پارامترهای ژنتیکی واریانس باقیمانده برخی صفات وزن بدن در بره های سنگسری

Background and Objectives: The majority of selection and breeding programs for sheep focus on improving weight and growth traits. Body weight uniformity is a crucial economic characteristic within the industry of sheep breeding. Birth weight is an important trait that has an optimal limit, and extremely high or extremely low birth weight can cause difficulties during birth, stillbirth, and problems such as dystocia, stillbirths, and diminished growth of lambs. The existence of genetic variability for the residual variance is a necessary condition for improving this trait through breeding. Recent studies have demonstrated that the residual variance is governed by additive genetic influences. The enhancement of uniformity in body weight traits can be achieved as an economic advantage by employing selection strategies that take into account the genetic variability of the residual variance. The present study was undertaken to ascertain variance components and genetic parameters pertaining to the residual variance of birth weight and weaning weight traits in Sangsari sheep. Also, the genetic correlation between these traits and their residual variance was calculated.
Materials and Methods: To carry out this study, the phenotypic data of 5986 lambs obtained between 1986 and 2016 from the Sangsari sheep breeding station were utilized. The traits examined comprised birth weight, weaning weight, and the residual variance associated with them. To incorporate fixed factors into the model and assess their impact on the examined traits, a least-squares analysis was conducted utilizing the GLM procedure of SAS software. The estimation of variance components and genetic parameters was conducted using the average information restricted maximum likelihood (AI-REML) algorithm implemented in the DMU software, employing the DHGLM method. Firstly, an animal model was applied to estimate the residual variance of birth weight and weaning weight traits. Following that, a bivariate model was employed to examine the influence of additive genetic effects on the residual variance.
Results: The additive genetic standard deviations for the residual variance of birth weight and weaning weight were determined to be 7.74 and 17.90, respectively, consequently, a decrease of one standard deviation in the breeding value of residual variance resulted in a reduction of 7.74 and 17.90% in the uniformity of the respective traits. The heritability estimates for the mean and residual variance of birth weight and weaning weight were found to be 0.26 ± 0.09, 0.059 ± 0.003, 0.23 ± 0.06, and 0.037 ± 0.001, respectively. The genetic coefficient of variation was determined to be 0.58 for birth weight and 0.48 for weaning weight. The genetic correlation ranged from -0.16 for birth weight and residual variance to 0.11 for weaning weight and residual variance.
Conclusion: The results of the current study offer crucial information regarding the presence of additive genetic variation for the residual variance of birth weight and weaning weight within Sangsari sheep herds, which can be utilized to enhance uniformity by selecting for reduced residual variance. The selection for increasing birth weight resulted in a reduction of the breeding value of residual variance and an enhancement of herd uniformity, owing to the favorable genetic correlation observed between birth weight and its residual variance. The genetic correlation for weaning weight was found to be unfavorable, indicating that selecting for higher weaning weights led to increased heterogeneity within the herd.