Different models for genetic evaluation of growth rate and efficiency-related traits in Iran-Black sheep

Six univariate animal models, including various combinations of the maternal effects, were used to estimate the (co)variance components and genetic parameters for growth rates from birth to weaning (GR1), weaning to six months of age (GR2) and weaning to 12 months of age (GR3), and the corresponding Kleiber ratios (KR1, KR2, KR3), efficiencies of growth (EF1, EF2, EF3) and relative growth rates (RGR1, RGR2, RGR3) in Iran-Black sheep. The most appropriate model for each trait was identified by the Akaike Information Criterion (AIC). In addition, bivariate analyses were used to estimate the (co)variance components between traits. Estimated values of the direct heritability (±S.E.) were 0.08±0.03, 0.07±0.03 and 0.05±0.03 for GR1, GR2, and GR3; 0.25±0.07, 0.05±0.02, and 0.01±0.01 for KR1, KR2 and KR3; 0.05±0.03, 0.04±0.02 and 0.00±0.01 for EF1, EF2 and EF3; and 0.09±0.04, 0.05±0.02 and 0.00±0.01 for RGR1, RGR2 and RGR3, respectively. There was little additive genetic variation in growth rate and efficiency-related traits in Iran-Black sheep and therefore, a small genetic progress would be expected through selection. All the studied traits were affected by maternal effects. Estimates of the maternal heritability (m2) ranged from 0.02 (GR3) to 0.13 (EF1) and estimates of the ratio of maternal permanent environmental variance to phenotypic variance (c2) ranged from 0.03 (GR2, GR3, KR2) to 0.09 (GR1, EF3). Genetic correlations between the studied traits varied from -0.63 (KR1 and EF3) to 0.99 (KR2 and EF3), and the phenotypic correlations ranged from -0.65 (GR1 and EF3) to 0.98 (EF2 and RGR2 and EF3 and RGR3). The study also showed the importance of inclusion of efficiency-related traits in selection programs to improve the biological characteristics of Iran-Black sheep.