Decoding Bull Fertility Under Tropical Conditions: Multivariate Predictive Modelling From 25,512 Field Spermiograms

Area of Science:

• Veterinary Science
• Animal Reproduction
• Quantitative Genetics

Background:

• Assessing breeding soundness in bulls is crucial for efficient livestock management.
• Tropical conditions present unique challenges affecting bull fertility and reproductive health.
• A data-driven approach is needed to objectively evaluate breeding soundness in large bull populations.

Purpose of the Study:

• To identify key factors determining breeding soundness in Nellore bulls under tropical conditions.
• To develop a reliable risk stratification framework for breeding soundness evaluation.
• To create a practical tool for sire selection and management decisions.

Main Methods:

• Evaluated 25,512 Nellore bulls using macro- and microscopic semen traits, clinical data, and epidemiological factors.
• Employed multivariable logistic regression and internal validation to identify predictors of breeding unfitness.
• Utilized principal component and clustering analyses to define phenotypic profiles and developed a Breeding Soundness Risk Index (BSRI).

Main Results:

• Progressive motility, major sperm defects, and scrotal circumference were independent predictors of breeding unfitness (ROC AUC 0.84).
• Three distinct phenotypic profiles (optimal, intermediate, critical) were identified.
• The BSRI demonstrated high discriminative performance and stability, enabling objective risk stratification.

Conclusions:

• Progressive motility, major sperm defects, and scrotal circumference are essential indicators of breeding soundness in tropical field settings.
• The BSRI provides an objective, field-applicable score for sire selection and management.
• Advancing age and heat load negatively impact breeding soundness, highlighting the effects of tropical stress.