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The nutrition of the early weaned lamb. IV. Effects on growth rate, food utilization and body composition of changing from a low to a high protein diet

Summary About sixty male lambs were fed ad libitum from 6 weeks of age on low- or highprotein diets based on barley or barley and fishmeal and containing respectively 120 and 200 g crude protein/kg dry matter. Some lambs were changed from one diet to the other when they reached 28 kg live weight. All were slaughtered as they attained a predetermined series of live weights ranging from 20 to 75 kg. Throughout the experiment, rates of live-weight gain were substantially higher with the high-protein (HP) than with the low-protein (LP) diet, but were highest after a change from low to high protein (LHP). The feed consumption of the LHP lambs did not exceed that of the HP lambs, but the former showed a substantial superiority in feed conversion ratio at the same live weight. At similar empty body weights, the LP lambs contained more fat and less water in the empty body than the HP lambs. Although the percentage differences decreased at higher weights, differences were still apparent at 70 kg live weight. The LHP lambs showed dramatic and rapid changes in body composition, particularly in water and fat content. By 40 kg live weight, their composition approached that of the HP lambs. The ratio of water to protein was consistently lower for the LP lambs. The ratio of protein to ash also differed between LP and HP lambs. It was initially highest for the HP lambs, at about 40 kg live weight it was the same, and at 70 kg live weight it was highest for the LP lambs. The ash content of the LHP lambs remained virtually constant during the period of rapid growth and rapid deposition of water, protein and fat which took place immediately after the change of diet, and only showed compensatory increases after 35 kg live weight. This finding was supported by the pattern of changes in weight and specific gravity of the femur and tibia + fibula. Use was made of separate relationships between live weight and body composition for the LP, HP and LHP lambs to estimate rates of accretion of crude protein, fat and water in the empty body. There was a particularly striking increase in the rate of accretion of water immediately following the change of diet. There was an increase in the water content of empty-body gain and a reduction in the ratio of gain in fat to gain in protein. Derived estimates of the ratio of metabolizable energy intake above maintenance to the energy content of empty-body gain gave some suggestion of an improvement in efficiency of utilization of metabolizable energy for gain following the change from low to high protein. It is concluded however that the improvement in food conversion ratio following the change is attributable mostly to difference in the composition of gain.