The effect of diet on volatile fatty acid production in the rumen with special reference to the synthesis of milk constituents

The basic adaption of herbivores to diets of high crude fibre content is the enlargement of a portion or portions of the alimentary tract. Such enlarged areas in the Equidae, are the caecum and colon, whilst the Ruminatia have a modified capacious gastric system located at the head of the digestive tract (Elsden and Phillipson, 1948). This compound stomach, the larger part of which is the Rumen, occupies nearly three-quarters of the abdominal cavity of the cow (Sisson and Grossman, 1953). With the absence of celluases in the gastric juices of mammals, fibre degradation is brought about by micro-organisms. In this respect the rumen is known to support a large and diverse microflora. The suitability of the rumen environment for the maintenance of the microbial population is due to (i) the relatively constant temperatures, about 39 degrees Centigrade, and (ii) the pH, which is usually slightly acid. The latter is held relatively constant by the influx of food, water and heavily buffered saliva. There is also a tendency toward equilibrium between the rumen ingesta and the bloodstream in regard to hydrogen ions (Bryant, 1959). Therefore, all rations fed to this class of animal become subjected to rumen fermentation, the more important products of such fermentation being the volatile fatty acids (VFA’s). The following experiment was designed to compare rumen and VFA production when a group of dairy cows was fed markedly different types of rations. Further, an attempt was made to relate these products to rumen fermentation to the synthesis of milk constituents. Since the VFA’s are the major source of energy to the ruminant (Huffman, 1958), gross differences in the concentrations of the individual VFA’s present in the rumen mixture, provided results obtained are interpreted correctly.