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 compo sition in ruminants. The relative proportions in which VFA are produce d, are influenced by a number of factors, including substrate composit ion, substrate availability and rate of depolymerization, and microbia l species present. Interactions between these factors hamper conclusio ns with respect to the effect of one single factor. Molar proportions of VFA in rumen fluid are generally assumed to represent the proportio ns in which they are produced. Evidence is presented that individual V FA absorption rates vary with changes in pH or VFA concentration and h ence, that this assumption need not be valid. Attempts to predict the supply of VFA based on substrate degradation in the rumen and stoichio metric parameters have not been satisfactory. Future attempts should i nclude the differential VFA absorption rates, as well as provisions fo r the effect of amount and rate of degradation, pH and microbial speci es preferences on type of VFA produced. Such improvements, together wi th simplifications of the current detailed models of digestion and met abolism, and improved characterization of animals, should then allow b etter predictions of ruminant performance.