Fermentation in the rumen of the sheep. II. The production and absorption of volatile fatty acids during the fermentation of wheaten hay and lucerne hay in the rumen.

Although the composition of the mixture of volatile fatty acids in the rumen of the sheep may be determined, it can throw no light on the composition of the mixture actually formed in the fermentation unless the relative rates of absorption of the acids are known, for it has been established that they are readily absorbed through the rumen wall into the blood stream (Barcroft, McAnally & Phillipson, 1944). Some attempts have been made (Barcroft et al. 1944; Danielli, Hitchcock, Marshall & Phillipson, 1945; Gray, 1947, 1948) to determine these relative rates of absorption, but it seems certain that the results must remain of doubtful significance in view of the fact that the experimental conditions under which they were obtained were too far removed from the normal physiological state of the animal. In fact, while Gray (1947) concluded that propionic acid was more rapidly absorbed than the other acids, the English School, whose work is summarized by Elsden & Phillipson (1948) arrived at the opposite conclusion, namely that acetic acid is more rapidly absorbed than either propionic or butyric acid. The only practicable approach to the problem of the amounts of the fatty acids actually formed in the rumen is by an examination of the fermentation in vitro. The results of experiments in this field have been reported in Part I of the present studies, and it is clear that the findings imply certain relative rates of absorption of the acids in vivo. It is important that these rates should be checked, if possible, by evidence derived from the animal in its normal state. It is known that during the digestion of the natural fodders in the rumen there may be a considerable increase in the concentration of fatty acid there immediately after feeding, followed later by a decrease to the original level. It might be expected that if fatty acids are produced in the proportions found in vitro, proportions very different from those in the rumen fluid, then the composition of the rumen fluid should undergo certain changes during the fermentation. In the period of an increasing rate of production of acid these changes should reflect the difference in composition between the mixture formed and the mixture previously present in the