Genetic parameters for liveweight and live animal ultra-sound fat and eye muscle dimensions in a synthetic terminal sire breed

摘要

During 1993-97,～7000 lambs were weighed at weaning, reweighed and ultrasonically scanned for subcutaneous fat depth and eye-muscle dimensions at 6 months of age. Animals belonged to Landcorp's "Lamb Supreme" terminal sire breeding programme, derived in 1989-91 from mainly Romney ewes mated to Romney, Wiltshire, Coopworth, Texel and Poll Dorset sires selected for fast growth and leanness. Records representing the selection criteria were analysed by univariate and multivariate animal model Restricted Maximum Likelihood methods able to take account of the effects of selection on variance components. Heritabilities were higher for body weight at reweighing (0.33±0.03) than for weaning weight (0.28±0.03), and litter contributions (as a proportion of phenotypic variance) were lower (0.29±0.02 and 0.44±0.02 respectively). Heritability estimates for all traits were similar in the 2 sexes except for eye-muscle width, which was higher in females (0.34±0.05) than in males (0.23±0.05). Averaged across sexes, estimates were as follows: fat depth, 0.30±0.03; eye-muscle width, 0.29±0.04; eye-muscle depth, 0.33±0.03. Litter contributions were lower for fat depth (0.13±0.02) than for eye-muscle width (0.22±0.01) and eye-muscle depth (0.19±0.02). Eye-muscle depth was closely related genetically to eye-muscle width (0.97±0.01), but less closely to body weight at reweighing (0.44±0.06) than was eye-muscle width (0.57±0.05). The 2 muscle traits showed similar genetic correlations with fat depth (0.48±0.07 for width; 0.43±0.07 for depth). Parameter estimates indicated positive genetic trends for body weight at reweighing (regression of estimated breeding values on year of birth) more than 2-fold higher than those previously published, and with little concomitant increase in fat depth.