Many mammals are herbivorous and consume plant material high in cellulose. Consequently, these animals have evolved a close symbiotic relationship with the microorganisms which reside in their gut which aid the digestion of highly fibrous plant material for the host. The ruminant animal has evolved a specially adapted digestive system to enable, for the best part, relatively efficient breakdown of feedstuffs and is split into four different compartments, the reticulum, rumen, omasum and abomasum. Therumen is the main site of microbial digestion and is perhaps best described as a large fermentation vat which contains a complex array of different microorganisms which act synergistically to break down feed for the host animal. After extensive fermentation by the resident microbes, the products of fermentation, mainly organic volatile fatty acids (VFAs) and microbial protein then become available to the host. Up to 80% of an animal's energy requirements may be met by the production of VFAs and, depending on the diet, microbial protein leaving the rumen may account from between 50 and 90% of the protein that enters the small intestine which is available to the host. Conditions in the rumen are strictly anaerobic, although small trace amounts of oxygen may be found, particularly in close proximity to the rumen wall and in ruminal gas. Temperature is maintained between 38 to 42°C which enables optimum growth of the microbes present and if animals are fed a balanced ration of forage and grain the pH lies between 5.8 and 6.4 which is a favourable environment for the growth of a wide variety of different microorganisms. Problems may occur however, if there is a sharp decrease in pH which may cause a marked change in the composition of the microflora. This may have significant consequences for the productivity and health of the host animal and in some instances these effects may not be realized until several weeks after the initial change. The aim of this paper is to describe the different microorganisms present in the rumen, their nutrient requirements and metabolism, what roles they play and how a perturbation or an imbalance in the microbial population may lead to several metabolic disorders which can have a direct impact on productivity and health.
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