THERMAL STRESS AND ITS CONSEQUENCES ON BULL SPERMATOGENESIS AND SEMEN QUALITY

Abstract

Heat stress is a significant environmental stressor with detrimental effects on the male reproductive system in cattle, but the physiological, endocrine, oxidative, and sperm functional responses have not been fully elucidated. Spermatogenesis and semen quality were studied in 30 bulls exposed to heat stress, using a within-subject paired design. The animals were evaluated under thermoneutral (THI ≈ 67) and heat stress (THI ≈ 83) conditions, and the physiological and hormonal profiles, oxidative stress biomarkers, and semen quality traits were recorded in both periods. Heat stress resulted in higher scrotal and internal temperatures and heart rate, along with increased cortisol concentration and reduced testosterone levels in endocrine analysis. The activities of antioxidant enzymes in seminal plasma (SOD, CAT, and GPx) were markedly decreased, and malondialdehyde levels were significantly increased, suggesting oxidative stress. Semen analysis revealed significant decreases in ejaculate volume, sperm concentration, motility, and morphology, hematocrit, viability, acrosome integrity, and membrane integrity under HS, but an elevation in the DNA fragmentation index. A negative regression between THI and total and progressive motility, lipid peroxidation, and sperm functional tests was corroborated by means of regression models. These results substantiate earlier studies showing an association between heat stress and reduced bull fertility and further elucidate the mechanistic pathways involved in endocrine disruption, oxidative stress, and sperm DNA damage. These findings underscore the susceptibility of bull spermatogenesis to elevated environmental heat loads and the importance of applied management, nutrition, and genetic strategies for maintaining reproductive efficiency in cattle under global warming conditions.