Ruminants have evolved to utilise diets high in ‘plant structural cell biomass’ of which most
of the energy is present in the structural carbohydrates and to a lesser extent simple sugars. The
content of structural carbohydrates depends on the maturity of the plant and these complex
carbohydrates are also only slowly degraded by microbial enzymes. The rumen digestive tract
has evolved to capably ferment a wide range of feed materials with the production of microbial
biomass and a number of fermentation end products including methane and the volatile organic
acids. Microbial cells, synthesised in the rumen, provide the majority of the animal’s requirements
for essential amino acids when microbes are washed out of the rumen and are digested and
absorbed from the intestines. The supply of microbial protein from the rumen often requires
augmentation with dietary bypass protein to ensure optimal animal production from a particular
feed. The volatile fatty acids and plant fats (which are relatively unchanged in the rumen) provide
the majority of the energy substrates for maintenance and synthesis of new tissue (see Preston
and Leng 1985).