Genetic and environmental effects on carcass characteristics of Southdown × Romney lambs: II. Genetic and phenotypic variation.
Approximately 4400 Southdown × Romney lambs were slaughtered at approx. 18, 23 and 28 wk of age over a 16-yr period. Body weight, carcass measurements and chemical carcass composition were analysed. Heritabilities for preslaughter body weight and postweaning gain were about 0.20, for fat and water percentages in the carcass adjusted for age about 0.35, for kidney fat percentage 0.53, for fat depth and muscle measurements 0.21-0.37, for crutch depth 0.73 and for cannon bone length 0.74. The genetic correlation between carcass fat and fat-free carcass weight was 0.47 when lambs were slaughtered at a constant age. Carcass weight was poorly correlated (0.01-0.04) with percentages of fat, water and protein when lambs were slaughtered at the same age. Carcass measurements plus carcass weight increased the accuracy of selection for fat-free weight at a constant age very little compared with using only carcass weight. However, the addition of carcass measurements to selection criteria would result in correlated responses in carcass composition that more closely resemble direct selection for fat-free weight. Carcass weight alone would be of little value in selection for reduced fat weight adjusted for carcass weight. Direct measurement of carcass composition increased the predicted response for reduced fat weight by 1.6-2.6 times more than any combination of carcass weight and a single fat depth measurement. [For Pt. I see pp. 1856-1863 of the same number of the journal].