Effect of Nutrient Supply on Ruminal Microbial Populations

Many mammals are herbivores, and most herbivorous mammals live on a diet that makes cellulose digestion essential. The ruminants, which include some of our most important domestic meatand milk-producing animals cattle, sheep and goats have a specialized digestive tract which is highly adapted and has evolved to digest highly fibrous plant materials which nonruminants are unable to utilize. In ruminants, the stomach consists of several compartments; the abomasum, which is the true digestive stomach is preceded by the reticulum, rumen and omasum. The rumen serves as a large vat in which the food, mixed with saliva, undergoes extensive fermentation by the resident microbial population. The products of fermentation, mainly volatile fatty acids and microbial protein then become available to the host. Volatile fatty acids can account for up to 80% of the host animal’s requirements and microbial protein leaving the rumen may account from between 50 and 90% of the protein, depending on diet, entering the small intestine. Ruminants are one of the few animals in which microbial derived protein is nutritionally available to the host. The rumen microbial population contains a highly diverse and complex mixture of bacteria, protozoa, fungi, archaea, viruses (phage) and mycoplasmas. The roles of the bacteria, protozoa and anaerobic fungi in dietary material breakdown, have been extensively studied and are particularly well defined for the bacteria. In comparison, little is known about phage and mycoplasmas which are thought not to aid in feed breakdown but are believed to have a more opportunistic parasitic role living off the bacterial population. Phage however can lyse their bacterial host, thus contributing to microbial recycling and due to their large numbers (10 particles/ml; Klieve and Swain, 1993) they can have a significant impact upon the dynamics of the bacterial population and consequently upon ruminal fermentation.