Testicular temperature gradient and vascular perfusion as predictors of semen quality in summer-stressed Murrah bulls

Heat stress during summer poses a major challenge to reproductive efficiency in Murrah buffalo bulls by disrupting testicular thermoregulation and vascular perfusion, ultimately impairing semen quality. This study aimed to evaluate the relationships among testicular temperature gradient (TG), testicular blood flow, and semen parameters in twelve adult Murrah bulls maintained under summer stress (average THI 80.6). Infrared thermography (IRT) was performed fortnightly to measure scrotal surface temperatures at proximal and distal poles, and TG was calculated as the difference. Colour Doppler ultrasonography was used monthly to assess blood perfusion at the testicular vascular cone subjectively (vascularity score, 1-5) and objectively (percentage area of Doppler signal). Semen was collected using artificial vagina fortnightly and analyzed for volume, concentration, mass motility, progressive motility, viability, acrosome integrity, hypo-osmotic swelling test (HOST) response, and abnormality rate Bulls were stratified into two TG-based groups (>3 °C and <3 °C), with six animals in each group. Bulls with TG > 3 °C exhibited significantly higher testicular blood flow (52.7 ± 1.6% vs. 40.6 ± 2.1%) and vascularity scores (3.3 ± 0.1 vs. 2.5 ± 0.1). They also showed significant improvements in progressive motility (79.3 ± 1.6% vs. 71.1 ± 1.2%), viability (82.9 ± 0.9% vs. 76.4 ± 1.7%), acrosome integrity (86.6 ± 0.2% vs. 84.0 ± 0.3%), and HOST response (68.0 ± 0.7% vs. 63.0 ± 0.5%), along with significantly lower abnormal spermatozoa (10.8 ± 0.2% vs. 13.3 ± 0.3%). Correlation and regression analyses confirmed strong associations between TG, perfusion metrics, and semen traits, while elevated THI negatively influenced testicular function. IRT and Doppler assessments were a reliable and non-invasive biomarkers for detection of sperm quality impairment in heat-stressed Murrah bulls, with potential applications for efficient reproductive management to reduce the adverse effect of heat stress.