • Hassanat F.
      • Gervais R.
      • Julien C.
      • Masse D.I.
      • Lettat A.
      • Chouinard P.Y.
      • Petit H.V.
      • Benchaar C.
      Replacing alfalfa silage with corn silage in dairy cow diets: Effects on enteric methane production, ruminal fermentation, digestion, N balance, and milk production.
      ;
      • Gislon G.
      • Colombini S.
      • Borreani G.
      • Crovetto G.M.
      • Sandrucci A.
      • Galassi G.
      • Tabacco E.
      • Rapetti L.
      Milk production, methane emissions, nitrogen, and energy balance of cows fed diets based on different forage systems.
      ).
      • Rotz C.A.
      • Montes F.
      • Chianese D.S.
      The carbon footprint of dairy production systems through partial life cycle assessment.
      reported that increasing the ratio of corn silage to alfalfa silage in dairy cow diets resulted in N being used more efficiently, which resulted in a small decrease in excreted N and in emissions of N2O.
      • Uddin M.E.
      • Aguirre-Villegas H.A.
      • Larson R.A.
      • Wattiaux M.A.
      Carbon footprint of milk from Holstein and Jersey cows fed low or high forage diet with alfalfa silage or corn silage as the main forage source.
      reported a 2.5% decrease in CO2e per kilogram of fat- and protein-corrected milk for corn silage compared with alfalfa silage in the diet of lactating dairy cows.
      • Little S.M.
      • Benchaar C.
      • Janzen H.H.
      • Kröbel R.
      • McGeough E.J.
      • Beauchemin K.A.
      Demonstrating the effect of forage source on the carbon footprint of a Canadian dairy farm using whole-systems analysis and the Holos model: Alfalfa silage vs. corn silage.
      showed that, although replacing alfalfa silage with corn silage in the diet of lactating dairy cows lowered enteric CH4 yield by 10%, differences in CO2e emission intensity between the 2 forage systems were minimal when soil carbon was accounted for. Thus, feeding high-starch forages to reduce enteric CH4 emissions is not recommended unless substantiated by an LCA that includes soil carbon changes, an area of knowledge that is currently evolving. The greatest potential for high-starch forages to reduce total GHG emissions may take place when replacing another annual forage crop.

      High-Sugar Grasses

      High-sugar cultivars of perennial ryegrass (Lolium perenne L.) have elevated water-soluble carbohydrate (WSC) concentrations (250 to 350 g/kg of DM;
      • Rivero M.J.
      • Keim J.P.
      • Balocchi O.A.
      • Lee M.R.F.
      In vitro fermentation patterns and methane output of perennial ryegrass differing in water-soluble carbohydrate and nitrogen concentrations.
      ), mainly at the expense of CP and, in some cases, NDF concentration. The greater concentration of readily available carbohydrates decreases the acetate-to-propionate ratio in the rumen (
      • Rivero M.J.
      • Keim J.P.
      • Balocchi O.A.
      • Lee M.R.F.
      In vitro fermentation patterns and methane output of perennial ryegrass differing in water-soluble carbohydrate and nitrogen concentrations.
      ). In vitro studies report less CH4 production for high- versus low-sugar grasses (
      • Lovett D.K.
      • McGilloway D.
      • Bortolozzo A.
      • Hawkins M.
      • Callan J.
      • Flynn B.
      • O'Mara F.P.
      In vitro fermentation patterns and methane production as influenced by cultivar and season of harvest of Lolium perenne.
      ). Using modeling approaches,
      • Ellis J.L.
      • Dijkstra J.
      • France J.
      • Parsons A.J.
      • Edwards G.R.
      • Rasmussen S.
      • Kebreab E.
      • Bannink A.
      Effect of high-sugar grasses on methane emissions simulated using a dynamic model.
      estimated that an increase in WSC concentration of 40 g/kg of DM or more may be required to decrease in vivo CH4 yield, and the mitigation potential also depends on concomitant changes in CP and NDF concentration and digestibility.
      • Zhao Y.G.
      • O'Connell N.E.
      • Yan T.
      Prediction of enteric methane emissions from sheep offered fresh perennial ryegrass (Lolium perenne) using data measured in indirect open-circuit respiration chambers.
      fed fresh perennial ryegrass to sheep and reported moderate inverse correlations (r = −0.44 to −0.54) between WSC concentration and various expressions of CH4 production. A meta-analysis of 27 in vivo experiments found that for every 10 g/kg (DM) increase in WSC content, CH4 yield decreased by 0.311 g/kg of DMI (
      • Vera N.
      • Ungerfeld E.M.
      Effect of water-soluble carbohydrates content in Lolium perenne on enteric methane emissions: Meta-analysis.
      ). The CH4 mitigation potential of high-sugar grasses appears to be reduced when the forage is conserved as hay or silage (
      • Staerfl S.M.
      • Amelchanka S.L.
      • Kalber T.
      • Soliva C.R.
      • Kreuzer M.
      • Zeitz J.O.
      Effect of feeding dried high-sugar ryegrass (‘AberMagic’) on methane and urinary nitrogen emissions of primiparous cows.
      ).
      An inverse relationship was reported between CP and WSC content across 195 samples of perennial ryegrass genotypes including conventional and high-WSC cultivars from 49 studies (N. Vera, Instituto de Investigaciones Agropecuarias, Vilcún, Chile; personal communication). A lower ratio of CP to WSC in high-WSC grasses improves rumen microbial protein synthesis, with less ammonia-N absorbed and excreted as urea in urine (
      • Foskolos A.
      • Moorby J.
      The use of high sugar grasses as a strategy to improve nitrogen utilization efficiency: A meta-analysis.
      ), potentially resulting in lower ammonia and N2O emissions. Importantly, even if CP and WSC are genetically negatively related across conventional and high-WSC perennial ryegrass cultivars, lush swards generally have elevated contents of both CP and WSC because of their vegetative phenological stage, compared with more mature swards of the same cultivar. An LCA of milk production indicated that CO2e per kilogram of milk was 3% less when dairy cows were fed high-sugar versus conventional ryegrass pastures (
      • Soteriades A.D.
      • Gonzalez-Mejia A.M.
      • Styles D.
      • Foskolos A.
      • Moorby J.M.
      • Gibbons J.M.
      Effects of high-sugar grasses and improved manure management on the environmental footprint of milk production at the farm level.
      ).
      A mechanistic model developed by
      • Ellis J.L.
      • Dijkstra J.
      • France J.
      • Parsons A.J.
      • Edwards G.R.
      • Rasmussen S.
      • Kebreab E.
      • Bannink A.
      Effect of high-sugar grasses on methane emissions simulated using a dynamic model.
      predicted a 3.3% average increase in DMI with increased WSC concentration (+39 g/kg of DM) of grass, leading to increased milk yield. However, a meta-analysis indicated that feeding dairy cattle high-sugar grasses did not increase milk production although urinary N excretion was decreased by 26% (
      • Foskolos A.
      • Moorby J.
      The use of high sugar grasses as a strategy to improve nitrogen utilization efficiency: A meta-analysis.
      ). An aspect to be considered is that the lower CP concentration of high-sugar grasses may negatively affect productivity of high-producing ruminants if protein requirements are not met (
      • Staerfl S.M.
      • Amelchanka S.L.
      • Kalber T.
      • Soliva C.R.
      • Kreuzer M.
      • Zeitz J.O.
      Effect of feeding dried high-sugar ryegrass (‘AberMagic’) on methane and urinary nitrogen emissions of primiparous cows.
      ), and would require balancing for dietary protein. An LCA of diet changes may be required, also considering the forage yield of the different cultivars. Most of the research on high-sugar grass cultivars has been limited to the United Kingdom, Netherlands, and New Zealand. In vivo studies are needed to quantify the effects of high-sugar grasses on CH4 production, forage crop yields, and animal performance in various production systems. The effect of selection for the high-sugar trait on fungal diseases and insect attack also requires further assessment.

      Pastures and Grazing Management

      Grazing systems vary with climate, plant species, soil types, and livestock, and include season-long continuous grazing, rest-rotation grazing, deferred rotational grazing, and intensively managed grazing. These systems manage pastures to provide forage resources for animals, attempting to balance livestock nutritional requirements with herbage availability and quality while promoting rapid pasture regrowth and long-term pasture resilience. Grazing management can enhance herbage quantity and quality, leading to increased animal production per hectare (
      • Congio G.F.S.
      • Batalha C.D.A.
      • Chiavegato M.B.
      • Berndt A.
      • Oliveira P.P.A.
      • Frighetto R.T.S.
      • Maxwell T.M.R.
      • Gregorini P.
      • Da Silva S.C.
      Strategic grazing management towards sustainable intensification at tropical pasture-based dairy systems.
      ;
      • Savian J.V.
      • Schons R.M.T.
      • Marchi D.E.
      • Freitas T.S.
      • da Silva Neto G.F.
      • Mezzalira J.C.
      • Berndt A.
      • Bayer C.
      • Carvalho P.C.F.
      Rotatinuous stocking: A grazing management innovation that has high potential to mitigate methane emissions by sheep.
      ), with increased soil carbon stocks and decreased CH4 intensity (
      • Guyader J.
      • Janzen H.H.
      • Kroebel R.
      • Beauchemin K.A.
      Forage use to improve environmental sustainability of ruminant production.
      ). Some pasture species also contain phytocompounds such as condensed and hydrolyzable tannins and saponins that may reduce enteric CH4 production (
      • MacAdam J.W.
      • Villalba J.J.
      Review: Beneficial effects of temperate forage legumes that contain condensed tannins.
      ;
      • Kozłowska M.
      • Cieślak A.
      • Jóźwik A.
      • El-Sherbiny M.
      • Stochmal A.
      • Oleszek W.
      • Kowalczyk M.
      • Filipiak W.
      • Szumacher-Strabel M.
      The effect of total and individual alfalfa saponins on rumen methane production.
      ). In addition to traditional pasture-based systems, silvopastoral systems that incorporate trees and shrubs in pastures increase the amount of biomass per unit of area and provide other ecosystem services. Silvopastoral systems promote sustainable intensification of land, potentially increasing biodiversity, water use efficiency and biomass production, while promoting animal welfare by providing shade to alleviate heat stress (
      • Mauricio R.M.
      • Ribeiro R.S.
      • Paciullo D.S.C.
      • Cangussú M.A.
      • Murgueitio E.
      • Chará. M. X. J.
      • Estrada F.
      Silvopastoral systems in Latin America for biodiversity, environmental, and socioeconomic improvements.
      ).
      Grazing management for CH4 mitigation considers pregrazing and postgrazing sward height and biomass to maximize herbage nutritional quality (
      • Muñoz C.
      • Letelier P.A.
      • Ungerfeld E.M.
      • Morales J.M.
      • Hube S.
      • Pérez-Prieto L.A.
      Effects of pre grazing herbage mass in late spring on enteric methane emissions, dry matter intake, and milk production of dairy cows.
      ;
      • Congio G.F.S.
      • Batalha C.D.A.
      • Chiavegato M.B.
      • Berndt A.
      • Oliveira P.P.A.
      • Frighetto R.T.S.
      • Maxwell T.M.R.
      • Gregorini P.
      • Da Silva S.C.
      Strategic grazing management towards sustainable intensification at tropical pasture-based dairy systems.
      ;
      • Savian J.V.
      • Schons R.M.T.
      • Marchi D.E.
      • Freitas T.S.
      • da Silva Neto G.F.
      • Mezzalira J.C.
      • Berndt A.
      • Bayer C.
      • Carvalho P.C.F.
      Rotatinuous stocking: A grazing management innovation that has high potential to mitigate methane emissions by sheep.
      ). Grazing management can lower enteric CH4 intensity, but total CH4 production has not changed in most studies (
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      ;
      • Vargas J.
      • Ungerfeld E.
      • Muñoz C.
      • DiLorenzo N.
      Feeding strategies to mitigate enteric methane emission from ruminants in grassland systems.
      ), and it may increase if DMI is increased. In turn, changes in CH4 production per unit of grazing area depend on changes in CH4 production per animal and stocking rate. The extent to which grazing management lowers CH4 intensity is extremely variable depending upon the production system and local conditions. For example, rotational grazing based on sward pre- and postgrazing heights increased digestible OM intake of sheep grazing Italian ryegrass, reducing CH4 intensity by 63% and CH4 production per hectare by 39%, although CH4 production per animal was increased by 12% (
      • Savian J.V.
      • Schons R.M.T.
      • Marchi D.E.
      • Freitas T.S.
      • da Silva Neto G.F.
      • Mezzalira J.C.
      • Berndt A.
      • Bayer C.
      • Carvalho P.C.F.
      Rotatinuous stocking: A grazing management innovation that has high potential to mitigate methane emissions by sheep.
      ). For dairy cattle, optimizing grazing management improved milk production efficiency by 51%, while decreasing CH4 intensity by 20% and CH4 yield by 18%, although CH4 emissions per hectare increased by 29% (
      • Congio G.F.S.
      • Batalha C.D.A.
      • Chiavegato M.B.
      • Berndt A.
      • Oliveira P.P.A.
      • Frighetto R.T.S.
      • Maxwell T.M.R.
      • Gregorini P.
      • Da Silva S.C.
      Strategic grazing management towards sustainable intensification at tropical pasture-based dairy systems.
      ). Dairy cows grazing swards differing in pregrazing herbal mass had similar total CH4 production, but the increase in DMI and milk yield with low herbage mass (lower in NDF concentration) resulted in 10% less CH4 yield (
      • Muñoz C.
      • Letelier P.A.
      • Ungerfeld E.M.
      • Morales J.M.
      • Hube S.
      • Pérez-Prieto L.A.
      Effects of pre grazing herbage mass in late spring on enteric methane emissions, dry matter intake, and milk production of dairy cows.
      ). For beef cattle, CH4 production was greater for light versus heavy continuous grazing because plants were at a more advanced stage of maturity, but the additional CH4 was more than offset by greater soil carbon sequestration (
      • Alemu A.W.
      • Janzen H.
      • Little S.
      • Hao X.
      • Thompson D.J.
      • Baron V.
      • Iwaasa A.
      • Beauchemin K.A.
      • Kröbel R.
      Assessment of grazing management on farm greenhouse gas intensity of beef production systems in the Canadian Prairies using life cycle assessment.
      ). Therefore, optimum grazing management needs to consider the productivity of animals as well as pastures and soil, and LCA is needed to account for all GHG emissions and removals (changes in soil carbon), and other ecosystem services provided by grassland ecosystems also need to be considered.
      Implementation of improved pasture management by farmers can be hindered by additional costs (e.g., fences, water troughs, moving cattle, tree plantation) and lack of long-term, regionally relevant research. Extension services supported by public policies (e.g., payment for environmental services) may be needed to encourage adoption.

      Forage Preservation and Processing

      Ensiling forage causes losses in DM and changes in nutritive value, but good management practices can be used to ensure excellent quality silage. Thus, the effect of ensiling forage on CH4 production is expected to be highly variable depending upon the resulting forage quality and ensiling practices. Few in vivo studies have examined the direct effects of forage preservation method on CH4 production (
      • Knapp J.R.
      • Laur G.L.
      • Vadas P.A.
      • Weiss W.P.
      • Tricarico J.M.
      Invited review: Enteric methane in dairy cattle production: Quantifying the opportunities and impact of reducing emissions.
      ). The impact of preservation methods on CH4 emissions is mainly due to effects on animal performance, which affects CH4 intensity (
      • Evans B.
      The role ensiled forage has on methane production in the rumen.
      ).
      Processing of forage by grinding and pelleting reduces particle size, which increases ruminal passage rate, decreases OM degradation in the rumen, and shifts fermentation toward propionate production with less CH4 production.
      • Johnson D.E.
      • Ward G.W.
      • Ramsey J.J.
      Livestock methane: Current emissions and mitigation Potential.
      reported a 20 to 40% decrease in CH4 yield when forage was ground or pelleted compared with feeding long forage. However, total CH4 production is likely not decreased or is even increased (
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      ), by forage processing in animals fed ad libitum due to increased DMI, especially for low-quality forages (
      • Hironaka R.
      • Mathison G.W.
      • Kerrigan B.K.
      • Vlach I.
      The effect of pelleting of alfalfa hay on methane production and digestibility by steers.
      ). A faster ruminal passage rate can also result in a reduction in forage digestibility if structural carbohydrates are not digested in the lower tract.
      Forage preservation and processing increase the use of fuel for machinery and associated emissions compared with grazing fresh herbage. Moreover, reduced NDF digestibility due to processing can lead to increased manure emissions of CH4 (
      • Knapp J.R.
      • Laur G.L.
      • Vadas P.A.
      • Weiss W.P.
      • Tricarico J.M.
      Invited review: Enteric methane in dairy cattle production: Quantifying the opportunities and impact of reducing emissions.
      ), depending on how the manure is managed. Before recommending a change in forage preservation or processing for CH4 mitigation, additional inputs required, effects on animal productivity, and whole-farm GHG emissions need to be considered.

      ACTION ON RUMEN FERMENTATION

      Ionophores

      Ionophores are polyether compounds that increase permeability of cell membranes to ions in gram-positive bacteria and protozoa, resulting in retarded growth and death. Most targeted microorganisms produce H2 and, in this way, ionophores decrease the availability of H2 for methanogenesis, and shift fermentation from acetate to propionate (
      • Duffield T.F.
      • Rabiee A.R.
      • Lean I.J.
      A meta-analysis of the impacts of monensin in lactating dairy cattle. Part 1: Metabolic effects.
      ). Reports on microbial adaptation to ionophores are conflicting (
      • Appuhamy J.A.D.R.N.
      • Strathe A.B.
      • Jayasundara S.
      • Wagner-Riddle C.
      • Dijkstra J.
      • France J.
      • Kebreab E.
      Antimethanogenic effects of monensin in dairy and beef cattle: A meta-analysis.
      ).
      Monensin is routinely used in feedlot cattle production in many countries, but its effects on CH4 production are generally small. The meta-analysis by
      • Appuhamy J.A.D.R.N.
      • Strathe A.B.
      • Jayasundara S.
      • Wagner-Riddle C.
      • Dijkstra J.
      • France J.
      • Kebreab E.
      Antimethanogenic effects of monensin in dairy and beef cattle: A meta-analysis.
      reported average decreases in total CH4 production of between 3.6 and 10.7% in dairy cows and beef steers, respectively. Additionally, monensin improves feed conversion efficiency (
      • Duffield T.F.
      • Rabiee A.R.
      • Lean I.J.
      A meta-analysis of the impacts of monensin in lactating dairy cattle. Part 2: Production effects.
      ), which decreases GHG emissions from feed production needed to sustain animal production.
      Monensin inclusion in manure at concentrations resulting from recommended inclusion in dairy cow diets did not affect manure CH4 production (
      • Arikan O.A.
      • Mulbry W.
      • Rice C.
      • Lansing S.
      The fate and effect of monensin during anaerobic digestion of dairy manure under mesophilic conditions.
      ). Monensin decreases the concentration of rumen ammonium, but there are contradictory results about its effects on N metabolism and release to the environment (
      • Duffield T.F.
      • Rabiee A.R.
      • Lean I.J.
      A meta-analysis of the impacts of monensin in lactating dairy cattle. Part 1: Metabolic effects.
      ). Increases in emissions associated with manufacturing and transport of ionophores are small because they are typically included in the diet at concentrations of 50 mg/kg of DM or less. Use of monensin in beef and dairy animals is permitted in some countries and prohibited in others. Adoption is favored by intensive systems where animals are fed or supplemented daily, but slow-release forms, suitable for use in grazing cattle, are commercially available. It has been questioned whether widespread usage of monensin can contribute to antibiotic resistance, but these antimicrobials are presently not used in human medicine. Overall, ionophores can help achieve minor mitigation of enteric CH4 production and intensity of ruminant products, and they have favorable effects on animal productivity. The use of ionophores in ruminant diets is already approved in many regions of the world, but with growing concerns over antimicrobial resistance, their use may become more limited in the future.

      3-Nitrooxypropanol

      3-Nitrooxypropanol is a molecule that when included in small doses (60 to 200 mg/kg of DMI) in ruminant feeds inhibits CH4 production in the rumen. Chemical inhibitors of methanogens have been studied in vitro and in vivo since the 1960s. Research on some compounds was abandoned because of risks of toxicity, passage to animal products, volatility, or transient in vivo effects. 3-Nitrooxypropanol for CH4 mitigation was patented in 2012 (
      • Duval S.
      • Kindermann M.
      Use of nitrooxy organic molecules in feed for reducing enteric methane production in ruminants, and/or to improve ruminant performance. International Patent WO2012084629A1.
      ) and has been comprehensively investigated in silico, in pure enzyme-substrate systems, in in vitro pure and mixed cultures (
      • Duin E.C.
      • Wagner T.
      • Shima S.
      • Prakash D.
      • Cronin B.
      • Yáñez-Ruiz D.R.
      • Duval S.
      • Rümbeli R.
      • Stemmler R.T.
      • Thauer R.K.
      • Kindermann M.
      Mode of action uncovered for the specific reduction of methane emissions from ruminants by the small molecule 3-nitrooxypropanol.
      ), and in vivo (e.g.,
      • Yu G.
      • Beauchemin K.A.
      • Dong R.
      A review of 3-nitrooxypropanol for enteric methane mitigation from ruminant livestock.
      ). 3-Nitrooxypropanol targets methyl-coenzyme M reductase, which catalyzes the last step of methanogenesis in methanogenic archaea. Its mechanism of action is established, as are the products resulting from its metabolism in the rumen (
      • Duin E.C.
      • Wagner T.
      • Shima S.
      • Prakash D.
      • Cronin B.
      • Yáñez-Ruiz D.R.
      • Duval S.
      • Rümbeli R.
      • Stemmler R.T.
      • Thauer R.K.
      • Kindermann M.
      Mode of action uncovered for the specific reduction of methane emissions from ruminants by the small molecule 3-nitrooxypropanol.
      ).
      On average and at typical inclusion levels in beef (144 ± 82.3 mg/kg of DM; mean ± SD) and dairy (81 ± 41.2 mg/kg of DM; mean ± SD) diets, 3-NOP decreases CH4 production by 30% (
      • Dijkstra J.
      • Bannink A.
      • France J.
      • Kebreab E.
      • van Gastelen S.
      Short communication: Antimethanogenic effects of 3-nitrooxypropanol depend on supplementation dose, dietary fiber content, and cattle type.
      ;
      • Kim H.
      • Lee H.
      • Baek Y.
      • Lee S.
      • Seo J.
      The effects of dietary supplementation with 3-nitrooxypropanol on enteric methane emissions, rumen fermentation, and production performance in ruminants: A meta-analysis.
      ), although decreases of 80% or greater have been obtained in some studies with high-concentrate diets (
      • Yu G.
      • Beauchemin K.A.
      • Dong R.
      A review of 3-nitrooxypropanol for enteric methane mitigation from ruminant livestock.
      ). The effect of 3-NOP on CH4 production is related to its level of inclusion in the diet (
      • Yu G.
      • Beauchemin K.A.
      • Dong R.
      A review of 3-nitrooxypropanol for enteric methane mitigation from ruminant livestock.
      ) and is negatively affected by dietary concentration of NDF (
      • Dijkstra J.
      • Bannink A.
      • France J.
      • Kebreab E.
      • van Gastelen S.
      Short communication: Antimethanogenic effects of 3-nitrooxypropanol depend on supplementation dose, dietary fiber content, and cattle type.
      ;
      • Yu G.
      • Beauchemin K.A.
      • Dong R.
      A review of 3-nitrooxypropanol for enteric methane mitigation from ruminant livestock.
      ).
      Long-term in vivo inhibition of enteric CH4 production by 3-NOP was initially reported by
      • Hristov A.N.
      • Oh J.
      • Giallongo F.
      • Frederick T.W.
      • Harper M.T.
      • Weeks H.L.
      • Branco A.F.
      • Moate P.J.
      • Deighton M.H.
      • Williams S.R.O.
      • Kindermann M.
      • Duval S.
      An inhibitor persistently decreased enteric methane emission from dairy cows with no negative effect on milk production.
      and has since been confirmed in various studies (
      • Yu G.
      • Beauchemin K.A.
      • Dong R.
      A review of 3-nitrooxypropanol for enteric methane mitigation from ruminant livestock.
      ). Although in most long-term studies, 3-NOP effectiveness has remained constant, a couple of studies have shown that 3-NOP effectiveness declined slightly over time, which might be related to the low dose used (
      • Yu G.
      • Beauchemin K.A.
      • Dong R.
      A review of 3-nitrooxypropanol for enteric methane mitigation from ruminant livestock.
      ).
      In the meta-analysis by
      • Jayanegara A.
      • Sarwono K.A.
      • Kondo M.
      • Matsui H.
      • Ridla M.
      • Laconi E.B.
      • Nahrowi
      Use of 3-nitrooxypropanol as feed additive for mitigating enteric methane emissions from ruminants: A meta-analysis.
      , 3-NOP did not affect DMI of dairy and beef cattle, whereas the later meta-analysis by
      • Kim H.
      • Lee H.
      • Baek Y.
      • Lee S.
      • Seo J.
      The effects of dietary supplementation with 3-nitrooxypropanol on enteric methane emissions, rumen fermentation, and production performance in ruminants: A meta-analysis.
      reported a decrease in DMI with 3-NOP in beef but not dairy animals. Both meta-analyses found increases or tendencies to increase milk fat and protein percentages with 3-NOP supplementation, although
      • Kim H.
      • Lee H.
      • Baek Y.
      • Lee S.
      • Seo J.
      The effects of dietary supplementation with 3-nitrooxypropanol on enteric methane emissions, rumen fermentation, and production performance in ruminants: A meta-analysis.
      reported a decrease in milk yield with 3-NOP. Feed conversion efficiency has been shown to be improved in beef cattle supplemented with 3-NOP (
      • Jayanegara A.
      • Sarwono K.A.
      • Kondo M.
      • Matsui H.
      • Ridla M.
      • Laconi E.B.
      • Nahrowi
      Use of 3-nitrooxypropanol as feed additive for mitigating enteric methane emissions from ruminants: A meta-analysis.
      ). Digestibility of various dietary fractions was unaffected (
      • Jayanegara A.
      • Sarwono K.A.
      • Kondo M.
      • Matsui H.
      • Ridla M.
      • Laconi E.B.
      • Nahrowi
      Use of 3-nitrooxypropanol as feed additive for mitigating enteric methane emissions from ruminants: A meta-analysis.
      ) or improved (
      • Kim H.
      • Lee H.
      • Baek Y.
      • Lee S.
      • Seo J.
      The effects of dietary supplementation with 3-nitrooxypropanol on enteric methane emissions, rumen fermentation, and production performance in ruminants: A meta-analysis.
      ), the latter of which might be due to decreased DMI (
      • Illius A.
      • Allen M.
      Assessing forage quality using integrated models of intake and digestion by ruminants.
      ). Heterogeneity among studies or the interactions between the experiment effect and the effect of 3-NOP supplementation were not reported by
      • Jayanegara A.
      • Sarwono K.A.
      • Kondo M.
      • Matsui H.
      • Ridla M.
      • Laconi E.B.
      • Nahrowi
      Use of 3-nitrooxypropanol as feed additive for mitigating enteric methane emissions from ruminants: A meta-analysis.
      or
      • Kim H.
      • Lee H.
      • Baek Y.
      • Lee S.
      • Seo J.
      The effects of dietary supplementation with 3-nitrooxypropanol on enteric methane emissions, rumen fermentation, and production performance in ruminants: A meta-analysis.
      , so it was not determined whether the observed effects were consistent across studies.
      Because the recommended dietary concentration of 3-NOP is very low, CO2 emissions associated with its manufacture and transport are also very low. Based on CO2 emissions from 3-NOP production (
      • Feng X.
      • Kebreab E.
      Net reductions in greenhouse gas emissions from feed additive use in California dairy cattle.
      ) and the average DMI, CH4 production, and 3-NOP dose, it is estimated that the additional emissions of GHG associated with manufacturing and transporting 3-NOP would represent between 1.8 and 5.3% of the decrease in CH4 emissions that it would elicit (calculations not shown). No effects on manure emissions of GHG as a consequence of feeding 3-NOP were observed by
      • Nkemka V.N.
      • Beauchemin K.A.
      • Hao X.
      Treatment of feces from beef cattle fed the enteric methane inhibitor 3-nitrooxypropanol.
      and
      • Owens J.L.
      • Thomas B.W.
      • Stoeckli J.L.
      • Beauchemin K.A.
      • McAllister T.A.
      • Larney F.J.
      • Hao X.
      Greenhouse gas and ammonia emissions from stored manure from beef cattle supplemented 3-nitrooxypropanol and monensin to reduce enteric methane emissions.
      , although
      • Weber T.L.
      • Hao X.
      • Gross C.D.
      • Beauchemin K.A.
      • Chang S.X.
      Effect of manure from cattle fed 3-nitrooxypropanol on anthropogenic greenhouse gas emissions depends on soil type.
      found soil-dependent effects. Hence, the effects of 3-NOP on manure emissions need further examination.
      Chemical inhibitors can be easily combined with other mitigation strategies. Their adoption requires them to pass safety tests for animals, consumers, and the environment. In the animal, 3-NOP carbon is largely metabolized to CO2, carbohydrates, fatty acids, and amino acids, with less than 5% of the original compound excreted in urine (
      • Thiel A.
      • Rümbeli R.
      • Mair P.
      • Yeman H.
      • Beilstein P.
      3-NOP: ADME studies in rats and ruminating animals.
      ). Mutagenic and genotoxic potential were not found (
      • Thiel A.
      • Schoenmakers A.C.M.
      • Verbaan I.A.J.
      • Chenal E.
      • Etheve S.
      • Beilstein P.
      3-NOP: Mutagenicity and genotoxicity assessment.
      ). Chemical inhibitors of methanogenesis need approval by government agencies, which has been recently granted for 3-NOP in Brazil, Chile, and the European Union, and is under consideration in other countries (
      • Yu G.
      • Beauchemin K.A.
      • Dong R.
      A review of 3-nitrooxypropanol for enteric methane mitigation from ruminant livestock.
      ).
      Research on the discovery of new chemical inhibitors is ongoing (
      • Henderson G.
      • Cook G.M.
      • Ronimus R.S.
      Enzyme and gene-based approaches for developing methanogen-specific compounds to control ruminant methane emissions: A review.
      ). The greatest hurdles for the widespread adoption of 3-NOP or other chemical inhibitors that may be discovered in the future are the additional feeding cost from their inclusion in animal diets, if no consistent benefits in productivity are obtained, and the difficulty of delivering the required dose to grazing ruminants in extensive production systems in a format that works over extended periods (
      • Hegarty R.S.
      • Cortez Passetti R.A.
      • Dittmer K.M.
      • Wang Y.
      • Shelton S.
      • Emmet-Booth J.
      • Wollenberg E.
      • McAllister T.
      • Leahy S.
      • Beauchemin K.
      • Gurwick N.
      An evaluation of emerging feed additives to reduce methane emissions from livestock. Edition 1. A report coordinated by Climate Change, Agriculture and Food Security (CCAFS) and the New Zealand Agricultural Greenhouse Gas Research Centre (NZAGRC) initiative of the Global Research Alliance (GRA).
      ;
      • Ungerfeld E.M.
      • Beauchemin K.A.
      • Muñoz C.
      Current perspectives on achieving pronounced enteric methane mitigation from ruminant production.
      ).

      Macroalgae

      Macroalgae (seaweeds) have highly variable chemical composition, depending upon species, time of collection, and growth environment, and they can contain bioactive components that inhibit methanogenesis. Red seaweeds such as Asparagopsis taxiformis and Asparagopsis armata accumulate halogenated compounds, of which bromoform is the most abundant (
      • Machado L.
      • Magnusson M.
      • Paul N.A.
      • Kinley R.
      • de Nys R.
      • Tomkins N.
      Identification of bioactives from the red seaweed Asparagopsis taxiformis that promote antimethanogenic activity in vitro.
      ). Methane halogenated analogs react with vitamin B12 to block the last step of methanogenesis in methanogenic archaea (
      • Wood J.M.
      • Kennedy F.S.
      • Wolfe R.S.
      The reaction of multihalogenated hydrocarbons with free and bound reduced vitamin B12.
      ). Other seaweeds contain polysaccharides, proteins, peptides, bacteriocins (produced by surface-associated bacteria), lipids, phlorotannins, saponins, and alkaloids that are known to decrease CH4 production by suppressing archaea and protozoa, and in some cases cause an undesirable decrease in nutrient degradability (
      • Abbott D.W.
      • Aasen I.M.
      • Beauchemin K.A.
      • Grondahl F.
      • Gruninger R.
      • Hayes M.
      • Huws S.
      • Kenny D.A.
      • Krizsan S.J.
      • Kirwan S.
      • Lind V.
      • Meyer U.
      • Ramin M.
      • Theodoridou K.
      • von Soosten D.
      • Walsh P.
      • Waters S.
      • Xing X.
      Seaweed and seaweed bioactives for mitigation of enteric methane: Challenges and opportunities.
      ).
      In vivo studies with sheep, steers, and dairy cows reported dose- and diet-dependent decreases between 9 and 98% of CH4 production by supplementing Asparagopsis to the diet (
      • Li X.
      • Norman H.C.
      • Kinley R.D.
      • Laurence M.
      • Wilmott M.
      • Bender H.
      • de Nys R.
      • Tomkins N.
      Asparagopsis taxiformis decreases enteric methane production from sheep.
      ;
      • Roque B.M.
      • Salwen J.K.
      • Kinley R.
      • Kebreab E.
      Inclusion of Asparagopsis armata in lactating dairy cows' diet reduces enteric methane emission by over 50 percent.
      ,
      • Roque B.M.
      • Venegas M.
      • Kinley R.D.
      • de Nys R.
      • Duarte T.L.
      • Yang X.
      • Kebreab E.
      Red seaweed (Asparagopsis taxiformis) supplementation reduces enteric methane by over 80 percent in beef steers.
      ;
      • Kinley R.D.
      • Martinez-Fernandez G.
      • Matthews M.K.
      • de Nys R.
      • Magnusson M.
      • Tomkins N.W.
      Mitigating the carbon footprint and improving productivity of ruminant livestock agriculture using a red seaweed.
      ;
      • Stefenoni H.A.
      • Räisänen S.E.
      • Cueva S.F.
      • Wasson D.E.
      • Lage C.F.A.
      • Melgar A.
      • Fetter M.E.
      • Smith P.
      • Hennessy M.
      • Vecchiarelli B.
      • Bender J.
      • Pitta D.
      • Cantrell C.L.
      • Yarish C.
      • Hristov A.N.
      Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
      ). A substantial decrease in CH4 yield for cattle was confirmed in a meta-analysis (
      • Lean I.J.
      • Golder H.M.
      • Grant T.M.D.
      • Moate P.J.
      A meta-analysis of effects of dietary seaweed on beef and dairy cattle performance and methane yield.
      ). Efficacy of Asparagopsis for CH4 mitigation depends on its concentration of bromoform, which ranges from 3.0 to 51.0 mg/kg of DMI (
      • Kinley R.D.
      • Martinez-Fernandez G.
      • Matthews M.K.
      • de Nys R.
      • Magnusson M.
      • Tomkins N.W.
      Mitigating the carbon footprint and improving productivity of ruminant livestock agriculture using a red seaweed.
      ;
      • Roque B.M.
      • Salwen J.K.
      • Kinley R.
      • Kebreab E.
      Inclusion of Asparagopsis armata in lactating dairy cows' diet reduces enteric methane emission by over 50 percent.
      ,
      • Roque B.M.
      • Venegas M.
      • Kinley R.D.
      • de Nys R.
      • Duarte T.L.
      • Yang X.
      • Kebreab E.
      Red seaweed (Asparagopsis taxiformis) supplementation reduces enteric methane by over 80 percent in beef steers.
      ;
      • Stefenoni H.A.
      • Räisänen S.E.
      • Cueva S.F.
      • Wasson D.E.
      • Lage C.F.A.
      • Melgar A.
      • Fetter M.E.
      • Smith P.
      • Hennessy M.
      • Vecchiarelli B.
      • Bender J.
      • Pitta D.
      • Cantrell C.L.
      • Yarish C.
      • Hristov A.N.
      Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
      ). Additionally, Asparagopsis may be more effective at decreasing CH4 production with high-concentrate than with high-forage diets (
      • Roque B.M.
      • Venegas M.
      • Kinley R.D.
      • de Nys R.
      • Duarte T.L.
      • Yang X.
      • Kebreab E.
      Red seaweed (Asparagopsis taxiformis) supplementation reduces enteric methane by over 80 percent in beef steers.
      ). There are preliminary concerns about Asparagopsis losing effect in the long term (
      • Hristov A.N.
      • Melgar A.
      • Wasson D.
      • Arndt C.
      Symposium review: Effective nutritional strategies to mitigate enteric methane in dairy cattle.
      ). Studies on the efficacy of other seaweeds on CH4 production are mostly limited to in vitro conditions (
      • Abbott D.W.
      • Aasen I.M.
      • Beauchemin K.A.
      • Grondahl F.
      • Gruninger R.
      • Hayes M.
      • Huws S.
      • Kenny D.A.
      • Krizsan S.J.
      • Kirwan S.
      • Lind V.
      • Meyer U.
      • Ramin M.
      • Theodoridou K.
      • von Soosten D.
      • Walsh P.
      • Waters S.
      • Xing X.
      Seaweed and seaweed bioactives for mitigation of enteric methane: Challenges and opportunities.
      ), although interest is growing.
      Dietary supplementation with Asparagopsis reduced feed intake in a dose-dependent manner in most but not all experiments. A meta-analysis of experiments supplementing Asparagopsis or brown algae reported no effects on DMI or ADG and, depending on the estimation method, a significant or numerical decrease in the feed to body mass gain ratio and increase in milk yield (
      • Lean I.J.
      • Golder H.M.
      • Grant T.M.D.
      • Moate P.J.
      A meta-analysis of effects of dietary seaweed on beef and dairy cattle performance and methane yield.
      ). Asparagopsis supplementation was reported to increase feed efficiency in some small-scale beef studies (
      • Kinley R.D.
      • Martinez-Fernandez G.
      • Matthews M.K.
      • de Nys R.
      • Magnusson M.
      • Tomkins N.W.
      Mitigating the carbon footprint and improving productivity of ruminant livestock agriculture using a red seaweed.
      ;
      • Roque B.M.
      • Venegas M.
      • Kinley R.D.
      • de Nys R.
      • Duarte T.L.
      • Yang X.
      • Kebreab E.
      Red seaweed (Asparagopsis taxiformis) supplementation reduces enteric methane by over 80 percent in beef steers.
      ). There were no effects of Asparagopsis inclusion in the diet on carcass quality, meat quality, or taste (
      • Kinley R.D.
      • Martinez-Fernandez G.
      • Matthews M.K.
      • de Nys R.
      • Magnusson M.
      • Tomkins N.W.
      Mitigating the carbon footprint and improving productivity of ruminant livestock agriculture using a red seaweed.
      ;
      • Roque B.M.
      • Venegas M.
      • Kinley R.D.
      • de Nys R.
      • Duarte T.L.
      • Yang X.
      • Kebreab E.
      Red seaweed (Asparagopsis taxiformis) supplementation reduces enteric methane by over 80 percent in beef steers.
      ), although a possible decrease in beef shelf life was reported at a high dose of Asparagopsis inclusion (
      • Bolkenov B.
      • Duarte T.
      • Yang L.
      • Yang F.
      • Roque B.
      • Kebreab E.
      • Yang X.
      Effects of red macroalgae Asparagopsis taxiformis supplementation on the shelf life of fresh whole muscle beef.
      ). The effect of Asparagopsis inclusion on manure emissions is unknown.
      Long-term oral exposure of animals to high concentrations of bromoform can cause liver and intestinal tumors; hence, the classified the compound in Group B2: probable human carcinogen. Within the dietary concentrations used (<0.5% of seaweed/DMI), bromoform residues were not detected in milk, meat, fat, organs, or feces from sheep and beef or dairy cattle fed Asparagopsis (
      • Li X.
      • Norman H.C.
      • Kinley R.D.
      • Laurence M.
      • Wilmott M.
      • Bender H.
      • de Nys R.
      • Tomkins N.
      Asparagopsis taxiformis decreases enteric methane production from sheep.
      ;
      • Kinley R.D.
      • Martinez-Fernandez G.
      • Matthews M.K.
      • de Nys R.
      • Magnusson M.
      • Tomkins N.W.
      Mitigating the carbon footprint and improving productivity of ruminant livestock agriculture using a red seaweed.
      ;
      • Roque B.M.
      • Salwen J.K.
      • Kinley R.
      • Kebreab E.
      Inclusion of Asparagopsis armata in lactating dairy cows' diet reduces enteric methane emission by over 50 percent.
      ,
      • Roque B.M.
      • Venegas M.
      • Kinley R.D.
      • de Nys R.
      • Duarte T.L.
      • Yang X.
      • Kebreab E.
      Red seaweed (Asparagopsis taxiformis) supplementation reduces enteric methane by over 80 percent in beef steers.
      ). In contrast,
      • Muizelaar W.
      • Groot M.
      • Van Duinkerken G.
      • Peters R.
      • Dijkstra J.
      Safety and transfer study: Transfer of bromoform present in Asparagopsis taxiformis to milk and urine of lactating dairy cows.
      , with no control animals in their study, reported passage of bromoform to milk in nonadapted dairy cows. However, bromoform was not detected in milk after 10 d of continuously feeding Asparagopsis at any level of supplementation, raising the possibility that microbial adaptation may play a role in reducing the flow of bromoform into milk. In an in vitro study, bromoform was degraded within 12 h of incubation, with dibromomethane as the main degradation product (
      • Romero P.
      • Belanche A.
      • Hueso R.
      • Ramos-Morales E.
      • Salwen J.K.
      • Kebreab E.
      • Yañez-Ruiz D.R.
      In vitro rumen microbial degradation of bromoform and the impact on rumen fermentation.
      ). Accumulation of iodine and bromide in milk (
      • Stefenoni H.A.
      • Räisänen S.E.
      • Cueva S.F.
      • Wasson D.E.
      • Lage C.F.A.
      • Melgar A.
      • Fetter M.E.
      • Smith P.
      • Hennessy M.
      • Vecchiarelli B.
      • Bender J.
      • Pitta D.
      • Cantrell C.L.
      • Yarish C.
      • Hristov A.N.
      Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
      ) and iodine in meat (
      • Roque B.M.
      • Venegas M.
      • Kinley R.D.
      • de Nys R.
      • Duarte T.L.
      • Yang X.
      • Kebreab E.
      Red seaweed (Asparagopsis taxiformis) supplementation reduces enteric methane by over 80 percent in beef steers.
      ) has been reported in animals fed Asparagopsis. Assuming a milk iodine concentration of 3 mg/L, as reported by
      • Stefenoni H.A.
      • Räisänen S.E.
      • Cueva S.F.
      • Wasson D.E.
      • Lage C.F.A.
      • Melgar A.
      • Fetter M.E.
      • Smith P.
      • Hennessy M.
      • Vecchiarelli B.
      • Bender J.
      • Pitta D.
      • Cantrell C.L.
      • Yarish C.
      • Hristov A.N.
      Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
      for cows supplemented Asparagopsis,
      • Lean I.J.
      • Golder H.M.
      • Grant T.M.D.
      • Moate P.J.
      A meta-analysis of effects of dietary seaweed on beef and dairy cattle performance and methane yield.
      estimated an iodine consumption 15-fold greater than the maximum tolerable for children younger than 3 yr drinking 1 L/d of milk. Further residue and safety studies are needed, including effects on organ histology of treated animals (
      • Glasson C.R.K.
      • Kinley R.D.
      • de Nys R.
      • King N.
      • Adams S.L.
      • Packer M.A.
      • Svenson J.
      • Eason C.T.
      • Magnusson M.
      Benefits and risks of including the bromoform containing seaweed Asparagopsis in feed for the reduction of methane production from ruminants.
      ). For other seaweeds, potential toxicity and residues in meat and milk will depend on the content of toxic minerals and the level of inclusion of seaweeds in the diet.
      The GHG emissions of growing, harvesting, processing (drying), storing, and transporting seaweeds on a large scale need to be considered in an LCA to determine the net effect on GHG intensity of meat and milk production. There is also potential to purify or extract seaweed bioactives, which would decrease emissions related to drying and transportation. The potential global depletion of stratospheric ozone was estimated to be relatively small for Australian Asparagopsis growth conditions (
      • Jia Y.
      • Quack B.
      • Kinley R.D.
      • Pisso I.
      • Tegtmeier S.
      Potential environmental impact of bromoform from Asparagopsis farming in Australia.
      ), but impacts on aquatic biodiversity would need to be considered if Asparagopsis were harvested directly from the ocean. On the other hand, seaweed cultivation can result in net CO2 fixation and export part of the carbon to the deep sea, where it can be buried in sediments (
      • Duarte C.M.
      • Wu J.
      • Xiao X.
      • Bruhn A.
      • Krause-Jensen D.
      Can seaweed farming play a role in climate change mitigation and adaptation?.
      ).
      • Ridoutt B.
      • Lehnert S.A.
      • Denman S.
      • Charmley E.
      • Kinley R.
      • Dominik S.
      Potential GHG emission benefits of Asparagopsis taxiformis feed supplement in Australian beef cattle feedlots.
      estimated that inclusion of Asparagopsis in Australian feedlot diets could substantially contribute to decrease net emissions of GHG from the feedlot sector in that economy.
      Consequently, adoption of Asparagopsis depends on the ability to sustainably grow the algae in aquaculture or marine systems with consistent concentration of the active compounds, which need to be maintained throughout transporting, handling, storage, and animal feeding. Concentrations of minerals such as iodine need to be controlled so that transfer to animal products does not exceed safe limits. In addition, the feeding of Asparagopsis may need to be approved by regulatory bodies before widespread adoption. Inclusion of other seaweeds in ruminant diets may be acceptable to consumers if there is no risk of toxicity and no off-flavors in meat or milk. More in vivo research is needed to determine CH4 mitigation and productivity changes under different diet and management conditions for both bromoform-containing and other seaweeds (
      • Lean I.J.
      • Golder H.M.
      • Grant T.M.D.
      • Moate P.J.
      A meta-analysis of effects of dietary seaweed on beef and dairy cattle performance and methane yield.
      ). Use of macroalgae as an antimethanogenic strategy may be feasible in confined and mixed systems, but it is likely to be challenging to implement in extensive systems. Animal delivery mechanisms that do not reduce the efficacy of the bioactive compounds of macroalgae need to be designed for supplementing animals in extensive systems.

      Alternative Electron Acceptors

      Alternative electron acceptors are organic (e.g., fumarate, malate) and inorganic (e.g., nitrate, sulfate) compounds that draw electrons away from methanogenesis and incorporate them into alternative pathways. Organic electron acceptors are rumen fermentation intermediates that are metabolized to VFA (mainly propionate), which can be absorbed and used by the ruminant host (
      • Carro M.D.
      • Ungerfeld E.M.
      Utilization of organic acids to manipulate ruminal fermentation and improve ruminant productivity.
      ). When completely reduced to ammonium, nitrate is incorporated into microbial protein, and also absorbed through the rumen wall and converted into urea in the liver and kidneys (
      • Yang C.
      • Rooke J.A.
      • Cabeza I.
      • Wallace R.J.
      Nitrate and inhibition of ruminal methanogenesis: Microbial ecology, obstacles, and opportunities for lowering methane emissions from ruminant livestock.
      ). Sulfate is reduced to hydrogen sulfide (dissimilatory reduction) and expelled, and is also incorporated into the synthesis of microbial sulfur amino acids (assimilatory reduction;
      • Drewnoski M.E.
      • Pogge D.J.
      • Hansen S.L.
      High-sulfur in beef cattle diets: A review.
      ).
      In general, in vivo effects of malate and fumarate on enteric CH4 production range from no effects in some studies to mild or moderate effects (10 to 23%) in others (
      • Carro M.D.
      • Ungerfeld E.M.
      Utilization of organic acids to manipulate ruminal fermentation and improve ruminant productivity.
      ). The average decrease in CH4 production in 56 treatment means from 24 studies in which nitrate was supplemented was estimated to be 13.9% at the mean nitrate dose (16.7 g/kg of DM), with greater efficacy in dairy than in beef cattle, in which the difference might be caused by the encapsulated nitrate used in most beef studies (
      • Feng X.Y.
      • Dijkstra J.
      • Bannink A.
      • van Gastelen S.
      • France J.
      • Kebreab E.
      Antimethanogenic effects of nitrate supplementation in cattle: A meta-analysis.
      ). The meta-analysis by
      • Arndt C.
      • Hristov A.N.
      • Price W.J.
      • McClelland S.C.
      • Pelaez A.M.
      • Cueva S.F.
      • Oh J.
      • Dijkstra J.
      • Bannink A.
      • Bayat A.R.
      • Crompton L.A.
      • Eugène M.A.
      • Enahoro D.
      • Kebreab E.
      • Kreuzer M.
      • McGee M.
      • Martin C.
      • Newbold C.J.
      • Reynolds C.K.
      • Schwarm A.
      • Shingfield K.J.
      • Veneman J.B.
      • Yáñez-Ruiz D.R.
      • Yu Z.
      Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5°C target by 2030 but not 2050.
      found that the inclusion of fumarate and nitrate as organic and inorganic electron acceptors, respectively, decreased total CH4 production by 16 and 17%, respectively. Nitrate supplementation decreased CH4 intensity for growth and milk production by 12 and 15%, respectively, as well as causing a slight decrease in DMI of 3% without affecting animal production. Fumarate supplementation did not affect CH4 intensity of milk production, and there was no information on its effect on CH4 intensity of body mass gain.
      Nitrate can be partially converted to N2O in the rumen and expelled (
      • Petersen S.O.
      • Hellwing A.L.F.
      • Brask M.
      • Højberg O.
      • Poulsen M.
      • Zhu Z.
      • Baral K.R.
      • Lund P.
      Dietary nitrate for methane mitigation leads to nitrous oxide emissions from dairy cows.
      ). Nitrous oxide is a very potent GHG; hence, supplementation with nitrate to mitigate CH4 emissions can have effects on the emissions of other GHG. Furthermore, if nitrate is supplemented to an N-sufficient diet, the extra N will be voided to the environment and increase N2O emissions to the atmosphere and contaminate ground water with nitrate. Nitrate supplementation does not benefit animal productivity unless added to an N-deficient diet (
      • Yang C.
      • Rooke J.A.
      • Cabeza I.
      • Wallace R.J.
      Nitrate and inhibition of ruminal methanogenesis: Microbial ecology, obstacles, and opportunities for lowering methane emissions from ruminant livestock.
      ), as is often the case in tropical and subtropical regions. In that regard,
      • Nguyen S.H.
      • Barnett M.C.
      • Hegarty R.S.
      Use of dietary nitrate to increase productivity and reduce methane production of faunated and defaunated lambs consuming protein-deficient chaff.
      reported an improvement with nitrate supplementation in DMI and ADG of lambs fed N-deficient chaff.
      Nitrite is an intermediate in the reduction of nitrate that can be absorbed through the rumen wall and react with hemoglobin to form methemoglobin, which cannot transport oxygen. This condition can be fatal, although it is possible to gradually adapt the rumen to nitrate supplementation (
      • Lee C.
      • Beauchemin K.A.
      A review of feeding supplementary nitrate to ruminant animals: Nitrate toxicity, methane emissions, and production performance.
      ;
      • Yang C.
      • Rooke J.A.
      • Cabeza I.
      • Wallace R.J.
      Nitrate and inhibition of ruminal methanogenesis: Microbial ecology, obstacles, and opportunities for lowering methane emissions from ruminant livestock.
      ). Traces of nitrate can pass to milk (
      • Guyader J.
      • Janzen H.H.
      • Kroebel R.
      • Beauchemin K.A.
      Forage use to improve environmental sustainability of ruminant production.
      ) and tissues (
      • Doreau M.
      • Arbre M.
      • Popova M.
      • Rochette Y.
      • Martin C.
      Linseed plus nitrate in the diet for fattening bulls: Effects on methane emission, animal health and residues in offal.
      ) but are considered safe for consumers. Nitrate feeding to animals is not approved in North America (
      • Beauchemin K.A.
      • Ungerfeld E.M.
      • Eckard R.J.
      • Wang M.
      Fifty years of research on rumen methanogenesis: Lessons learned and future challenges for mitigation.
      ), but carbon credits can be obtained by feeding nitrate to beef in Australia (https://www.legislation.gov.au/Details/F2015C00580). Due to the risks of acute toxicity, nitrate supplementation can only be recommended in production systems where feed intake is closely managed.
      The use of nitrate (e.g., calcium nitrate) as a source of nonprotein N is usually more expensive than urea (
      • Callaghan M.J.
      • Tomkins N.W.
      • Benu I.
      • Parker A.J.
      How feasible is it to replace urea with nitrates to mitigate greenhouse gases emissions from extensively managed beef cattle?.
      ). At present, the adoption of nitrate as an antimethanogenic strategy might be feasible in some cases but is mostly dependent on carbon market pricing, mitigation of ammonia and N2O emissions from manure, and the availability of safe in-feed delivery procedures.
      There are few studies of nitrate supplementation to grazing animals, with most being in mixed systems with nitrate offered mixed with substantial amounts of concentrates (
      • van Wyngaard J.D.V.
      • Meeske R.
      • Erasmus L.J.
      Effect of dietary nitrate on enteric methane emissions, production performance and rumen fermentation of dairy cows grazing kikuyu-dominant pasture during summer.
      ,
      • van Wyngaard J.D.V.
      • Meeske R.
      • Erasmus L.J.
      Effect of dietary nitrate on enteric methane emissions, production performance and rumen fermentation of dairy cows grazing ryegrass pasture during spring.
      ;
      • Granja-Salcedo Y.T.
      • Fernandes R.M.
      • Araujo R.C.
      • Kishi L.T.
      • Berchielli T.T.
      • Resende F.D.
      • Berndt A.
      • Siqueira G.R.
      Long-term encapsulated nitrate supplementation modulates rumen microbial diversity and rumen fermentation to reduce methane emission in grazing steers.
      ). Supplementing nitrate in a molasses lick block to grazing beef cows resulted in lower and more variable intake of the nitrate N supplement than urea blocks, resulting in lower free conceptus mass and BCS (
      • Callaghan M.J.
      • Tomkins N.W.
      • Hepworth G.
      • Parker A.J.
      The effect of molasses nitrate lick blocks on supplement intake, bodyweight, condition score, blood methaemoglobin concentration and herd scale methane emissions in Bos indicus cows grazing poor quality forage.
      ). High-nitrate-containing forages have mostly been investigated from the perspective of them causing toxicity, but there may also be merit in exploring their ability to lower enteric CH4 emissions in temperate climates.
      In general, the effects of fumarate and malate on animal productivity have been inconsistent. Malate stimulates Selenomonas ruminantium and can help prevent lactate acidosis by promoting lactate metabolism (
      • Carro M.D.
      • Ungerfeld E.M.
      Utilization of organic acids to manipulate ruminal fermentation and improve ruminant productivity.
      ). Fumarate and malate are natural intermediates of rumen fermentation regarded as safe and registered as feed ingredients in the European Union and the United States (
      • Carro M.D.
      • Ungerfeld E.M.
      Utilization of organic acids to manipulate ruminal fermentation and improve ruminant productivity.
      ). Feeding fumarate and malate to ruminants is largely limited by cost because of the relatively high levels of inclusion needed and the relatively small effects on CH4.

      Essential Oils

      Essential oils are complex mixtures of volatile lipophilic secondary metabolites that are responsible for a plant's characteristic flavor and fragrance (
      • Benchaar C.
      • Greathead H.
      Essential oils and opportunities to mitigate enteric methane emissions from ruminants.
      ). When extracted and concentrated, or chemically synthesized, essential oils may exert antimicrobial activities against bacteria and fungi (
      • Chao S.C.
      • Young D.G.
      • Oberg C.J.
      Screening for inhibitory activity of essential oils on selected bacteria, fungi and viruses.
      ). Chemically, essential oils are variable mixtures of principally terpenoids, and a variety of low-molecular-weight aliphatic hydrocarbons, acids, alcohols, aldehydes, acyclic esters or lactones, and, exceptionally, N- and S-containing compounds, coumarins, and homologs of phenylpropanoids (
      • Dorman H.J.D.
      • Deans S.G.
      Antimicrobial agents from plants: Antibacterial activity of plant volatile oils.
      ).
      Most essential oils exert their antimicrobial activities by interacting with processes associated with the bacterial cell membrane, including electron transport, ion gradients, protein translocation, phosphorylation, and other enzyme-dependent reactions (
      • Dorman H.J.D.
      • Deans S.G.
      Antimicrobial agents from plants: Antibacterial activity of plant volatile oils.
      ). Gram-positive bacteria appear to be more susceptible to the antibacterial properties of essential oils than gram-negative bacteria. However, phenolic compounds [e.g., thymol and carvacrol contained in some essential oils (e.g., thyme and oregano)] can inhibit the growth of gram-negative bacteria by disrupting the outer cell membrane (
      • Helander I.M.
      • Alakomi H.-L.
      • Latva-Kala K.
      • Mattila-Sandholm T.
      • Pol L.
      • Smid E.J.
      • Gorris L.G.M.
      • von Wright A.
      Characterization of the action of selected essential oil components on Gram-negative bacteria.
      ). Rumen gram-positive bacteria are involved in fermentation processes coupled with the production of CH4 through the release of H2 (
      • Owens F.N.
      • Goetsch A.L.
      Ruminal fermentation.
      ).
      Essential oils such as oregano, thyme, garlic oil, and its derivatives have been shown to decrease CH4 production in vitro (
      • Cobellis G.
      • Trabalza-Marinucci M.
      • Yu Z.
      Critical evaluation of essential oils as rumen modifiers in ruminant nutrition: A review.
      ) but results from in vivo studies have been far less conclusive (
      • Benchaar C.
      • Greathead H.
      Essential oils and opportunities to mitigate enteric methane emissions from ruminants.
      ;
      • Hristov A.N.
      • Melgar A.
      • Wasson D.
      • Arndt C.
      Symposium review: Effective nutritional strategies to mitigate enteric methane in dairy cattle.