Comparative evaluation of physiological responses, milk composition and yield, and biomarkers in Montbéliard, Brown Swiss, and Holstein cows under heat stress condition.

This study examined the impact of heat stress on physiological responses, milk yield and composition, and heat shock biomarkers in 45 dairy cows (Holstein (HO, n = 15), Montbéliard (MT, n = 15), and Brown Swiss (BS, n = 15)) at the same lactation stage and reared under uniform management and climatic conditions in Jendouba, North Tunisia. During a three-week summer period (July - August 2024), cows were exposed to high temperatures and humidity with THI levels ranging from 70.11 to 80.60. Significant breed-specific differences (P < 0.05) were observed in rectal temperature, respiration rate, heart rate, and panting score, with HO cows showing the highest stress indicators. BS cows spent more time lying (69.99%) than MT (48.88%) and HO (39.99%). Although HO had the highest average daily milk yield (25.97 ± 5 L/day) across the study period, production declined significantly from week 1 to 3 by 11.58%, while BS maintained stable output (P > 0.05) for a decrease of 2.79%. Milk composition also varied, with HO showing reduced fat and protein levels under stress, whereas BS remained more stable. Plasma cortisol levels increased across all breeds by week 2 (up to 3.84 ng/ml), but HO had the lowest HSP70 concentrations (4.64 - 6.53 ng/ml), indicating lower thermotolerance. Overall, BS cows demonstrated greater resilience to heat stress, maintaining physiological stability and milk performance, underscoring the importance of breed choice for climate-adaptive dairy farming.