Different THI metrics differentially explain semen kinematics and seminal plasma parameters in Simmental Bulls

Heat stress impairs male reproductive performance, reducing semen quality through acute and cumulative effects. In this study, a Bayesian model assessed the impact of the Temperature Humidity Index (THI) during the 60 days preceding ejaculation, considering average THI, daily THI range (max–min difference), and hours with THI > 78 as indicators of heat stress intensity and duration across spermatogenesis phases (7–10-day periods). The one-year trial involved 114 calves (12 months old) at the Genetic Station of the Italian Simmental Breeders Association. Semen collected after training was analyzed for concentration, morphology, motility, and viability, while seminal plasma was assessed for Advanced Oxidation Protein Products (AOPP), carbonyl groups, thiols and testosterone (T). Greater daily THI variation improved semen volume, morphology, and viability. In contrast, longer exposure to THI > 78 and higher average THI reduced the percentage of normal sperm and increased abnormalities. Unexpectedly, TM, PM, LIN, and WOB increased with higher average THI but decreased with wider THI fluctuations. Conversely, BCF and ALH increased with THI variation, whereas, together with VCL, they declined after prolonged exposure to THI > 78. AOPP increased, and thiols decreased with higher THI, with the strongest effects observed from 60 to 30 days before ejaculation, indicating acute oxidative stress. Protein carbonylation increased with THI fluctuations. T concentrations rose during periods of greater THI variability, suggesting recovery phases that may support Leydig cell activity. Overall, hours with THI > 78 and THI range emerged as key indicators of heat stress, effectively capturing both exposure duration and recovery dynamics (Dual Breeding–Phase 2, PSRN 2014/2022, MIPAAF).