The relative growth and development of the carcass tissues of Merino and crossbred rams and wethers

Abstract

One hundred and twenty lambs were used to study the effects of breed (Merino or Dorset Horn x (Border Leicester x Merino)), sex (entire or castrate male) and liveweight (birth to 50 kg) on carcass conformation and composition. Merinos were lighter at birth and grew more slowly than crossbreds in both the pre- and post-weaning phases (P < 0.01). Rams grew significantly faster than wethers only after weaning (P < 0.01). Merinos exceeded crossbreds in height at withers and chest depth (P < 0.01), but the reverse was true for width at 'hooks' (P < 0.01) and length of pelvis (P < 0.05). Sex differences were confined to chest girth, which was greatest in wethers (P < 0.01). Eye muscle width and area were greater in crossbreds than Merinos (P < 0.01). Growth coefficients for bone and muscle calculated on the basis of both half-carcass weight and half-carcass muscle-plus-bone weight were comparable with reports in the literature. For fat, however, the coefficients were considerably higher than has been reported previously (1.938-2.010 and 2.364-2.563 for the half-carcass and the muscle-plus-bone bases respectively). When their values were averaged over the range of slaughter weights studied, crossbreds yielded more bone (P < 0.05), muscle and fat (P < 0.01) than Merinos. Rams yielded more bone (P < 0.05) and less fat (P < 0.01) than wethers. Variation in yields of muscle and fat were due largely to differences in tissue deposition in animals slaughtered at 40 and 50 kg liveweight. Partial correlation analysis indicated that changes in carcass composition were largely accounted for by differences in liveweight.