Milk replacers for preruminant lambs: protein and carbohydrate interactions

Abstract

Twenty-one male crossbred lambs, aged 2–5 days at the start of the experiments, were used. Experiment I. Groups of three lambs were each given one of three low protein diets (0.10 of the total energy as protein) based on isolated soybean protein. Carbohydrate was omitted from one diet, but in the other two it was added at the expense of fat to provide 0.10 or 0.25 of the total dietary energy. In lambs given the carbohydrate-free diet there was a significant reduction in nitrogen balance, in the retention of the apparently digested nitrogen, in liveweight gain and in plasma glucose levels, but a significant increase in plasma urea nitrogen and in the urinary excretion of nitrogen, urea, ammonia, sodium and potassium. The dry matter content of the faeces of lambs given the diet containing 0.10 carbohydrate (0.450) was significantly higher than that of lambs given the 0.25 carbohydrate diet (0.294). There were no other significant differences in performance between these two groups. Experiment 2. Twelve lambs were given diets based on casein, supplemented with DL-methionine, in which protein supplied 0.100, 0–175 or 0.250 of the total gross energy and, at each protein level, carbohydrate supplied 0.025,0.050, 0.100 or 0.200. There was a significant linear increase in liveweight gain, nitrogen balance and apparently digested nitrogen retained as the carbohydrate concentration at each protein level was increased, and a highly significant effect of the level of dietary protein on all parameters measured. It was concluded that the beneficial effect of carbohydrate on nitrogen balance and liveweight gain is of sufficient consequence to warrant the highest inclusion of carbohydrate in milk replacers for lambs that is compatible with the absence of diarrhoea.