Heat Stress In Beef Cattle
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Dr. Grant Dewell
ISU Beef Extension Veterinarian
As temperatures heat up during the summer cattle producers need to assess the heat stress that their cattle are under. Typically pastured cattle are not as susceptible to heat stress as feedlot cattle. Pastured cattle have the ability to seek shade, water and air movement to cool themselves. In addition, radiant heat from dirt or concrete surface is increased for feedlot cattle. At temperatures above 80 degrees Fahrenheit cattle endure physiologic stress trying to deal with their heat load. Although cattle at this temperature are not at risk of dying they will have an increased maintenance requirement to cope with the heat. Feedlot operators should have a plan to manage heat to not only prevent death loss but also performance loss from decreased efficiency and feed intake.
Compared to other animals cattle cannot dissipate their heat load very effectively. Cattle do not sweat effectively and rely on respiration to cool themselves. A compounding factor on top of climatic conditions is the fermentation process within the rumen generates additional heat that cattle need to dissipate. Since cattle do not dissipate heat effectively they accumulate a heat load during the day and dissipate heat at night when it is cooler. During extreme weather conditions with insufficient environmental cooling at night cattle will accumulate heat that they cannot disperse. Therefore, a temperature-humidity index (THI) alone may not predict cattle heat stress because it does not account for accumulated heat load. Another short fall of THI is that it does not account for solar radiation and wind speed which can affect heat load of cattle.
Cattle should not be worked during times of extreme heat and only early in morning when it is hot. Working cattle will elevate their body temperature. Cattle should not wait in processing areas longer than 30 minutes when it is hot. Do not work cattle in the evening even if it has cooled off a little. Cattle’s core temperature peaks 2 hours after peak environmental temperature. It also takes at least 6 hours for cattle to dissipate their heat load. Therefore, if peak temperature occurred at 4:00 pm cattle will not have recovered from that heat load until after 12:00 am and it will be later than that before cattle have fully recovered from the entire days heat load. Feedlots should evaluate cattle daily, especially during July and August, for evidence of heat stress. Special attention should be paid to cattle with increased risk of heat stress including heavy cattle, black cattle and respiratory compromised animals.
Heavy cattle cannot handle heat stress compared to lighter weight cattle. Increased fat deposition prevents cattle from regulating their heat effectively. Solar radiation is a critical component that can lead to death loss from heat stress. Typically, proportionality more black hided cattle die during heat waves then other hide colors. Since cattle rely on respiration as a method to manage heat respiratory function is important. Cattle that had severe respiratory disease early in the feeding period will have decreased ability to regulate their heat load.
Managing Heat Stress
The water requirements of cattle increases during heat stress. Cattle lose water from increased respiration and perspiration. Additionally, consumption of water is the quickest method for cattle to reduce their core body temperature. Therefore, water consumption will be greater than typical metabolic requirements. Rule of thumb is that cattle need 3 inches of linear water space per head during the summer. Extra water tanks should be introduced prior to extreme heat events so that cattle become accustomed to them. Waterers need to be kept clean to encourage cattle to consume adequate water. The water supply should be able to deliver 1.1% of body weight of the cattle per hour. A 1000 pound animal needs about 1.5 gallons of water per hour.
Heat production from feed intake peaks 4 to 6 hours after feeding. Therefore heat production in cattle fed in the morning will peak in the middle of the day when environmental temperatures are also elevated. Cattle should receive a least 70% of their feed 2 to 4 hours after peak ambient temperature. Changing the ration has been controversial but research indicates that lowering the energy content of diet will decrease the heat load. The general recommendation is to reduce the diet energy content by 5 to 7%.
Shade can be critical in determining whether cattle die during extreme heat events, especially for black cattle. To be effective there needs to be 20 to 40 square feet of shade per animal. If the shade structure has an east-west orientation then ground under the shade will remain cooler. However, if mud is an issue then a north-south orientation will increase drying as the shade moves across the ground during the day. The height of the shade structure should be greater than 8 feet tall to allow sufficient air movement under the shade.
Increasing the air flow can help cattle cope with extreme heat events. Wind speed has been shown to be associated with ability of cattle to regulate their heat load. Although we cannot influence wind speed, feedlots can increase ability for cattle to be exposed to air movement. Utilize temporary wind breaks in winter to allow maximal air movement in summer. Remove tall vegetation within 150 feet of the feedlot pens. Tall earthen mounds will allow cattle more exposure to air movement. Feedlots should assess their feedlot and know which pens have poor air movement. Avoid using these pens for cattle that will be approaching slaughter weights in mid to late summer.
Another factor that feedlots can address is to control flies. Biting flies cause cattle to bunch up which decreases cooling. Minimizing breeding areas for flies and applying insecticides to decrease fly populations prior to heat stress times is worthwhile.
Sprinklers can be used to cool cattle during times of stress. Sprinklers increase evaporative cooling and can reduce ground temperature. Sprinklers should thoroughly wet the animal and not just mist the air in order to cool the animal. Before installing a system make sure the water supply is adequate to provide drinking water and sprinklers. Sprinkle intermittently to avoid mud and increased humidity. Sprinklers should be placed away from feed bunks and waterers. Cattle need to be introduced to sprinklers prior to extreme heat. Cattle not used to sprinklers will try to avoid the spray. Additionally, sprinklers need to be used before cattle are in extreme stress. Thermal shock from cold water can kill cattle that are extremely stressed. Once sprinklers are utilized they need to be continued until the heat event is over and cattle can manage on their own.
Feedlots need to monitor environmental temperatures throughout the summer. Any time the Temperature-Humidity Index (THI) is above 80 cattle will be under heat stress. Hot weather following precipitation can increase the THI dramatically. Finally if overnight temperatures are above 70°F cattle will have increased heat stress. The USDA-ARS and NOAA forecasts heat stress that feedlots can use to make management decisions at http://www.ars.usda.gov/Main/docs.htm?docid=20426.
During times of increased heat stress cattle should be observed closely to identify if additional strategies need to be implemented. Initially feed intake will drop off and cattle become restless. As heat stress increases cattle will begin to slobber and respiration rates will increase. Eventually, cattle will begin to group together. In severe heat stress cattle will be open mouth breathing with a labored effort. Feedlots need to monitor for heat stress and implement strategies to minimize impact on cattle to prevent severe death from heat stress.