Effects of heat stress on close-up dairy cows and newborn heifer calf weights in a humid continental climate

In humid continental climates, dairy cattle are exposed to episodic periods of high temperature and humidity for several months each year. This environment is characterized by large fluctuations in temperature between season and even within the same day creating unique challenges for maintaining thermal balance and animal performance. However, the impact of prenatal heat stress during the close-up period on calf birth weight in this type of climate has not been well established. The objective of this observational study was to determine whether close-up cows exposed to summer conditions of a humid continental climate show physiological signs of heat stress, and whether their heifer calves born to cows gestating under humid summer conditions have lower birth weights. Close-up Holstein dairy cows (82 nulliparous, 51 parous) were enrolled at 2 commercial dairy farms in Ontario, Canada during the close-up period (-21 d to calving) between May and August of 2024. Cow gestation length averaged 276 ± 5 d (mean ± SD). The farms were visited 3x/wk for the study period, with cow respiration rate, rectal temperature, and skin temperature recorded at each visit for a subset of 102 cows that were bred with sexed semen (71 nulliparous, 31 parous). Heifer calves (n = 133) were weighed after birth, with calves averaging 1.1 ± 0.9 d of age at weighing; calves were weighed on the day of calving (d 0; n = 39), on d 1 (n = 57), on d 2-4 (n = 37). Data loggers, placed within pen just above cow level, recorded ambient temperature and relative humidity at 10-min intervals. Temperature-humidity indices (THI) were calculated for each cow's close-up period, including average daily THI, maximum THI, and average h/d for which THI exceeded the thresholds of 62, 65, 68, and 72. Daily THI averaged 67.0 ± 3.25 (range = 59.5-73.1). The daily maximum THI averaged 72.7 ± 3.07 (range = 65.1-78.7). The average time per day (and range) above the THI thresholds of 62, 65, 68, and 72, were 19.0 ± 4.19 h/d (7.2-24.0 h/d), 15.3 ± 5.05 h/d (2.7-23.8 h/d), 11.5 ± 5.36 h/d (0.43-23.0 h/d), and 5.3 ± 3.60 h/d (0.0-13.8 h/d), respectively. Cow respiration rate and both average and maximum skin temperatures were positively associated with all THI metrics. Calf birth weight averaged 41.0 ± 4.5 kg (range = 31.3-53.5 kg). For each 1-unit increase in daily average THI and maximum THI, calf birth weight (±SE) was reduced by 0.45 ± 0.18 kg and 0.44 ± 0.17 kg, respectively. Each 1 h increase in time (h/d) THI exceeded 62, 65, 68, and 72, birth weight decreased by 0.24 ± 0.12 kg, 0.30 ± 0.12 kg, 0.29 ± 0.11 kg, and 0.47 ± 0.17 kg, respectively. Overall, the results indicate that close-up cows in moderate, yet humid continental climate summer conditions experience negative consequences, specifically physiological signs of heat stress and reduced calf birth weight; this highlights the need to implement cooling strategies for close-up cows even in temperate regions.