Selection Criteria for Intensive Market Lamb Production: Growth Traits1

Data on 2,227 pedigreed Suffolk, Hampshire, Dorset, Rambouillet, Targhee, Corriedale and Coarse Wool lambs weaned at 8 to 12 or 10 to 14 weeks of age and 894 ram lambs by 117 sires fed to market ages were collected in 1969, 1970 and 1971. Growth data were weights (W) at 0, 10, 14, 18, 22 and 26 weeks of age and average daily gains (G) by periods. Heritabilities (h2), genetic and phenotypic correlations and variances were estimated from hierarchical co-variance analysis, adjusted for the large, fixed type of birth-rearing and linear and quadratic age of dam effects. Lambs born and reared as singles and those from 3- to 7-year-old ewes were heaviest at all ages (W), with rapid gains (G) to weaning and from 10 to 18 weeks but slower gains from 18 to 22 weeks. The higher G18–22 for twins and triplets or lambs from very young or very old ewes indicates a compensatory effect for the slower early growth due to limiting maternal environment. Genetic and phenotypic parameter estimates for growth traits indicated that G14–22 (h2 = 46%) and W22 (h2 = 44%) should be the most effective selection criteria for improving post-weaning and preweaning growth with limited effects on birth weight. G14–22 and W22 are easily measured when lambs are weaned early and full-fed, but W22 requires adjustment for type of birth-rearing and age of dam effects.