Microbiology and Physiology of the Rumen and Its Response to Different Feeding Strategies

Abstract

The rumen is inhabited by a complex and highly competitive microbial population composed predominantly of strictly anaerobic bacteria (1010 to 1011/g ruminal contents) and protozoa (HP to 107/g) with smaller numbers (~104/g) of anaerobic fungi and facultatively anaerobic bacteria. These microorganisms are responsible for the degradation of ingested feedstuffs to fermentation acids (primarily the volatile fatty acids: acetate, propionate and butyrate) which are absorbed by the host through the ruminal epithelium and serve as the dominant carbon and energy sources for growth, maintenance and production. The ruminant animal then is a symbiotic association between mammal and microorganisms which evolved to enable the animal to live on high fiber diets (19). The ruminal regulatory systems developed by the animal were intended to cope with the microbial fermentation of forage-based rations and they operate very effectively for this purpose. These regulatory systems include temperature control, ruminal pH control through the buffering action of saliva, provision of extra nutrients such as urea or phosphate which enters the rumen also via the saliva, removal of inhibitory soluble (end) products through absorption, and removal of indigestible solids through passage to the lower tract (28). The animal can regulate the activities of the ruminal microorganisms only in so far as it can vary these processes. For the rest, the microorganisms are controlled only by the limitations of their growth physiology and by synergism and competition between species.

With the practice of high grain feeding, the ruminal fermentation has become a liability to the host animal. When diets high in readily fermentable carbohydrates, i.e., diets high in starchy cereal grains, are fed to ruminants, caloric intake and substrate availability are no longer controlled by physical constraints such as fiber particle size and ruminal volume. The animal can now ingest more calories in a shorter time because of the smaller sized, energy-dense nature of cereal grains compared to cellulosic materials. As a result, members of the microbial community are released from substrate limitation restraints. For example, the activities of the amylolytic (starch-degrading) and the saccharolytic (sugar-degrading) bacteria are no longer limited by the relatively slow degradation of fibrous polymers to fermentable oligomers and thus starch degradation and fermentation can occur at very rapid rates.