Although breed substitution is a very common method of rapidly altering the characteristics of a livestock population, within-breed selection for lamb carcass characters is becoming more widely used in the terminal sire breeds in Britain. Such selection schemes require accurate and applicable genetic parameters for the carcass traits of interest. This paper reports a genetic analysis of 11 traits measured on 3592 lamb carcasses using a derivative-free restricted maximum likelihood technique. Because of the trial design it was possible to estimate the genetic parameters of the traits at comparable fat cover, carcass weight and age at slaughter, using appropriate within-subclass covariates.
Heritability estimates for muscle depth (0·23, s.e. 0·04), muscle width (0·32, s.e. 0·05), fat depth (0·31, s.e. 0·05), caliper fat (0·23, s.e. 0·04) and conformation (0·29, s.e. 0·05) were similar in the weight and age-corrected results. Heritabilities for fat cover (0·35, s.e. 0·05), daily carcass weight gain (0·63, s.e. 0·07), and leg conformation (0·44, s.e. 0·05) were higher with slaughter at fixed weight than at fixed age. The heritabilities of traits at comparable fat cover were similar to those at comparable age at slaughter with the exception of daily carcass weight gain, fat depth, caliper fat and leg conformation.
The different measures of fat were highly correlated with each other. Conformation was moderately correlated with external fat score, carcass length and muscle depth. Muscle depth was moderately correlated with carcass length. Differences were found between genetic correlations calculated at equal weight, a common point of comparison in breeding schemes, and equal fat cover, a common end-point for slaughter lambs.