PRODUCTION AND ABSORPTION OF VOLATILE FATTY-ACIDS IN THE RUMEN

Volatile fatty acids (VFA), produced by fermentation of organic matter in the rumen, can have a major effect on production and product composition in ruminants. The relative proportions in which VFA are produced, are influenced by a number of factors, including substrate composition, substrate availability and rate of depolymerization, and microbial species present. Interactions between these factors hamper conclusions with respect to the effect of one single factor. Molar proportions of VFA in rumen fluid are generally assumed to represent the proportions in which they are produced. Evidence is presented that individual VFA absorption rates vary with changes in pH or VFA concentration and hence, that this assumption need not be valid. Attempts to predict the supply of VFA based on substrate degradation in the rumen and stoichiometric parameters have not been satisfactory. Future attempts should include the differential VFA absorption rates, as well as provisions for the effect of amount and rate of degradation, pH and microbial species preferences on type of VFA produced. Such improvements, together with simplifications of the current detailed models of digestion and metabolism, and improved characterization of animals, should then allow better predictions of ruminant performance.