ABSTRACT
Early workers in the field of ruminant physiology were aware of the presence of formic, acetic, propionic and butyric acids (Tappeiner, 1884), and of smaller amounts of higher members of the saturated fatty acid series (Mangold, 1934), in the rumen fluid. Accurate data were not reported, however, until recently, when partition chromatography became available as an analytical procedure for the analysis of complex mixtures of these acids (Elsden, 1946). By this means acetic, propionic and butyric acids were established as the main components of the mixture of fatty acids in the rumen fluid of sheep and of other ruminants. The butyric acid fraction, however, was thought to contain some higher acid or acids, and it was claimed that formic acid was not a component of the mixture (Elsden, 1945). Using modifications of the older partition and distillation methods Gray (1946) found similar proportions of the three main acids, but also stated that small amounts of formic acid were normally present.
It is interesting that the feeding of two very different fodders such as lucerne hay and wheaten hay should lead to much the same mixture of acids in the rumen, for there is strong evidence that the proportions of various acids actually produced are by no means the same with these two substrates (Gray et al. 1951). The same phenomenon has been noticed by other workers (Elsden, Hitchcock, Marshall & Phillipson, 1946; McClymont, 1951) for fodders of even wider difference; moreover, it has been shown that the same general composition applies for different animals and for different parts of the alimentary tract where plant structures are digested. It is a matter for speculation whether this mixture represents an equilibrium brought about by the factors which govern the relative rates of absorption of the acids through the wall of the gut. There is a clear—but maybe quite fortuitous— parallel between the proportions of the acids in the mixture and their distribution coefficients between oil and water.
