Genetic Analysis of Survival from Birth to 6 Month-old of Sanjabi Lambs by REML and Bayes Methods

In order to determine the contribution of genetics for survival of lambs as well as use in the breeding program, it is necessary to estimate the covariance components and genetic parameters. Therefore, the aim of the current study was to estimate genetic parameters for survival from birth to six months of Sanjabi lambs.

Survival records of Sanjabi lambs, supplied by Mehrgan station from 2016 to 2022, were used. To find significant fixed effects, birth year, sex, birth type (single and multiple) and dam age were investigating based on the GLM procedure. Significant environmental factors were included in the final model of genetic analysis. To estimate the genetic parameters of survival traits, both restricted maximum likelihood and Bayesian methods via Gibbs sampling were used. In order to investigate the effect of maternal factors, six animal models with and without maternal genetic and permanent environmental effects were taken into account, and the appropriate model was selected based on Akaike's criterion (AIC). In addition, genetic parameters of survival traits were calculated based on threshold model and Bayes method.

Maternal genetic effect for survival from birth to one and two months of age was significant (p<0.05). Maternal heritability for survival from birth to one and two months of age was 0.43 and 0.18, respectively. Based on univariate analysis, direct heritability for survival traits from birth to one, two, three and six months of age based on the linear model was 0.38, 0.82, 0.017 and 0.043, and based on threshold model was 0.045, 0.020, 0.033 and 0.049, respectively. Based on the REML method, genetic correlation ranged from 0.433 (between traits of survival from birth to one month and survival from birth to two months) to 0.993 (between traits of survival from birth to three months and survival from birth to six months). Also based on the Bayesian method via Gibbs sampling ranged from 0.427 (between survival from birth to one month and survival from birth to two months) to 0.989 (between survival from birth to three months with survival from birth to six months).

Estimated genetic parameters based on Bayes method was more accurately than REML method. The negative covariance between direct and maternal genetic effects made the high value of direct heritability for survival from birth to two months of age. Low to medium direct heritability show the slow genetic progress of survival traits, therefore-paying attention to non-genetic factors along with genetic factors will increase survival of lambs. The genetic correlation between survival traits was positive and high in such a way that the selection for survival at a certain age will cause the correlated response for survival at other ages. Considering the importance of reproductive traits such as the number of lambs per ewe, the genetic improvement of survival is of particular importance and can be included in the breeding goals of Sanjabi sheep.