Genotype by environment interaction and model comparison for growth traits of Santa Ines sheep

The objectives of the present study were to compare alternative models for the genetic evaluation and assess the importance of genotype by environment interaction (GxE) in the estimation of genetic parameters and genetic evaluation of birth weight (BW), weight at 60days of age (W60) and weight at 180days of age (W180) of Santa Ines sheep. Data comprise 7622 BW, 4673W60 and 2830W180 records from animals born in 44 Brazilian herds. Four models were used for the analyses: animal model (AM) with homogeneous residual variance (1), or heterogeneous residual variance (2), hierarchical reaction norms model (HRNM) with homogeneous (1) or heterogeneous residual variance (2). The models that best fit the BW, W60 and W180 data were AM2, HRNM1 and HRNM2 respectively. Thus, models for genetic evaluation that consider heterogeneity of variances are recommended to evaluate growth traits of sheep. The correlation between intercept and slope of the HRNM was higher than 0.70 for all traits studied, indicating that animals with higher average breeding values responded better to improvement in environmental conditions, a fact characterizing the scale effect of GxE. Therefore, GxE is an important factor to be considered in the estimation of genetic parameters and genetic evaluation of growth traits of sheep.