Estimates of genetic parameters and genetic trends for pre-weaning growth traits in Sardi sheep

The aim of the present study was to estimate (co)variance components for weights at birth (BW), 30 days (W30d) and 90 days (W90d) of age, as well as average daily gain from birth to 30 days (ADG1) and from 30 to 90 days (ADG2) of Sardi sheep. Records of 3709 lambs that descended from 1170 dams and 85 sires were collected over a period of 21 years in the Kra Kra sheep farm. (Co) variance components and the corresponding genetic parameters were estimated under Restricted Maximum Likelihood (REML) method by fitting of six different animal models with ignoring or including maternal effects including maternal genetic or maternal permanent environmental effects. The best model was chosen based on Akaike's Information Criterion (AIC). Bivariate analysis was performed using the most appropriate models obtained in univariate analysis. Age of dam, type of birth, sex of lamb and year of birth showed significant effects on studied traits, so they were considered as fixed effects in analyzing models. Direct heritability estimates for BW, W30d, W90d, ADG1 and ADG2 were 0.07 ± 0.02, 0.09 ± 0.03, 0.05 ± 0.02, 0.01 ± 0.01 and 0.05 ± 0.02, respectively. Maternal heritability estimates were 0.13 ± 0.03 and 0.21 ± 0.04 for BW and W30d, respectively. These results indicate that maternal effects on BW and W30d of Sardi lambs need to be considered in any selection programme undertaken in this breed. Maternal permanent environmental effects accounted for 11%, 8% and 8% of phenotypic variance for W90d, ADG1 and ADG2, respectively. The direct genetic correlations between studied traits were positive in all cases and ranged from 0.23 for BW-ADG1 to 0.98 for W90d-ADG2. The corresponding phenotypic correlations varied from −0.01 for BW-ADG1 to 0.92 for W30d- ADG1. The maternal additive genetic correlation between BW and W30d was 0.66. Also, the environmental correlations among studied traits varied from −0.04 for BW-ADG1 to 0.93 for W30d-ADG1. Annual genetic gains for direct and maternal additive genetic effects of studied traits were close to zero. Thus to realize a genetic progress, selection should be based on additive genetic values instead of phenotypic characteristics.