Characterization of milk losses and recovery in response to heat wave in dairy cows.

This retrospective observational cohort study aimed to evaluate how a summer heat wave affected dairy cows performance using resilience indices that capture the amount and pattern of change in milk yield. Daily records from 305 cows on 4 commercial farms Arborea (Italy), all of which experienced the August 2022 heat wave, were analyzed. The 7-d pre heat-wave mean milk yield was defined as the baseline; responses were summarized for d 0-7 as the heat-wave period and for d 8-14 as the recovery period. Comparisons were made along 3 axes expected to shape response and recovery: parity (primiparous vs. multiparous), lactation stage (early, mid, and late), and baseline milk production level (low, medium, and high; classified by the pre heat-wave average). Differences between primiparous and multiparous cows were small overall; however, multiparous cows showed higher peak recovery milk yield, whereas immediate milk-yield responses and timing measures (milk loss, cumulative loss, minimum yield, time to minimum, time to recovery, and total heat-stress duration) were similar between parity groups. In contrast, lactation stage showed consistent patterns: early-lactation cows had the smallest milk loss and area milk loss percent, maintained higher minimum milk yield during the heat wave and higher mean milk yield at the end of the heat wave, achieved greater milk recovery percent at the end of the heat wave, and returned to baseline faster than mid- and late-lactation cows. Late-lactation cows were most affected in terms of depth of decline (lower minima and larger cumulative losses) and end-of-wave status (lower end-of-wave mean yield and less recovery by wave end), with mid-lactation cows generally intermediate. Baseline production level also structured outcomes: higher-producing cows had larger area milk loss but also maintained higher absolute performance, with higher minimum milk yield during the heat wave, higher peak recovery milk yield, and higher mean milk yield at the end of the heat wave than cows in the medium and low groups. Despite these differences in levels and cumulative effects, the temporal dynamics of response (time to minimum milk yield and recovery time to baseline) were similar across production classes, indicating broadly parallel timing once heat wave began. Overall, lactation stage and initial production level, more than parity, structured the level and trajectory of heat-related change and recovery in milk yield.