Once the temperature rises above 90ºF, cattle (especially fat cattle) start to absorb more heat than can be dissipated though the normal mechanisms of sweating and respiration, leading to heat stress. Combining excessive heat and high humidity (heat index) can mean trouble for feeder cattle.

What to think about

You should keep in mind the color, weight and health status of your cattle when planning for heat stress events. Cattle with darker hides will absorb more heat than lighter colored cattle. Heavier cattle have greater difficulty dissipating heat than lighter cattle. Like us, cattle use perspiration as their primary means of temperature regulation.

During conditions of high temperatures and humidity, cattle rely on respiration to aid in reduction of excess body temperature. Cattle that are debilitated have a difficult time regulating body temperature.

For example, cattle that have had a bout of respiratory disease will be less efficient at removing excess body heat through respiration than cattle that have always been healthy. Finally, as cattle ruminate they generate heat, and hotter rations generate more heat.

What to look for

Initial signs of heat-stressed cattle include: increased respiratory rate, restlessness and increased time standing.The severity of these signs will increase as the heat stress increases. The respiratory rate will continue to increase. The majority of the group of animals will be standing and cattle will start to drool.

In severe conditions, cattle may become isolated from the group. They may have their tongue extended and their head down, mouth open and neck extended. You will see a significant abdominal component to their breathing. Do not add undo stress to heat-stressed cattle. These cattle need to be cooled slowly by providing shade, cool drinking water and a cool water sprinkler. 

Management starts early with preparation

Although the temperature-humidity index (THI) can be useful in predicting the risk of heat stress, THI is not an accurate predictor of the magnitude of heat stress placed on individuals. THI does not account for the color of cattle or wind speed. Dark cattle will absorb more heat, while wind can contribute to cooling.

Use heat stress forecast maps provided by USDA-ARS and NOAA to plan for heat stress events. Provide shade; 20-40 square feet per animal. Shades should be tall enough to allow sufficient air movement. Provide sprinklers to wet the animals and cool the ground around them. Start sprinkling early, before the animals become stressed. 

Cattle require two to three times more drinking water during periods of heat stress. Cattle should be provided 3 inches of linear water space per head. Adding extra water tanks will prevent a few calves from dominating the water, thus making it available to more animals. Water tanks should be clean and provide cool water at a rate of approximately 2 gallons of water per 100 pounds of body weight. 

Reduce cattle activity during the hottest part of the day. Ride pens, pull sick cattle, treat and process in the early morning while it is cool. Feed earlier in the morning before it heats up and later in the evening when the ambient temperature is cooling.

Long-term fixes include movable windbreaks that can be opened or removed in the summer, allowing maximum airflow. In the upper plains, loafing mounds with a 3-4 degree slope will increase airflow.  

Troy A. Brick is assistant professor of veterinary diagnostics and production animal medicine at Iowa State University College of Veterinary Medicine. Email Troy A. Brick.

PHOTO: Cattle require two to three times more drinking water in times of heat stress than in regular weather. Staff photo.