Genetic variation in lamb growth and carcass composition

The results of two experiments designed to examine the effects of genetic and environmental factors on the liveweight growth and carcass composition of cross-bred lambs are reported.In the first experiment, data were available for the progeny of Dorset Down, lie de France, Oldenburg, Oxford, Suffolk and Texel sires out of Border Leicester * Scottish Blackface and ABRO Dam Line (5 strains) x Scottish Blackface ewes. Analyses were made for (a) growth traits to 12 weeks for 2585 lambs, the progeny of 102 sires; (b) growth traits to slaughter at fixed weights of 35 kg and 40 kg for 1884 lambs (79 sires); and (c) half carcass dissection traits for 956 lambs (65 sires.Sire breed, year, sex, rearing type, ewe age and other environ­mental factors significantly affected liveweight growth traits, percentage carcass composition and the distribution of dissectible lean tissue and subcutaneous fat between eight standard joints.Paternal half-sib estimates of the heritability of liveweight growth rates to slaughter were low (0.10 ± 0.06). Moderate heritability estimates were recorded for percentage lean in the carcass (0.41 ± 0.13), lean tissue and subcutaneous fat distribution. Estimates of the genetic and phenotypic correlations between traits are also presented.In the second experiment the range of sire breeds was extended/to include the Southdown and Cotswold. Data for 511 lambs, the progeny of 4 to 8 sires/breed and serially slaughtered at 13, 17, 21,25, 29, 33, 37, 41, 51 and approximately 63 weeks of age, were analysed using the allometric equation. Analyses of the growth of dissectible carcass' tissues relative to age, liveweight and side weight, of dissectible subcutaneous, intermuscular and kidney knob and channel fat relative to lean tissue weight, and of lean tissue relative to bone weight, are reported.In general, the effects of breed, year, ewe age, rearing type and sex upon differential growth patterns were not significant (P > 0.05). However, significant (P < 0.05) effects of sire breed and of environmental factors on intercepts are reported