Minimizing heat stress in beef cattle

Source: Alberta Agriculture and Forestry, http://www.agric.gov.ab.ca/

Soaring summer temperatures not only affect humans, but cattle as well. Heat stress is hard on livestock, especially in combination with high humidity.

Heat stress is defined as any combination of temperature, humidity, radiation and wind producing conditions higher than the animal’s thermal neutral zone. The upper limit of which is the so called upper critical temperature. Beef cattle cool themselves primarily through a combination of respiratory tract (most important) and skin evaporative loss (sweating).

Heat stress occurs when the body temperature is elevated due to excessive heat production or high ambient temperatures, or reduced heat loss. High temperatures (above 28°C (82°F)) coupled with high humidity can cause heat stress in cattle, which can lead to a reduced breeding efficiency, milk production, feed intake, and weight gains. In the worst case, heat stress may increase the chance of illness and may even cause death.

Cattle are more sensitive to high temperatures than humans. Humidity is an additional key element in heat tolerance. The Livestock Temperature Humidity Index* (THI Table 1) was introduced by American animal scientists to alert producers of potential heat stress periods for livestock. The THI combines the effects of temperature and humidity into one value. The Livestock Safety Index (LSI) contains three stress categories (temperature given in Celsius (°C):

• Livestock Alert – LSI 24 -25.5: when the index reaches this range, heat stress will first appear. Precautionary measures should be taken to reduce heat stress conditions in confinement housing or livestock trailers.
• Livestock Danger – LSI of 26-28: an index in this category is dangerous for confined animals.
• Livestock Emergency – LSI of 29 or higher: These conditions are most likely to occur when air temperature exceeds 32°C (90°F). No cloud cover and little air movement are additional hazards found in such heat stress weather. Livestock should not be worked or shipped when the index reaches this level.

The lower the humidity the greater the ambient air temperature that livestock can withstand. Factors other than temperature and relative humidity can impact heat stress. The addition of direct sunlight can add 3 or 4 degrees Celsius to the THI. Wind can lower the THI by a few degrees due to its bringing cooler air to the animal and carrying away excess heat. During a typical summer day, the LSI moves from one category to another, depending on time of day, temperature and humidity. Night cooling (temperatures dropping below 20°C) can reduce the Temperature Humidity Index. Certain animal factors can increase the like hood of heat stress, such as increased activity level and whether or not the cattle are acclimatized to hot weather conditions.

Use the following to evaluate the potential for an impending heat stress crisis for your cattle:

1. Predicted hot weather following precipitation. It is the combined temperature and humidity that determines the severity of heat stress. Days above 27°C following a heavy rainstorm (higher humidity level) can be a warning of a heat stress crisis for cattle, especially if the wind becomes calm or the cattle are held in confined housing situations or during transport.
2. The greatest probability of heat stress occurs between early July and mid-August when ambient air temperature is greater than 30°C and the relative humidity (RH) is above 30%. An emergency heat stress situation can occur if ambient air temperature is greater than 36°C (98°F) and RH is greater than 30%.
3. A potential heat stress crisis situation exists for cattle when there is little or no night cooling (evening temperatures stay above 20°C).
4. Observing the cattle will tell you when they are becoming uncomfortable from heat.

Table 1. Livestock Temperature Humidity Index* (THI) at specific temperatures and relative humidity levels.

* The Livestock THI was adapted from the human Humidex Chart, which can be found at : http://www.ccohs.ca/oshanswers/phys_agents/humidex.html.

Cattle lose excess heat primarily through breathing, rather than sweating. They sweat only about 10% of what humans do. Observing the cattle will tell you when they are becoming uncomfortable from heat.

The cattle will start to move or walk around the pen looking for an area of the pen, or confined housing that is more comfortable. Cattle that are showing mild signs of heat stress will have shallow, rapid breathing.

Animals that are experiencing advanced heat stress may exhibit increased open mouth panting (respiratory rate will increase above 100 breaths per minute) and will have increased salivation or slobbering. They may stumble while walking and may look for a cool place, including water to lie down in.

Cattle will position their body to minimize their exposure to the sun; generally this is facing the sun. Cattle that are overheated tend to group themselves together to seek shade provided by other animals. They may crowd the water trough, not only to drink, but in an attempt to cool themselves. If body temperature increases, respiration becomes shallow, weak, and animals may collapse, convulse and slip into a coma, with death occurring shortly after.

1. Have ample water available. The most important practice is to provide cattle with sufficient quantities of cool drinking water. Water will keep an animal’s body temperature within normal limits, as well as help to improve feed consumption. Animals in confined feeding systems require water at a rate of 1.1% of body weight per hour. Increased water intake often increases production of urine and subsequent loss of certain minerals, such as sodium (salt), potassium and magnesium. Cattle should be provided with salt licks or loose salt in locations that animals frequent.
2. Provide shade. Provide shade for fatter animals and those with dark-colored coats. Trees, buildings or sunshades can provide shade but should not reduce airflow. If animals are housed spraying rooftops with water can further decrease the temperature within the building. Fans and breezes from open windows can provide a source of ventilation. In feedlot pens tall earth mounds or bedding mounds can allow cattle to move to the airflow and shade.
3. Change your feeding patterns. Feed cattle less in the morning and more in the evening to encourage greater feed intake. Cattle on pasture will adapt and increase the time spent grazing from 2 hours pre-dusk to 2 hours after dawn. For confined cattle deliver 70% of the days scheduled feed two to four hours after the peak ambient temperature of the day. This may help decrease the roller coaster intake patterns often observed during hot weather.
4. Avoid handling cattle if possible. Work feedlot cattle between midnight and 8 Am. Never handle, work or move susceptible cattle during heat stress conditions.

The following table is a guideline for daily water intake of beef cattle in 26°C (80°F) and 32°C (88°F) weather:

Table 2. Daily water Intake for different classes of beef cattle*

Not all cattle producing areas of Alberta experience yearly heat waves. Over a period of time, consecutive hot days are worse than intermittent hot days. Also when cattle can’t cool off at night the effects of the daytime highs are exacerbated and cattle are more prone to heat stress. Confined animals are more prone to heat stress than animals on range or pasture. Watching for stress signs and using common sense to ease heat stress can result in reduced production losses and healthier cattle.

If you have questions or require further assistance on this topic, please call the Ag-Info Center at 310-FARM.