Estimation of genetic parameters for sexual dimorphism in body weight of Baluchi sheep

The objective of this study was to estimate genetic parameters for sexual dimorphism (SD) in birth weights (BW), weaning weights (WW), 6-month weight (W6), 9-month weight (W9) and 12-month weight (W12) of Baluchi sheep. Data were collected during 1981-2011 from Baluchi sheep flock (flock No. 1) of Abbas Abad Sheep Breeding Station located in Mashhad, Iran. The data set included 7339, 7303, 5884, 5162 and 4691 records of birth weight, weaning weight, 6-month weight, 9-month weight and 12-month weight, respectively. Six bivariate animal models were used to estimate variance components and corresponding genetic parameters in male and female lambs separately. The SD levels (ratio of Male/Female) increased from 1.06 at birth to 1.13 at yearling age. Except for W9 in which additive genetic variance and direct heritability (𝒉^𝟐_𝒂) were greater in males, for the other studied traits, females experienced higher additive genetic variance and 𝒉^𝟐_𝒂. In both sexes, maternal effects contributed significantly to phenotypic variations in body weights with greater contribution to body weights measured early in life. The maternal genetic effect was greater in females but the maternal permanent environmental effect was greater in males, which together with higher estimates of residual variances revealed greater sensitivity of male lambs to environmental conditions. Cross-sex additive genetic correlation (ra) was 0.86 for BW and 1.00 for the other studied traits. Maternal correlations between sexes were close to 1.00 indicating lack of sexual dimorphism for maternal effects on body weights of Baluchi lambs. Based on an approximate 95% confidence intervals, the difference between sexes regarding direct heritability were not significant, indicating no need or opportunity for different selection strategies in two sexes. Therefore, a model including the lambs of both sexes and with sex effects included in the model can be used for genetic evaluation.