Effect of semen collection timing on physiological responses and sperm functional parameters in Thai native bulls...

This study investigated the effect of semen collection timing (06:00-08:00 h [morning] vs. 12:00-14:00 h [afternoon]) on semen quality in Thai native bulls under tropical heat stress conditions. Physiological responses, endocrine profiles, and sperm lipid peroxidation were also evaluated. Nine bulls were subjected to a multiple crossover design, with semen collection alternated between morning and afternoon periods across 12 sessions per animal. Environmental parameters, including temperature-humidity index (THI), were continuously monitored. Afternoon collections occurred under higher ambient temperature and THI (>81) than morning collections (77-80), accompanied by increased heart rate (93.88 vs. 79.75 beats/min; P < 0.001) and elevated average scrotal surface temperature (35.08 vs. 33.90 °C; P < 0.001). Serum cortisol concentrations did not differ between collection periods (2.36 vs. 2.18 ng/mL; P > 0.05), whereas testosterone levels were significantly higher in the afternoon (2.23 vs. 1.99 ng/mL; P < 0.05). Afternoon-collected semen exhibited higher total motility (82.80% vs. 76.02%; P < 0.05) and viability (83.68% vs. 77.24%; P < 0.05). However, acrosome integrity was significantly lower in afternoon collections than in morning collections (83.90% vs. 87.40%; P < 0.05), accompanied by higher malondialdehyde concentrations (1.15 vs. 0.84 µM/mL; P < 0.05). Midpiece abnormalities and bent tails with retained droplets were more prevalent in afternoon samples. These findings suggest a physiological trade-off between enhanced sperm motility and increased structural damage to sperm during afternoon semen collection under tropical heat stress. Morning collection may better preserve acrosome integrity and sperm morphology despite lower motility.