Impact of heat stress on dry matter intake and residual feed intake in mid-lactation dairy cows.

The rise in global temperature is affecting livestock production by limiting crop yields and exposing animals to more frequent heat-stress events. Heat-abatement technologies are widely used but increase production costs. Therefore, genetic selection for thermotolerance is an interesting tool to overcome the effects of heat stress. The goal of this study was to evaluate the effect of heat stress on both DMI and residual feed intake in lactating Holstein cows. Data consisted of 388,629 daily DMI and 54,353 weekly residual feed intake records from 6,333 cows collected on 6 research stations in Wisconsin, Michigan, and Iowa between 2007 and 2024. Heat stress was assessed using temperature-humidity index (THI), based on the measurements taken at weather stations. Average THI per day and per week were used to analyze DMI and residual feed intake, respectively. The effect of heat stress was also evaluated as the number of hours a cow was exposed to heat stress for DMI, and the number of days within a week a cow was exposed to heat stress for residual feed intake. Multitrait random regression models were used to estimate variance components for daily DMI and weekly residual feed intake, considering the first 3 lactations as different traits. The models included cohort (trial-treatment) and DIM effects as fixed, and general (intercept) and thermotolerance (slope) additive genetic and permanent environmental effects as random. Heritability estimates at the heat-stress level ranged from 0.16 to 0.33 for DMI and from 0.15 to 0.21 for residual feed intake. These results suggest substantial genetic variability underlying DMI and residual feed intake when cows are exposed to heat-stress conditions. Estimated genetic correlations between thermoneutral (intercept) and thermotolerant (slope) additive effects ranged from -0.06 to -0.36 for average THI and from 0.10 to -0.43 for hours exposed to heat stress. For residual feed intake, estimated genetic correlations between thermoneutral (intercept) and thermotolerant (slope) additive effects were negative and ranged from -0.17 to -0.48. The Pearson correlations among EBVs calculated in thermoneutral and thermal-stress conditions in different lactations were generally high. The Pearson correlation between the first and second lactations were higher than correlations between the first and third or second and third lactations. Note that most cows did not have records in multiple lactations, and the third group included animals in later lactations, which may have contributed to the lower correlations. In conclusion, DMI and residual feed intake are traits susceptible to heat stress. The negative genetic correlations observed between thermoneutral conditions and thermal-stress conditions suggest cows that consume more feed and are less efficient are more susceptible to heat stress.