The nutrition of the early weaned lamb:III. The effect of ad libitum intake of diets varying in protein concentration on performance and on body composition at different live weights

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Abstract
SUMMARY: 1. Fifty lambs weaned at about 5 weeks of age were fed ad libitum on mixtures of barley and fish meal containing either 11·0, 15·7 or 19·4% crude protein in dry matter. Male and female lambs on each diet were killed at intervals, starting after they had been on the diets for 3 weeks. The last to be killed had attained a live weight of about 55 kg.2. The mean rate of voluntary feed consumption was less at all live weights for the lambs on the diet containing 11·0% crude protein than for those on the other diets and the difference was statistically significant at live weights of 30 and 35 kg. Mean rates of live-weight gain on the low, medium and high protein diets were respectively 191, 270 and 330 g per day for the male lambs and 177, 225 and 301 g per day for the females, the increase with protein concentration being highly significant. Feed conversion rate (kg feed/kg gain) over the whole experiment increased with increasing weight at slaughter. After adjustment for this effect the mean values showed significant dietary effects, and were least on the highest protein concentration.3. The percentages (y) of nitrogen or of ether extract in the dry matter of the carcass or of the whole empty body were found to be related non-linearly to the empty-body weight (x) at time of slaughter. The relationships were satisfactorily described by equations of the form y = A + Be-Cx, where A, B and C were fitted constants. The relationships for males and females had to be fitted separately, but a common value of A could be used with each of the three groups of male lambs and similarly for the three female groups.4. These relationships were used to obtain estimates of the nitrogen, ether extract and energy content of empty-body gain per unit live-weight gain. The estimated percentages of energy retained as fat were of the order of 76–83%, in agreement with ARC estimates (1965), except for male lambs between 14 and 20 kg live weight on medium or high-protein diets, for which the estimates were only 62 and 63%.5. The implications of these relationships are discussed with particular reference to the finding of dietary effects on body composition during the growth period of the lambs and to the tendency for these differences in body composition to diminish as mature live weight was approached. The differences in body composition between male and female lambs showed no such tendency to diminish.

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