Fermentation in the Rumen of the Sheep Iv. the Nature and Origin of the Volatile Fatty Acids in the Rumen of the Sheep

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. Earlier communications in this series (Gray, Pilgrim & Weller, 1951; Gray & Pilgrim, 1951) dealt with the production of acetic, propionic and butyric acids during the rumen fermentation in vitro and in vivo. A more detailed analysis of the fatty acids of the rumen fluid is reported in the present work, and an attempt has been made to trace the origins of some of these acids by incorporating C-labelled acetic and propionic acids into the inoculum used to initiate the fermentation of wheaten hay and lucerne hay in vitro. The investigation was prompted by the observation that when such labelled acids were introduced into the normal rumen, some of the labelled carbon appeared later in the ' butyric' fraction of the fatty acid mixture. EXPERIMENTAL