Fisiologia térmica de vacas leiteiras mantidas confinadas em sistema free stall durante a primavera e verão em clima subtropical úmido

Abstract

Heat stress is one of the main limiting factors in milk production in intensive systems, compromising thermal homeostasis, animal welfare, and the economic viability of the activity. The objective of this study was to evaluate the impact of heat stress on the physiological responses and milk production of high- and low-producing Holstein cows kept in a free-stall system. The experiment was conducted on a commercial dairy farm, using two distinct groups: Group 1 (high production) and Group 2 (low production). Environmental variables, including air temperature, relative humidity, black globe temperature, and Black Globe Temperature-Humidity Index (BGHI), as well as physiological parameters (surface temperature, heart rate, and respiratory rate) and daily milk production, were monitored from October 2024 to January 2025. Statistical analysis included Student's t-test, linear regression, and chi-square test with Monte Carlo simulation. The average THI (Temperature-Humidity Index) observed was 72, below the critical threshold for thermal stress; however, approximately 28.3% of the days presented some level of thermal challenge. Lot 2 experienced a higher frequency of days under thermal alert conditions (~32% vs. ~20%), associated with higher relative humidity (p = 0.0346), while Lot 1 experienced more severe episodes of stress, reaching the extreme danger category in approximately 7% of the days. The physiological responses showed differentiated thermoregulatory strategies: Lot 1 presented a higher respiratory rate (54.42 bpm vs. 48.68 bpm; p < 0.001), prioritizing respiratory evaporative dissipation, while Lot 2 demonstrated a higher heart rate (92.72 bpm vs. 88.92 bpm; p < 0.001), indicating greater cardiovascular effort. Surface temperature did not differ between the lots (p = 0.6062). The productive impact was 0.31 kg/ITGU. It is concluded that high-producing cows are more vulnerable to heat stress, prioritizing respiratory evaporative dissipation mechanisms, and that monitoring respiratory rate is a sensitive tool for early identification of heat stress, reinforcing the need to prioritize investments in cooling systems for elite production herds in intensive dairy production systems.