During 1993-97, approximately 7000 lambs were weighed at weaning (WW), re-weighted (LW) and ultrasonically scanned for subcutaneous fat depth (FD) and eye muscle dimensions (depth (MD) and width (MW)) 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 REML methods able to take account of the effects of selection on variance components. Heritabilities were higher for LW (0.33 ± 0.03) than WW (0.28 ± 0.03) and litter contributions (as a proportion of phenotypic variance) lower (0.29 ± 0.02 and 0.44 ± 0.02, respectively). Heritability estimates for all traits were similar in both sexes except for MW, which was higher in females (0.34 ± 0.05) than males (0.23 ± 0.05). Averaged across sexes, estimates were: FD (0.30 ± 0.03), MW (0.29 ± 0.04) and MD (0.33 ± 0.03), litter contributions being lower for FD (0.13 ± 0.02) than MW (0.22 ± 0.01) or MD (0.19 ± 0.02). MD was closely related genetically with MW (0.97 ± 0.01), but less closely associated with LW (0.44 ± 0.06) than was MW (0.57 ± 0.05). Both muscle traits showed similar genetic correlations with FD (0.48 ± 0.07 for MW; 0.43 ± 0.07 for MD). Parameter estimates indicated positive genetic trends for LW (regression of estimated breeding values on year of birth) more than two-fold higher than those previously published, and with little concomitant increase in FD.
Proceedings of the New Zealand Society of Animal Production, Volume 58, , 140-142, 1998
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