Effects of index selection on the carcass composition of sheep given either ad libitum or controlled amounts of food

Sheep of a line (S) selected on an index to increase lean weight and decrease fatness at an age, and a control line (C), were given a high quality food at different levels including ad libitum. Live performance 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 feeding did not affect the extent to which S was superior to C in either the level of fatness in the carcass (0.86 as much) or the ratio of lean to fat (1.28 as much). The lean to bone ratio was slightly greater in S (1.028 of the value of C; P < 0.05) and was higher on the lowest level of feeding compared with the two higher levels used (P < 0.05 in one experiment on females and P < 0.001 in another on males). On ad libitum feeding the S line grew 1.19 times as fast and was 1.17 times as efficient compared with C. These advantages to S decreased as level of feeding decreased to become virtually zero at the lowest level of feeding used, which allowed C to grow at only 0.53 of the rate seen on ad libitum feeding. On ad libitum feeding growth was well described by a Gompertz growth function of the form W = (Z/B) exp(-exp (G(0) -B t)). The maximum growth rate is (Z/e). Line S had a value of Z that was 1.10 that of C averaged across the two sexes. A Spillman function W = W-0 + (A-W-0) (1-exp (-k F)) was used to describe weight, W, in terms of cumulative intake, F. It worked well for ad libitum feeding and for the two restricted regimes used. The value of the combined parameter (A k) varied across treatments in the same way as efficiency did.