Genetic evaluation and genetic trend of growth traits of Zandi sheep in semi-arid Iran using random regression models

A number of 5573 body weight records of 1523 lambs from 164 sires and 875 dams were used to estimate additive genetic, direct and maternal permanent environmental effects on growth from 1 to 300 days of age. The data were collected during the period 1993-2007 from the rearing and breeding station of Zandi sheep in Tehran, Iran and analyzed fitting the random regression (RR) models. Fixed effects in models included age of dam, sex of lamb, type of birth and year of birth. The independent variables were Legendre polynomials of age at weighing and orders of fit from 2 to 5 were considered. The assumptions about the distribution of the residual variance were compared. The variances increased along the trajectory from 0.02 to 25.86, 0.21 to 5.46 and 0.09 to 3.90 for direct additive genetic, direct and maternal permanent environmental effect, respectively. The direct heritability ranged from 0.06 at 1 days of age to 0.52 at 300 days of age. The genetic changes of birth weight from 1991 to 2007 appear to be flat. For the other selected weight (150-d and 300-d weights), plots of genetic trends following similar increasing patterns. The Zandi sheep growth was adequately modeled using RR models despite the limited data. This study has demonstrated the possibility of application of RR models for routine genetic evaluation of Zandi sheep in Iran.