Rumen microbes and digestion of plant cell walls

The role of bacteria, protozoa and fungi in rumen digestion of plant cell wall material is briefly summarized. Literature evidence is presented suggesting a substantial contribution of entodiniomorph protozoa to fibre digestion in the rumen. Interactions between various bacteria degrading plant cell walls to volatile fatty acids, methane and CO₂ are described, as well as stoichiometry of fermentation. Net microbial growth yields in the rumen are discussed as net results of microbial synthesis and degradation (lysis). It is shown that the absence of protozoa lowers degradation and thus increases efficiency of synthesis. The effect of rumen defaunation on microbial protein supply to the animal is discussed. It is suggested that this effect is the result of: (1) a decrease in rumen fibre digestion; (2) an increase in efficiencies of microbial protein synthesis. Conversion of fibrous diets to soluble carbohydrates is suggested as a means of obtaining an optimal effect of defaunation on microbial protein supply to the animal.