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

Farm animals are now exposed to more heat-stress events due to an overall increase in global temperature than ever before, and the dairy industry is highly affected by climate change because Holstein cows, which comprise the vast majority of the global dairy population, are very sensitive to high temperatures. Heat-abatement technologies are often used on dairy farms to mitigate the effects of heat stress. The most common cooling methods are fans, sprinkler systems, ridge-roof opening, and natural ventilation. Despite the positive effect in reducing heat stress, the costs of installation, maintenance, and energy consumption can limit their use on farms, which is often experienced in South Africa. To that effect, genetic selection for thermo-tolerance could be an interesting alternative. Genetic gains obtained with selection are cumulative and permanent, which makes genetic selection for thermo-tolerance potentially a very cost-effective strategy. Genetic studies have concentrated mainly on the effect of heat stress on milk production and not on feed intake. Those that have, have been very small studies using data collected in chambers. Thus, the objective in the study cited here, was to evaluate the effect of heat stress on both dry matter intake (DMI) and residual feed intake in lactating Holstein cows using data collected on six research farms for approximately 20 years in the US.

The data consisted of 388 629 daily DMI and 54 353 weekly residual feed intake records. 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 analyse 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. Multi-trait random regression models were used to estimate variance components for daily DMI and weekly residual feed intake, considering the first three lactations as different traits.

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 thermo-neutral and thermo-tolerant 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

thermo-neutral and thermo-tolerant additive effects were negative and ranged from −0.17 to −0.48. The Pearson correlations among estimated breeding values (EBVs) calculated in thermo-neutral 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.

In conclusion, DMI and residual feed intake are traits susceptible to heat stress. The negative genetic correlations observed between thermo-neutral conditions and thermal-stress conditions suggest cows that consume more feed and are less efficient are more susceptible to heat stress.