Effects of summer thermal conditions on physiological responses, lying behavior, and weight gain in preweaning Holstein dairy heifer calves in a humid continental climate.

Dairy calves raised in humid continental climates experience intermittent exposure to high temperatures and humidity during several months of the year. The objectives of this observational study were to determine whether summer heat intensity and persistency in a humid continental climate trigger physiological responses indicative of heat stress in preweaned calves, and whether this heat exposure leads to changes in lying behavior and reductions in calf growth. Holstein heifer calves (n = 138) were enrolled at birth from 2 commercial dairy farms in Ontario, Canada between June and August of 2024. Calves were housed in indoor pens or individual outdoor hutches at farm 1 and 2, respectively. Farms were visited 3 x/wk and calves were monitored for the first 9 wk of life. Calf respiration rate, rectal temperature, and skin temperature were measured at each visit and averaged per calf per wk. Accelerometers recorded individual calf lying behavior for the duration of the study period. Calves were weighed weekly until weaning to determine average daily gain (ADG). Data loggers, placed within pen at calf level, recorded ambient temperature and relative humidity at 10-min intervals. Climatic data were used to determine the temperature-humidity index (THI) for each 10-min interval. Various THI measures were calculated and averaged per wk, including average and maximum daily THI and h/d THI exceeded 65, 69, and 72. To assess thermal variability, a coefficient of variation (CV) was also determined for each of these weekly THI measures, in addition to within-d THI CV. Weekly data were analyzed using repeated measures, mixed-effect linear regression models. Weekly THI averaged 65.3 ± 4.7 (mean ± SD), ranging from 49.4 to 74.3. Weekly maximum THI averaged 73.5 ± 4.6, ranging from 58.9 to 82.9. Calf respiration rate and average and maximum skin temperature were positively associated with all THI measures. All THI measures were associated with changes in lying behavior, including decreased daily lying time, more frequent lying bouts/d, and reduced durations of lying bouts. Greater thermal variability was also associated with respiration rate, skin temperature, and lying behavior. Calf weekly ADG averaged 0.8 ± 0.55 kg/d. Average daily THI tended to be associated with reduced ADG, such that for each 1-unit increase in the average THI, ADG was reduced by 0.01 kg/d. Maximum daily THI and h/d THI >72 were also associated with reduced ADG. For each 1-unit increase in maximum THI, ADG was reduced by 0.01 kg/d. For every 1 h increase in the h/d THI >72, ADG was reduced by 0.02 kg/d. These findings demonstrate that preweaned dairy calves in a humid continental climate are exposed to THI at or above thresholds associated with heat stress, and that heat intensity and variability are associated with physiological, behavioral, and growth responses. These results highlight the importance of heat mitigation strategies for preweaned calves in temperate regions where heat stress has historically been overlooked.