Ruminant Digestion Notes

Ruminant Digestion

Cattle Digestion in General

• Cattle digestion is a two-step process:

  • Ruminal digestion

    • The bulk of the feed is digested by microbes in the rumen.

    • This is a non-enzymatic digestion.

    • It accounts for a major part of nutrient digestion.

  • Post-ruminal digestion

    • Microbes and any undigested residues are digested by the animal itself.

    • This accounts for the rest of the nutrients.

    • It involves enzymatic digestion in the abomasum and ‘ small intestine.

    • Rumen products are digested in this stage.

Digestion of Crude Protein ‘

Ruminal Digestion of Crude Protein

• Crude protein (CP) is digested by a variety of bacteria.

• There are 3 stages of digestion:

  1. Protein to Oligopeptides

  2. Oligopeptides to Dipeptides

  3. Dipeptides to $NH_3$

• Prevotella ruminicola occupies a central position due to its peptidase enzymes active against a wide range of substrates. Other bacteria involved include Butyrivibrio fibrisolvens and Selenominas ruminantium, as well as protozoa and potentially the fungus Neocalimastix frontalis.

Enzymes

• Bacterial proteases:

  • Mainly cell-bound, located in the cell membrane or glycocalyx.

• Protozoal proteases:

  • Intracellular.

• Optimum pH for protease activity is 6-7.

• Their action resembles that of pancreatic proteases.

• Activity does not change much with substrate concentration changes.

• Deaminases:

  • Probably inducible enzymes.

  • Optimum pH is 6.9.

  • Deamination occurs via the Stickland reaction, where two amino acids participate: one as a hydrogen donor and the other as a hydrogen acceptor.

  • This process yields VFA, $CO<em>2$, and $NH</em>3$.

Ammonia's Role

• Ammonia ($NH_3$) plays a central role in nitrogen metabolism in the rumen.

  • It's an end-product of bacterial fermentation of crude protein.

  • It serves as the starting point for microbial synthesis of bacterial protein and amino acids.

  • Many bacteria, especially fiber-digesters, require ammonia.

  • It can be synthesized “from scratch,” bacteria can synthesize protein from any substrate yielding ammonia in the rumen, including NPN (urea).

• Urea is converted to ammonia by bacterial ureases, which are abundant in the rumen, resulting in a very quick conversion.

• This process depends on available energy.

• Ammonia is effectively utilized only when converted to microbial protein.

• Excess ammonia is absorbed and metabolized in the liver, where it's converted to urea.

  • Urea is recycled to the rumen via saliva or diffusion across the rumen wall (40-80% of urea produced in the liver).

  • It can also be excreted via urine but doing so is energy demanding.

  • There's a risk of ammonia poisoning if the detoxifying capacity of the liver is exceeded.

Post-Ruminal Digestion of Crude Protein

• Microbial protein (microbes):

  • Approximately 1.8 kg/day (80% bacteria, 20% protozoa).

  • Consists of true protein (60-70% of total N) and non-protein N (in bacterial AA and indigestible cell wall components like peptidoglycans).

  • Microbes are killed in the highly acidic abomasum, leading to some acid hydrolysis and breakdown of the microbes.

  • Bacterial cell walls contain diaminopimelic acid (DAPA), a unique amino acid used to estimate microbial protein quantities.

  • Protozoa contain aminoethyl-phosphoric acid, another unique amino acid used for estimation.

  • Contains sufficient essential amino acids for maintenance and survival of ruminants but is insufficient for high-yielding animals (growing, lactating).

  • Deficient in several amino acids, including methionine and lysine.

  • Supplementation of rumen-protected amino acids is sometimes necessary (e.g., Smartamine M, ML, Mepron M 85).

• By-pass protein (non-degradable, RUP):

  • Many dietary proteins have a more favorable amino acid composition than microbial protein.

  • These proteins would be used more efficiently if they bypassed the rumen and went directly to the small intestine for digestion

  • RUP vs. PDP

  • The extent to which dietary protein is digested by rumen microorganisms depends on:

    • Chemical and physical composition of the feed (55-80% of dietary CP is degraded).

    • The nature of the microbial population and the rate of passage of undigested feed through the rumen.

  • Treatments used to increase the resistance of proteins to rumen digestion:

    • Heat treatment

    • Controlled non-enzymatic browning (reaction of soybean meal with xylose).

    • Sodium hydroxide treatment

    • Coating with Ca salts of fatty acids

• Microbial protein + RUP undergo digestion in the abomasum and small intestine via abomasal, pancreatic, and intestinal enzymes.

  • This results in a different AA profile than dietary protein.

  • Loss of feed protein occurs through its absorption as $NH_3$ from the rumen.

  • $NH_3$ from the digestion of endogenous secretions and some remnants of rumen bacterial cells may be absorbed from the hindgut.

Protein Digestion Summary

• Dietary protein and NPN are either degraded or undegraded.

• Degraded components form $NH_3$, which can be used for microbial protein synthesis or converted to urea in the liver.

• Urea can be excreted or recycled via saliva.

• Microbial protein, along with undegraded dietary protein, is digested in the small intestine.

• Approximately 20% of dietary N is converted to microbial protein.

Digestion of Lipids

Ruminal Digestion of Lipids

• Natural diets are generally low in lipids.

  • Forages contain galactolipids, phospholipids, waxes, pigments, and essential oils.

  • Grains contain triacylglycerides (TAG), which are prevalent in modern feeding systems.

• Rumen microbes use fat inefficiently due to the absence of emulsifying agents and pancreatic lipases.

  • High dietary fat content (above 5%) can decrease fiber digestibility by coating fiber particles and inhibiting microflora.

  • Unsaturated fatty acids (UFA) are toxic to bacteria.

• Lipolysis – release of fatty acids from TAG = FFA + glycerol

• Hydrogenation - UFA SFA

  • UFA are converted to more stable saturated FA (SFA) due to the reducing conditions in the rumen (high H).

  • UFA are converted to more stable trans-forms.

  • Formation of odd- and branched-chain FA

  • Linoleic acid is converted to several conjugated fatty acids (CLA), which are intermediates in the conversion of linoleic and linolenic acid to stearic acid; CLAs are of interest in human health.

  • Free fatty acids (80-90%) are mainly stearic (C18:0, predominant in bacteria) and palmitic (C16:0, predominant in protozoa).

  • Salts (soaps) of FA are neutralized at rumen pH forming potassium and calcium salts of FA.

  • Synthesis of odd- and branched-chain FA helps maintain the optimal fluidity of microbial cell membranes.

Postruminal Digestion of Lipids

• Free fatty acids are absorbed in the small intestine due to the evolved lysolecitine system, which releases FA from feed particles and microbes.

• Salts (soaps) of FA are absorbed directly in the small intestine.

• Dairy cow diets are designed to protect some fatty acids against microbial digestion in the rumen, making them available for intestinal resorption to improve energy and the quality of animal products.

  • Fat protection methods include using Ca salts and chemical treatments.

Lipid Digestion Summary

• Dietary fat is either degraded or undegraded in the rumen.

• Degraded fats yield glycerol and undergo saturation to form VFAs.

• Undegraded fats and the products of degradation are absorbed in the small intestine, contributing to energy and fat synthesis in the liver and peripheral tissues.

Digestion of Saccharides

• Classification according to structure

  • Non-structural saccharides

  • Structural saccharides

  • Non-fibre saccharides

Ruminal Digestion of Non-Fibre Saccharides

• Non-fiber carbohydrates (NFC) include simple sugars and starch such as glucose, fructose, lactose, and sucrose.

• Under physiological conditions, these are fermented into propionic acid.

• Pathologically, they can be converted into lactic acid.

• Serve as a source of energy for microorganisms, leading to the production of ATP.

• Only a minor amount of simple saccharides reaches the intestine.

• Starch is degraded by microbial enzymes, specifically amylase, however amylase has restricted activity.

  • An excess of grains may result in starch appearing in the feces.

• Starch-digesting (amylolytic) bacteria include Streptococcus bovis, Ruminobacter amylophilus, Prevotella ruminicola, B. fibrisolvens, Succinomonas amylolytica, and S. ruminantium.

  • These bacteria attach to feed particles and starch granules, producing amylases.

  • The process converts starch to maltose to glucose; saccharolytic bacteria then convert glucose to ATP + pyruvate (which is converted to VFA, mainly propionic acid), $CO_2$, and methane.

• The digestion of soluble carbohydrates (starch) is rate-limited by protozoa.

  • Protozoa consume starch granules, making them unavailable for bacterial digestion.

Ruminal Digestion of Structural Saccharides

• Structural saccharides include cellulose, hemicellulose, pectins, and lignin, which are sources of crude fiber.

• Fiber is essential for:

  • Proper rumen function.

  • Enabling rumination.

• During rumination, a large amount of saliva (150 L/day) is produced.

  • Saliva acts as a buffer, maintaining the correct pH (6.2-6.8) in the rumen.

• Effective fiber requires a right particle size: minimum 1.3 cm, optimal 2.5 cm and larger.

• Feed intake occurs for 2-4 hours, and rumination lasts 5-10 hours.

• Particles are reduced to approximately 1 mm before passing to the abomasum.

• The amount of fiber needed depends on production level; efficiency is high when fiber is less.

• Ruminant microbes employ anaerobic digestion, specifically, fermentation.

  • Fermentation involves breaking down a substrate without using oxygen.

  • This allows ruminants to use energy from non-structural carbohydrates (CHO).

  • It isn’t a competition with humans.

  • The microbe-composition is influenced by feed; changes in feed lead to changes in microbes.

Plant Cell Wall Digestion

• The primary bacterial species involved are Ruminococcus albus, R. flavefaciens, and Fibrobacter succinogenes, which are the most important fibrolytic (cellulolytic) bacteria.

• Adherence to the plant cell wall varies:

  • Some bacteria adhere immediately to plant cell walls, with their cellulases remaining membrane-bound.

  • Others adhere using finger-like projections (fimbriae) from their cell membranes, which contain specific molecules that attach to the substrate, including binding domains which are part of their glycocalyx (mucopolysaccharide-rich coating on the cell membrane).

  • These structures are associated with the digestive enzymes.

• Protozoa have their own cellulases; cellulose digestion improves when protozoa are added to defaunated rumen.

• Fungi colonize the vascular tissue of forages and even lignified material (straw), possessing an enzyme system to degrade cellulose and hemicellulose to monosaccharides.

Bacterial Cellulases – 3 Types

• Endo-β 1-4 glucanase: acts randomly within the cellulose molecule.

• Exo – β 1-4 glucanase: acts at the end of the molecule, releasing cellobiose units.

• β -D glucoside glucohydrolase: splits cellobiose molecule into its constituent glucoses.

• The process is:

  • Cellulose to cellobiose to glucose.

  • saccharolytic bacteria then convert glucose to ATP + pyruvate (which is converted to VFA, mainly acetic), $CO_2$, and methane.

Ruminal Digestion Summary

• Cellulose, hemicellulose, pectin, fructans, and starch are broken down into cellobiose, pentoses, uronic acid, galactose, sucrose, and maltose.

• These are further metabolized to fructose and glucose.

• Glucose is then converted to pyruvate and oxaloacetic acid, leading to the production of lactate and Acetyl CoA.

• Acetyl CoA generates methane, propionate, acetate, and butyrate.

Volatile Fatty Acids (VFA)

• VFAs are the end products of fermentation.

  • They can meet 60-85% of the animal’s energy requirements.

• Acetic acid (Acetate) – over 50%

  • A dairy cow produces about 1.5 kg/day.

  • It forms Acetyl CoA (used in ketogenesis, lipogenesis, and the Krebs cycle).

  • Most acetate comes from cellulose and is important for milk fat in dairy cows.

• Propionic acid (Propionate) – over 20 %

  • About 1 kg/day is produced.

  • It is used in gluconeogenesis to produce glucose and glycogen.

  • Most propionate comes from starch.

• Butyric acid (Butyrate) – over 10 %

  • About 0.5 kg/day is produced.

  • It is involved in ketogenesis in rumen walls, producing ketone bodies.

  • It is derived from acetic acid.

• Other VFAs include valeric acid, isovaleric acid, caproic acid, isobutyric acid, and lactic acid.

• Acetic, propionic, and butyric acids account for 95% of VFA production (3.5 kg/day).

• Maximum VFA concentration occurs 3-4 hours after feeding.

• VFAs cross the rumen wall and become the major sources of energy for the cow.

Fermentation

• Other products of fermentation include gases:

  • $CO_2$ (Carbon dioxide): 40-80% (conversion of pyruvic acid to acetic acid).

  • $CH_4$ (Methane): 20-40%.

  • $N_2$ (Nitrogen): 15%.

  • $H<em>2$, $O</em>2$, $H_2S$, etc.

• These gases are excreted by eructation.

• A small portion is absorbed into the blood and eliminated during respiration.

• Ionophore Feed additives:

  • Increase the production of propionic acid.

  • Decrease the production of acetic acid.

  • Reduce methane production, thus lessening the greenhouse effect.

Postruminal Digestion of Saccharides

• No glucose passes to the intestine, as everything is fermented by microorganisms.

• Blood sugar is low.

• Glucose is made from glucoplastic amino acids and propionic acid in the liver through gluconeogenesis.

• Highly productive dairy cows experience catabolism and have a high energy demand, leading to underproduction of glucose, which is caused by an underproduction of propionic acid.

• This condition can lead to ketosis.

Carbohydrate Digestion Summary

• Fiber and starch are either fermented in the rumen or undegraded.

• Fermentation produces VFAs, which are used for energy and fat synthesis in the liver and peripheral tissues.

• Undegraded carbohydrates are processed in the small intestine.

• Methane is also produced as a byproduct

Summary of End Products of Ruminant Digestion

• VFAs: Main energy source for cows.

• Gases: $CO<em>2$, $CH</em>4$ (Methane).

• $NH_3$ (Ammonia).

• Saturated fatty acids.

• Microbial protein (= microorganisms).