Genetic appraisal of growth traits in Iranian native Ghezel sheep using random regression models

Abstract

This research was carried out to estimate the genetic parameters of the body weight traits of the Ghezel sheep using three different random regression models. The used data were 39,288 test-day records collected from 13,378 animals at Ghezel Sheep Breeding Station in Miandoab, West Azerbaijan Province, Iran from 1994 to 2012. Fixed effects in models included type of birth (single, twin) and year of birth (1994-2012). The weight as a function of age in the days of weighting was considered as the fixed regression of orthogonal polynomial. The random effects of the basic model were direct genetic and direct permanent environment. Maternal genetic and maternal permanent environment effects were added to it respectively. In the basic model, increasing the order of fit for the fixed regression, direct genetic and direct permanent environment effects, increased the accuracy of the model. Adding the maternal permanent environmental effect to the model and increasing the order of fit increased the accuracy of the model. However, adding the maternal additive genetic effect to the model and increasing the order of fit for this effect, decreased the model accuracy. Direct heritability for birth weight and weight at 90, 180, 270, and 360 days of age were: 0.30, 0.22, 0.39, 0.38, and 0.55, respectively. These heritability estimates reduced after birth and reached the lowest amount for body weight at 20 days of age; however, after that, they increased as the age increased (this increase was not uniform and had some fluctuations). The direct additive genetic correlations between the traits of weight at 90 days of age and the weight at 180 days of age was the highest (0.955) and between the birth weight and 180 days of age was the lowest (0.072). These correlations between adjacent age groups were closer than the remote age groups. The value of the direct genetic, direct permanent environment, maternal permanent environment, and the phenotypic variances for birth weight was the lowest and increased by increasing of the age (maximum was for weight of 360th day of age).