On the Mechanism of Non-Protein-Nitrogen Utilization by Ruminants

Abstract

Defaunated growing lambs utilize the nitrogen of urea-containing rations to about the same extent as normal growing lambs. The average digestibility of the nitrogen in a ration, in which 83, percent of the total nitrogen was in the form of urea, was 68.4 and the biological value 49. Protozoa may be separated from paunch contents in a fairly high state of purity by mechanical procedures of filtration and centrifugation. The dried protozoa fraction, isolated from paunch contents, contained 54.75 percent crude protein. Protozoal protein, when fed to rats, had a true digestibility of 86.2 percent and a biological value of 68. A fraction of bacteria was isolated from paunch contents less satisfactorily than were the protozoa. The bacterial fraction isolated contained 44.50 percent when dried, while a bacterial fraction, obtained by culture on synthetic media, contained 58.81 percent protein. The digestibility of the protein in the purer preparation was 82.4 percent. The biological value of bacterial protein, when fed to rats at a level similar to that at which the protozoa were fed, was 66 percent. Methane production in sheep is highest during the first hour following feeding, decreases for three to four hours quite rapidly, and then slowly. Nine hours after feeding either a urea or casein ration to growing lambs the methane production had decreased about 50 percent or more. Bacterial and protozoal counts, in paunch contents made at intervals for 24 hours following feeding, showed the greatest number of bacteria and the fewest protozoa one hour after feeding. At succeeding intervals, the numbers of bacteria decreased while the numbers of protozoa increased more or less regularly for 16 hours. One hour after feeding, 6,500,000 bacteria and 450,000 protozoa per cc. of paunch contents were counted; 16 hours later but 500,000 bacteria and 840,000 protozoa per cc. were counted. A yeast' like organism was also observed to be present in paunch contents in very large numbers. The data presented here are in agreement with the hypothesis that in ruminants much of the food nitrogen, whether of protein or of non-protein nature, is first synthesized by the bacteria into their own cellular proteins, and then, at least to a considerable extent, the protozoa utilize bacterial protein in their own growth, and finally the host digests the protozoal protein, and remaining bacterial protein. Thus up to the maximum amount of nitrogen capable of utilization by the microorganisms in the paunch, all food nitrogen would exhibit a biological value characteristic of the mixed microorganisms reaching the abomasum and duodenum. This biological value seems to approximate 60. Nitrogen consumed above that required by the microorganisms should exhibit a biological value approximating (probably somewhat less) that of a non-ruminant of similar requirements.