DIRECT AND MATERNAL EFFECTS ON BIRTH AND WEANING WEIGHT IN SHEEP

Weights at birth (BW) and weaning (120W) of Rambouillet, Targhee and Columbia lambs adjusted for sex, age of dam, year and type of birth and rearing, were used to study genetic relationships between direct and maternal effects. Data were analyzed to estimate variances among and within paternal and maternal half-sibs, variances among full-sibs and cousins and the covariances offspring-dam and offspring-sire. Heritability estimates for BW and 120W were 0.33 and 0.34 (paternal half-sib), 0.38 and 0.20 (regression offspring-dam), and 0.17 and 0.08 (regression offspring-sire), respectively. Observed variances and covariances were equated to their theoretical expectations for additive and dominance direct and maternal effects using five different statistical models. Each model was applied to a data set grouping animals with specific relationships. For BW heritability estimates of direct effects ranged between 0.20 and 0.34, for maternal effects they ranged between 0.30 and 0.65 depending on the model applied. Corresponding estimates for 120W ranged between 0.13 and 0.21, and between 0.02 and 0.47. Estimated genetic correlations between additive direct and additive maternal effects for BW ranged between -0.18 and -0.74 and for 120W ranged between 0.55 and -0.88 depending on the model applied. These results indicate that additive direct and additive maternal effects for BW and 120W were important, and that generally a negative genetic correlation between additive direct and maternal effects was observed. Therefore, the portion of the selection differential realized in a maternally influenced trait, is expected to be low.