It has recently been well established that the overall utilization of protein by ruminants represents the result of two opposing processes of ammonia utilization: part of the ammonia produced by the decomposition of food protein may be excreted as urea in the urine, and thus lost to the animal, and part may be built into valuable microbial protein and utilized by the host animal. However, only a few assessments of the nutritive value of proteins supplied in different types of feeds have been conducted on the basis of the metabolic fate of proteins in the ruminant.It was found in this laboratory (Tagari, Ascarelli & Bondi, 1962) that a decrease in solubility of different protein feeds as a result of heat treatment, such as the toasting of soya-bean meal, is accompanied by improved utilization (cf. Chalmers, 1961 b ; Sherrod & Tillman, 1962). The reason given for this was that highly soluble proteins such as those of untoasted soya-bean meal are degraded in the rumen to NH, at a rate too rapid for efficient utilization.Two further aspects of this problem are considered in this paper. Supplying excess protein to cattle is typical of the feeding practice prevailing in many countries. This is so during the greater part of the year in Israel, where protein-rich feeds, such as leguminous forage are more plentiful during the winter than are carbohydrate-rich foods. Most practical rations that could be formulated to meet the energy requirements of ruminants for that season would of necessity include a considerable excess of protein. The influence of giving a surplus of proteins on the metabolic reactions occurring in the rumen has therefore been studied. Lewis, 1957Lewis, , 1962; Phillipson, Dobson, Blackburn & Brown, 1962) noted that the addition of readily available carbohydrates to protein-rich diets considerably depressed the NH, concentration in the rumen liquor. We, therefore, made experiments to determine whether the utilization of surplus proteins could be improved by substituting starch for the carbohydrates of the roughage in the diet and whether the concentration of protein-breakdown products in the rumen, particularly the concentration of NH, in the rumen liquor and of urea in the blood, could be used for estimating the efficiency of protein utilization.
I964 E X P E R I M E N T A L A N D RESUlLTS
Animals and treatmentLambs and adult male and female sheep of the Awassi breed were used. The content of digestible crude protein and the starch equivalent of the various foods in the diets are given in Table I. T h e animals had free access to water and mineral lick. Tables 1-3 on the assumption that the energy content of the rations was adjusted to the theoretical requirements with cottonseed hulls, grain or starch. Sampling. Samples of rumen liquor were removed by suction through a Polythene tube of internal diameter 5 mm. Samples of blood for urea estimation were withdrawn from the jugular vein before the morning feeding and also later, after different time intervals.The analysis of rumen liquor and blood wa...
