Ruminal Volatile Fatty Acid Production in Relation to Animal Production from Grass.

T h e studies of Sir Joseph Barcroft and his colleagues (Phillipson & McAnalIy, 1942; Barcroft, McAnally & Phillipson, 1944) at Cambridge in the early 1940s established that the volatile fatty acids (VFAs) produced during the fermentation of foodstuffs within the rumen of sheep and cattle are absorbed through the rumen wall and metabolized by the tissues. I t has since been shown (see Annison & Lewis, 1959) that the VFAs arise largely from fermentation of dietary carbohydrates, and of glycerol liberated on hydrolysis of triglycerides, and that only a small part of the soluble carbohydrates escapes degradation. Also, in addition to the major VFAs present in the rumen-acetic, propionic and n-butyric acids---n-valeric acid and branched-chain isomers of butyric and valeric acids (isobutyric, a-methylbutyric and isovaleric) have been found in low concentrations, more particularly in animals on protein-rich diets (el-Shazly, 1952a,b; Annison, 1954). Formic acid, although undoubtedly an intermediate in the fermentation processes, is rapidly dissimilated in the rumen and is present only in very low concentrations (Johns, 1955 ; Bensadoun, Paladines & Reid, I 962). Only traces of lactic acid are found in rumen liquor, with the exception of transient high levels shortly after feeding in animals given diets containing large amounts of cooked, starchy foods (Balch & Rowland, 1957).