Thermo-physiological responses and hutch microclimate of outdoor hutch-housed dairy heifers with or without continuous ventilation during a continental summer

Heat stress compromises the growth, health, and welfare of dairy heifers from an early age. With the rising frequency of summer heat-stress days, innovative and effective mitigation strategies are increasingly needed. This study evaluated the effects of continuous active ventilation on thermo-physiological responses and hutch microclimate in outdoor-housed dairy heifers during a continental summer (June to August, average temperature-humidity index = 71 ± 7.6). Holstein heifers (n = 32) were enrolled at birth and sequentially assigned to passive (PASS; rear window opened ∼20 degrees, and vents opened 50%) or active (ACT; solar-powered fan redirecting airflow via a custom opening in the hutch rear window, and rear vents open at 50%) ventilation. Fans operated continuously from 3 to 28 d of age, then were turned off. Heifers were monitored until 56 d, corresponding to 1-wk postweaning. During the ventilation treatment period, all heifers were restricted using fencing to 1 h outside (unshaded) and 1 h inside their respective hutches between 1100 and 1300 h 3 times per week. After each restriction, rectal temperature (RT), sweating rate (SR; measured at the neck and rump), skin temperature (ST; measured at the neck and rump), and the difference between "outdoor" and "indoor" values were analyzed. Hutch air speed (AS), particulate matter, formaldehyde, and ammonia levels were monitored inside the hutch. Calf BW and stature were recorded at 7, 14, 21, and 28 d (during ventilation treatment period) and at d 42 and 56 (postventilation period). Airborne bacterial counts inside hutches were assessed before occupancy and on d 28. Health scores and lung ultrasounds were conducted at age-dependent frequencies throughout the study. The ACT hutches had higher internal AS than PASS (1.11 vs. 0.07 ± 0.02 m/s). Both groups showed reductions in SR, ST, and RT after re-entering the hutch, but ACT hutches led to greater reductions in heifers' thermal indices. Ventilation type had no effect on health. Growth parameters, grain intake, and feed efficiency were similar between groups during the ventilation period, but ACT heifers were 4 and 5.5 kg lighter at d 42 and 56, respectively. Although the time spent inside the hutches during the day (regardless of age) was similar between ventilation groups, ACT heifers spent more time lying down when outside the hutch. By d 28, ACT hutches tended to have lower ammonia, formaldehyde, and decreased airborne bacteria (3,466 vs. 29,800 cfu/m³). Providing active ventilation improved the internal microclimate of the hutch and heifer thermoregulation. Additional research is warranted to determine the value of extended ventilation beyond 28 d during persistent hot weather.