Rumenal Flora Studies in the Sheep I. The Nutritive Value of Rumen Bacterial Protein

Abstract

Two large samples of "mixed" rumen bacteria, virtually free from protozoa and feed residues, were prepd. from abattoir sheep, one from sheep coming from "green" feed conditions and the other from "dry"-fed sheep. The "true" digestibilities and biological values of the crude protein of these prepns. were detd. by N-balance method with young growing rats and compared with "standard" casein. The cystine, cysteine and methionine contents of the rumen bacteria samples and of "whole" protein prepns. made from them were also detd. The avg. "true" digestibilities of the protein of the "green"-fed and "dry"-fed rumen bacteria were 62.1 and 64.8, respectively. These were not significantly different from each other but very much lower than that obtained for the casein, namely 101.2. The mean biological values found for the "dry"-fed rumen bacterial protein, the "green"-fed rumen bacterial protein, and the casein were 77.9, 79.9, and 79.6, respectively, when fed as the sole source of N in the ration at levels of 9.2, 9.7, and 9.5% crude protein (N x 6.25). The crude protein content of the "green"-fed bacterial sample was 47.6% and of the "dry"-fed 50.9% on the dry basis. These samples and their protein prepns. were found to be very, similar in cystine content in relation to the total N present, but the former was appreciably richer in methionine. These findings were compared with those of other workers with similar material, with other micro-organisms, and with other sources of protein, and are discussed in relation to the problem of the utilization of microbial protein by the ruminant. Judged by results with growing rats, rumen bacterial protein must be regarded as low in digestibility, relatively high in biological value, but mildly deficient in methionine.