This large-scale study evaluated 25,512 Nellore bulls raised under tropical field conditions to elucidate multivariate determinants of breeding soundness and to derive a data-driven risk stratification framework based on BSE outcomes. Macro- and microscopic semen traits (e.g., progressive motility, sperm morphology including major defects, scrotal circumference, vigour and mass activity), recorded clinical alterations and epidemiological factors (age and seasonality) were integrated using multivariable modelling and internal validation. Logistic regression identified progressive motility, proportion of major defects and scrotal circumference as independent predictors of BSE-based unfitness, with a cross-validated ROC AUC of 0.84 (95% CI 0.83-0.85), indicating high discriminative performance. Principal component and clustering analyses revealed three reproducible phenotypic profiles (optimal, intermediate and critical) combining functional, morphological and anatomical attributes. These components were synthesised into a Breeding Soundness Risk Index (BSRI) showing consistent calibration and stability under five-fold cross-validation, enabling objective stratification of bulls by probability of BSE-based unfitness. Advancing age and seasonal heat load were associated with increased odds of BSE-based unfitness, consistent with cumulative thermal and physiological stress under tropical management. Collectively, these results support progressive motility, major sperm defects and scrotal circumference as cornerstone indicators of breeding soundness in large-scale field settings, and the BSRI provides an objective, field-applicable score to support sire selection and management decisions under tropical conditions.
Fassano et al. (2026) studied this question.