Utilization of proteins from milk and raw or acid-treated Toprina yeast by newly born ruminant lambs and growing rats

Abstract

SUMMARYFour experiments were carried out to study the possibility of replacing milk proteins for lambs with Toprina yeast (G) grown on hydrocarbons. In Expt 1 the effect of replacing all the milk protein with Toprina was studied in the presence of milk fat or a mixture of lard and coconut fat. In Expt 2 the maximum level of Toprina yeast that could be used was studied. In Expt 3 the effect of treatment of the yeast with acid on the digestion of Toprina proteins in the small intestine of pre-ruminant lambs (fitted with re-entrant cannulae in the terminal ileum) was examined. In Expt 4 the effect of the supplementation of acid-treated yeasts with tryptophane and methionine was studied using growing rats.The results of Expt 1 showed that lambs given milk diets grew faster (145 v. 80g/day) and converted feed more efficiently than those given Toprina yeast. Apparent digestibility of nitrogen of skim milk was higher (93%) than that of Toprina yeast (84%). The poorest results were obtained from lambs given lard and coconut as the source of fat and Toprina yeast as the source of protein. In Expt 2 it was found that progressively replacing the casein protein by yeast protein (0, 33, 67 and 100%) resulted in a linear reduction in daily live-weight gain, feed conversion and nutrient digestibility, but at 33% replacement of milk protein any effect on lamb performance appeared to be small. In Expt 3 the flow rates of dry matter, nitrogen and its fractions (soluble and insoluble in water) and fats were highest with the raw yeast, intermediate with acid-treated yeasts and lowest with skim milk. Apparent digestibility of nitrogen up to the ileal cannula was 67, 77 and 88% for raw yeast, acid-treated yeast and skim milk respectively.In Expt 4, raw yeast supplemented with methionine was found to be as good a protein source as casein for growing rats. The supplementation of acid-treated yeasts with tryptophane did not improve rat performance, but supplementation with methionine increased growth rate and improved efficiency of feed and protein utilization. Acid hydrolysis of yeast also improved its nutritive value, but the full improvement was only obtained when the diet was supplemented with methionine.It is concluded that yeast protein can provide only about one third of the protein in milk replacers for lambs; prehydrolysis of the yeast may help to increase this proportion without reduction in animal performance.