Application of structural equation models for genetic evaluation of lifetime reproductive traits and age at first lambing in Moghani sheep

The present study aimed to investigate the advantages of structural equation modeling for genetic evaluation of lifetime reproduction traits and age at first lambing in Moghani sheep. Data collected between 1988 and 2011 were retrieved from the Jafarabad Breeding Station of Moghani sheep, located in the north-west of Iran. The investigated traits were age at first lambing (AFL) and lifetime reproductive traits, including overall litter size at birth (OLSB), overall litter size at weaning (OLSW), overall litter weight at birth (OLWB), and overall litter weight at weaning (OLWW). Two multivariate animal models, including standard (SMM) and Inductive Causation algorithm-based structural equation models (IC-SEM), were compared in terms of deviance information criterion (DIC) and predictive ability measures, including the mean square of error (MSE) and Pearson's correlation coefficient between the observed and predicted values (r(y,yˆ)) of records. In general, IC-SEM performed better than SMM in terms of DIC, MSE, and r(y,yˆ) in such a way that the lowest DIC and MSE, and the highest r(y,yˆ) were obtained under IC-SEM. All the structural coefficients estimated by IC-SEM were statistically significant. Heritability estimates for the traits studied were low values of 0.11, 0.03, 0.07, 0.03, and 0.09 for AFL, OLSB, OLSW, OLWB, and OLWW, respectively, implying that the studied traits were mainly controlled by non-additive genetic and environmental effects. AFL had negative and statistically significant (95% highest posterior density (HPD) intervals did not include zero) genetic correlations with other traits studied, ranging from − 0.45 for AFL-OLSB to − 0.69 for AFL-OLWW; meaning that a fast-growing ewe lamb can achieve higher reproductive performance. Genetic correlations among lifetime reproductive traits were positive and statistically significant (99% HPD did not include zero), and varied from 0.36 (OLSB-OLWW) to 0.81 (OLSB-OLSW). According to the causal structure detected by the inductive causation (IC) algorithm, it can be concluded that OLSB, as a parent trait, had indirect causal effects on OLWW, as a composite reproductive trait, mediated via both OLSW and OLWB. AFL had no phenotypic causal impact on the lifetime reproductive traits of Moghani ewes but was negatively associated with these traits in terms of additive genetic effects.