Abstract
Bulls in humid tropical climates are constantly exposed to heat stress, affecting their reproductive performance. This study evaluated the effect of variables associated with season, especially temperature and humidity, on semen quality of Brahman bulls. A total of 39 ejaculates were analyzed from 12 Brahman bulls with ages at collection between 24 and 30 months. The temperature-humidity index (THI) was modeled as a continuous predictor variable; whereas, ejaculates were grouped into two THI cohorts for secondary exploratory analyses. These were based on sample medians of ≤80.1 and >80.1 points in a consistently high-THI tropical environment. Season was associated with variation in total motility (TM), progressive motility (PM), and sperm morphology. TM and PM were highest during the transitional season, whereas the percentage of normal sperm morphology was highest during the rainy season. Sperm curvilinear velocity (VCL), straight-line velocity (VSL), average path velocity (VAP), linearity index (LIN), straightness index (STR), and wobble (WOB) were significantly higher (p<0.05) during the dry season. Sperm viability did not differ significantly among seasons. Under the THI cohort comparison, significant differences (p<0.05) were observed in VCL, VAP, LIN, STR, and WOB, whereas TM, PM, viability, normal morphology, and VSL were not significantly affected. In conclusion, season was associated with variation in sperm motility, kinematic patterns, and morphology in Brahman bulls, whereas sperm viability remained relatively stable across seasons. The results do not suggest a beneficial effect of heat stress, but rather highlight the complexity of interpreting seminal quality responses under tropical conditions. In the continuous THI analysis, THI showed significant positive associations with TM and PM, whereas no significant associations were detected for the remaining kinematic variables, viability, or normal morphology.