Selection Criteria for Intensive Market Lamb Production: Carcass and Growth Traits1

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Abstract
Alternative selection criteria to improve age-constant carcass value were evaluated from genetic parameters of growth and carcass traits. These were estimated in data from 584 pedigreed Suffolk, Hampshire, Dorset, Rambouillet, Targhee, Corriedale and Coarse Wool spring ram lambs, by 85 sires, born in 1969, 1970 and 1971. Lambs were weaned at 8 to 14 weeks of age and fed in dry lot to slaughter at ages of 22 to 33 weeks. Data included gains by periods (G), weights (W) at 0, 10, 14, 18, 22 and 26 weeks and slaughter (S) and age-constant carcass measures of fatness, conformation and predicted yields of trimmed and of boneless-trimmed retail cuts. Phenotypic correlations (rp) plus heritabilities (h2) and genetic correlations (rH) were estimated from hierarchial covariance analysis, adjusted for large fixed effects of type of birth-rearing, age of dam and slaughter age. Lambs reared as singles exceeded twins by 3.4 kg in carcass weight, 2.3 kg in trimmed cuts and 1.4 kg in boneless cuts at 25 weeks of age, and in leg and carcass conformation, quality grade and dressing %, but had more back and kidney fat and lower cutability. Lambs from mature ewes had the least kidney fat. Older lambs were fatter and lower in cutability but had better conformation and heavier cuts. Expected improvement (in genetic σ's) of boneless cuts from 25-week-old lambs per σ of mass selection differential was nearly as large for selection on 22-week live weight (.63) or 14- to 22-week gain (.52) as for hypothetical direct selection for boneless cut weight (.66). Selection for 22-week weight also should increase leg and carcass conformation (.5) and quality (.4) grades as much as direct selection without increasing backfat, but is likely to increase kidney fat (.3). Copyright © 1976. American Society of Animal Science . Copyright 1976 by American Society of Animal Science.

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