Objective measures of muscularity were calculated for 1602 Romney and Romney-cross carcasses which were the progeny of 102 sires. Muscularity was defined as a measure of muscle thickness relative to carcass length. Phenotypic and genetic parameters were calculated from REML estimates of variance and covariance components. Different measures of muscularity had genetic correlations with one another between .48 and .84 and phenotypic correlations between .30 and .62. Measures of muscularity and high heritability (.40 to .68), positive genetic correlations with weight of carcass lean (.32 to .53) and carcass weight (.08 to .32) and negative genetic correlations with carcass fat (-.06 to -.33), and carcass length (-.11 to -.30). Single trait selection on muscularity was expected to result in increased lean and decreased fat. Selection on some of the muscularity measures was expected to result in increased subcutaneous fat. When the breeding objective gave positive emphasis to lean weight and negative emphasis to fat weight and the selection criteria were carcass weight and a muscularity measure, the muscularity measure that gave the greatest economic response to selection was that which used the longissimum muscle width (A) as a measure of muscle thickness.
Proceedings of the New Zealand Society of Animal Production, Volume 51, , 465-468, 1991
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