Effects of index selection on the performance and carcass composition of sheep given foods of different protein concentrations ad libitum

Sheep of a line selected on an index to increase carcass lean content at 150 days of age (selected (S); no. = 90), and an unselected control line (control (C); no. = 90), were given ad libitum foods of three different protein concentrations (192, 141 and 120 g/kg dry matter). Growth was measured from about 21 to 114 kg live weight. The carcasses of each line were analysed for lean, fat and bone at three widely varying weights in both males and females. Level of protein did not affect the extent to which S was superior to C in the content of fat (0·86 as much) or lean (1·08 as much) in the carcass. The fat concentration of the carcass increased, and the lean concentration decreased, as dietary protein concentration was reduced (P < 0·01). On the highest level of protein used, the S line grew 1·17 times as fast and was 1·10 times as efficient compared with C. The extent to which growth rate in S exceeded that in C was greater on the highest level of protein used (92·3 g/day) than on the two lower protein diets (26·4 g/day). The difference of 65·9 (s.e. 18·4) g/day was significant (P < 0·01). On the diet of highest protein concentration, growth was well described by a Gompertz function. The S line had an estimated maximum growth rate 1·25 times that of the C when averaged across males and females. A Spillman function was used to describe weight in terms of cumulative intake. It worked well for all three levels of dietary protein concentration. S sheep performed better than unselected sheep on foods differing in protein concentration and over a wide range of live weights, suggesting benefits are likely within the diverse farming environments found in practice.