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Symposium review: Effects of carbohydrate digestion on feed intake and fuel supply

Open AccessPublished:November 07, 2022DOI:https://doi.org/10.3168/jds.2022-22420

      ABSTRACT

      Carbohydrates are the primary energy source for lactating dairy cows, and dairy diets are usually formulated for certain concentrations of forage neutral detergent fiber (NDF) and starch due to their direct effects on dry matter intake and milk production. Forage NDF exerts greater filling effects in the rumen than other dietary components and can limit maximum voluntary feed intake of lactating dairy cows. Since an analytical method for NDF was developed more than a half century ago, it has been used widely to characterize forages and diets for dairy cows. However, because NDF is a chemical measurement varying in its digestibility, in vitro digestibility measurements were developed as a biological approach to assess forage quality. Research efforts over the last several decades led to the development of forage cultivars or hybrids with enhanced in vitro NDF digestibility, such as brown midrib, and management practices considering differences in NDF digestibility of forages. In addition, in vitro NDF digestibility and undigested NDF are commonly measured in commercial labs, and estimated rates of digestion are used in dynamic models in an effort to improve the accuracy and precision of diet formulation. Starch digestion in the rumen also varies among starch sources, being affected by grain type, extent of processing, and conservation method. The site and rate of starch digestion affect dry matter intake and nutrient partitioning in dairy cows by modifying temporal supply of fuel. In addition, dietary starch content and its fermentability can affect digestion rates of starch itself and NDF in the rumen. Previous research has increased our understanding of dietary carbohydrates, but its application for diet formulations requires integrated approaches accounting for factors affecting the filling effects of forage NDF, starch digestion, and temporal fuel supply.

      Key words

      INTRODUCTION

      Carbohydrates are approximately 70% of diets fed to lactating dairy cows and serve as the primary source of energy. The Dairy
      • NASEM (National Academies of Sciences, Engineering, and Medicine)
      Nutrient Requirements of Dairy Cattle.
      separated carbohydrates into the following 4 fractions based on the difference in digestibility: NDF, starch, neutral detergent soluble fiber, and water-soluble carbohydrates. Starch and NDF were relatively lower in digestibility among them, and the rate of digestion is greatly variable depending on their sources, affecting feed intake, fuel supply, and fuel utilization (
      • NASEM (National Academies of Sciences, Engineering, and Medicine)
      Nutrient Requirements of Dairy Cattle.
      ). Forages are more filling in the rumen than concentrates, and filling effects of forages are related to their NDF content (
      • Van Soest P.J.
      Symposium on factors influencing the voluntary intake of herbage by ruminants: Voluntary intake in relation to chemical composition and digestibility.
      ). Neutral detergent fiber has been used more than 50 years as a quality parameter of forages and diets fed to dairy cows, and dietary NDF content is also used as an independent variable to predict DMI of dairy cows (
      • Mertens D.R.
      Predicting intake and digestibility using mathematical models of ruminal function.
      ). However, forage NDF greatly varies in its digestion characteristics and NDF alone, as a chemical measurement, is not sufficient to predict DMI responses of lactating dairy cows (
      • Allen M.S.
      Effects of diet on short-term regulation of feed intake by lactating dairy cattle.
      ). Starch digestibility is affected by grain type, endosperm type, processing method, and conservation method, and the extent of starch digestion in the rumen can affect DMI (
      • Allen M.S.
      Effects of diet on short-term regulation of feed intake by lactating dairy cattle.
      ). The objective of this paper is to review factors affecting filling effects of forage NDF and starch digestibility of grain, and to discuss effects of carbohydrate digestion on feed intake and fuel supply.

      FILLING EFFECTS OF FORAGE NDF

      Digesta is selectively retained in the rumen depending on its particle size. In early work using sheep or cattle (
      • Evans E.W.
      • Pearce G.R.
      • Burnett J.
      • Pillinger S.L.
      Changes in some physical characteristics of the digesta in the reticulo-rumen of cows fed once daily.
      ;
      • Poppi D.P.
      • Norton B.W.
      • Minson D.J.
      • Hendricksen R.E.
      The validity of the critical size theory for particles leaving the rumen.
      ,
      • Poppi D.P.
      • Minson D.J.
      • Ternouth J.H.
      Studies of cattle and sheep eating leaf and stem fractions of grasses. 3. The retention time in the rumen of large feed particles.
      ), the fraction of particles retained on the 1.18-mm sieve was proposed as the critical particle size to be selectively retained in the rumen.
      • Maulfair D.D.
      • Fustini M.
      • Heinrichs A.J.
      Effect of varying total mixed ration particle size on rumen digesta and fecal particle size and digestibility in lactating dairy cows.
      reported that diet particle size affects rumen digesta particle size for particles retained on ≥3.35-mm sieves, but not for <3.35-mm sieves, and suggested that the critical particle size to be retained in the rumen is likely greater than the particles retained on the 1.18-mm sieve. In high-producing dairy cows, the fraction retained on the 2.36-mm sieve was selected as the threshold for passage based on sieving analysis of duodenal digesta (
      • Voelker Linton J.A.
      • Allen M.S.
      Nutrient demand affects ruminal digestion responses to a change in dietary forage concentration.
      ;
      • Kammes K.L.
      • Allen M.S.
      Rates of particle size reduction and passage are faster for legume compared to cool-season grass, resulting in lower rumen fill and less effective fiber.
      ). Another factor affecting retention of rumen digesta is its location relative to the reticulo-omasal orifice.
      • Voelker Linton J.A.
      • Allen M.S.
      Nutrient demand affects ruminal digestion responses to a change in dietary forage concentration.
      fed diets with corn silage as the primary forage and showed that the rate of passage is much slower for potential digestible NDF (pdNDF) compared with indigestible NDF (iNDF), and attributed it to the difference in buoyancy. Digestible NDF is associated with fermentation gas, and more buoyant in the rumen and lower in specific gravity, which makes it form the fiber mat in the rumen and away from the reticulo-omasal orifice.
      • Kammes K.L.
      • Allen M.S.
      Rates of particle size reduction and passage are faster for legume compared to cool-season grass, resulting in lower rumen fill and less effective fiber.
      suggested that entrapment by the fiber mat can slow down digesta passage regardless of potential difference in buoyancy or specific gravity for cows fed alfalfa silage or grass silage. As such, particle size reduction does not necessarily mean its passage out of the rumen, but it is considered as a prerequisite to ruminal passage. It is important to understand factors affecting particle size of ruminal digesta as they are related to filling effects of forage NDF.

      Particle Size of Forages

      Effects of forage particle size on DMI are not consistent; for example, some reported that cows fed forages with short particles increased DMI (
      • Mooney C.S.
      • Allen M.S.
      Physical effectiveness of the neutral detergent fiber of whole linted cottonseed relative to that of alfalfa silage at two lengths of cut.
      ;
      • Kononoff P.J.
      • Heinrichs A.J.
      The effect of reducing alfalfa haylage particle size on cows in early lactation.
      ;
      • Haselmann A.
      • Zehetgruber K.
      • Fuerst-Waltl B.
      • Zollitsch W.
      • Knaus W.
      • Zebeli Q.
      Feeding forages with reduced particle size in a total mixed ration improves feed intake, total-tract digestibility, and performance of organic dairy cows.
      ), whereas others reported no effects of forage particle size (
      • Krause K.M.
      • Combs D.K.
      • Beauchemin K.A.
      Effects of forage particle size and grain fermentability in midlactation cows. I. Milk production and diet digestibility.
      ;
      • Beauchemin K.A.
      • Yang W.Z.
      • Rode L.M.
      Effects of particle size of alfalfa-based dairy cow diets on chewing activity, rumen fermentation, and milk production.
      ) or the opposite effects (
      • Krause K.M.
      • Combs D.K.
      Effects of forage particle size, forage source, and grain fermentability on performance and ruminal pH in midlactation cows.
      ). The inconsistent effects are partly attributed to mastication activity during eating.
      • Schadt I.
      • Ferguson J.D.
      • Azzaro G.
      • Petriglieri R.
      • Caccamo M.
      • Van Soest P.
      • Licitra G.
      How do dairy cows chew?—Particle size analysis of selected feeds with different particle length distributions and of respective ingested bolus particles.
      fed hay components that were retained on each screen of the Penn State Particle Separator, and measured particle size of swallowed digesta. They reported mean particle size of the swallowed digesta was similar at 8.1 to 10.8 mm regardless of initial particle size (
      • Schadt I.
      • Ferguson J.D.
      • Azzaro G.
      • Petriglieri R.
      • Caccamo M.
      • Van Soest P.
      • Licitra G.
      How do dairy cows chew?—Particle size analysis of selected feeds with different particle length distributions and of respective ingested bolus particles.
      ), indicating that the differences in forage particle size is substantially reduced by mastication during eating. If the swallowed digesta becomes similar in particle size regardless of the initial differences in the diet, it may not affect digestion kinetics in the rumen. Digestion rate of pdNDF and passage rate of iNDF or pdNDF were not affected by particle size of dietary forage for cows fed alfalfa silage (
      • Kammes K.L.
      • Ying Y.
      • Allen M.S.
      Nutrient demand interacts with legume particle length to affect digestion responses and rumen pool sizes in dairy cows.
      ) or grass silage (
      • Kammes K.L.
      • Allen M.S.
      Nutrient demand interacts with grass particle length to affect digestion responses and chewing activity in dairy cows.
      ).
      Another possible reason accounting for the inconsistent animal responses in DMI to forage particle size is whether rumen fill is the dominant factor in regulation of DMI.
      • Dado R.G.
      • Allen M.S.
      Intake limitations, feeding behavior, and rumen function of cows challenged with rumen fill from dietary fiber or inert bulk.
      showed that rumen fill affected DMI for cows fed a high NDF diet, but not for cows fed a low NDF diet.
      • Beauchemin K.A.
      • Farr B.I.
      • Rode L.M.
      • Schaalje G.B.
      Effects of alfalfa silage chop length and supplementary long hay on chewing and milk production of dairy cows.
      reported a significant interaction between dietary forage allocation and forage particle size on DMI, in which greater forage particle size decreased DMI only for cows fed a 65% forage diet, but not for a 35% forage diet. Particle size of forages may not exert consistent effects on particle size of ruminal digesta, filling effects of forage NDF, or DMI.

      In Vitro NDF Digestibility

      Forages with enhanced in vitro NDF digestibility can increase DMI of lactating dairy cows.
      • Oba M.
      • Allen M.S.
      Effects of brown midrib 3 mutation in corn silage on dry matter intake and productivity of high yielding dairy cows.
      ,
      • Oba M.
      • Allen M.S.
      Effects of brown midrib 3 mutation in corn silage on productivity of dairy cows fed two concentration of dietary neutral detergent fiber: 1. Feeding behavior and nutrient utilization.
      ) showed that feeding brown midrib corn silage, which is higher in in vitro NDF digestibility than control corn silage, increased DMI of lactating dairy cows. Some recent studies consistently reported positive effects of in vitro NDF digestibility of forages on DMI for cows fed alfalfa hay (
      • Fustini M.
      • Palmonari A.
      • Canestrari G.
      • Bonfante E.
      • Mammi L.
      • Pacchioli M.T.
      • Sniffen C.J.
      • Grant R.J.
      • Cotanch K.W.
      • Formigoni A.
      Effect of undigested neutral detergent fiber content of alfalfa hay on lactating dairy cows: Feeding behavior, fiber digestibility, and lactation performance.
      ) or corn silage (
      • Miller M.D.
      • Lanier J.S.
      • Kvidera S.K.
      • Dann H.M.
      • Ballard C.S.
      • Grant R.J.
      Evaluation of source of corn silage and trace minerals on lactational performance and total tract nutrient digestibility of Holstein cows.
      ;
      • Miller M.D.
      • Kokko C.
      • Ballard C.S.
      • Dann H.M.
      • Fustini M.
      • Palmonari A.
      • Formigoni A.
      • Cotanch K.W.
      • Grant R.J.
      Influence of fiber degradability of corn silage in diets with lower and higher fiber content on lactational performance, nutrient digestibility, and ruminal characteristics in lactating Holstein cows.
      ). A meta-regression of treatment means from the literature showed a positive relationship between DMI and in vitro NDF digestibility within forage family (i.e., grass or legume;
      • Oba M.
      • Allen M.S.
      Evaluation of the importance of the digestibility of neutral detergent fiber from forage: Effects on dry matter intake and milk yield of dairy cows.
      ). However, DMI and in vitro NDF digestibility were related negatively across forage family (
      • Oba M.
      • Allen M.S.
      Evaluation of the importance of the digestibility of neutral detergent fiber from forage: Effects on dry matter intake and milk yield of dairy cows.
      ). Similarly, a recent meta-analysis by
      • Johansen M.
      • Lund P.
      • Weisbjerg M.R.
      Feed intake and milk production in dairy cows fed different grass and legume species: A meta-analysis.
      showed that feeding legumes increased both DMI and milk yield compared with grasses. These findings indicate that in vitro NDF digestibility may be a good indicator of filling effects for comparisons within forage families but cannot be used as an overall indicator of filling effects of forages that works for all comparison of forages.

      Fragility

      The filling effects of forage NDF are affected by fragility of forage particles (
      • Allen M.S.
      Effects of diet on short-term regulation of feed intake by lactating dairy cattle.
      ). Forage fragility can be determined by measuring electrical energy required to grind samples (
      • Minson D.J.
      • Cowper J.L.
      An integrating wattmeter for measuring the energy used to grind pasture samples.
      ) or the difference in the fraction of particles retained on the 1.18-mm sieve before and after ball milling (
      • Farmer E.R.
      • Tucker H.A.
      • Dann H.M.
      • Cotanch K.W.
      • Mooney C.S.
      • Lock A.L.
      • Yagi K.
      • Grant R.J.
      Effect of reducing dietary forage in lower starch diets on performance, ruminal characteristics, and nutrient digestibility in lactating Holstein cows.
      ). Fragility should be distinguished from digestibility as ruminal digesta does not need to be completely digested to disappear from the rumen, but its particle size can be also reduced by mastication and detrition to become small enough, allowing for passage. As such, fragility that affects the rate of particle size reduction in the rumen can be an independent factor affecting filling effects of forages.
      • Voelker Linton J.A.
      • Allen M.S.
      Nutrient demand affects ruminal digestion responses to a change in dietary forage concentration.
      determined fragility by measuring the reduction rate of iNDF from large particles (≥2.36 mm) to small particles (<2.36 mm) in the rumen based on a pool and flux method, where intake, duodenal flow, and rumen pool of iNDF in ≥2.36-mm particles were used for the calculation. In their study (
      • Voelker Linton J.A.
      • Allen M.S.
      Nutrient demand affects ruminal digestion responses to a change in dietary forage concentration.
      ), greater dietary forage NDF content (27.3 vs. 19.9% of dietary DM) increased rumen pH (6.00 vs. 5.86), digestion rate of pdNDF (4.74 vs. 3.32%/h), ruminal NDF digestibility (40.1 vs. 29.3%), and rate of particle size reduction (6.92 vs. 3.81%/h), indicating that fragility is related to NDF digestion parameters.
      Although in vitro NDF digestibility and iNDF are expected to be related to intrinsic differences in fragility of forages, this may not hold true for comparisons between legume and grass; for example, alfalfa silage had a greater rate of particle size reduction compared with orchardgrass silage (7.16 vs. 4.67%/h), although it was lower in in vitro NDF digestibility (38.3 vs. 53.3%) and higher in iNDF content (23.0 vs. 16.1%;
      • Kammes K.L.
      • Allen M.S.
      Rates of particle size reduction and passage are faster for legume compared to cool-season grass, resulting in lower rumen fill and less effective fiber.
      ). However, the digestion rate of pdNDF was higher for cows fed alfalfa silage (7.27 vs. 4.74%/h;
      • Kammes K.L.
      • Allen M.S.
      Nutrient demand interacts with forage family to affect digestion responses in dairy cows.
      ), which is consistent with the faster rate of particle size reduction (
      • Kammes K.L.
      • Allen M.S.
      Rates of particle size reduction and passage are faster for legume compared to cool-season grass, resulting in lower rumen fill and less effective fiber.
      ). As discussed above, the positive relationship between rate of NDF digestion and fragility was also shown when forage NDF sources were the same (
      • Voelker Linton J.A.
      • Allen M.S.
      Nutrient demand affects ruminal digestion responses to a change in dietary forage concentration.
      ). These findings collectively indicate that fragility is affected more by the rate of NDF digestion rather than NDF digestibility or pdNDF content.
      • Kammes K.L.
      • Allen M.S.
      Rates of particle size reduction and passage are faster for legume compared to cool-season grass, resulting in lower rumen fill and less effective fiber.
      demonstrated that legume, which is greater in fragility than grass, decreased the ruminal pool of large NDF particles and increased the passage rate of small NDF particles. Fragility may play a direct role in reducing filling effects of forage NDF, and a common analytical approach for fragility measurement needs to be established.

      STARCH DIGESTIBILITY OF GRAIN

      Over the last 35 years, factors affecting starch digestibility in the rumen and effects of greater starch digestion on DMI have been extensively studied. Grain type affects starch digestibility, and its ruminal degradability is in the order of oats, wheat, barley, corn, and milo, from high to low (
      • Herrera-Saldana R.E.
      • Huber J.T.
      • Poore M.H.
      Dry matter, crude protein, and starch degradability of five cereal grains.
      ). Researchers in Illinois showed that steam-rolled barley is more fermentable in the rumen compared with dry ground corn, and cows fed barley grain decreased DMI by more than 3 kg/d (
      • McCarthy Jr., R.D.
      • Klusmeyer T.H.
      • Vicini J.L.
      • Clark J.H.
      • Nelson D.R.
      Effects of source of protein and carbohydrates on ruminal fermentation and passage of nutrients to the small intestine of lactating cows.
      ;
      • Overton T.R.
      • Cameron M.R.
      • Elliott J.P.
      • Clark J.H.
      • Nelson D.R.
      Ruminal fermentation and passage of nutrients to the duodenum of lactating cows fed mixtures of corn and barley.
      ).
      • Silveira C.
      • Oba M.
      • Beauchemin K.A.
      • Helm J.
      Effect of grains differing in expected ruminal fermentability on productivity of lactating dairy cows.
      also reported that cows fed barley grain decreased DMI (21.6 vs. 23.6 kg/d). However, effects of grain type on DMI have not been consistent, and the discrepancies can be attributed to the differences in the level of dietary grain allocation, dietary starch concentration, and availability of physically effective fiber among studies. For example,
      • Grings E.E.
      • Roffler R.E.
      • Deitelhoff D.P.
      Evaluation of corn and barley as energy sources for cows in early lactation fed alfalfa-based diets.
      and
      • Doepel L.
      • Cox A.
      • Hayirli A.
      Effects of increasing amounts of dietary wheat on performance and ruminal fermentation of Holstein cows.
      reported that grain type did not affect DMI in their studies, but they fed low-starch (<22%) diets.
      Conservation method (high-moisture corn vs. dry ground corn) has consistent effects on DMI. Cows fed high-moisture corn had greater starch digestibility in the total-tract (
      • Krause K.M.
      • Combs D.K.
      • Beauchemin K.A.
      Effects of forage particle size and grain fermentability in midlactation cows. I. Milk production and diet digestibility.
      ;
      • Krause K.M.
      • Combs D.K.
      Effects of forage particle size, forage source, and grain fermentability on performance and ruminal pH in midlactation cows.
      ) or in the rumen (
      • Oba M.
      • Allen M.S.
      Effects of corn grain conservation method on ruminal digestion kinetics for lactating dairy cows at two dietary starch concentrations.
      ;
      • Allen M.S.
      • Ying Y.
      Effects of corn grain endosperm type and conservation method on site of digestion, ruminal digestion kinetics, and flow of nitrogen fractions to the duodenum in lactating dairy cows.
      ), and all these researchers reported lower DMI for cows fed high-moisture corn. However, it is noteworthy that
      • Oba M.
      • Allen M.S.
      Effects of corn grain conservation method on feeding behavior and productivity of lactating dairy cows at two dietary starch concentrations.
      reported an interaction effect between dietary starch content and conservation method of corn grain, in which high-moisture corn decreased DMI (20.8 vs. 22.5 kg/d) only for cows fed a high-starch (32%) diet but not a low-starch (21%) diet. In their study (
      • Oba M.
      • Allen M.S.
      Effects of corn grain conservation method on ruminal digestion kinetics for lactating dairy cows at two dietary starch concentrations.
      ), feeding high-moisture corn increased the rate of starch digestion compared with dry ground corn to a greater extent in the high-starch diet (28.2 vs. 14.6%/h) than in the low-starch diet (16.8 vs. 12.2%/h), which is consistent with animal responses in DMI.
      Grain processing can increase starch digestibility in the rumen. In steam rolling/flaking, heat and moisture are added to the grain to gelatinize starch and denature protein matrix that protects starch granules, and the extent of processing is indicated by the specific gravity with lower values meaning greater processing. Although greater extent of processing increased total-tract starch digestibility, it did not decrease DMI of lactating dairy cows in the literature (
      • Plascencia A.
      • Zinn R.A.
      Influence of flake density on the feeding value of steam-processed corn in diets for lactating cows.
      ;
      • Yu P.
      • Huber J.T.
      • Santos F.A.P.
      • Simas J.M.
      • Theurer C.B.
      Effects of ground, steam-flaked, and steam-rolled corn grains on performance of lactating cows.
      ;
      • Yang W.Z.
      • Beauchemin K.A.
      • Rode L.M.
      Effects of barley grain processing on extent of digestion and milk production of lactating cows.
      ). Grinding is another common processing method of grain, which increases starch digestibility by increasing the surface area of grain and exposing more of the endosperm for microbial attachment and enzyme access. Fine-grinding increased starch digestibility in the total-tract (
      • Knowlton K.F.
      • Allen M.S.
      • Erickson P.S.
      Lasalocid and particle size of corn grain for dairy cows in early lactation. 1. Effect on performance, serum metabolites, and nutrient digestibility.
      ) or in the rumen (
      • Rémond D.
      • Cabrera-Estrada J.I.
      • Champion M.
      • Chauveau B.
      • Coudure R.
      • Poncet C.
      Effect of corn particle size on site and extent of starch digestion in lactating dairy cows.
      ;
      • Allen M.S.
      • Longuski R.A.
      • Ying Y.
      Effects of corn grain endosperm type and fineness of grind on site of digestion, ruminal digestion kinetics, and flow of nitrogen fractions to the duodenum in lactating dairy cows.
      ) compared with coarse-grinding, but it did not affect DMI. Dry ground corn is relatively low in ruminal starch digestibility, and even with fine-grinding may not have increased starch digestibility to an extent to affect DMI; it was 58.6% (
      • Rémond D.
      • Cabrera-Estrada J.I.
      • Champion M.
      • Chauveau B.
      • Coudure R.
      • Poncet C.
      Effect of corn particle size on site and extent of starch digestion in lactating dairy cows.
      ) or 50.6% (
      • Allen M.S.
      • Longuski R.A.
      • Ying Y.
      Effects of corn grain endosperm type and fineness of grind on site of digestion, ruminal digestion kinetics, and flow of nitrogen fractions to the duodenum in lactating dairy cows.
      ), which is far lower than ruminal starch digestibility of steam-rolled barley grain (
      • McCarthy Jr., R.D.
      • Klusmeyer T.H.
      • Vicini J.L.
      • Clark J.H.
      • Nelson D.R.
      Effects of source of protein and carbohydrates on ruminal fermentation and passage of nutrients to the small intestine of lactating cows.
      ;
      • Overton T.R.
      • Cameron M.R.
      • Elliott J.P.
      • Clark J.H.
      • Nelson D.R.
      Ruminal fermentation and passage of nutrients to the duodenum of lactating cows fed mixtures of corn and barley.
      ) or high-moisture corn (
      • Oba M.
      • Allen M.S.
      Effects of corn grain conservation method on ruminal digestion kinetics for lactating dairy cows at two dietary starch concentrations.
      ;
      • Allen M.S.
      • Ying Y.
      Effects of corn grain endosperm type and conservation method on site of digestion, ruminal digestion kinetics, and flow of nitrogen fractions to the duodenum in lactating dairy cows.
      )
      Endosperm type of corn grain (floury vs. vitreous) also affects starch digestibility in the rumen (
      • Taylor C.C.
      • Allen M.S.
      Corn grain endosperm type and brown midrib 3 corn silage: Site of digestion and ruminal digestion kinetics in lactating cows.
      ;
      • Allen M.S.
      • Longuski R.A.
      • Ying Y.
      Effects of corn grain endosperm type and fineness of grind on site of digestion, ruminal digestion kinetics, and flow of nitrogen fractions to the duodenum in lactating dairy cows.
      ) or in the total-tract (
      • Allen M.S.
      • Ying Y.
      Effects of corn grain endosperm type and conservation method on site of digestion, ruminal digestion kinetics, and flow of nitrogen fractions to the duodenum in lactating dairy cows.
      ), but has less consistent effects on DMI. Feeding floury corn did not affect DMI in one study (
      • Allen M.S.
      • Ying Y.
      Effects of corn grain endosperm type and conservation method on feed intake, feeding behavior, and productive performance of lactating dairy cows.
      ), tended to decrease DMI in another study (
      • Allen M.S.
      • Longuski R.A.
      • Ying Y.
      Effects of corn grain endosperm type and fineness of grind on feed intake, feeding behavior, and productive performance of lactating dairy cows.
      ), and decreased DMI only for cows fed corn silage with lower in vitro NDF digestibility in the other study (
      • Taylor C.C.
      • Allen M.S.
      Corn grain endosperm type and brown midrib 3 corn silage: Site of digestion and ruminal digestion kinetics in lactating cows.
      ). Other dietary factors such as primary forage source or starch source might have exerted greater effects on rumen fermentation, masking effects of endosperm type of grain.

      EFFECTS OF FEEDING SUGAR

      Sugar (water-soluble carbohydrate) is another carbohydrate fed in the diet of lactating dairy cows, and ferments faster than starch in general (
      • Oba M.
      Review: Effects of feeding sugars on productivity of lactating dairy cows.
      ). Although replacing dietary starch with sugar is expected to increase ruminal fermentation, it often increases rumen pH (
      • Chamberlain D.G.
      • Robertson S.
      • Choung J.J.
      Sugars versus starch as supplements to grass silage: Effects on ruminal fermentation and the supply of microbial protein to the small intestine, estimated from the urinary excretion of purine derivatives, in sheep.
      ;
      • Heldt J.S.
      • Cochran R.C.
      • Stokka G.K.
      • Farmer C.G.
      • Mathis C.P.
      • Titgemeyer E.C.
      • Nagaraja T.G.
      Effects of different supplemental sugars and starch fed in combination with degradable intake protein on low-quality forage use by beef steers.
      ;
      • Penner G.B.
      • Oba M.
      Increasing dietary sugar concentration may improve dry matter intake, ruminal fermentation, and productivity of dairy cows in the postpartum phase of the transition period.
      ). It is partly because sugar provides approximately 10% less carbon than starch per unit of mass due to less glycosidic bonds (
      • Hall M.B.
      • Herejk C.
      Differences in yields of microbial crude protein from in vitro fermentation of carbohydrates.
      ). In addition, sugar fermentation in the rumen increases butyrate production rather than propionate (
      • Vallimont J.E.
      • Bargo F.
      • Cassidy T.W.
      • Luchini N.D.
      • Broderick G.A.
      • Varga G.A.
      Effects of replacing dietary starch with sucrose on ruminal fermentation and nitrogen metabolism in continuous culture.
      ;
      • Ribeiro C.V.D.M.
      • Karnati S.K.R.
      • Eastridge M.L.
      Biohydrogenation of fatty acids and digestibility of fresh alfalfa or alfalfa hay plus sucrose in continuous culture.
      ;
      • Gao X.
      • Oba M.
      Effect of increasing dietary nonfiber carbohydrate with starch, sucrose, or lactose on rumen fermentation and productivity of lactating dairy cows.
      ), leading to less fermentation acid production per unit of fermented hexose (
      • Oba M.
      Review: Effects of feeding sugars on productivity of lactating dairy cows.
      ) and faster absorption (
      • Dijkstra J.
      • Boer H.
      • van Bruchem J.
      • Bruining M.
      • Tamminga S.
      Absorption of volatile fatty acids from the rumen of lactating dairy cows as influenced by volatile fatty acid concentration, pH, and rumen liquid volume.
      ). Furthermore, sugar would decrease fermentation acid production in the rumen if it is used for microbial glycogen storage (
      • Hall M.B.
      • Weimer P.J.
      Sucrose concentration alters fermentation kinetics, products, and carbon fates during in vitro fermentation with mixed ruminal microbes.
      ).
      Although sugar ferments quickly in the rumen, feeding sugar in place of starch often increased DMI of dairy cows possibly due to preference for sweet taste (
      • Nombekela S.W.
      • Murphy M.R.
      • Gonyou H.W.
      • Marden J.I.
      Dietary preferences in early lactation cows as affected by primary tastes and some common feed flavors.
      ) or decreasing propionate flux to the liver (
      • Oba M.
      Review: Effects of feeding sugars on productivity of lactating dairy cows.
      ).
      • Broderick G.A.
      • Luchini N.D.
      • Reynal S.M.
      • Varga G.A.
      • Ishler V.A.
      Effect on production of replacing dietary starch with sucrose in lactating dairy cows.
      fed experimental diets where corn starch was replaced by sucrose at 4 increments (0, 2.5, 5.0, and 7.5% of dietary DM), and reported that DMI increased linearly from 24.5 to 26.0 kg/d as dietary sugar contents increased from 2.7 to 10.0%.
      • Penner G.B.
      • Oba M.
      Increasing dietary sugar concentration may improve dry matter intake, ruminal fermentation, and productivity of dairy cows in the postpartum phase of the transition period.
      also found that sucrose supplementation at 4.7% of dietary DM increased DMI of fresh cows (18.3 vs. 17.2 kg/d).
      • Gao X.
      • Oba M.
      Effect of increasing dietary nonfiber carbohydrate with starch, sucrose, or lactose on rumen fermentation and productivity of lactating dairy cows.
      reported feeding sucrose or lactose at 5.5% of dietary DM in place of corn grain increased DMI (27.5 vs. 26.2 kg/d).

      PREDICTION OF DMI

      • Allen M.S.
      • Sousa D.O.
      • VandeHaar M.J.
      Equation to predict feed intake response by lactating cows to factors related to the filling effect of rations.
      developed the following equation to predict DMI of lactating dairy cows from filling effects of diets, which was adopted by Dairy
      • NASEM (National Academies of Sciences, Engineering, and Medicine)
      Nutrient Requirements of Dairy Cattle.
      :
      DMI (kg/d) = 12.0 − 0.107 × forage NDF + 8.17 × ADF/NDF + 0.0253 × in vitro or in situ forage NDF digestibility (FNDFD) − 0.328 × (ADF/NDF − 0.602) × (FNDFD − 48.3) + 0.225 × milk yield + 0.00390 × (FNDFD − 48.3) × (milk yield − 33.1).


      The equation contained dietary forage NDF content, forage type indicated by the ADF/NDF ratio (0.8 for legumes, 0.6 for grasses), and FNDFD along with its interactions with forage type and milk yield. Total dietary NDF content was not included in the DMI prediction equation as effects of non-forage NDF on DMI have not been consistent in the literature. Animal responses to non-forage NDF are likely affected by type of feedstuffs replaced by non-forage NDF sources (
      • Allen M.S.
      Effects of diet on short-term regulation of feed intake by lactating dairy cattle.
      ). For example, effects of feeding soyhulls, a common non-forage NDF source, on DMI are different when it replaces grain or forage in the diet (
      • Ipharraguerre I.R.
      • Clark J.H.
      Soyhulls as an alternative feed for lactating dairy cows: A review.
      ). In addition, non-forage NDF sources vary in neutral detergent soluble fiber and water-soluble carbohydrate contents, which confound effects of non-forage NDF on DMI.
      Some interactions affect the filling effects of forage NDF; for instance, higher producing cows respond more positively to legume (
      • Voelker Linton J.A.
      • Allen M.S.
      Nutrient demand interacts with forage family to affect intake and digestion responses in dairy cows.
      ;
      • Kammes K.L.
      • Allen M.S.
      Nutrient demand interacts with forage family to affect digestion responses in dairy cows.
      ) or a forage with greater in vitro NDF digestibility (
      • Oba M.
      • Allen M.S.
      Effects of brown midrib 3 mutation in corn silage on dry matter intake and productivity of high yielding dairy cows.
      ). In addition, animal responses to enhanced in vitro NDF digestibility are more positive in grass than legume (
      • Allen M.S.
      • Sousa D.O.
      • VandeHaar M.J.
      Equation to predict feed intake response by lactating cows to factors related to the filling effect of rations.
      ). These findings are also reflected in the DMI prediction equation of Dairy
      • NASEM (National Academies of Sciences, Engineering, and Medicine)
      Nutrient Requirements of Dairy Cattle.
      .
      Several research groups measure undigested NDF after 240-h in vitro incubation (uNDF) for forages and other feedstuffs and use it as a diet formulation parameter related to NDF digestion characteristics (
      • Cotanch K.W.
      • Grant R.J.
      • Van Amburgh M.E.
      • Zontini A.
      • Fustini M.
      • Palmonari A.
      • Formigoni A.
      Applications of uNDF in ration modeling and formulation.
      ). In vitro NDF digestibility and uNDF are negatively related within forage families; specifically, forages higher in 24-h in vitro NDF digestibility were lower in uNDF for comparison within alfalfa hay (
      • Fustini M.
      • Palmonari A.
      • Canestrari G.
      • Bonfante E.
      • Mammi L.
      • Pacchioli M.T.
      • Sniffen C.J.
      • Grant R.J.
      • Cotanch K.W.
      • Formigoni A.
      Effect of undigested neutral detergent fiber content of alfalfa hay on lactating dairy cows: Feeding behavior, fiber digestibility, and lactation performance.
      ) or corn silage (
      • Miller M.D.
      • Kokko C.
      • Ballard C.S.
      • Dann H.M.
      • Fustini M.
      • Palmonari A.
      • Formigoni A.
      • Cotanch K.W.
      • Grant R.J.
      Influence of fiber degradability of corn silage in diets with lower and higher fiber content on lactational performance, nutrient digestibility, and ruminal characteristics in lactating Holstein cows.
      ). The uNDF has several advantages compared with in vitro NDF digestibility as a quality parameter of forages. As uNDF is an end-point measurement, it is associated with fewer analytical errors, and less variation is expected even when the same sample is analyzed in multiple commercial labs. In addition, dietary uNDF is easily calculated by the sum of uNDF content of individual feedstuffs.
      However, uNDF may not be a good indicator of filling effects of forage NDF. As it is an end-point measurement, it does not tell how NDF is digested nor indicate digestion rate or fragility, which has a direct effect on the filling effects of forage NDF.
      • Fustini M.
      • Palmonari A.
      • Canestrari G.
      • Bonfante E.
      • Mammi L.
      • Pacchioli M.T.
      • Sniffen C.J.
      • Grant R.J.
      • Cotanch K.W.
      • Formigoni A.
      Effect of undigested neutral detergent fiber content of alfalfa hay on lactating dairy cows: Feeding behavior, fiber digestibility, and lactation performance.
      fed alfalfa hay differing in in vitro NDF digestibility at 2 dietary uNDF contents, and reported that DMI was greater for cows fed the hay with greater in vitro NDF digestibility, but was not affected by dietary uNDF content. In addition, legume is greater in uNDF content than grass, but it is more fragile and less filling in the rumen (
      • Kammes K.L.
      • Allen M.S.
      Rates of particle size reduction and passage are faster for legume compared to cool-season grass, resulting in lower rumen fill and less effective fiber.
      ). As such, uNDF content may not be a good overall parameter related to filling effects of forage that works across different forage types. Development of an integrated system universally accounting for filling effects of forage NDF is warranted.
      Factors affecting starch digestibility of grain do not have consistent effects on DMI, as summarized in this review. The lack of consistent effects on DMI indicates that DMI is not affected by starch digestibility per se, but affected by a variable associated with starch digestibility, such as propionate metabolism in the liver (
      • Allen M.S.
      • Bradford B.J.
      • Oba M.
      Board-invited review: The hepatic oxidation theory of the control of feed intake and its application to ruminants.
      ). Positive effects of feeding sugar on DMI also suggest that greater ruminal fermentation per se does not necessarily decrease DMI. Dairy
      • NASEM (National Academies of Sciences, Engineering, and Medicine)
      Nutrient Requirements of Dairy Cattle.
      did not include starch digestion characteristics or dietary sugar content in the DMI prediction equation, and more research is warranted to accurately predict DMI from digestion characteristics of carbohydrates.

      DIGESTIBILITY AND FUEL SUPPLY

      It is not appropriate to use in vitro NDF digestibility to estimate in vivo NDF digestibility of forages. As discussed above, higher in vitro NDF digestibility is related to greater fragility, which reduces the ruminal pool of large NDF particles and decreases the retention time of ruminal digesta. This may contribute to greater DMI, but decreases NDF digestion in the rumen. Recent meta-analyses showed that greater DMI decreases the total-tract NDF digestibility (
      • White R.R.
      • Roman-Garcia Y.
      • Firkins J.L.
      • VandeHaar M.J.
      • Armentano L.E.
      • Weiss W.P.
      • McGill T.
      • Garnett R.
      • Hanigan M.D.
      Evaluation of the National Research Council (2001) dairy model and derivation of new prediction equations. 1. Digestibility of fiber, fat, protein, and nonfiber carbohydrate.
      ;
      • de Souza R.A.
      • Tempelman R.J.
      • Allen M.S.
      • Weiss W.P.
      • Bernard J.K.
      • VandeHaar M.J.
      Predicting nutrient digestibility in high-producing dairy cows.
      ). In addition, in vitro NDF digestibility is to be measured under an optimal condition where concentrations and activities of enzymes do not limit the digestion process, whereas NDF digestion in the rumen often occurs at a suboptimal environment for fibrolytic microbes. Daily mean rumen pH is positively related to digestion rate of pdNDF in the rumen (
      • Oba M.
      • Allen M.S.
      Effects of corn grain conservation method on ruminal digestion kinetics for lactating dairy cows at two dietary starch concentrations.
      ), and cows fed high starch diets have lower NDF digestibility (
      • Ferraretto L.F.
      • Crump P.M.
      • Shaver R.D.
      Effect of cereal grain type and corn grain harvesting and processing methods on intake, digestion, and milk production by dairy cows through a meta-analysis.
      ;
      • de Souza R.A.
      • Tempelman R.J.
      • Allen M.S.
      • Weiss W.P.
      • Bernard J.K.
      • VandeHaar M.J.
      Predicting nutrient digestibility in high-producing dairy cows.
      ). To account for these negative effects on in vivo NDF digestibility, Dairy
      • NASEM (National Academies of Sciences, Engineering, and Medicine)
      Nutrient Requirements of Dairy Cattle.
      discounts NDF digestibility when DMI exceeds 3.5% of BW and when dietary starch content exceeds 26% of dietary DM.
      Greater dietary starch content may increase ruminal starch digestibility via greater amylolytic enzyme activity (
      • Oba M.
      • Allen M.S.
      Effects of corn grain conservation method on ruminal digestion kinetics for lactating dairy cows at two dietary starch concentrations.
      ;
      • Voelker J.A.
      • Allen M.S.
      Pelleted beet pulp substituted for high-moisture corn: 2. Effects on digestion and ruminal digestion kinetics in lactating dairy cows.
      ), but decrease the total-tract starch digestibility due to faster passage and limited capacity of intestinal digestion (
      • de Souza R.A.
      • Tempelman R.J.
      • Allen M.S.
      • Weiss W.P.
      • Bernard J.K.
      • VandeHaar M.J.
      Predicting nutrient digestibility in high-producing dairy cows.
      ). Greater DMI also decreases total-tract starch digestibility (
      • Ferraretto L.F.
      • Crump P.M.
      • Shaver R.D.
      Effect of cereal grain type and corn grain harvesting and processing methods on intake, digestion, and milk production by dairy cows through a meta-analysis.
      ;
      • de Souza R.A.
      • Tempelman R.J.
      • Allen M.S.
      • Weiss W.P.
      • Bernard J.K.
      • VandeHaar M.J.
      Predicting nutrient digestibility in high-producing dairy cows.
      ). Dairy
      • NASEM (National Academies of Sciences, Engineering, and Medicine)
      Nutrient Requirements of Dairy Cattle.
      discounts starch digestibility when DMI exceeds 3.5% of BW, but no adjustments with dietary factors are made for starch digestibility due to insufficient data.
      The type of absorbed fuels is affected by carbohydrate digestion. Carbohydrate type affects fermentation end products (
      • Murphy M.R.
      • Baldwin R.L.
      • Koong L.J.
      Estimation of stoichiometric parameters for rumen fermentation of roughage and concentrate diets.
      ); particularly, fiber digestion increases absorption of fuel as acetate, whereas fermentation of starch and sugar in the rumen increases absorption of propionate and butyrate, respectively. In addition, site of starch digestion affects the type of absorbed fuel. When a slowly fermentable starch source, such as dry ground corn, is fed to dairy cows, ruminal starch digestibility is decreased substantially but total-tract starch digestibility is not affected as much due to partial compensatory digestion in the intestines (
      • Oba M.
      • Allen M.S.
      Effects of corn grain conservation method on ruminal digestion kinetics for lactating dairy cows at two dietary starch concentrations.
      ), indicating that substantial amounts of starch are digested in the intestines. If starch is digested enzymatically in the small intestine, it is absorbed as glucose, which increases glucose or lactate flux to the liver (
      • Allen M.S.
      Effects of diet on short-term regulation of feed intake by lactating dairy cattle.
      ). Type of absorbed fuels (propionate, glucose, or lactate) affects the extent of hepatic oxidation and feed intake (
      • Allen M.S.
      • Bradford B.J.
      • Oba M.
      Board-invited review: The hepatic oxidation theory of the control of feed intake and its application to ruminants.
      ). As such, temporal pattern of fuel supply from carbohydrate digestion needs to be integrated in predictions of DMI and fuel metabolism.

      CONCLUSIONS

      The filling effects of forage NDF are affected by in vitro NDF digestibility, fragility, and forage type. Starch digestibility is affected by grain type, processing method, conservation method, and endosperm type. Type of carbohydrate and its digestive characteristics can affect DMI and absorbed fuels. Previous research has increased our understanding of dietary carbohydrates, but its application for diet formulation requires integrated approaches accounting for factors affecting filling effects of forage NDF, starch digestion, and temporal fuel supply.

      ACKNOWLEDGMENTS

      The authors gratefully acknowledge M. S. Allen for his helpful suggestions for preparation of this manuscript. This work received no external funding. The authors have not stated any conflicts of interest. Because no live human or animal subjects were used in this review, ethical approval by IRB or IACUC was not required.

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