Heat stress factors
Heat stress costs you money. Heat stress leads to reduced dry matter intake, milk production and pregnancy rates, and increased number of days open, cull and death rates. Cows experience heat stress when they have trouble getting rid of body heat. Cows begin to experience heat stress when respiration rates rise above 30 breaths per minute (bpm). Recording respiration rates is a good way to see how much heat stress your cows are feeling.
Most dairy producers are well aware of the temperature-humidity index (THI) that is commonly used to indicate the amount of heat stress cows experience. Table 1 summarizes the heat stress levels and the corresponding THI values, respiration rates and body temperatures. High producing cows start to have milk yield losses of one 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 if they can have several hours a day without heat stress.
Many factors beyond THI affect heat stress. Common methods used to reduce heat stress indicate some of the factors. Providing shade cuts the heat load from sunshine. Fans increase heat loss to the air. Sprinklers that wet the cows increase evaporative heat loss and misters reduce air temperature. When managing heat stress it is important to understand more of the factors and their importance. A new computer model is helping researchers understand how much the factors affect heat stress.
For example, sunshine on a hot day makes a big difference in heat stress for cows on pasture or open lots. A cow that is in the sun on a day when the THI is 78 or higher and already in the mild-moderate stress level will have a respiration rate that is 40 to 50 bpm higher than a cow in the shade. A cow in the sun can reach the severe heat stress level.
Many producers know that heat stress is harder on their better producing cows. A cow producing 77 pounds of milk per day when the THI is 78 would be expected to have a respiration rate around 68 bpm and a body temperature of 101.8° F. Another cow producing 99 pounds of milk per day would be expected to have a respiration rate of 85 bpm and body temperature of 102.7° F. This means that the high producing cow is more stressed. She probably will reduce dry matter intake to lower metabolic heat production to make it easier for her to balance her heat gains and losses.
A common way to help cows deal with heat stress is to increase the air velocity past her. Ventilation is needed to remove heat and moisture that cows give off to avoid a barn from becoming hotter and more humid. Mixing fans, tunnel-ventilating and cross-ventilating systems are used to increase the air velocity past cows. A cow in a barn when the THI is 78 and the air velocity past her is 60 feet per minute (fpm) will be expected to have a respiration rate of 83 bpm and a body temperature of 102.6° F. If the air velocity past her is increased to 177 fpm, her respiration rate decreases by 7 bpm and body temperature decreases by about 0.1° F. If that cow is in air moving at 530 fpm, her respiration rate drops to 66 bpm and her body temperature to 101.8° F. The higher air velocity takes her from the top of the mild-moderate stress level to the threshold stress level. Increasing the air velocity to 885 fpm her respiration rate drops by another 5 bpm and her body temperature drops by almost 0.2° F. The extra air velocity helps reduce the heat stress some but at a diminishing rate. More analysis is needed to assess the costs of adding fans and paying for more electricity, and benefits in cow production needed to increase the air velocities from 530 fpm to 885 fpm.
Crowding cows together is another practice that increases the amount of heat stress they experience. Crowding is common when cows are in holding areas before being milked in a parlor. Crowding reduces the surface area of the cow exposed to the air blowing past her and available for getting rid of heat. When the surface area a cow has exposed drops from 100% to 80%, her respiration rate can increase by 14 bpm and her body temperature increases by 0.5° F. If her surface area is decreased to 60%, her respiration rate can become 108 bpm and her body temperature is raised to 103.8° F.
Heat stress is becoming a bigger challenge. Our understanding about heat exchange needs to improve in order to design and manage better systems to help cows deal with heat stress.
|Heat stress level||THI||Respiration rate
(breaths per minute)
|Threshold||68 – 71||>60||>101.3|
|Mild-moderate||72 – 79||>75||>102.2|
|Moderate-severe||80 – 89||>85||>104.4|
|Severe||90 – 99||120 – 140||>105.8|