Onset of heat stress and effects of warm temperature on daily milk yield in Swedish dairy cattle.

Climate change is associated with increases in the frequency, intensity, and duration of heat events, exposing even temperate regions to prolonged warm periods that can induce heat stress in dairy cattle, affecting milk production. This observational study investigated how warm weather affects milk yield in Nordic dairy cattle breeds kept under Swedish conditions. Deviations from the target lactation curve were analyzed to identify the onset of heat stress and predictors of sensitivity to heat stress using daily milk records from milking robots. The target curve was defined as the cow-specific expected lactation curve modeled using fourth-order polynomial quantile regression. Data included 907,403 daily milk yield (DMY) records for 1,056 Holstein, 566 Swedish Red, and 56 Jersey multiparous cows across 10 herds. Deviations from the target curve were quantified as residuals (RES), calculated as the difference between observed and expected milk yield, and were assumed to reflect responses to environmental disturbances, including heat stress. Heat stress was quantified by analyzing the effect of a 5-d average of maximum temperature (T5d) on DMY and RES. The T5d was used as a smoother term in a generalized additive model (GAM) for each breed. Heat stress thresholds were determined based on the point where the fitted values of the smoother for T5d began to decline, which occurred at ∼21°C for all breeds. After identifying this threshold, a heat load function was calculated and included as a linear covariate in a GAM model for all breeds. The results showed that Holstein cows were the most sensitive to heat stress, and Jersey cows were the least affected. This study demonstrates that RES are a reliable approach to estimate the effects of temperature on milk yield, allowing identification of heat stress onset. High-producing breeds were more susceptible to heat stress, and cows in mid-lactation and their second and third lactations were more affected by high temperatures.