Genetic analysis of number of lambs at weaned per lambing of Lori - Bakhtiari sheep using B-spline random regression models

Improving female reproductive performance is an important objective for increasing the profitability of sheep. Random regression model is a method to estimate variance components. This model provides behavior over time continuous observation and be able to combine heterogeneous (co)variance over time by reducing the number of parameters. The objective of this study was to estimate the variance components and heritability for the number of lambs at weaned per lambing trait, with B-spline random regression models in Lori-Bakhtiari sheep. 1
A number of 6146 lambing records of Lori-Bakhtiari sheep, collected during years 1990 to 2013 were utilized from a research flock at the Lori-Bakhtiari sheep breeding station. The flock was managed under a semimigratory or village system. The lambs were weaned at 90 ± 5 days of age. The fixed factors affecting the studied trait were identified using general linear model analysis. Variance components were estimated as based on linear, quadratic and linear-quadratic B-spline random regression models with 4 to 6 knots, using WOMBAT software.
The mean and standard deviation for number of lambs at weaned per lambing trait was 1.17 ± 0.42. The effects of lambing year and age of ewe at lambing had highly significant effects on studied trait. The models were compared using AIC and BIC indices, out of which model LLL4 (linear for fixed regression, additive genetic and permanent environmental effects with 4 knots) was found out as more suitable than the others. According to this model, estimates of additive genetic, permanent environmental and phenotypic variances this trait was in the ranges of 0.003 to 0.090, 0.002 to 0.066 and 0.081 to 0.267, respectively. The range of heritability and the ratio of permanent environmental variance to phenotypic variance estimate for the number of lambs at weaned per lambing trait were 0.02 to 0.34 and 0.01 to 0.25, respectively.
The heritability estimates for the number of lambs at weaned per lambing at 14 to 25 months of age were higher than those for the other ages. Therefore, based on the heritability and standard error, accuracy of breeding values for the number of lambs at weaned per lambing trait is higher in the 14 to 32 months of dam age.