Estimation of (co) variance components and genetic parameters for growth traits in Arman sheep

(Co) variance components and genetic parameters for growth traits in Arman sheep were estimated, using data collected during an 11-year period (1999-2010), by applying the restricted maximum likelihood (REML) procedure under univariate and bivariate animal models. The studied traits were body weight of lamb at birth (BW), body weight at 3 months of age as weaning weight (WW), body weight at 6 months (6MW), body weight at 9 months (9MW), yearling weight (YW), average daily gain from birth to weaning (ADG) and the Kleiber ratio (KR) from birth to weaning. Significant random effects for each trait were determined by AIC test fitting the additive direct genetic effect, additive maternal effect, covariance between additive direct and additive maternal effect, maternal permanent environmental and maternal temporary environmental (common litter) effects under nine animal models. Univariate analyses were carried out under the most appropriate model, determined by AIC test. Direct heritability estimates for BW, WW, ADG, KR, 6MW, 9MW and YW were 0.03 ± 0.02, 0.15 ± 0.02, 0.16 ± 0.02, 0.04 ± 0.03, 0.15 ± 0.04, 0.08 ± 0.04 and 0.16 ± 0.02, respectively. Maternal additive genetic effect was fitted only for BW, WW and ADG; corresponding estimates of 0.20 ± 0.02, 0.13 ± 0.01 and 0.07 ± 0.03 were obtained for maternal heritability of BW, WW and ADG, respectively. Maternal permanent environmental effects had a small contribution in expression of pre-weaning growth traits and 6MW, and led to estimates of 0.05 ± 0.02, 0.06 ± 0.04, 0.12 ± 0.03, 0.07 ± 0.02 and 0.06 ± 0.03 for maternal permanent environmental variance as a proportion of phenotypic variance (c2) of BW, WW, ADG, KR and 6MW, respectively. The magnitude of ratio of common litter variance to phenotypic variance (l2) was 0.07 ± 0.02 and 0.09 ± 0.02 for BW and WW, respectively. Direct genetic correlations were positive and ranged from 0.08 for KR-YW to 0.83 for WW-ADG; the phenotypic ones ranged from 0.19 for KR-9MW to 0.96 for WW-ADG. The results showed that the inclusion of maternal effects in genetic evaluation of early growth traits in Arman sheep is of crucial importance.