Carcass composition in weight-selected and control bulls from a serial slaughter experiment

Abstract

Bull calves were sampled at random at weaning from the 17th calf crop of an Angus herd selected for yearling weight (AS1) and from its contemporary control herd (ACO). Thirty animals per herd were slaughtered in groups of five per herd at intervals, from 7 to 25 months of age. Live weight and scrotal circumference were taken on the day before slaughter, along with a blood sample for analysis of plasma insulin-like growth factor-1 (IGF-1) and testosterone concentration. The left side of each carcass was dissected into saleable meat, bone and trimmed fat components. Live weights and hot carcass weights of AS1 animals were, on average, 16.8 and 17- 6% greater respectively than those of ACO animals. Allometric comparisons of the carcass composition data (log-log regressions) showed no significant herd differences in the rate at which saleable meat, bone or trimmed fat weights increased with carcass weight. Adjusted weights of bone, however, were significantly greater in the AS1 than ACO herds (P < 0.05). There were significant herd differences in the weights of kidney, liver and testes, in scrotal circumference and in the concentration of IGF-1, but these disappeared after adjustment for live weight. On an adjusted basis, heart weight was 6.1% greater in the AS1 than the ACO herd (P < 0.05).