CHEMISTRY AND MICROBIOLOGY OF THE RUMEN
1. What composes the microbial population in the rumen?
2. What are the VFAs produced by microbial fermentation and where are they absorbed?
3. What happens to glucose in the rumen?
4.
What happens to protein in the rumen?5. What happens to lipids in the rumen?
6. What vitamins are synthesized in the rumen? Which vitamin requires cobalt?
7. Note advantages of “up-front” fermentation.
Fermentation that occurs in the rumen and reticulum of ruminants is accomplished by the action of bacterial and protozoan microorganisms. Bacteria account for about 80% of the rumen metabolism (about 1011 bacteria/mL of content). Protozoa account for about 20% of the rumen metabolism (about 106 protozoa/mL of rumen content). These microorganisms are anaerobic, meaning that they thrive in the absence of oxygen.
Both bacteria and protozoa produce short-chain VFAs, carbon dioxide, and methane from their fermentation of foodstuffs. The principal VFAs are acetic, propionic, and butyric acids (see Figure 12-36). These are mostly absorbed from the rumen before the ingesta reaches the duodenum. The usual proportions of VFAs in the rumen are about 60% to 70% acetic acid, 15% to 20% propionic acid, and 10% to 15% butyric acid. The concentration of propionic acid increases when the diet contains large quantities of soluble sugars or starch and decreases when animals are fed poorquality hay. The acetic acid concentration varies in an inverse direction. The propionate-to-acetate ratio is increased by the presence of certain substances. For example, monensin, an ionophore antibiotic, inhibits certain organisms (H2 producers) and favors others (succinate producers) and the propionate to acetate ratio increases.
The products of the fermentation of most carbohydrates are simple mixtures of VFAs with carbon dioxide.
The rumen epithelium can absorb glucose as well as VFAs so that some ingested glucose or intermediary might be absorbed before fermentation. It seems likely, however, that most of the glucose yields VFAs.The microorganisms of the rumen are also involved in the hydrolysis of protein. Hydrolysis occurs through the breakdown of peptides of decreasing chain length to free amino acids, which are largely destroyed by fermentative deamination with the production of carbon dioxide, ammonia, and VFAs. Some peptides and amino acids pass directly into bacterial cells, but it seems that many of the rumen bacteria can synthesize their nitrogenous cell constituents using ammonia as a principal source of nitrogen. Ammonia is the principal soluble nitrogenous constituent of rumen fluid. The ammonia can be derived from dietary protein, urea from saliva, and urea that diffuses through the rumen wall. Rumen fluid has urease activity, so that urea entering rapidly hydrolyzes to ammonia and carbon dioxide.
Triglycerides undergo hydrolysis in the rumen to glycerol and fatty acids. The hydrolysis is caused by rumen microorganisms and the glycerol is fermented further, mostly to propionic acid. The fatty acids continue into the duodenum for further digestion. Some of the unsaturated fatty acids might be hydrogenated in the rumen to saturated fatty acids.
Rumen bacteria can synthesize the B complex vitamins. B vitamin deficiencies are not observed in adult ruminants, except for vitamin B12. Vitamin B12 requires cobalt (a trace mineral) for its synthesis - thus, a cobalt deficiency can result in vitamin B12 deficiency.
The types of digestion accomplished by microorganisms for ruminants are also carried out in the large intestine of nonruminant herbivores (see the previous subsection, Microbial Digestion in the Large Intestine). Its occurrence in the forestomach of ruminants instead of the large intestine has certain advantages:
1. Microbial products of value to the host (VFAs and B vitamins) are presented to efficient absorptive sites, both in the rumen and in the small intestine.
2. Ammonia and substances metabolized to ammonia are used by microbes to synthesize high- quality microbial protein, which is subsequently subjected to abomasal and small intestine digestion and absorption.
3. Selective retention of particles at the reticulo-omasal orifice and the added opportunity for mechanical breakdown of fibers during rumination enhance digestion of coarse foods.
4. The large quantities of gas that are produced can be readily released from the system by eructation.
5. The large input of saliva provides a buffered fluid that permits effective mixing by rumen contractions.
6. Some toxic dietary substances can be rendered nontoxic by fermentation in the rumen, and small intestine absorption of toxin is thereby prevented.
■