Effects of protein and energy supply on the growth and carcass compositionof lambs from differing nutritional histories

Effects of dietary energy and protein supply on liveweight (LW) gain and ga in of protein, fat and ash in the carcass, and weight and gain of non-carca ss organs were determined in 118 weaned crossbred lambs from two nutritiona l histories at Camden, NSW in 1991. Half of the lambs were fed to achieve a nd maintain LW at 35 kg (LOW group) and half of the lambs were fed ad libit um until they attained 50 kg LW (HIGH group), during a preliminary period o f 126 days. In the subsequent experimental period, lambs were allocated to treatments providing 500, 800, 1200 or 1500 g/day of pelleted diets (123 g crude protein, 10 MJ ME/kg dry matter). Diets at each intake contained eith er 0, 30, 60 or 90 g of formaldehyde-treated casein (rumen escape protein, REP). This resulted in an experiment comparing LOW and HIGH group lambs at four energy intakes, within which were four rates of inclusion of REP. Duri ng the 90-day experimental period, LOW group lambs had higher rates of gain of LW, carcass weight and all non-carcass components than did HIGH lambs ( P < 0.001). At any rate of carcass gain, LOW lambs contained a significantl y lower proportion of fat in carcass gain than did HIGH lambs (P < 0.05). A fter adjustment to a common carcass weight, the carcass of LOW lambs contai ned a significantly lower mass of fat than did that of HIGH lambs at slaugh ter (P < 0.05). Carcass fat gain in the experimental period was not affected by LW at the s tart of that period or by nutritional history once initial LW was accounted for as a covariate. Data were consistent with fat deposition being princip ally controlled by energy intake over the immediate pre-slaughter period. I n contrast, responses to energy intake in the rate of gain of carcass muscl es, ash, liver, head and feet and gut tissue were significantly greater in lambs of LOW compared to HIGH nutritional history. A significant component of this effect of nutritional history was attributable to LW differences be tween LOW and HIGH lambs; however, nutritional history still had a signific ant effect on these parameters once initial LW was accounted for as a covar iate. Nutritional history may also have modified carcass composition by cha nging the partial efficiency of use of available energy for protein deposit ion without changing the partial energetic efficiency of fat deposition.