The effect of aging on genetic parameters of boar semen traits

The main objective of this study was to investigate whether the genetic parameters of porcine semen traits change with the age of the boar. The dataset included records from 449,966 ejaculates collected for artificial insemination from 5,692 boars from a commercial line between 7 and 60 mo of age. These records included 12 semen quantity and quality traits measured with a Computer-Assisted Sperm Analysis system. First, an animal repeatability model was applied to the complete dataset to predict the effect of the age of the boar at collection on semen traits. Based on the predicted effects of age, the dataset was divided into 3 age classes: 7 to 13 mo, 14 to 23 mo, and 24 to 60 mo. Traits measured in these age classes were analyzed using a trivariate model to estimate variance components and genetic correlations between traits at different ages. The unadjusted raw averages of semen quality traits with age differed considerably from the model predictions, likely because boars with low semen quality were culled at younger ages and were absent from the data at older ages. Model predictions showed that semen quantity traits increased early in production and plateaued around 18 mo, while sperm motility peaked at 13 mo and gradually declined. Sperm morphology was lowest at 11 mo and gradually increased after. Genetic correlations between the traits at different age classes were close to unity, indicating that, genetically, semen traits at different ages are identical traits. Heritabilities were stable across age classes for all semen traits. Additive genetic variances remained stable for semen quality traits with age class but increased for semen quantity, indicating that the effects of genetic variants affecting semen quantity traits increase with the age of the boar.