Determination of the genetic and non-genetic variations in growth curve of Zandi lambs by random regression models

The aim of this study was to model the variances and covariances of body weight in Zandi sheep from 60 to 365 days of age using random regression models (RRM). Legendre polynomials of different orders were used to model the direct and maternal covariances. Mean trends were also modeled through a quadratic regression on orthogonal polynomials of age. Homogeneity and heterogeneity of the residual variance were considered along the growth trajectory. Different models were compared by log-likelihood ratio test (LRT) and Akaike’s information criterion (AIC). Results showed that simple repeatability model in which orders of 1 were used for all random effects could not adequately model variations in growth curve of Zandi lambs. A RRM with Legendre polynomials of orders 3, 3, 3, and 3 for direct additive genetic, individual permanent environment, maternal additive genetic and maternal permanent environmental effects was selected as the most parsimonious model. The power of the parsimonious model decreased when maternal effects were excluded from the analysis, indicating the necessity of including maternal effects in the model for genetic evaluation of Zandi lambs. Considering the heterogeneity of residual variance along with the growth trajectory improved the overall properties of the model. Direct heritability (h2) decreased from 0.3 at 60 days of age to 0.15 at about 120 days and then increased with age gradually and reached 0.39 at 365 days of age. The individual permanent environmental effect (p2) decreased from 0.43 at 60 days of age to 0.23 at 180 days of age and fixed between 0.25 and 0.30 thereafter. Maternal heritability (m2) was 0.03 at 60 days of age, increased to a peak around 240 days of age (0.22) and decreased with age thereafter. The ratio of maternal permanent environmental variance to phenotypic variance (c2) was below 0.03 throughout the trajectory. Estimates of coefficients of variation (CV) revealed the presence of considerable genetic and environmental variability in growth curve of Zandi sheep which can be exploited for breeding purposes. Both direct and maternal correlations were positively high between adjacent weighs but decreased as the distance between ages increased.

Keywords:

sheep , body weight , random regression models , heritability , genetic correlation