Environmental stress and stallion fertility: mechanistic insights, evidence gaps, and functional implications.

Environmental stress is increasingly recognized as an important determinant of reproductive performance in breeding stallions, particularly in the context of rising temperatures, climate variability, and increasingly intensive management systems. Stallions are especially susceptible because spermatogenesis is highly sensitive to temperature and reproductive activity is regulated by seasonal physiological changes. This review critically evaluates the current understanding of the molecular and cellular mechanisms through which environmental stress affects stallion fertility, with particular emphasis on heat stress, oxidative imbalance, endocrine dysregulation, and sperm functional competence. The available evidence indicates that oxidative stress is currently supported by the strongest stallion-specific data linking environmental stress with impaired reproductive function. In contrast, apoptosis, mitochondrial dysfunction, epigenetic regulation, and fertilization-related signaling pathways are supported primarily by mechanistic studies in other mammalian species and remain insufficiently validated under equine-specific conditions. Environmental stress may compromise not only spermatogenesis but also sperm functional competence, including processes required for successful fertilization, potentially resulting in subfertility despite apparently normal conventional semen parameters. Although mechanistic understanding has advanced considerably, important evidence gaps remain because stallion-specific studies are limited and many proposed mechanisms rely on carefully interpreted cross-species evidence. At present, management-based strategies, including optimization of thermal conditions, housing, and breeding management, provide the strongest evidence for mitigating the adverse effects of environmental stress in stallions. By comparison, antioxidant supplementation, nutritional interventions, molecular biomarkers, and other targeted approaches remain promising but require further validation under equine-specific conditions. Future progress will depend on well-designed stallion-specific studies integrating molecular biomarkers, endocrine profiling, thermophysiological assessment, and comprehensive evaluation of sperm functional competence. Such studies will be essential for improving early detection of reproductive dysfunction and for developing evidence-based strategies to maintain stallion fertility under increasingly challenging environmental conditions.