Dairy herd response to annual heat load mainly differs depending on geographical location, calving distribution, and...

Heatwaves increasingly threaten dairy production stability in the context of global warming. Although the effects of heat stress on cows are well documented, the responses of herds to such hazards are poorly described. The main objectives of this study were to assess whether annual heat load has a quantifiable impact on the annual performance of dairy herds and to establish which herd characteristics influence the herd-scale response. Using a bottom-up approach linking hypotheses on animal performance to herd-scale outcomes, we quantified herd response and capacity to thrive during heat waves (resilience) in 3,393 commercial dairy herds located in 4 contrasting climatic areas in France. Herd data were available from 12 to 24 years during the period of 2000 to 2023. The effects of heat load on annual herd performance variables (milk yield, fat and protein contents, duration of lactation, calving interval, recalving number, productive longevity of cows, and herd size) were studied using the cumulative annual temperature-humidity index (CATHI). This index, which assesses the annual heat load level at the farm location, was developed specifically for this study. A multivariate regression was used to study the marginal effects of CATHI on herd performance according to the herd's main calving season, geographical area, average milk production level, primiparous rate, and herd size. We found that on an annual scale, heat load negatively affected herds in terms of several key performance measures: average milk yield, lactation duration, recalving number, productive longevity, and herd size. Herds located in areas with low exposure to heat waves (lower CATHI values) showed declines in performance at least twice as large compared with herds located in areas more often exposed to heat stress, suggesting climatic acclimatization or climate adaptation. Calving seasonality also influenced the response of herds to annual heat load: herds with year-round calving showed 40% less performance loss than herds with grouped winter or fall calving. As expected, the effect of annual heat load on herd performance was greater for higher-producing herds. Breed composition, the proportion of primiparous cows in the herd, and herd size had little or no impact on herd response to annual heat load. These results demonstrate that both regional climatic conditions and calving distribution over the year modulated dairy herd response to annual heat load. Understanding how the coping abilities of cows and management flexibility interact in the herd could provide actionable levers for designing dairy herds that are more resilient to climate change.