Multivariate association between αS1- and κ-casein variants and semen quality traits in Murciano-Granadina bucks in...

This study evaluated the association between αS1- and κ-casein genotypes and semen quality traits in Murciano-Granadina bucks. The aim was to assess their potential relevance in dual-purpose breeding programs integrating milk production and reproductive performance. A total of 6,868 ejaculates from 115 bucks collected between 2010 and 2019 were analyzed under standardized collection and evaluation protocols. Semen traits exhibited substantial variability, particularly in ejaculate volume (mean 1.34 mL; range 0-5.0), sperm concentration (mean 3,619 × 10⁶/mL), and motility parameters. Multivariate analyses revealed significant genotype-associated differentiation patterns for both casein systems. For αS1-casein, 2 canonical functions explained over 90% of the total multivariate variation, with sperm concentration, progressive motility, and membrane functionality (HOST) contributing most strongly to genotype discrimination (P < 0.0001). Bucks carrying AA and AB genotypes were associated with higher sperm output and ejaculate volume, whereas BB genotypes showed comparatively higher values for motility-related traits and acrosome integrity. For κ-casein, a single canonical function accounted for most of the multivariate discrimination (89.0%), primarily driven by progressive and total motility (P < 0.0001). Similarly, AA and AB genotypes were linked to higher sperm production, while BB genotypes exhibited relatively enhanced motility stability and acrosomal integrity. Complementary CHAID classification analyses supported these patterns and achieved moderate classification accuracy (∼66%). However, balanced accuracy remained low (17.9-35.2%), reflecting genotype imbalance. Standardized effect sizes derived from mixed models were small (partial R² range: 0.002-0.037), consistent with the polygenic nature of reproductive traits. Overall, the results demonstrate statistically significant multivariate associations between casein genotypes and semen quality traits. While these findings do not imply causal relationships, they suggest that casein variants may provide complementary information for multi-trait selection schemes. However, their individual effects are modest, and further validation in independent populations and functional studies is required before practical implementation in breeding programs.