Validation of a positive pressure tube ventilation system for heat abatement of outdoor preweaning dairy heifers

Even though dairy calves are susceptible to heat stress, they are commonly overlooked for heat abatement strategies. The objectives of this study were to examine the effectiveness of a positive pressure tube ventilation (PPTV) system adapted for preweaning dairy calves housed outdoors during a subtropical summer (June 30 to July 6, average temperature-humidity index [THI] >79). Two-week old Holstein heifers were individually housed in wire pens under a shade cloth structure with a PPTV system at ground level to provide consistent horizontal airflow with daytime misting (PPTV-CL group, n = 24) or without the PPTV system (control heat-stressed group, CON-HS; n = 21). Thermal indices were assessed 3 times per week at 0700 and 1600 h for all 45 heifers. From a subset of pens (PPTV-CL, n = 12; CON-HS, n = 6), airspeed (rear, center, and front of the pen) and sand bedding temperature (adjacent to the calf) were measured 6 times during a 30-h time period (2000, 0200, 0800, 1400, 2000, and 0200 h). At the same 30-h time period, calf thermal indices were measured after a 45-min location restriction to the front of the pen (further from PPTV), after a 45-min restriction to the rear of the pen (closer to PPTV), and after 45 min blocking the PPTV outlet (to mimic heat stress). The microenvironment of the wire pens (ambient temperature, relative humidity, and THI) was measured every 5 min and averaged hourly across the 30-h time period. Standing and lying duration and location within the wire pens were quantified using cameras and computer vision. Airspeeds in the PPTV-CL pens were 1.2 m/s faster than the CON-HS pens. The PPTV system reduced the microenvironment ambient temperature and THI within the pen, leading to reductions in sand bedding temperature without affecting relative humidity. The PPTV system decreased respiratory rates by 19 breaths per minute and skin temperature by 0.4 to 0.7°C. Rectal temperatures were not different between groups. Heifers in the PPTV-CL group spent more time standing during nighttime hours and spent more time in the rear of the pen (closer to PPTV) across all times of the day compared with CON-HS heifers. The thermoregulatory outcomes (reduction in respiration frequency and skin temperature) of the PPTV system were most pronounced when heifers were restricted to the rear of the pen (closer to PPTV), but still evident when restricted in the front of the pen. This system provided continuous horizontal airflow combined with daytime misting, leading to cooler pen microclimates and lower thermoregulatory indices in preweaning dairy heifers during summer. This cooling system shows potential to mitigate hyperthermia in young calves and may support improved health, growth, and well-being during rising temperatures.