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NUTRIENTS, DIGESTION AND ABSORPTION | Fermentation in the Rumen
NUTRIENTS, DIGESTION AND ABSORPTION
Contents Fermentation in the Rumen Fiber Digestion in Pasture-Based Cows Small Intestine of Lactating Ruminants Absorption of Minerals and Vitamins
Fermentation in the Rumen M R Murphy, University of Illinois at Urbana-Champaign, Champaign, IL, USA ª 2011 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by N. R. Merchen, Volume 4, pp 2112–2120, ª 2002, Elsevier Ltd.
Introduction Nutrients consumed by weaned dairy cattle are subject to fermentation by microbes in their rumen or, more properly, their reticulorumen. This pregastric fermentation also affects the dynamics of digestion and absorption in subsequent segments of their digestive tract and, ultimately, the amount and pattern of nutrients available for metabolism. The rumen microbial ecosystem in cattle fed typical diets is both open and complex, composed mainly of strictly anaerobic bacteria, archaea (single-celled microorganisms that have an evolutionary history and biochemistry that differ from bacteria), protozoa, and fungi. Symbiotic and other interactions of these microbes with each other and with the host animal are also nutritionally relevant. Anaerobic fermentation partially metabolizes nutrients consumed by the host, providing energy and nutrients needed by the microbes. Some end products of microbial metabolism (e.g., volatile fatty acids (VFAs) or short-chain fatty acids, primarily acetate, propionate, and butyrate, but not CO2 and CH4) and the microorganisms themselves then provide nutrients for the host. This relationship allows the host to capitalize on the ability of some ruminal microbes to digest plant cell wall carbohydrates (cellulose, hemicellulose, and pectin) otherwise unavailable to mammals. Additional benefits of pregastric fermentation are that the host animal can digest microbial protein passing from the reticulorumen, obtain vitamins synthesized by the
980
microbes, and reduce the effects of some antinutrients (e.g., phytate and gossypol). The interaction of rumination with fermentation is also nutritionally important: chewing during rumination accelerates microbial colonization and fermentation, and as a result digesta particles become more fragile and susceptible to comminution during rumination. Direct microscopic counts of bacteria in the rumen are 3- to 10-fold those of viable (i.e., culturable) bacteria. Recent data based on studies of rRNA confirm that many more species of bacteria are present in the rumen than have been cultured or identified to date. Some uncultivated organisms are abundant; however, isolated bacteria and archaea are able to perform most of the major transformations known to occur in the reticulorumen and provide a model of the ecosystem. At 1–5 mm in size, and assuming an individual cell volume of 1 mm3 (or 10 9 ml), ruminal bacteria account for about 1% of fluid volume when present at 1010 ml 1 or 10% of fluid volume if counts reach 1011 ml 1. The larger ruminal protozoa vary in size from 20 to 200 mm and are normally present at concentrations of 104–105 ml 1; therefore, they can account for approximately half of the total microbial cell contents in the rumen. Counts of protozoa and bacteria in the rumen tend to be inversely related because the former prey on the latter. Anaerobic fungi are also part of this microbial ecosystem. Although they are difficult to quantify because of their complex life cycle, they have been estimated to contribute about 6% of total biomass.
Nutrients, Digestion and Absorption | Fermentation in the Rumen 981
A lactating, mature, 680-kg Holstein cow consuming a sample diet will provide the framework for this discussion of fermentation in the reticulorumen. It is assumed that the cow produced 45 kg of milk per day with 3.5% fat and 3.0% true protein; lived in a thermoneutral environment; and ate (on a dry matter basis) 7.08 kg of steam-flaked maize, 6.16 kg of immature legume hay, 5.61 kg of maize silage, 2.53 kg of whole cottonseed (with lint), 2.26 kg of sorghum-sudan silage, 1.41 kg of solvent-extracted soybean meal (48% crude protein), 0.98 kg of midmaturity grass (C3) hay, 0.87 kg of coastal Bermudagrass hay, 0.51 kg of a vitamin and mineral premix, 0.23 kg of ring-dried blood meal, 0.12 kg of sodium chloride, 0.09 kg of calcium carbonate, and 0.04 kg of monosodium phosphate (monohydrate) per day – a total of 27.9 kg of feed dry matter, 4.1% of body mass. The ration, as consumed, also included 18.9 kg of water and the cow drank an additional 115.6 kg of water per day; therefore, total water intake was 134.5 kg per day and total nutrient consumption was 162.4 kg per day. Ignoring contributions by saliva and scurf (sloughed epithelial cells from the digestive tract), the profile of nutrients entering the reticulorumen was then estimated using tabulated feed composition data
(Figure 1). Nonnutritive lignin was accounted for; it and similar but unquantified compounds (e.g., tannins and acid-insoluble ash) are discussed separately. Once swallowed and upon entering the reticulorumen, nutrients become potential substrates for microbes to utilize; their various fates are now considered in turn.
Water Considerably more water than dry matter is consumed by the lactating cow, 4.8 times as much in our example. Even more would have been consumed during periods of high environmental temperature. Although often an overlooked nutrient, water is clearly an extremely important one and consumption of such large amounts also impacts fermentation in the reticulorumen. Water is required for fermentation itself because the enzymatically catalyzed chemical reactions by which complex molecules in carbohydrates, protein, and fat are converted into assimilable forms, that is, hydrolysis, involve water. A portion of imbibed water, perhaps 5–18%, bypasses the reticulorumen; however, the rest equilibrates with the 60–80 l of fluid normally present. Water in the
Figure 1 The profile of nutrients entering the reticulorumen: (a) nutrient consumption; (b) dry matter consumption; and (c) carbohydrate consumption.
982 Nutrients, Digestion and Absorption | Fermentation in the Rumen
reticulorumen (either drunk, from saliva, or in feed) also exchanges rapidly with body water; the half-life of water molecules in the reticulorumen is about 1 h. The fractional dilution rate in the reticulorumen is typically 10–17% h 1; in our example, the value is likely toward the high end of this range. Unless a microbial species is continually reinoculated, it must multiply at least as quickly as the fractional dilution rate, attach to digesta particles having a slower fractional passage rate, or perhaps (if motile) sequester in a location having a slower turnover rate to avoid being removed from the reticulorumen. Microbes growing more quickly also grow more efficiently because a smaller proportion of their energy is utilized for maintenance.
metabolism of pyruvate leads to the production of VFAs, CO2, and CH4. Methane is produced from CO2 and hydrogen by the archaea (e.g., Methanobrevibacter ruminantium). Some methanogens are found attached to protozoa (exosymbionts) where they derive hydrogen directly from specialized protozoal organs (hydrogenosomes). The CH4 cannot be utilized by the cow and represents a loss of 5–7% of the gross energy in her ration. That said, production of CH4 does dispose of hydrogen and allow anaerobic fermentation to proceed. Although CH4 production by a mature lactating cow consuming the sample ration would be nearly maximal in terms of liters per day, CH4 production per kilogram of milk yield would be approaching its minimum; that is, the emission of ‘anthropogenic’ CH4 per unit of human food produced is lower than it would be in cows producing less milk.
Carbohydrates The sample ration provided 18.8 kg day 1 of carbohydrates. The carbohydrate fraction of feeds is a complex mixture of monomers and polymers making up about 68% of dietary dry matter (Figure 1(b)). Carbohydrates can be fractionated, based on various analytic procedures related to chemical composition and rumen fermentability, into starches, cellulose, hemicellulose, pectins, sugars, and organic acids. Although various ruminal microbes are adapted to utilize all carbohydrate fractions, the dynamics of fermentation vary considerably. There are also known interactions between carbohydrate fractions; for example, too much or too rapid a fermentation of starch can cause ruminal acidosis that inhibits fiber (cellulose, hemicellulose, and pectin) digestion. The overall scheme of carbohydrate fermentation by ruminal microbes is similar regardless of source (Figure 2). If not already in hexose form, carbohydrates are hydrolyzed and converted to hexose monomers. These are metabolized to pyruvate via Embden–Meyerhof glycolysis in bacteria and the Entner–Doudoroff pathway in archaea. Subsequent
Starches Fifty to ninety percent of the 7.0 kg day 1 of starch entering the rumen of the cow would be expected to be fermented in her reticulorumen, the large range indicating potential effects of many factors affecting starch degradation. The rate of starch fermentation varies with its source and processing. Starch in oats or wheat is degraded more quickly in the reticulorumen than starch in maize or sorghum; some of this difference is related to starch solubility and the ease with which proteins surrounding starch granules in the cereal’s endosperm are fermented. Starch degradation rate is increased by cracking or grinding, and steam flaking. These processes increase relative surface area and disrupt starch granules. As with other nutrients, feed consumption affects the proportion of consumed starch fermented because it influences the fractional passage rate of digesta from the reticulorumen. This means that, for the cow fed the sample diet and eating 4% of her body mass in dry feed per day, starch digestion in the reticulorumen would likely be closer to 50% than 90% of that consumed.
Figure 2 Overview of carbohydrate fermentation in the reticulorumen.
Nutrients, Digestion and Absorption | Fermentation in the Rumen 983
Predominant species of ruminal bacteria utilizing starch include Butyrivibrio fibrisolvens, Ruminobacter amylophilus, Prevotella sp., Streptococcus bovis, Succinomonas amylolytica, and some strains of Selenomonas ruminantium. Cellulose The sample ration provided 4.8 kg day 1 of cellulose to the reticulorumen and 30–60% of it would be hydrolyzed in the reticulorumen, albeit more slowly than starch. Among predominant ruminal bacteria able to use cellulose are Fibrobacter succinogenes, Ruminococcus albus, R. flavefaciens, and some strains of B. fibrisolvens. Hemicellulose and Pectins As with cellulose, 30–60% of the 3.2 kg day 1 of hemicellulose and 2.1 kg day 1 of pectins would be hydrolyzed in the reticulorumen. Butyrivibrio fibrisolvens and Prevotella sp. are predominant organisms using hemicellulose or pectins. Lachnospira multiparus is another predominant pectin fermenter. Sugars Fermentation of the 1.2 kg day 1 of sugars in the sample ration would be rapid; many ruminal microbes can utilize them. In cattle fed low-forage, high-grain diets, the halflives of both sugars and starch in the reticulorumen are about 4 h. Predominant species of ruminal bacteria utilizing sugars include B. fibrisolvens, S. ruminantium, S. bovis, Eubacterium ruminantium, and L. multiparus. Organic Acids The 0.6 kg day 1 of organic acids provided by the sample ration, probably mostly citrate and malate, would be rapidly fermented in the reticulorumen. Wolinella succinogenes is a species that can use malate.
Nitrogenous Compounds About 70% of the 4.5 kg day 1 of dietary protein and nonprotein N entering the reticulorumen of the cow fed the sample ration would be fermented there, leading to the production of microbial protein and ammonia. Considerable recycling of N can occur within the reticulorumen: between bacteria and protozoa, between one bacterium and another, and between the cow and the reticulorumen. Depending on the circumstances, these processes may be advantageous or disadvantageous to the nutrition of the cow. The advantages include microbial conversion of nonprotein N into protein that can then be digested to provide amino acids to the cow, conversion of
poor-quality dietary proteins into higher-quality microbial protein, and utilization of salivary urea to provide protein to the cow when N intakes are low. The processes can be disadvantageous when degradation of dietary protein exceeds synthesis of microbial protein or when unprotected proteins of higher quality than microbial protein are fed. Defaunation (i.e., removal of protozoa from the reticulorumen) reduces N recycling; however, this has a potential downside. By ingesting starch granules, some protozoa (entodinomorphs) buffer its rate of fermentation and reduce the potential for lactic acidosis. A similar benefit can be provided by those protozoa (holotrichs) that compete directly with bacteria for soluble sugars. Predominant rumen bacterial species utilizing protein, peptides, or amino acids are Clostridium aminophilum, C. sticklandii, Megasphaera elsdenii, Peptostreptococcus anaerobius, and Prevotella sp. Clostridium aminophilum, C. sticklandii, and P. anaerobius are all obligate amino acid fermenting bacteria; they cannot utilize carbohydrates for energy but deaminate amino acids much faster than other ruminal bacteria. Ammonia can be used by a majority of rumen bacteria for biosynthesis of amino acids and protein.
Minerals The 2.2 kg day 1 of minerals in the sample ration can affect fermentation in the reticulorumen. Fermentation can also alter both the dynamics of mineral metabolism in the cow and the ultimate availability of some of these elements to the host. Minerals soluble at the pH of the reticulorumen, normally 5.5–7.0, increase the osmolality of fluid in this compartment of the digestive tract and this can enhance the fractional dilution rate. Many of these elements (e.g., Cl, I, K, Mg, Na, S (in sulfide form), and Zn) can be absorbed from the reticulorumen. Carbonates also help buffer rumen pH and reduce the adverse effects of low pH (rapid fermentation) on fiber-degrading microbes. Provided that the ration contains adequate Co, fermentation in the reticulorumen supplies enough vitamin B12 (cobalamin) to meet requirements of the cow. A three-way interaction of Cu, Mo, and S affects the availability of Cu in ruminants. Briefly, fermentation of Mo and S in the reticulorumen produces thiomolybdates. Copper reacts with thiomolybdates in the rumen to form highly insoluble complexes that are poorly absorbed. Thiomolybdates can also be absorbed by the cow, reducing the availability of systemic Cu.
Lipids Most of the 1.2 kg day 1 of lipids ingested in the sample ration would undergo some form of
984 Nutrients, Digestion and Absorption | Fermentation in the Rumen
transformation by fermentation in the reticulorumen. For example, dietary triacylglycerols (fats and oils, like those in cottonseed) would be normally hydrolyzed completely to free fatty acids and glycerol. The latter would be rapidly fermented to produce propionate, whereas 70–90% of the unsaturated fatty acids released in this process would be extensively biohydrogenated. A predominant bacterial species that hydrolyzes dietary triacylglycerols to free fatty acids and glycerol with little accumulation of mono- or diacylglycerols is Anaerovibrio lipolytica. Fatty acids can interfere with fermentation in the reticulorumen both indirectly and directly. Their association with digesta particles may reduce microbial access to substrate. They can also directly inhibit microbial growth and metabolism.
Lignins Although a portion of the 1.2 kg day 1 of lignin phenolics in the sample ration would be digested, absorbed, metabolized, and excreted in urine as aromatic acids, most would pass, unaltered, through the reticulorumen. Any delignification of plant cell walls would increase the availability of cellulose and hemicellulose in the reticulorumen. See also: Nutrients, Digestion and Absorption: Absorption of Minerals and Vitamins; Fiber Digestion in
Pasture-Based Cows; Small Intestine of Lactating Ruminants.
Further Reading Firkins JL, Yu Z, and Morrison M (2007) Ruminal nitrogen metabolism. Perspectives for integration of microbiology and nutrition for dairy. Journal of Dairy Science 90(electronic supplement): E1–E16. Hespell RB (1987) Biotechnology and modifications of the rumen microbial ecosystem. Proceedings of the Nutrition Society 46: 407–413. Jenkins TC (1993) Lipid metabolism in the rumen. Journal of Dairy Science 76: 3851–3863. Leedle JAZ, Barsuhn K, and Hespell RB (1986) Postprandial trends in estimated ruminal digesta polysaccharides and their relation to changes in bacterial groups and ruminal fluid characteristics. Journal of Animal Science 62: 789–803. Leedle JAZ, Bryant MP, and Hespell RB (1982) Diurnal variations in bacterial numbers and fluid parameters in ruminal contents of animals fed low- or high-forage diets. Applied and Environmental Microbiology 44: 402–412. Munyard KA and Baker SK (2006) Size fractionation of a rumen microbial population by counter-flow centrifugal elutriation. Journal of Microbiological Methods 67: 566–573. National Research Council (2001) Nutrient Requirements of Dairy Cattle, 7th revised edn. Washington, DC: National Academy Press. Russell JB (2002) Rumen Microbiology and Its Role in Ruminant Nutrition. Ithaca, NY: Russell Publishing Co. Russell JB and Van Soest PJ (1984) In vitro fermentation of organic acids common in forage. Applied and Environmental Microbiology 47: 155–159. Silanikove N and Brosh A (1989) Lignocellulose degradation and subsequent metabolism of lignin fermentation products by the desert black Bedouin goat fed on wheat straw as a single-component diet. The British Journal of Nutrition 62: 509–520.
Contents Fermentation in the Rumen Fiber Digestion in Pasture-Based Cows Small Intestine of Lactating Ruminants Absorption of Minerals and Vitamins
Fermentation in the Rumen M R Murphy, University of Illinois at Urbana-Champaign, Champaign, IL, USA ª 2011 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by N. R. Merchen, Volume 4, pp 2112–2120, ª 2002, Elsevier Ltd.
Introduction Nutrients consumed by weaned dairy cattle are subject to fermentation by microbes in their rumen or, more properly, their reticulorumen. This pregastric fermentation also affects the dynamics of digestion and absorption in subsequent segments of their digestive tract and, ultimately, the amount and pattern of nutrients available for metabolism. The rumen microbial ecosystem in cattle fed typical diets is both open and complex, composed mainly of strictly anaerobic bacteria, archaea (single-celled microorganisms that have an evolutionary history and biochemistry that differ from bacteria), protozoa, and fungi. Symbiotic and other interactions of these microbes with each other and with the host animal are also nutritionally relevant. Anaerobic fermentation partially metabolizes nutrients consumed by the host, providing energy and nutrients needed by the microbes. Some end products of microbial metabolism (e.g., volatile fatty acids (VFAs) or short-chain fatty acids, primarily acetate, propionate, and butyrate, but not CO2 and CH4) and the microorganisms themselves then provide nutrients for the host. This relationship allows the host to capitalize on the ability of some ruminal microbes to digest plant cell wall carbohydrates (cellulose, hemicellulose, and pectin) otherwise unavailable to mammals. Additional benefits of pregastric fermentation are that the host animal can digest microbial protein passing from the reticulorumen, obtain vitamins synthesized by the
980
microbes, and reduce the effects of some antinutrients (e.g., phytate and gossypol). The interaction of rumination with fermentation is also nutritionally important: chewing during rumination accelerates microbial colonization and fermentation, and as a result digesta particles become more fragile and susceptible to comminution during rumination. Direct microscopic counts of bacteria in the rumen are 3- to 10-fold those of viable (i.e., culturable) bacteria. Recent data based on studies of rRNA confirm that many more species of bacteria are present in the rumen than have been cultured or identified to date. Some uncultivated organisms are abundant; however, isolated bacteria and archaea are able to perform most of the major transformations known to occur in the reticulorumen and provide a model of the ecosystem. At 1–5 mm in size, and assuming an individual cell volume of 1 mm3 (or 10 9 ml), ruminal bacteria account for about 1% of fluid volume when present at 1010 ml 1 or 10% of fluid volume if counts reach 1011 ml 1. The larger ruminal protozoa vary in size from 20 to 200 mm and are normally present at concentrations of 104–105 ml 1; therefore, they can account for approximately half of the total microbial cell contents in the rumen. Counts of protozoa and bacteria in the rumen tend to be inversely related because the former prey on the latter. Anaerobic fungi are also part of this microbial ecosystem. Although they are difficult to quantify because of their complex life cycle, they have been estimated to contribute about 6% of total biomass.
Nutrients, Digestion and Absorption | Fermentation in the Rumen 981
A lactating, mature, 680-kg Holstein cow consuming a sample diet will provide the framework for this discussion of fermentation in the reticulorumen. It is assumed that the cow produced 45 kg of milk per day with 3.5% fat and 3.0% true protein; lived in a thermoneutral environment; and ate (on a dry matter basis) 7.08 kg of steam-flaked maize, 6.16 kg of immature legume hay, 5.61 kg of maize silage, 2.53 kg of whole cottonseed (with lint), 2.26 kg of sorghum-sudan silage, 1.41 kg of solvent-extracted soybean meal (48% crude protein), 0.98 kg of midmaturity grass (C3) hay, 0.87 kg of coastal Bermudagrass hay, 0.51 kg of a vitamin and mineral premix, 0.23 kg of ring-dried blood meal, 0.12 kg of sodium chloride, 0.09 kg of calcium carbonate, and 0.04 kg of monosodium phosphate (monohydrate) per day – a total of 27.9 kg of feed dry matter, 4.1% of body mass. The ration, as consumed, also included 18.9 kg of water and the cow drank an additional 115.6 kg of water per day; therefore, total water intake was 134.5 kg per day and total nutrient consumption was 162.4 kg per day. Ignoring contributions by saliva and scurf (sloughed epithelial cells from the digestive tract), the profile of nutrients entering the reticulorumen was then estimated using tabulated feed composition data
(Figure 1). Nonnutritive lignin was accounted for; it and similar but unquantified compounds (e.g., tannins and acid-insoluble ash) are discussed separately. Once swallowed and upon entering the reticulorumen, nutrients become potential substrates for microbes to utilize; their various fates are now considered in turn.
Water Considerably more water than dry matter is consumed by the lactating cow, 4.8 times as much in our example. Even more would have been consumed during periods of high environmental temperature. Although often an overlooked nutrient, water is clearly an extremely important one and consumption of such large amounts also impacts fermentation in the reticulorumen. Water is required for fermentation itself because the enzymatically catalyzed chemical reactions by which complex molecules in carbohydrates, protein, and fat are converted into assimilable forms, that is, hydrolysis, involve water. A portion of imbibed water, perhaps 5–18%, bypasses the reticulorumen; however, the rest equilibrates with the 60–80 l of fluid normally present. Water in the
Figure 1 The profile of nutrients entering the reticulorumen: (a) nutrient consumption; (b) dry matter consumption; and (c) carbohydrate consumption.
982 Nutrients, Digestion and Absorption | Fermentation in the Rumen
reticulorumen (either drunk, from saliva, or in feed) also exchanges rapidly with body water; the half-life of water molecules in the reticulorumen is about 1 h. The fractional dilution rate in the reticulorumen is typically 10–17% h 1; in our example, the value is likely toward the high end of this range. Unless a microbial species is continually reinoculated, it must multiply at least as quickly as the fractional dilution rate, attach to digesta particles having a slower fractional passage rate, or perhaps (if motile) sequester in a location having a slower turnover rate to avoid being removed from the reticulorumen. Microbes growing more quickly also grow more efficiently because a smaller proportion of their energy is utilized for maintenance.
metabolism of pyruvate leads to the production of VFAs, CO2, and CH4. Methane is produced from CO2 and hydrogen by the archaea (e.g., Methanobrevibacter ruminantium). Some methanogens are found attached to protozoa (exosymbionts) where they derive hydrogen directly from specialized protozoal organs (hydrogenosomes). The CH4 cannot be utilized by the cow and represents a loss of 5–7% of the gross energy in her ration. That said, production of CH4 does dispose of hydrogen and allow anaerobic fermentation to proceed. Although CH4 production by a mature lactating cow consuming the sample ration would be nearly maximal in terms of liters per day, CH4 production per kilogram of milk yield would be approaching its minimum; that is, the emission of ‘anthropogenic’ CH4 per unit of human food produced is lower than it would be in cows producing less milk.
Carbohydrates The sample ration provided 18.8 kg day 1 of carbohydrates. The carbohydrate fraction of feeds is a complex mixture of monomers and polymers making up about 68% of dietary dry matter (Figure 1(b)). Carbohydrates can be fractionated, based on various analytic procedures related to chemical composition and rumen fermentability, into starches, cellulose, hemicellulose, pectins, sugars, and organic acids. Although various ruminal microbes are adapted to utilize all carbohydrate fractions, the dynamics of fermentation vary considerably. There are also known interactions between carbohydrate fractions; for example, too much or too rapid a fermentation of starch can cause ruminal acidosis that inhibits fiber (cellulose, hemicellulose, and pectin) digestion. The overall scheme of carbohydrate fermentation by ruminal microbes is similar regardless of source (Figure 2). If not already in hexose form, carbohydrates are hydrolyzed and converted to hexose monomers. These are metabolized to pyruvate via Embden–Meyerhof glycolysis in bacteria and the Entner–Doudoroff pathway in archaea. Subsequent
Starches Fifty to ninety percent of the 7.0 kg day 1 of starch entering the rumen of the cow would be expected to be fermented in her reticulorumen, the large range indicating potential effects of many factors affecting starch degradation. The rate of starch fermentation varies with its source and processing. Starch in oats or wheat is degraded more quickly in the reticulorumen than starch in maize or sorghum; some of this difference is related to starch solubility and the ease with which proteins surrounding starch granules in the cereal’s endosperm are fermented. Starch degradation rate is increased by cracking or grinding, and steam flaking. These processes increase relative surface area and disrupt starch granules. As with other nutrients, feed consumption affects the proportion of consumed starch fermented because it influences the fractional passage rate of digesta from the reticulorumen. This means that, for the cow fed the sample diet and eating 4% of her body mass in dry feed per day, starch digestion in the reticulorumen would likely be closer to 50% than 90% of that consumed.
Figure 2 Overview of carbohydrate fermentation in the reticulorumen.
Nutrients, Digestion and Absorption | Fermentation in the Rumen 983
Predominant species of ruminal bacteria utilizing starch include Butyrivibrio fibrisolvens, Ruminobacter amylophilus, Prevotella sp., Streptococcus bovis, Succinomonas amylolytica, and some strains of Selenomonas ruminantium. Cellulose The sample ration provided 4.8 kg day 1 of cellulose to the reticulorumen and 30–60% of it would be hydrolyzed in the reticulorumen, albeit more slowly than starch. Among predominant ruminal bacteria able to use cellulose are Fibrobacter succinogenes, Ruminococcus albus, R. flavefaciens, and some strains of B. fibrisolvens. Hemicellulose and Pectins As with cellulose, 30–60% of the 3.2 kg day 1 of hemicellulose and 2.1 kg day 1 of pectins would be hydrolyzed in the reticulorumen. Butyrivibrio fibrisolvens and Prevotella sp. are predominant organisms using hemicellulose or pectins. Lachnospira multiparus is another predominant pectin fermenter. Sugars Fermentation of the 1.2 kg day 1 of sugars in the sample ration would be rapid; many ruminal microbes can utilize them. In cattle fed low-forage, high-grain diets, the halflives of both sugars and starch in the reticulorumen are about 4 h. Predominant species of ruminal bacteria utilizing sugars include B. fibrisolvens, S. ruminantium, S. bovis, Eubacterium ruminantium, and L. multiparus. Organic Acids The 0.6 kg day 1 of organic acids provided by the sample ration, probably mostly citrate and malate, would be rapidly fermented in the reticulorumen. Wolinella succinogenes is a species that can use malate.
Nitrogenous Compounds About 70% of the 4.5 kg day 1 of dietary protein and nonprotein N entering the reticulorumen of the cow fed the sample ration would be fermented there, leading to the production of microbial protein and ammonia. Considerable recycling of N can occur within the reticulorumen: between bacteria and protozoa, between one bacterium and another, and between the cow and the reticulorumen. Depending on the circumstances, these processes may be advantageous or disadvantageous to the nutrition of the cow. The advantages include microbial conversion of nonprotein N into protein that can then be digested to provide amino acids to the cow, conversion of
poor-quality dietary proteins into higher-quality microbial protein, and utilization of salivary urea to provide protein to the cow when N intakes are low. The processes can be disadvantageous when degradation of dietary protein exceeds synthesis of microbial protein or when unprotected proteins of higher quality than microbial protein are fed. Defaunation (i.e., removal of protozoa from the reticulorumen) reduces N recycling; however, this has a potential downside. By ingesting starch granules, some protozoa (entodinomorphs) buffer its rate of fermentation and reduce the potential for lactic acidosis. A similar benefit can be provided by those protozoa (holotrichs) that compete directly with bacteria for soluble sugars. Predominant rumen bacterial species utilizing protein, peptides, or amino acids are Clostridium aminophilum, C. sticklandii, Megasphaera elsdenii, Peptostreptococcus anaerobius, and Prevotella sp. Clostridium aminophilum, C. sticklandii, and P. anaerobius are all obligate amino acid fermenting bacteria; they cannot utilize carbohydrates for energy but deaminate amino acids much faster than other ruminal bacteria. Ammonia can be used by a majority of rumen bacteria for biosynthesis of amino acids and protein.
Minerals The 2.2 kg day 1 of minerals in the sample ration can affect fermentation in the reticulorumen. Fermentation can also alter both the dynamics of mineral metabolism in the cow and the ultimate availability of some of these elements to the host. Minerals soluble at the pH of the reticulorumen, normally 5.5–7.0, increase the osmolality of fluid in this compartment of the digestive tract and this can enhance the fractional dilution rate. Many of these elements (e.g., Cl, I, K, Mg, Na, S (in sulfide form), and Zn) can be absorbed from the reticulorumen. Carbonates also help buffer rumen pH and reduce the adverse effects of low pH (rapid fermentation) on fiber-degrading microbes. Provided that the ration contains adequate Co, fermentation in the reticulorumen supplies enough vitamin B12 (cobalamin) to meet requirements of the cow. A three-way interaction of Cu, Mo, and S affects the availability of Cu in ruminants. Briefly, fermentation of Mo and S in the reticulorumen produces thiomolybdates. Copper reacts with thiomolybdates in the rumen to form highly insoluble complexes that are poorly absorbed. Thiomolybdates can also be absorbed by the cow, reducing the availability of systemic Cu.
Lipids Most of the 1.2 kg day 1 of lipids ingested in the sample ration would undergo some form of
984 Nutrients, Digestion and Absorption | Fermentation in the Rumen
transformation by fermentation in the reticulorumen. For example, dietary triacylglycerols (fats and oils, like those in cottonseed) would be normally hydrolyzed completely to free fatty acids and glycerol. The latter would be rapidly fermented to produce propionate, whereas 70–90% of the unsaturated fatty acids released in this process would be extensively biohydrogenated. A predominant bacterial species that hydrolyzes dietary triacylglycerols to free fatty acids and glycerol with little accumulation of mono- or diacylglycerols is Anaerovibrio lipolytica. Fatty acids can interfere with fermentation in the reticulorumen both indirectly and directly. Their association with digesta particles may reduce microbial access to substrate. They can also directly inhibit microbial growth and metabolism.
Lignins Although a portion of the 1.2 kg day 1 of lignin phenolics in the sample ration would be digested, absorbed, metabolized, and excreted in urine as aromatic acids, most would pass, unaltered, through the reticulorumen. Any delignification of plant cell walls would increase the availability of cellulose and hemicellulose in the reticulorumen. See also: Nutrients, Digestion and Absorption: Absorption of Minerals and Vitamins; Fiber Digestion in
Pasture-Based Cows; Small Intestine of Lactating Ruminants.
Further Reading Firkins JL, Yu Z, and Morrison M (2007) Ruminal nitrogen metabolism. Perspectives for integration of microbiology and nutrition for dairy. Journal of Dairy Science 90(electronic supplement): E1–E16. Hespell RB (1987) Biotechnology and modifications of the rumen microbial ecosystem. Proceedings of the Nutrition Society 46: 407–413. Jenkins TC (1993) Lipid metabolism in the rumen. Journal of Dairy Science 76: 3851–3863. Leedle JAZ, Barsuhn K, and Hespell RB (1986) Postprandial trends in estimated ruminal digesta polysaccharides and their relation to changes in bacterial groups and ruminal fluid characteristics. Journal of Animal Science 62: 789–803. Leedle JAZ, Bryant MP, and Hespell RB (1982) Diurnal variations in bacterial numbers and fluid parameters in ruminal contents of animals fed low- or high-forage diets. Applied and Environmental Microbiology 44: 402–412. Munyard KA and Baker SK (2006) Size fractionation of a rumen microbial population by counter-flow centrifugal elutriation. Journal of Microbiological Methods 67: 566–573. National Research Council (2001) Nutrient Requirements of Dairy Cattle, 7th revised edn. Washington, DC: National Academy Press. Russell JB (2002) Rumen Microbiology and Its Role in Ruminant Nutrition. Ithaca, NY: Russell Publishing Co. Russell JB and Van Soest PJ (1984) In vitro fermentation of organic acids common in forage. Applied and Environmental Microbiology 47: 155–159. Silanikove N and Brosh A (1989) Lignocellulose degradation and subsequent metabolism of lignin fermentation products by the desert black Bedouin goat fed on wheat straw as a single-component diet. The British Journal of Nutrition 62: 509–520.