Actively ventilating calf hutches using solar-powered fans: Effects on hutch microclimate and calf thermoregulation

Effects of thermal stress on calf welfare

Cold and heat stress present welfare challenges for dairy calves. The consequences of thermal stress on biological functioning have been well documented, and many housing and management strategies have been evaluated to mitigate those detrimental impacts. In cold weather, mitigation strategies have largely focused on nutritional interventions or limiting heat loss with resources such as bedding or jackets. In hot weather, heat abatement strategies such as supplemental shade, increased environmental air exchange through passive ventilation, and forced air movement through mechanical ventilation have been evaluated. Recently in Wisconsin's continental climate, our group evaluated how 2 aspects of calf welfare-the needs for thermal comfort and social contact (i.e., pair or group housing vs. individual housing)-may align or conflict in winter and summer, respectively. In both seasons, calves pair-housed in outdoor hutches preferred social proximity. When 2 calves shared a hutch, the heat load was greater than for a single calf, which may be beneficial for thermal comfort in winter. In summer, the potential detriments from the additional heat load of 2 calves was mitigated with passive hutch ventilation, which calves preferred. Nonetheless, knowledge gaps remain regarding the impacts of thermal stress on calves' affective states, and much remains unknown about their preferences and motivations for specific thermal stress mitigation resources. Future research to address these gaps could improve our understanding of calf welfare and inform best practices for calf management.

Thermal comfort and ventilation preferences of dairy calves raised in paired outdoor hutches during summertime

Dairy calves are social creatures who are highly motivated for access to a companion. Additionally, heat stress negatively affects the welfare and productivity of calves housed in outdoor hutches. However, no studies have examined the potential tradeoffs pair-housed calves face between competing motivations for social contact and thermal comfort. We evaluated the effects of hutch ventilation on thermoregulatory and behavioral responses of pair-housed calves in outdoor hutches during a Wisconsin summer. Fifty Holstein-Friesian heifer calves were pair-housed (n = 25 pairs) in adjacent hutches with a shared outdoor area. In each pair of hutches, 1 was ventilated (V) with 2 windows at the rear base and the rear bedding door propped open; the other had no rear windows and a closed bedding door (nonventilated, NV). Calves were exposed to 4 conditions for 1 h each (1100-1200 h and 1230-1330 h on 2 consecutive days during wk 4, 6, and 9 of life) in a 2 × 2 factorial design in a balanced order: individually or in pairs in the NV or V hutch. Immediately before and after the 1 h hutch restriction period, respiration rate (RR) and rectal temperature (RT) were recorded while calves were outside. On the subsequent 3 d in those weeks, the locations of each calf (outdoors or inside a hutch) were recorded at 15-min intervals using time-lapse cameras. Linear mixed models were used to evaluate the fixed effects of ventilation, number of calves inside the hutch, week of life, and their interactions, on change in temperature-humidity index (THI), RR, and RT after 1 h; pair of calves was the subject of the repeated statement. Within weeks, the proportion of time calves spent in each hutch and together were averaged across the 3 d of observation. One-sample t-tests were used to evaluate preferences compared with 50% (chance, no preference): (1) for the V (vs. NV) hutch and (2) to be together (or separate) in either the V or NV hutch as well as overall. The THI gain inside the V hutch after 1 h with calves present was lower relative to the NV hutch (0.90 vs. 1.79 units, respectively, standard error of the mean [SEM] = 0.16). Calves in wk 9 of life increased the hutch THI more than in wk 6 of life (1.81 vs. 0.72 units respectively, SEM = 0.16). After 1 h, RR decreased versus was unchanged, respectively, when calves were in the V versus NV hutch (-14.4 vs. -0.9 breaths/min, respectively, SEM = 1.4 breaths/min). No differences were detected in RT. Calves chose to be together most of the time regardless of location (wk 4, 6, and 9, respectively: 83.1% ± 2.4%, 80.3% ± 2.1%, and 78.0% ± 3.1%). Calves had no hutch preference during wk 4 but developed a preference for the V hutch as they aged (wk 4, 6, and 9, respectively: 47.3% ± 4.5%, 61.2% ± 5.1%, and 72.8% ± 4.3%). This is the first study to demonstrate passive ventilation improves animal welfare by reducing heat stress in pair-housed dairy calves in outdoor hutches.