How hot is too hot?

The summer of 2023 was hot and dry. However, until the third week in August, most farmers in my area reported little, if any, drop in milk production.
Cows did well because dairy producers have invested a lot of money in better cow housing and cooling systems. Another reason cows did well was the low relative humidity on most of the hot days, which happened because of prolonged, severe drought.
It is easy to cool cows when humidity is low because evaporative cooling works so well. However, Aug. 22-24 was different. It was hot and humid. The official high at the La Crosse, Wisconsin, office of the National Weather Service was 104 degrees the afternoon of Aug. 23, and the heat index peaked at 114 degrees. What does this mean to a cow?
Cows produce a lot of heat from digestion. A cow producing 100 pounds of milk produces about 1300 watts of excess energy. She can get rid of that excess heat through conduction, convection or evaporation. However, when the temperature of the cow’s environment approaches the body temperature of the cow, convection and conduction are not effective. When the relative humidity is very high, evaporative cooling is also not effective. Thus, that short August heat wave was particularly difficult for our cows. Fortunately, it only lasted for about 48 hours.
But, what if it hadn’t? Even one more day of that intense heat and humidity could have been devastating for cows on a lot of farms. Summers are getting hotter, more humid and longer. Thus, it is likely that we will see longer and more severe heat waves in the future. Combining this trend with the trend of increasing milk production over time, we should have concerns about how we are going to cool our cows in the near future.
Temperature-humidity index is the parameter most used to measure conditions to predict heat stress in dairy cows. However, THI does not include a measure of time. Most of us know from farm observations that the duration of a heat wave matters. A recent study in the Journal of Dairy Science (Vitali, et. at., 2020) used a different parameter called heat load index that includes time. HLI can be used to calculate a measure of accumulated heat, or accumulated heat load, where a positive value means a period where a cow is accumulating heat. When the AHL is positive, cows are gaining heat. When it is negative, they lose heat.
One way to lower the AHI during heat waves is to rapidly cool the cow as soon as the outside temperature and humidity will allow. Think of a magical barn where the entire structure disappears as soon as the sun goes down, giant fans turn on and cow soakers wet the cows. Removing the roof allows cows to cool by conduction to the cooler outside air. The fans cool cows by convection and evaporation, assuming the temperature and humidity are low enough. Now, think of your barns. You cannot remove the roof, but you can cool the barn by removing the warm and humid air if you have sufficient exhaust capability. Correctly designed, naturally ventilated barns will exhaust without fans. You can cool the cows by convection if you have sufficient air speed at the cow level.
Another recent study looked at the difference between cooling cows just in the daytime versus daytime and nighttime in a dry-lot dairy in Australia (Gaughan, 2023). Ducted air blowing onto cows was provided over the resting area for the nighttime cooling groups. There were four heat waves during the study: the first beginning Jan. 20 and the last ending March 26. Cows with nighttime cooling produced 4.5 pounds more milk over the course of the study. During the most severe heat wave, milk production of both groups was similar, but over the six days following the heat wave, the nighttime-cooled group produced 8 pounds more milk. The cows cooled at night lost accumulated heat more rapidly than the other cows, and thus could spend a greater proportion of each 24-hour period with normal body temperatures than cows that had to cool off only by conductive cooling to the cool night sky.
The trends of warmer summers and increasing production mean we have work to do regarding how we cool our cows. In the meantime, some things we can do are as follows:
Make sure cooling systems are correctly designed. This requires adequate air exchanges per hour, airflow at the cow level in free stalls in the parlor and the holding area, plus appropriate soaker systems in the barns, parlor and parlor exit. Many barns still have poorly designed systems today.
Make sure cooling systems are maintained. A fan pointing at an area 5 feet above the back of the cows, for example, is useless. Broken fans, slow fans and plugged soaker nozzles are all summertime emergencies.
Do not turn off your cooling systems until the cows have cooled down at night. This may mean running circulating fans all night. It is a real possibility we may need to perfect other cooling systems — stall surface cooling, for example — to protect our cows in the future, but right now, most farms have opportunities to help our cows on those long, hot spells. For help, talk with your veterinarian.
Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at [email protected] with comments or questions.