Some of the organic acids of lower molecular weight play an important role in the nutrition of ruminants. Recent studies in vitro by Woodman and Evans (15) indicated that lactic and pyruvic acids and the volatile fat ty acids, especially acetic acid, are the products of cellulose digestion in the rumen of cattle. Forage crops preserved by acid fermentation, such as corn, grass, and legume silages, are becoming an increasingly important part of cattle rations. These feeds contain appreciable quantities of lactic and acetic acids. I t is, therefore, of practical and theoretical value to know how efficiently the energy of these low-carbon organic acids is utilized by animals.