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Intensive research in the past decade has resulted in a better understanding of factors driving enteric methane (CH4) emissions in ruminants. Meta-analyses of large databases, developed through the GLOBAL NETWORK project, have identified successful strategies for mitigation of CH4 emissions. Methane inhibitors, alternative electron sinks, vegetable oils and oilseeds, and tanniferous forages are among the recommended strategies for mitigating CH4 emissions from dairy and beef cattle and small ruminants. These strategies were also effective in decreasing CH4 emissions yield and intensity. However, a higher inclusion rate of oils may negatively affect feed intake, rumen function, and animal performance, specifically milk components in dairy cows. In the case of nitrates (electron sinks), concerns with animal health may be impeding their adoption in practice, and potential emission trade-offs have to be considered. Tannins and tanniferous forages may have a negative effect on nutrient digestibility, and more research is needed to confirm their effects on overall animal performance in long-term experiments with high-producing animals. A meta-analysis of studies with dairy cows fed the CH4 inhibitor 3-nitrooxypropanol (3-NOP) at the Pennsylvania State University showed (1) a consistent 28 to 32% decrease in daily CH4 emissions or emissions yield and intensity; (2) no effect on dry matter intake, milk production, body weight, or body weight change, and a slight increase in milk fat concentration and yield (0.19 percentage units and 90 g/d, respectively); 3-NOP also appears to increase milk urea nitrogen concentration; (3) an exponential decrease in the mitigation effect of the inhibitor with increasing its dose (from 40 to 200 mg/kg of feed dry matter, corresponding to 3-NOP intake of 1 to 4.8 g/cow per day); and (4) a potential decrease in the efficacy of 3-NOP over time, which needs to be further investigated in long-term, full-lactation or multiple-lactation studies. The red macroalga Asparagopsis taxiformis has a strong CH4 mitigation effect, but studies are needed to determine its feasibility, long-term efficacy, and effects on animal production and health. We concluded that widespread adoption of mitigation strategies with proven effectiveness by the livestock industries will depend on cost, government policies and incentives, and willingness of consumers to pay a higher price for animal products with decreased carbon footprint.
Livestock enteric CH4 mitigation is an old feed energy efficiency problem with new dimensions. Governments and the public are interested in finding solutions to climate change, and it is believed that mitigation of agricultural greenhouse gas (GHG) emissions is part of the solution (
). In the United States, agricultural activities are responsible for the generation of GHG such as CO2, CH4, and N2O, with the latter 2 gases being of primary interest: agriculture contributed 39% (CH4) and 80% (N2O) of their total emissions in 2019, on a CO2-equivalent basis (
Methane mitigation has been identified as essential for addressing climate change, and a recent analysis suggested that strategies exist to decrease global CH4 emissions from human activities by 50% within the next decade (
). These authors estimated that immediate implementation of available mitigation measures could slow warming by about 30% in the next decade and argued that efforts aimed at benefiting climate in the near term, such as CH4 mitigation, should be pursued simultaneously with long-term goals of reducing global CO2 emissions.
A Food and Agriculture Organization–commissioned analysis of livestock non-CO2 GHG mitigation strategies identified several viable interventions that can decrease CH4 and N2O emissions from enteric fermentation and manure management (
Hegarty, R. S., R. A. Cortez Passetti, K. M. Dittmer, Y. Wang, S. Shelton, J. Emmet-Booth, E. Wollenberg, T. McAllister, S. Leahy, K. Beauchemin, and N. Gurwick. 2021. 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).
) outlined opportunities and limitations of practices aimed at mitigating enteric CH4. New developments in the field, such as research with the CH4 inhibitor 3-nitrooxypropanol (3-NOP;
Effects of 3-nitrooxypropanol and varying concentrate feed proportions in the ration on methane emission, rumen fermentation and performance of periparturient dairy cows.
Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
) necessitate an updated analysis of available enteric CH4 mitigation options. Therefore, the current review, which was part of the 2021 ADSA symposium titled “Production, Management and the Environment: Advances in Enteric Methane Mitigation in Dairy Cattle—The Last Decade and Future Prospects” (https://www.adsa.org/Portals/0/SiteContent/Docs/Meetings/PastMeetings/Annual/2021/107.pdf)aims to (1) discuss recent developments in the area of enteric CH4 mitigation by nutritional means; (2) summarize findings of a comprehensive meta-analysis of enteric CH4 mitigation strategies conducted under the GLOBAL NETWORK project; and (3) perform a meta-analysis of production and CH4 emission data from 3-NOP experiments conducted at the Pennsylvania State University. The current review is focused on enteric CH4 emissions; however, it is important to note that agricultural GHG mitigation strategies should not be limited to direct livestock emissions (from enteric fermentation or manure management) but should include emissions from crop production as well. Although beyond the scope of this review, GHG emissions and sinks from growing livestock feed are also an important part of the carbon cycle and balance in an agricultural production system and must be considered in the context of sustainable global food production (see
in: Cavallaro N. Shrestha G. Birdsey R. Mayes M.A. Najjar R.G. Reed S.C. Romero-Lankao P. Zhu Z. Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report. U.S. Global Change Research Program,
2018: 229-263
A BRIEF OVERVIEW OF EXTANT NUTRITIONAL MITIGATION STRATEGIES FOR ENTERIC METHANE
The intention of this review is not to comprehensively cover and discuss nutritional interventions that have the potential to decrease enteric CH4 emissions in cattle; several reviews have done that (
Hegarty, R. S., R. A. Cortez Passetti, K. M. Dittmer, Y. Wang, S. Shelton, J. Emmet-Booth, E. Wollenberg, T. McAllister, S. Leahy, K. Beauchemin, and N. Gurwick. 2021. 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).
). In this section, we will offer a brief summary on the current state of some enteric CH4 mitigation strategies that are not explicitly covered in the following sections.
Increasing forage digestibility and digestible forage intake has the potential for decreasing enteric CH4 emission intensity (iCH4; i.e., g/d of CH4 per kg of product—milk, ECM, or ADG in beef cattle) and was one of the recommended mitigation practices in the analysis by
Replacing alfalfa silage with corn silage in dairy cow diets: Effects on enteric methane production, ruminal fermentation, digestion, N balance, and milk production.
Linseed oil supplementation to dairy cows fed diets based on red clover silage or corn silage: Effects on methane production, rumen fermentation, nutrient digestibility, N balance, and milk production.
). The mitigation effect of legumes (vs. grasses), however, is less consistent than that of corn silage. With some species, specifically red clover, slight increases in CH4 yield (yCH4; i.e., g/d of CH4 per kg of DMI) have been reported (
). Those authors also concluded that legumes rich in tannins can decrease overall GHG emissions by mitigating both enteric CH4 and manure N2O emissions (by shifting N excretion from urine to feces). To properly compare the carbon footprint of different forages in dairy cow nutrition, however, one has to account for GHG from forage production and soil carbon sequestration (
), which is beyond the scope of the current analysis.
Another mitigation practice that has been consistently found to decrease daily CH4 emissions and yCH4 or iCH4 is increasing the inclusion of concentrate feeds in the diet (replacing forages). Implementation of this practice, however, can be limited by feed availability, cost, and the risk of rumen function disturbances. Further, market improvements in the emitted CH4 energy as a percent of gross energy intake (Ym factor) can be expected beyond 35 to 40% concentrate inclusion in the diet, but the effect also depends on the overall level of feed intake (
). Arndt et al. (2021) reported that decreasing the dietary forage-to-concentrate ratio decreased yCH4 by an average of 13% (over control treatments within studies), increased milk yield (17%), and decreased iCH4 (9%); these effects were not associated with decreased fiber digestibility, which is typically reported for higher concentrate inclusion rates (
). The effect of this category of compounds on CH4 emission in controlled studies, however, has been inconsistent, and the strategy was not recommended in the analysis by
. Many studies have reported mitigation effects of botanicals or EO in vitro, but it is widely accepted that in vitro systems, although well suited for screening of large number of treatments, are not very representative of live animal responses, and it is strongly recommended that in vitro data are confirmed in in vivo experiments (
Hegarty, R. S., R. A. Cortez Passetti, K. M. Dittmer, Y. Wang, S. Shelton, J. Emmet-Booth, E. Wollenberg, T. McAllister, S. Leahy, K. Beauchemin, and N. Gurwick. 2021. 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).
); the compounds generally have a broad spectrum of activities, in some cases negatively affecting the overall rumen fermentation, and it has been suggested that the field may benefit from a more targeted compounds approach (
A meta-analysis describing the effects of the essential oils blend agolin ruminant on performance, rumen fermentation and methane emissions in dairy cows.
Hegarty, R. S., R. A. Cortez Passetti, K. M. Dittmer, Y. Wang, S. Shelton, J. Emmet-Booth, E. Wollenberg, T. McAllister, S. Leahy, K. Beauchemin, and N. Gurwick. 2021. 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).
Edenhofer O. Pichs-Madruga R. Sokona Y. Farahani E. Kadner S. Seyboth K. Adler A. Baum I. Brunner S. Eickemeier P. Kriemann B. Savolainen J. Schlömer S. von Stechow C. Zwickel T. Minx J.C. Cambridge University Press,
2014
Latin America Methane Project Collaborators Enteric methane mitigation strategies for ruminant livestock systems in the Latin America and Caribbean region: A meta-analysis.
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
. Briefly, the database used for the analysis included treatment means from in vivo studies published from 1964 to 2018. The final database included 425 peer-reviewed publications with cattle (66% of the studies), small ruminants (sheep and goats; 31%), and other ruminant species (buffalo, deer, yak; 3% of the studies). Mitigation strategies were classified into 3 main categories: animal and feed management, diet formulation, and rumen manipulation, and up to 5 subcategories (a total of 99 mitigation strategy combinations were examined). Daily CH4 emissions were analyzed in 783 mean comparisons, yCH4 in 598, and iCH4 in 260. The analysis also examined the effect of mitigation strategies on DMI, milk production (or daily gain), and total-tract NDF digestibility but these results are not discussed here (see
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
). Analyses were run across all ruminant species and included main mitigation strategies and their respective subcategories as potential moderator fixed effects. Details on the statistical procedures can be found in
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
The analysis identified several CH4 mitigation practices, for which there are sufficient published data, that substantially decrease daily CH4 emissions and do not negatively affect DMI and animal production to the extent that yCH4 and iCH4 would increase. Methane inhibitors (3-NOP and bromochloromethane) had the largest CH4 mitigation effect (Table 1) and had no effect on DMI, fiber digestibility, milk production, or ADG. Since 2018 [the last year that data were included in the
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
analysis. An extensive study on a commercial feedlot in Alberta, Canada, for example, reported a 26% decrease in CH4 emissions and a slight (nonsignificant) decrease in DMI, which resulted in a trend for increased feed efficiency in beef cattle fed a backgrounding diet (
3-Nitrooxypropanol decreased enteric methane production from growing beef cattle in a commercial feedlot: Implications for sustainable beef cattle production.
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
Alternative electron sinks (fumarate, data from 11 studies; nitrates, data from 24 studies) were another effective CH4 mitigation strategy identified by
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
. These rumen modifiers decreased daily CH4 emissions and both yCH4 and iCH4 (Table 1) and appeared to slightly increase milk yield (by 3%; P = 0.001), despite decreased DMI (1.5%; P = 0.008). Similar mitigation effect for nitrates was reported by
in a recent meta-analysis but that study pointed to a differential response in beef compared with dairy cattle, attributing the lower mitigation effect in beef to inclusion of slow-release nitrate studies in the database. Although effective, the nitrate strategy has been criticized as unsafe for the animal, particularly if applied in commercial settings. Another potential issue with nitrates could be increased N excretion in urine and, consequently, increased NH3 and N2O emissions from manure during storage or following soil application, although this has not yet been systematically studied. A recent report by
found 49% of the 15N from labeled nitrate in urine 141 h after being fed to sheep, approximately 41% of which was recovered as urinary urea. This led the authors to conclude that plasma nitrate is reduced to NH3 and partially converted to urea in the liver and excreted in urine, which suggests the possibility of increased manure N emissions when nitrates are fed to ruminants. The latter could be particularly problematic when diets are high in overall CP and RDP and excretion of nitrate-N in urine is increased. There are also indications that the practice can increase, albeit not substantially, enteric N2O emissions (
Dietary lipids and lipid supplements are well known to decrease enteric CH4 emissions and, along with oilseeds, were one of the recommended mitigation strategies identified by
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
; Table 1). In that analysis, lipids and oilseeds decreased daily CH4 emissions (g/d) by an average of 19% and decreased yCH4 and iCH4 (by 15% and by 12 to 22% for milk and meat production, respectively). Feed intake and fiber digestibility, however, were also decreased [by 5.7% (P < 0.001) and 4.2% (P = 0.005), respectively] but this did not result in decreased animal production. Lipids decrease iCH4 by providing a highly digestible source of energy to the host animal that can boost milk production (
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
indicated that within the oils and fats category, the CH4 mitigation effect was significant for vegetable oils but not for animal fat, which confirmed previous reports (
). Care must be taken when feeding vegetable oils to ruminants because of its detrimental effect, specifically that of PUFA, on ruminal fermentation, often leading to milk fat depression (
, “… lipids are effective in reducing enteric CH4 emission, but the feasibility of this mitigation practice depends on its cost-effectiveness and potential effects on feed intake (negative), productivity (negative) and milk fat content in lactating animals (positive or negative).” The
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
analysis indicated that oilseeds have a CH4 mitigation effect similar to that of oils, with perhaps the advantage that the oil may be released at a slower rate in the rumen, thus having a less harmful effect on the microbial ecosystem. Some by-product feeds routinely fed to dairy cows, such as extruded oilseed meals or high-oil distiller grains, could also decrease CH4 emissions (
). The effect of these feeds on CH4 emissions and animal performance, however, appears to be inconsistent. As an example, substitution of solvent-extracted soybean meal (1.3% oil) with extruded soybean meal (8.6% oil) had no effect on daily CH4 emissions or yCH4/iCH4 in dairy cows, despite a 25% increase in dietary ether extract concentration (
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
report. The mitigation effect was lower relative to the other recommended strategies (Table 1), and the authors indicated that fiber digestibility was also decreased (6.9%; P = 0.02), which could increase manure CH4 emissions. Among the forages included in the database, Sericea lespedeza (Lespedeza cuneata) reduced daily CH4 emissions by 32% (3 publications; P < 0.001). Other tanniferous forages that had some effect on CH4 emissions were Leucaena and Lotus (L.corniculatus and L. pedunculatus) with an average reduction of 8% (7 and 2 studies at P = 0.10 and P = 0.01, respectively). Studies with these forages did not report data on animal performance; therefore, effects on iCH4 could not be elucidated.
Full adoption of strategies to mitigate enteric methane emissions by ruminants and how they can help to meet the 1.5°C climate target by 2030 but not 2050.
reported that supplementation of the diet with condensed tannins did not affect daily CH4 emissions but decreased yCH4 by 12% (7 publications; P = 0.01) and fiber digestibility by 12% (5 publications; P = 0.003). An earlier meta-analysis reported that the relationship of dietary tannins with CH4 emissions was linear, but apparent digestibility of dietary OM and fiber were reduced (
). As discussed above, tannins (from grazing tanniferous forages or through dietary supplementation of condensed tannins) have the ability to shift N excretion from urine to feces, which would decrease N2O (and NH3) emissions from manure. Feeding of or grazing tanniferous forages is particularly attractive for extensive and pasture-based production systems, but more long-term research with high-producing animals is needed to conclude with confidence that tannins and tanniferous forages are a viable and cost-effective mitigation strategy for decreasing livestock GHG emissions. Due to these uncertainties,
Hegarty, R. S., R. A. Cortez Passetti, K. M. Dittmer, Y. Wang, S. Shelton, J. Emmet-Booth, E. Wollenberg, T. McAllister, S. Leahy, K. Beauchemin, and N. Gurwick. 2021. 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).
rated the CH4 reduction potential of tannins as low and the confidence in their efficacy at 2 (low agreement and medium evidence).
Meta-Analysis of Penn State's Experiments with 3-NOP
A meta-analysis of Penn State's 3-NOP data with lactating dairy cows (Table 2) was conducted to elucidate effects of the inhibitor on CH4 emissions and animal production variables. Cows involved in these studies were both primi- and multiparous Holsteins and in mid-lactation (with the exception of
, where cows entered the study within 3 d after calving). Average milk production was >40 kg/d, with milk components typical for the Penn State dairy herd. Diets had, on average, 31% amylase-treated NDF (aNDF), 44% NFC, and 5.3% ether extract.
Table 2Descriptive statistics of animal and diet data from Penn State experiments with dairy cows used in the 3-nitrooxypropanol (3-NOP) meta-analysis
) was a crossover design. The inclusion rate of 3-NOP was from 40 to 200 mg/kg of TMR DM. In all studies, 3-NOP was included in a treatment premix, with the control cows receiving a placebo premix. Feeding was once a day, in the morning, and diets included feed ingredients typically used at Penn State University's Dairy Teaching and Research Center (corn silage, alfalfa haylage, corn grain, canola meal, whole roasted soybeans, and others). Methane emission was measured using the GreenFeed system (C-Lock Inc.) following established protocols for tiestall and freestall barn settings (
, respectively). For the current meta-analysis, data were analyzed using the Comprehensive Meta-Analysis software (version 3.3.070; Biostat) with a random-effects model and values weighted by inverse variance (
). Means, standard deviations, and number of observations were used to compute the effect sizes. Heterogeneity between observed effect sizes was examined with the Cochran's Q test and the I2 statistic. Funnel plots were used to visually assess potential publication bias. The pooled effect size is reported with its 95% confidence interval (Table 3), where positive values indicate an increment and negative values indicate a reduction compared with the control.
Table 3A meta-analysis of the effect of 3-nitrooxypropanol (3-NOP) on production variables and enteric gas emissions in Penn State's experiments with dairy cows
The analysis showed no effect of 3-NOP on DMI, milk yield, ECM yield, feed efficiency (uncorrected milk and ECM bases), or BW and BW change of the cows (Table 3). Milk true protein and lactose concentrations and yield were also not affected by 3-NOP. Concentration of milk fat, however, was increased (P = 0.04) by 0.19 percentage units, and fat yield tended to increase (P = 0.06) by 90 g/d by 3-NOP compared with the control. Across studies, MUN concentration was also increased (P = 0.03) by 3-NOP compared with the control. Heterogeneity in the production responses to 3-NOP was negligible (I2 ≤ 29%).
Milk fat concentration was numerically higher for 3-NOP than for controls in all 5 experiments included in the meta-analysis; however, the difference was statistically significant in 3 (
) of the 5 studies. A likely explanation for the milk fat effect is the observed increase in rumen butyrate concentration (one of the main precursors for de novo synthesized fatty acids in milk) with 3-NOP and consequent increase in short-chain fatty acids, accompanied by a decrease in trans fatty acids, in milk fat (
). Our hypothesis for explaining the MUN effect of 3-NOP is based on the observed consistent increase in ruminal butyrate concentration by the inhibitor (
). Thus, increased absorption of NH3 would likely explain the increase in MUN in several studies from our laboratory included in the current meta-analysis (
The average reduction (P < 0.001) in daily CH4 emissions by 3-NOP was 123 g/d (or 28%; 310 vs. 433 g/d, 3-NOP and control, respectively; Table 3). The reductions (P < 0.001) in yCH4 and iCH4 were 4.91 g/kg of DMI (or 28%; 12.3 and 17.2 g/kg of DMI, respectively) and 3.74 g/kg of ECM (or 32%; 8.0 and 11.8 g/kg of ECM, respectively). Emissions of H2 were largely increased (P < 0.001; 1.41 vs. 0.13 g/d, respectively) by 3-NOP, but daily CO2 emissions and emission yield were not affected by the inhibitor. The intensity of CO2 emissions, however, was slightly decreased (by 18 g/kg of ECM, or 5%; P = 0.03) by 3-NOP compared with the control. Heterogeneity in the gas emission responses to 3-NOP was negligible (I2 ≤ 19%), except for yCH4 and H2 emission (I2 = 67 and 80%, respectively). The substantial heterogeneity in the H2 emission data can be explained by the large variability in the response among experiments, as these emissions are negligible or zero in control animals and dramatically increase with 3-NOP inhibition (see
The relationship between 3-NOP intake and yCH4 was exponential (Figure 1; P = 0.03), with the mitigation effect diminishing beyond 3-NOP intake of 2 to 2.4 g/cow per day. As discussed in
, although a numerically maximum mitigation effect was achieved at 150 mg of 3-NOP/kg of DMI (corresponding to an average 3-NOP intake of 3.7 g/cow per day), there was no difference in CH4 emission among 100, 150, and 200 mg/kg of DMI. Similarly, daily CH4 emission and iCH4 were exponentially decreased by 3-NOP intake (data not shown), and there was no further improvement in the mitigation effect beyond 100 mg of 3-NOP/kg of DMI.
Figure 1Relationship of 3-nitrooxypropanol (3-NOP) intake and enteric methane emission yield (g/kg of DMI) in dairy cows. Data are treatment means from
). At the end of that experiment, cows receiving 3-NOP in phase 2 of the main experiment were switched to the control treatment, and cows that were on the control treatment were fed 3-NOP for another 3 wk (experimental phase 3), and CH4 emissions were measured during wk 3. As Figure 2 shows, control cows in phase 3 (that were on 3-NOP in phase 2) had yCH4 similar to that of control cows in phase 2 (14.3 vs. 15.1 g/kg of DMI, respectively). Likewise, cows that were on the control treatment in phase 2 decreased their yCH4 to levels similar to those of 3-NOP cows in phase 2 (12.4 vs. 10.3 g/kg of DMI, respectively) when they received 3-NOP in phase 3. These data suggest that the mitigation effect of 3-NOP disappears rapidly after the compound is removed from the feed and also that the effect may be taking place within 2 wk of treatment. Indeed, the first published dairy cow study with 3-NOP (
) reported a sharp decline in CH4 emissions immediately after the inhibitor was administered through the rumen cannula, but the effect disappeared rapidly (within 2 to 3 h) after dosing. As pointed out in
Short communication: Relationship of dry matter intake with enteric methane emission measured with the GreenFeed system in dairy cows receiving a diet without or with 3-nitrooxypropanol.
, due to its chemical structure and solubility, the residence time of 3-NOP in the rumen is likely short. Those authors observed marked differences in the mitigation effect of 3-NOP during a 24-h feeding cycle; CH4 emissions were similar between control and 3-NOP 2 h before feeding, but 3-NOP cows had 45% lower CH4 emissions compared with the control immediately after feeding, with the mitigation effect decreasing to 13% at 4 h before feeding. These trends were clearly related to DMI (i.e., 3-NOP intake) by the cows. Thus,
Short communication: Relationship of dry matter intake with enteric methane emission measured with the GreenFeed system in dairy cows receiving a diet without or with 3-nitrooxypropanol.
concluded that the CH4 mitigation effect of 3-NOP is highest immediately after feeding (highest 3-NOP intake) and lowest before feeding (lowest 3-NOP intake).
Figure 2Enteric methane yield (g/kg of DMI) of dairy cows receiving 3-nitrooxypropanol (3-NOP). Data are from
and represent treatment means and associated SEM. In experimental phase 1, treatment cows received 3-NOP at 60 mg/kg of DMI for 15 wk, and data shown in graph are from experimental wk 15. In phase 2, control cows from phase 1 received 3-NOP at 60 mg/kg of DMI for 3 wk, and methane emissions were measured during wk 3. Cows receiving 3-NOP in phase 1 were control cows in phase 2.
An earlier meta-analysis of 3-NOP data (11 experiments) in dairy and beef cattle concluded that the CH4 mitigation effect of the inhibitor was less pronounced in beef (22% reduction in daily CH4 emissions) than in dairy (39% reduction) cattle and that NDF content of the diet impairs the effect of the additive (
). Another more recent meta-analysis (18 experiments) reported a trend for decreased DMI in beef (but not dairy) cattle and trends for decreased milk yield and increased concentration of milk fat and protein in dairy cows (
The effects of dietary supplementation with 3-nitrooxypropanol on enteric methane emissions, rumen fermentation, and production performance in ruminants: A meta-analysis.
The combined effects of supplementing monensin and 3-nitrooxypropanol on methane emissions, growth rate, and feed conversion efficiency in beef cattle fed high forage and high grain diets.
3-Nitrooxypropanol decreased enteric methane production from growing beef cattle in a commercial feedlot: Implications for sustainable beef cattle production.
Effects of 3-nitrooxypropanol and varying concentrate feed proportions in the ration on methane emission, rumen fermentation and performance of periparturient dairy cows.
. In that study with lactating dairy cows, the mitigation effect of 3-NOP (applied at around 48 mg/kg of DMI) disappeared after 8 wk of treatment, when cows were fed a high-forage/low-concentrate diet [70% forages; 40% aNDF on OM basis (aNDFOM) and 25% starch, feed DM basis], but not on the low-forage/high-concentrate diet (34% aNDFOM and 31% starch). This observation was in line with the analysis by
Effects of 3-nitrooxypropanol and varying concentrate feed proportions in the ration on methane emission, rumen fermentation and performance of periparturient dairy cows.
hypothesized that lower DMI and increased rumen retention time with the high-forage/low-concentrate diet resulted in enhanced H2 accumulation, which may have played a role in the diminishing mitigation effect of 3-NOP over time. Similarly,
reported a 7.5 to 26.1% (depending on the measurement method used) decrease over 90 d in the enteric CH4 emission reduction by 3-NOP (fed at 125 mg/kg of feed DM) in beef cattle on a commercial feedlot. Other studies from the same group, however, did not report signs of adaptation to 3-NOP over periods of 112 (
3-Nitrooxypropanol decreased enteric methane production from growing beef cattle in a commercial feedlot: Implications for sustainable beef cattle production.
). The averaged mitigation effect of 3-NOP (expressed as percentage reduction in yCH4 compared with the control within experiment) was regressed over the duration of the experiments as shown in Figure 3. There was an apparent linear decrease [adjusted R2 = 0.58 and 0.81; P = 0.15 and 0.02 for
study decreased from 31% during wk 3 to 24% during wk 15; in the first experiment, the rate of decrease was 0.61 g/kg of DMI per week and, in the second experiment, the decrease was 0.56 g/kg of DMI per week. These observations suggest a possible transient, or inconsistent, effect of 3-NOP over time in diets that are around 28 to 31% (or higher, as implied by
Effects of 3-nitrooxypropanol and varying concentrate feed proportions in the ration on methane emission, rumen fermentation and performance of periparturient dairy cows.
Figure 3Enteric methane yield of dairy cows receiving 3-nitrooxypropanol (3-NOP) treatments expressed as percent of the control within experiment. Data are from
documented a strong antimethanogenic effect of the red alga Asparagopsis taxiformis in sheep. Research groups around the globe have screened red, brown, and green macroalgae for antimethanogenic effects (
) and, although some species have shown promising results, Asparagopsis spp. (A.taxiformis and A.armata) appear to be the only ones with a confirmed mitigating effect in in vivo experiments with dairy and beef cattle (
Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
). Our current understanding is that the antimethanogenic activity of Asparagopsis spp. is based on its content of low-molecular-weight halogenated compounds, of which the brominated halomethane bromoform is dominant (
Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
Hegarty, R. S., R. A. Cortez Passetti, K. M. Dittmer, Y. Wang, S. Shelton, J. Emmet-Booth, E. Wollenberg, T. McAllister, S. Leahy, K. Beauchemin, and N. Gurwick. 2021. 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).
Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
). As our data clearly show, decreasing bromoform concentration and its intake will linearly diminish the mitigation potential of A. taxiformis (Figure 4; data from
Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
and unpublished data from A. N. Hristov). Based on these data, yCH4 will decrease by 1.5 to 2.0 g/kg of DMI for every 100 mg/d increase in bromoform intake. Long-term effects of seaweeds on animal productivity, health, reproduction, and milk quality need to be studied, and the economics of mass application in the global dairy and beef industries are unclear. In addition, data from a recent continuous design experiment indicated that the yCH4 mitigating effect of Asparagopsis taxiformis may be transient, disappearing by experimental week 9 (unpublished data by D. Wasson and A. N. Hristov). As a result of these uncertainties,
Hegarty, R. S., R. A. Cortez Passetti, K. M. Dittmer, Y. Wang, S. Shelton, J. Emmet-Booth, E. Wollenberg, T. McAllister, S. Leahy, K. Beauchemin, and N. Gurwick. 2021. 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).
rated the confidence in Asparagopsis spp. efficacy in their meta-analysis at 1 (low agreement and limited evidence). Other seaweed species have also shown promising preliminary results (Figure 5; unpublished data by D. Wasson and A. N. Hristov) that need to be confirmed in animal experiments. Research in this novel and exciting field will certainly continue in the near future, but its long-term impact on livestock GHG emissions is difficult to predict.
Figure 4Relationship of bromoform intake from Asparagopsis taxiformis and enteric methane emission yield in dairy cows [data from
Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
and unpublished data from D. Wasson and A. N. Hristov, the Pennsylvania State University]. Data are individual animal observations. Overall relationship: CH4 yield, g/kg of DMI = 17.4733 (SE = 0.8772) − 0.0198 (SE = 0.0020) × bromoform intake, mg/d (R2 = 0.59, P < 0.001); solid line (D. Wasson and A. N. Hristov, unpublished): CH4 yield, g/kg of DMI = 15.1197 (SE = 1.3051) − 0.0186 (SE = 0.0026) × bromoform intake, mg/d (R2 = 0.74, P < 0.001); long-dashed line (D. Wasson and A. N. Hristov, unpublished): CH4 yield, g/kg of DMI = 15.4765 (SE = 3.2812) − 0.0154 (SE = 0.0058) × bromoform intake, mg/d (R2 = 0.41, P = 0.02); short-dashed line (
Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
Figure 5Dose titration effect of seaweed X on methane and hydrogen emissions in vitro. Data are means and associated SE (unpublished data by D. Wasson and A. N. Hristov).
One element that is often missing in enteric CH4 discussions is the cost of implementing mitigation strategies with proven effectiveness. Our intention here is not to comprehensively discuss the literature on this topic but rather to bring the attention of the reader to this very important detail that may be a limiting factor in many national mitigation programs. The reader is referred to several analyses where the economics of mitigating livestock GHG emissions are discussed in detail (
, although the technical mitigation potential for the livestock sector may be large, the share that can be achieved at a reasonable economic cost is likely to be much smaller. As an example, the current price of wild-harvest, freeze-dried Asparagopsis taxiformis (discussed above) from one source (seaExpert, Feteira, Ilha Do Faial, Portugal) was €167.2/kg, excluding transportation. At a feeding rate of around 0.5% of dietary DM (
Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.
), inclusion of this seaweed in a dairy cow diet will cost the producer around $24/d, which is obviously unrealistic. It should be noted, however, that this price is expected to rapidly decrease if and when technologies for aquaculture production of Asparagopsis are developed and global supplies increase. Thus, it is safe to assume that without revolutionary improvements in technology or government and market interventions (i.e., carbon credits/market or tax), efficacy of a mitigation strategy alone will not be enough to ensure its adoption by the livestock industries. In addition, several key factors on the global scale need to be considered in the various mitigation scenarios, including emissions leakage (increasing livestock emissions in one region as a result of decreased animal production in another), rebound effects (i.e., increased demand for animal products as production efficiency increases and production costs decrease), emissions pricing, demand-side effects (including dietary changes), and others (
). Therefore, efficacy, although critically important, is only one piece of the complex puzzle of adoption of mitigation strategies for livestock GHG emissions.
CONCLUSIONS
Based on a recent meta-analysis, recommended enteric methane mitigation strategies included CH4 inhibitors, alternative electron sinks, vegetable oils and oilseeds, and tanniferous forages. These strategies were also effective in decreasing CH4 emissions yield and intensity, despite potential negative effects on DMI and fiber digestibility A meta-analysis of studies with lactating dairy cows fed the CH4 inhibitor 3-NOP at Penn State showed a consistent 28 to 32% decrease in daily CH4 emissions or emissions yield and intensity. There was no effect on DMI, milk production, or BW and BW change, but there was a slight increase in milk fat concentration and yield. Results with the red macroalga Asparagopsis taxiformis are encouraging, but more research, particularly long-term experiments, are needed before this mitigation practice can be recommended. We also conclude that widespread adoption by the livestock industries of mitigation strategies with proven efficacy will depend on cost, government policies and incentives, and willingness of consumers to pay a higher price for animal products with decreased carbon footprint.
ACKNOWLEDGMENTS
This work was supported by the USDA National Institute of Food and Agriculture (Washington, DC) Federal Appropriations under Project PEN 04539 and Accession number 1000803. The authors acknowledge the GLOBAL NETWORK project for generating part of the Arndt et al. (2021) database used in the current analysis. The GLOBAL NETWORK project (https://globalresearchalliance.org/research/livestock/collaborative-activities/global-research-project) is a multi-national initiative funded by the Joint Programming Initiative on Agriculture, Food Security and Climate Change and was coordinated by the Feed and Nutrition Network (https://globalresearchalliance.org/research/livestock/networks/feed-nutrition-network) within the Livestock Research Group of the Global Research Alliance on Agricultural GHG (https://globalresearchalliance.org; accessed February 18, 2022). A. Melgar was supported by the Government of Panama through the IFARHU-SENACYT (City of Knowledge, Panama) Scholarship Program and the Agricultural Innovation Institute of Panama (IDIAP, City of Knowledge, Panama). The authors have not stated any conflicts of interest.
Article info
Publication history
Published online: July 18, 2022
Accepted:
April 27,
2022
Received:
October 8,
2021
Footnotes
Presented as part of the Production, Management, and the Environment Fall Webinar: Advances in Enteric Methane Mitigation in Dairy Cattle—The Last Decade and Future Prospects at the ADSA Annual Meeting Webinar Series, September 2021.
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