Evaluation of the ovine Callipyge locus:: II.: Genotypic effects on growth, slaughter, and carcass traits

A resource flock of 362 F-2 lambs provided phenotypic and genotypic data to estimate effects of callipyge (CLPG) genotypes on growth, slaughter, and carcass traits. Lambs were serially slaughtered in six groups at 3-wk intervals starting at 23 wk of age to allow comparisons at different end points. Probabilities of CLPG genotypes were calculated at a position 86 cM from the most centromeric marker of chromosome 18. A contrast of CLPG genotypic effects, based on the paternal polar overdominance model, was used to evaluate callipyge and normal phenotypes. Relationships of traits with slaughter age, carcass weight, or 12th-rib fat depth for callipyge and normal phenotypic groups were estimated by regression. Callipyge and normal lambs did not differ for growth traits measured from birth to slaughter. Callipyge lambs produced 55.9% of live weight as chilled carcass weight compared with 51.7% for normal lambs at the same mean live weight of 48.32 kg. Lighter pelt, kidney-pelvic fat, and liver weights contributed to this advantage of callipyge lambs for dressing percentage (P < .001). Estimated accretion rates of carcass protein at the mean slaughter age were 12.5 and 10.2 g/d for callipyge and normal carcasses, respectively. Corresponding values for carcass fat were 35.2 and 42.1 g/d. Compositional differences in favor of callipyge carcasses were detected at constant values of slaughter age, carcass weight, and 12th-rib fat depth. Callipyge carcasses had 2.56 kg greater fat-free lean and 1.39 kg less fat than normal carcasses at the same mean age of 214.9 d (P < .001). The majority of these differences were established before the initial group was slaughtered and were maintained as age increased. Callipyge carcasses consisted of 24.3% fat and 71.3% fat-free lean, compared with 31.5 and 64.0% for normal carcasses at 25.6 kg of carcass weight. When evaluated at .49 cm of 12th-rib fat depth, callipyge lambs were 15.4 d older and produced 4.1 bg heavier carcasses with 4.3% less fat (P < .001). Effects of CLPG genotypic groups on carcass composition were greater than virtually all reported breed substitution effects. Use of the CLPG mutant allele in structured mating systems can dramatically increase production of lean lamb.