Volatile fatty acids (VFAs), principally acetate, propionate and butyrate but also lesser amounts of valerate, caproate, isobutyrate, isovalerate, 2-methylbutyrate and traces of various higher acids, are produced in the rumen as end-products of microbial fermentation | 6 Volatile Fatty Acid Production J. France1 and J. Dijkstra2 Centre for Nutrition Modelling Department of Animal Poultry Science University of Guelph Guelph Ontario N1G 2W1 Canada 2Animal Nutrition Group Wageningen Institute of Animal Sciences Wageningen University PO Box 338 6700 AH Wageningen The Netherlands Introduction Volatile fatty acids VFAs principally acetate propionate and butyrate but also lesser amounts of valerate caproate isobutyrate isovalerate 2-methylbutyrate and traces of various higher acids are produced in the rumen as end-products of microbial fermentation. During the fermentation process energy is conserved in the form of adenosine triphosphate and subsequently utilized for the maintenance and growth of the microbial population. As far as the microbes are concerned the VFAs are waste products but to the host animal they represent the major source of absorbed energy and with most diets account for approximately 80 of the energy disappearing in the rumen the remainder being lost as heat and methane and for 50-70 of the digestible energy intake in sheep and cows at approximately maintenance the range being 40-65 in lactating cows Sutton 1972 1979 1985 Thomas and Clapperton 1972 . Dietary carbohydrates . cellulose hemicellulose pectin starch and soluble sugars are the main fermentation substrates. They are degraded to their constituent hexoses and pentoses before being fermented to VFA via pyruvate Fig. . Pentoses are converted to hexose and triose phosphate by the transketolase and transaldolase reactions of the pentose cycle so that the majority of dietary carbohydrate metabolism proceeds via hexose which is metabolized to pyruvate almost exclusively by the Embden-Meyerhof glycolytic pathway. Acetyl CoA is an intermediate in the formation of both acetate and butyrate from pyruvate whilst propionate formation occurs mainly via succinate although an alternative pathway involving acrylate is also operative. The need to maintain redox balance
