Heritability of relative growth rate and its relationship with growth-related traits in Afshari sheep

Data on 1496 Afshari lambs collected from 1998 to 2005 was analysed to estimate heritability of relative growth rate in five growth phases, namely birth to weaning (RGR1), weaning to 6-month (RGR2), weaning to 9-month (RGR3), weaning to yearling (RGR4), and 6 months to yearling (RGR5) using a derivative-free algorithm combined with a series of six univariate linear animal models. The most appropriate animal model for each trait was determined based on Akaike's information criterion (AIC). Additionally, bivariate analyses were done to estimate (co)variance components between relative growth rate and corresponding absolute growth rates (AGR1, AGR2, AGR3, AGR4 and AGR5) and body weight at different ages (birth weight, BW; weaning weight, WW; 6-month weight, W6; 9-month weight, W9 and yearling weight, YW). Estimates of direct heritability (h2) were 0.15, 0.06, 0.03, 0.05 and 0.05 for RGR1, RGR2, RGR3, RGR4 and RGR5, respectively. Estimated values of additive coefficient of variations (CVA) were also low and ranged from 3.20% (RGR1) to 17.64% (RGR5), which revealed little additive vitiation in the variance of RGR. Genetic correlations ranged from −0.68 (RGR1-RGR4) to 0.98 (RGR1-AGR1) and the phenotypic correlations were in the range between −0.64 (RGR5-AGR1) to 0.82 (RGR2-AGR2). In general, results indicated that owing to low additive genetic variation, selection would not be very effective for modifying relative growth rate in body weight of lambs during pre- and post-weaning periods and that improvement in management condition and exploiting non-additive variances could be more effective strategy to improve RGR.