Genetic relationships between visual and objective measures of carcass composition in crossbred lambs

The aim of this study was toestimate genetic and phenotypic (coroariances between objective measures and carcass visual scores, as a test of the potential value afvisual scores in selection programmes to improve carcass composition in crossbred lambs. In each of 1986, 1987 and 1988, 22 Suffolk rams were chosen with either high or low scores on an indexdesigned to increase leangrowth rate. These rams were joined with 18 to20 crossbred ewes each and theirlambs were grov.J11 on grass to one of three target live weights (35·5, 41·5 and47·0 kg) for slaughter. The carcasses of 1881 lambs were visuallyscoredfor overall conformation andfatness using thestandard Meat and Livestock Commission methods. Additionally, a more detailed 1S-point scale assessment of conformation and a direct visual score of subcutaneous fat on the carcass were taken on 1252 lambs during the latter 2 years ofihe study. Carcass composition wasestimated by dissection ofa shoulder joint into lean, fat andbone. The possibility of combining data collected on lambs slaughtered at each a/the three target live weights, for the estimation, ofgenetic parameters wasinvestigated. Resultsindicatedtnmneruubilituestimates for a trait usingdata collected within each of the slaughter groups were homogeneous. Genetic correlations between records collected for a trait within each of the slaughter groups were not Significantly different from one. These results indicated that data collected at each of the target slaughter weights could justifiablybe combined. Heritability estimates were generally higher for shoulder tissue proportions (0,3) than for visual scores (0.2). Genetic correlations between all conformation scores and tissueproportions were not significantly different from 0 and therefore of little or no value inpredicting carcass composition. Genetic correlations between visualscores offat and both tissue proportions and ratios were generally high (around 0,65). These results suggest thaifat scores collected on crossbred animals could be valuable in purebred selection programmes where improving carcaes composition a/the crossbred generation is the underlying objective.