High-speed Air Flow Alleviates Circadian Heat Load and Stabilizes Peripheral Thermoregulation in Heifers

Heat stress represents a major challenge to livestock health and welfare under intensive housing systems in tropical and subtropical regions. Especially during the hottest periods of the day, when high temperatures and humidity intensify the physiological burden on dairy cows. This pilot study investigates the influence of varying high-speed airflow within the dairy barn on both the barn microclimate and the animals’ circadian heat load.

This pilot study investigated the short-term effects of varying high airspeed airflow on the barn microclimate and animal physiology to optimize maximum effectiveness. Airflow-based treatments were applied along the barn axis at targeted airflow speeds of ASc (control), AS1 (1.8 m/s), AS2 (3.6 m/s) and AS3 (4.8 m/s). At each airspeed treatment level, observations were recorded to evaluate diurnal physiological responses, specifically rectal temperature and respiration rate. The study involved six heifers (12-18 months) and both environmental and physiological measurements were taken three times daily during the hot-dry (HD) and hot-humid (HH) seasons.

THI ranged from 74–85 under hot-dry conditions and increased to 76–89 under hot-humid conditions, with relative humidity rising from 48–95%. Within each season, THI showed a slight decline with increasing air speed (ASc-AS3), indicating localized improvement in the shed microclimate. CO₂ concurrently decreased from 720–760 to 610–640 ppm (p<0.001). Rectal temperature peaked at 39.59°C and 39.98°C and respiration rate at 33.50 and 37.33 breaths/min, respectively, both significantly reduced at higher air speeds (p<0.001). Peripheral surface temperatures (forehead, head, eye, flank) were higher under humid heat but declined markedly with increasing airflow (3.6-4.8 m/s), with eye temperature decreasing from 39.79 to 39.20°C and 40.18 to 38.78°C. The skin temperature gradient increased from 2.76 to 3.34°C and from 2.36 to 3.25°C, reflecting enhanced heat dissipation. Overall, increasing air speed improved microclimate quality and effectively mitigated thermal and physiological stress.