ESTIMATION OF VARIANCE-COMPONENTS FOR LAMB WEIGHTS IN 3 SHEEP POPULATIONS

Variance components were estimated for lamb weight at birth, 50 d, and 100 d of age. Data from the Canadian flock recording program for lamb s born in 1977 to 1991 for Hampshires (n = 6,395) and Polled Dorsets ( n = 29,204) and 1982 to 1991 for Romanovs (n = 3,432) were studied. Ob served weights were pre-adjusted for the effects of age of dam, sex of lamb, birth-rearing type, month or quarter of year of birth, parity-l ambing interval, and age of dam at first lambing, using estimates deri ved from a fixed effects model including contemporary groups plus thes e factors. Pre-adjusting for nuisance variables reduced the number of equations in the model for variance component estimation. A single-tra it animal model with derivative-free restricted maximum-likelihood pro cedures was used. Random effects were additive direct and maternal gen etic, litter (common environmental), and error. An alternate model exc luded maternal genetic effects. Estimates of litter variance as a prop ortion of phenotypic variance were of moderate size (.12 to .43) and c onsistent across breeds and models. The mean correlation between direc t and maternal genetic effects, across traits and breeds, weighted by the number of animals, was -.40 (SE = .15). The maternal genetic varia nce or direct maternal genetic covariance component, or both, was diff erent from zero (P < .05) for all traits in Hampshires and Polled Dors ets, suggesting that maternal effects were important for weight of lam bs even at 100 d of age. Estimates of direct heritability ranged from .05 to .45, varying across traits, breeds, and models. In Romanovs, wi th the complex model, no estimate of direct or maternal heritability o r direct maternal genetic correlation was different from zero (P > .10 ), which emphasizes differences in these variance components across th e breeds and has implications for genetic evaluation programs.