Heat stress responses in dairy cows: a longitudinal study across multiple heat waves

Heat stress studies have focused on numerous traits in dairy cows, yet most do not consider that different heat events may trigger distinct responses even in the same animals. A first objective was therefore to follow the dynamics of several traits across different heat waves. To achieve this, weekly sampling was conducted from June to September 2025, with daily monitoring during heat waves, on a total of 57 lactating cows. Mixed models were applied to mitigate the impact of confounding effects and residuals were analysed. Some traits (udder surface temperature, milk protein percentage, milk magnesium concentration, dry matter intake and rumination time) showed relatively consistent direction of variation across heat waves but others, especially those related to
energy balance and activity, presented opposite variations depending on the heat wave. This highlight that cows’ responses to heat stress may depend on heat intensity and duration but probably also on the resistance or exhaustion levels of cow physiological adaptation mechanisms. A second objective was to identify favourable patterns by comparing cows classified as heat-tolerant or heat-sensitive. Thermotolerant cows exhibited a better energy balance during heat events based on associated markers and tended to better maintain production than thermosensitive cows. They also kept lower activity time during heat wave in which negative energy balance markers increased, which could have helped conserve energy. Overall, this study highlighted the central role of energy balance to cope with heat stress and the importance to account for heterogenous responses across heat events.