Model comparisons and genetic parameter estimates of growth and the Kleiber ratio in Shal sheep

Genetic and phenotypic parameters for lamb growth traits were estimated for the Shal sheep used as animal model. Data on lamb growth performance were extracted from available performance records at the Shal sheep Station in Qazvin, Iran. Studied traits were the body weight of lambs at birth, at three months of age as weaning weight, the six months weight, nine months weight, yearling weight, average daily gain from birth to weaning and Kleiber ratio from birth to weaning. Significant random effects for each trait were determined by fitting additive direct genetic effects, additive maternal effects, the covariance between additive direct and additive maternal effects, maternal permanent environmental and maternal temporary environmental (common litter) effects of twelve animal models. Univariate analyses were carried out under the most appropriate model, determined by the Akaike information criterion test. Direct heritability estimates for birth weight, weaning weight, average daily gain, Kleiber ratio, six months weight, nine months weight and yearling weight were 0.13, 0.19, 0.18, 0.05, 0.16, 0.18 and 0.19, respectively. Maternal additive genetic effects were fitted only for birth weight and weaning weight. Corresponding estimates of 0.12 and 0.10 were obtained for maternal heritability of birth weight and weaning weight, respectively. Maternal permanent environmental effects have low contribution to the expression of Kleiber ratio and lead to estimates of 0.06 and 0.06 for permanent maternal environmental variance as a proportion of phenotypic variance (c(2)) of these traits, respectively. All pre-weaning traits, except Kleiber ratio, were affected by litter effects. The magnitude of ratio of common litter variance to phenotypic variance (l(2)) was 0.05, 0.12 and 0.14 for birth weight, weaning weight and average daily gain, respectively. Direct genetic correlations were positive and ranged from 0.09 for Kleiber ratio-yearling weight to 0.80 for weaning weight-average daily gain. Phenotypic correlations ranged from 0.18 for Kleiber ratio-yearling weight to 0.87 for weaning weight-average daily gain.