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

Abstract

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.

Impact of maternal components on fitting of animal models in genetic parameters estimation for body weight traits in Rahmani lambs

“…Mohammadi et al (2013) Negative  am and r am are in consonance with the reports of María et al (1993) on BW of Romanov sheep; El-Awady (2011) on BW due to the reverse effect of ewes on the maternal ability of their females through offspring over nutrition; Shokrollahi and Baneh (2012) on growth traits in Arabi sheep. Negative association between direct and maternal genetic effect indicating high genetic antagonism between them which propose that these effects tend to amend for high or low weight in the correlated effect.…”

“…Direct heritability coefficients of WT6 were medium to high which were higher than those values obtained by, Gowane et al (2010) for BW which agreed with the results of Mohammadi et al (2013). Indeed, results obtained for WT3 were support the previous reports of Mohammadi et al (2013) estimates obtained from models 3 and 5 for all weight traits denotes that maternal genetic effect constitutes a negligible part of phenotypic variance which may resulted from absence of covariance between direct and maternal genetic effects suggesting slow genetic progress in case of direct selecting method.…”

“…increased faster than environmental variance components in the fourth and sixth models. Medium to high direct heritability estimates for body weight traits denotes that direct additive genetic effect constitutes the most part of phenotypic variation suggesting medium to high genetic progress would be expected through direct selecting method which contradicted with reports of Mohammadi et al (2013).…”

“…Body weight of lambs at different age stages influenced the development of sheep production enterprises (Mohammadi et al 2013). Birth weight is an economically important in any sheep production enterprise because it affects on growth rate before weaning period as published by Al-Shorepy (2001).…”

“…Permanent maternal environmental effect originated from level of feeding at last stage of pregnancy, uterine environment, maternal behavior, and litter size effect on milk yield of the ewe as mentioned by Mohammadi et al (2013).…”