Estimates of (co)variance components and genetic parameters of growth traits in Marwari sheep

The present investigation was undertaken to estimate the (co)variance components and genetic parameters for different growth traits in Marwari flock comprising records of 1649 animals distributed over a period of 12 years (1999-2010), maintained at Arid Region Campus of Central Sheep and Wool Research Institute, Bikaner, Rajasthan, India. The estimation was done by restricted maximum likelihood procedures, fitting six animal models with various combinations of direct and maternal effects. As per likelihood ratio test, direct heritability estimates from the best model for body weight at birth, weaning, 6, 9 and 12 months of age, and average daily gain during birth to weaning, weaning to 6 and 6-12 months of age were 0.28 +/- 0.058, 0.27 +/- 0.050, 0.28 +/- 0.049, 0.30 +/- 0.080, 0.29, 0.26 +/- 0.050, 0.16 +/- 0.040 and 0.31, respectively. Maternal genetic effect declined from 4% at 6 months weight to 1% at 9 months and was zero at 12 months of age. Maternal genetic effect on the post-weaning traits was a carryover effect of the maternal influences during pre-weaning age. Maternal permanent environmental effects contributed 19% of the total phenotypic variation in birth weight and 8% for weaning weight. The evidence for maternal genetic effect for average daily gain was observed only during 6-12 months of age where the additive maternal heritability was estimated as 8%. The genetic correlation between direct and maternal genetic effects was found significantly large and negative for all the traits, indicating antagonistic pleiotropy, which must be considered while formulating breeding plans. A modest rate of genetic progress seems possible in the flock through selection. Genetic correlations between body weight traits were positive and ranged from 0.23 between birth weight and weight at 6 months to 0.88 between weaning weight and weight at 9 months of age. The positive and high genetic correlation of weaning weight with weight at subsequent ages suggests that genetic gain in post-weaning weight will be maintained even if selection age is reduced to 3 months.