Cows generate heat by digesting feed and producing milk, and absorb solar heat when in sunshine. Heat stress occurs when cows generate and absorb more heat than they can easily get rid of by respiration, sweating and convection to air blowing by them. When heat stressed, their body temperature increases.
Heat stress leads to reductions in dry matter intake, milk production, pregnancy rates, calf birthweights and even milk production of heifers from heat-stressed close-up cows. Heat stress leads to increased respiration rates, body temperatures, sweating, time standing, days open and mortalities. Many dairy farmers implement multiple practices to help their cows manage their body temperature in warm sunny weather.
The temperature-humidity index (THI) is commonly used to indicate the amount of heat stress cows experience. Table 1 summarizes four heat stress levels and corresponding THI values, respiration rates and body temperatures. High-producing cows start to have milk yield losses of 1 pound of milk per day once THI levels are 65 or above. The amount of milk loss increases as the level and number of hours of heat stress increase. Many cows can recover from a few hours of heat if they have several hours each day without heat stress.
Many factors beyond THI affect heat stress. Common cow cooling methods indicate important factors. Providing shade cuts the heat gain from sunshine. Mixing fans increase heat loss to the air by convection. Sprinklers that wet the cows’ skin increase evaporative heat loss. Misters reduce air temperature.
Heat stress and the high-producing cow
High-producing cows eat more and generate more heat. They can begin to experience heat stress in well ventilated barns at air temperatures as low as 65°F. A cow producing 100 pounds of milk per day produces over 1,500 watts of heat, 26 percent more than a cow producing 70 pounds of milk per day and 1,200 watts of heat. To avoid heat stress, cows need to get rid of the heat they generate, while producing milk.
Cows without shade in a sunny pasture can begin to experience heat stress at temperatures below 65°F. The heat load from sunshine varies widely, but cows in the sun can easily need to get rid of 300 watts or more than a cow under a shade. Providing shade or housing cows in barns can greatly reduce this solar heat load.
Increasing the air velocity flowing past a cow can help her get rid of heat. Tunnel ventilation, cross-ventilation with baffles and high-speed mixing fans are ways to increase the air velocity past a cow. At a THI of 75 and an air speed of 3 miles per hour (mph), a cow producing 100 pounds per day would be expected to have a respiration rate around 68 breaths per minute (bpm), while a 70-pound-per-day cow would have a respiration rate around 53 bpm. Increasing the air speed past the 100-pound-per-day cow to 10 mph would lower her respiration rate to 57 bpm, below the heat stress threshold level.
Mixing fans can be mounted over freestalls and over feed alleys where cows stand to eat. Many fans generate air velocities over 10 mph past the cows. Mixing fans need to be mounted high enough to not be a hazard to people, cows or equipment moving underneath the fans. The fans should also be pointed downward toward the cows. A 20-degree downward angle works well.
Cooling with water
Sprinklers and misters help cows get rid of body heat. Sprinklers wet the cow’s skin, and her body heat is used to evaporate the liquid water on the skin. It takes roughly 1,040 BTUs to evaporate 1 pound of water. Misting systems cool the air by evaporating water droplets using heat in the air. Both methods increase the air’s relative humidity, but if the barn is well ventilated, the humidity levels should not become excessive. Sprinklers and misters are more effective in drier weather with lower dew point temperatures. Sprinklers need to cycle on and off to allow time for water evaporation. Avoid excessive sprinkling that causes water to run down and wet the udder.
Holding area cooling
Milking center holding areas need good ventilation and cooling in warm weather to avoid heat stress. Cows can be crowded in the holding area for up to an hour. A crowded pen reduces the cows’ exposed surface area and airflow past them. Reducing the exposed surface area by 20 percent can cause a 70-pound-per-day cow to increase her respiration rate from 53 to 85 bpm, which indicates she is nearing moderate to severe heat stress.
Measuring heat stress
Respiration rates and body temperature are both good ways to assess how well cows are managing their heat production and losses in warm weather. Individual cow responses will vary. And conditions change throughout the day. You can measure and record respiration rates or body temperatures at different times of the day to assess how well your cows are coping with the weather conditions you have and your cooling practices. Daily milk production can also be monitored, but declines in milk production are commonly delayed two or more days after cows begin to experience heat stress.
To prepare for warm and hot weather, it is a good idea to make sure that your ventilating and cooling equipment is cleaned and ready to go. Poorly maintained fans with dirty and rusty louvers and slipping belts can have airflow rates between 20 and 60 percent below their rated capacities. The fans run, but they don’t move as much air as they could if well maintained. Sprinkler systems should be checked to make sure that all the nozzles work properly and there are no leaks. Check sprinkler timers to make sure the system does not apply excess water without sufficient time to evaporate.
Good ventilation and effective cooling systems can help cows avoid heat stress and maintain production during warm and hot weather.
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Kevin Janni is and extension engineer for the University of Minnesota.