The digestibility of dietary protein in the ruminant

Abstract

1. Expts. were performed on goats, sheep and cows to obtain a figure for the endogenous excretion of N on a low-N diet. The metabolic N excretion on a normal diet approximated 0.45 gm. per 100 gm. dry matter intake. The effect of fiber on the N excretion was studied with results closely approaching those with the one-stomached animal, the greater the amount of indigestible dry matter the larger the fecal N. When this factor was taken into account, it was shown that the N of straw, which has an apparent digestibility of 30%, has a true digestibility of 100%. A study of the constant fraction, the N excretion during starvation, was undertaken. The urinary excretion of N during fasting and on a low-N diet was examined. During the fast the N excretion per kg. body weight varied from 0.088 to 0.13 gm. with the cows, and to 0.16 gm. with sheep. Further evidence that the fasting catabolism is merely an accelerated protein catabolism was obtained. The endogenous N excretion per kg. body weight varied from 0.063 with sheep, 0.044 with goata, to 0.039 with cows.[long dash]2. Expts. were performed on cows to discover the effect on N metabolism of realimentation following a prolonged fast. The fecal N excretion showed that the higher the biological value of a protein, the lower is the apparent digestibility; and the greater the requirement of the animal for protein, the higher is the apparent digestibility. The excess N excreted during those periods when the apparent digestibility was low was lost on heating at 180[degree]. The urinary N showed that, immediately following prolonged starvation, there is poor utilisation of absorbed N, owing either to increased demand for energy, or to impaired ability to retain absorbed N. N partition products in the urine showed that during active tissue anabolism a marked increase in creatine output occurs, whereas no alteration takes place when the absorbed N is deaminated and excreted. S excretion and the N:S ratio showed that in tissue anabolism S is retained in advance of N. Calculation of the biological value of the ingested protein showed that, following a prolonged fast, an 18% protein level had a value for tissue growth very little less than a 7% level.