Relative growth rate in sheep: heritability and relationship with absolute growth rate and body weight

Data and pedigree information collected between 1990 and 2005 at the Zandi Sheep Breeding Station were used to estimate heritability for relative growth rate (RGR) 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) and to estimate genetic and phenotypic correlations with 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). A derivative-free algorithm combined with a series of six univariate linear animal models was used to estimate heritability coefficients. Best model for each trait was determined with Akaike’s Information Criterion (AIC). (Co)variance components between relative growth rate, absolute growth rate and body weights were estimated using a multivariate analysis. Estimates of direct heritability were 0.13, 0.12, 0.15, 0.10, and 0.04 for RGR1, RGR2, RGR3, RGR4, and RGR5, respectively. Estimates of additive coefficient of variations (CVA ) ranged from 2.53% (RGR1) to 12.74% (RGR5). Genetic correlations ranged from -0.73 (RGR1-AGR5) to 0.99 (RGR1-AGR2) and the phenotypic correlations ranged from -0.75 (RGR1-AGR5) to 0.94 (RGR2-AGR2). In sum, results indicated that genes with additive effects contributed little in the phenotypic variation of RGR in the body weight of Zandi lambs and for this reason the magnitude of possible genetic change through selection would be low. Selection for pre-weaning RGR was recommended for changing the growth curve of Zandi sheep, as it negatively correlated with BW but positively with AGR1, AGR2, WW, and W6.