Dr. Grant Dewell
ISU Extension Beef Veterinarian
In August of 2010,
As the weather warms up in the summer, cattle farmers must measure the heat stress that their animals are experiencing. Heat stress is less common in pastured cattle than it is in feedlot cattle. Pastured calves have the ability to cool themselves by seeking shade, water, and air circulation. For feedlot cattle, radiant heat from the dirt or concrete surface is also increased. Cattle experience physiologic stress when temperatures exceed 80 degrees Fahrenheit as they strive to cope with their heat burden. Although cattle are not in risk of dying at this temperature, they will require more maintenance to cope with the heat. Feedlot operators should have a heat management plan in place to avoid not just death but also performance loss due to reduced efficiency and feed intake.
Cattle, in comparison to other animals, have a difficult time dissipating their heat load. Cattle do not sweat well and must rely on breathing to stay cool. The fermentation process within the rumen generates additional heat that cattle must disperse, which is a compounding problem on top of environmental conditions. Cattle retain heat throughout the day and dissipate heat at night when it is cooler because they do not dissipate heat well. Cattle will store heat that they cannot release during extreme weather conditions with minimal environmental cooling at night. As a result, because it does not account for accumulated heat load, a temperature-humidity index (THI) alone may not be able to predict cattle heat stress. Another flaw with THI is that it ignores solar radiation and wind speed, both of which might impact cattle’s heat burden.
Cattle should not be worked during periods of extreme heat and should only be worked early in the morning when the weather is warm. Cattle that are working raise their body temperature. When the weather is hot, cattle should not be kept in processing areas for more than 30 minutes. Even if it has cooled off a little, do not work cattle in the evening. The core temperature of cattle peaks two hours after the temperature of the surroundings peaks. Cattle must also disperse their heat load for at least 6 hours. As a result, if peak temperature occurs at 4:00 p.m., cattle will not recover from that heat load until after 12:00 a.m., and cattle will not fully recover from the entire day’s heat load until later than that. Feedlots should monitor cattle for signs of heat stress on a regular basis, especially in July and August. Cattle at risk of heat stress, such as hefty cattle, black cattle, and respiratoryly challenged animals, should be given special attention.
When compared to lighter weight cattle, heavy cattle cannot withstand heat stress. Increased fat accumulation makes it difficult for cattle to regulate their body temperature adequately. Heat stress is caused by a combination of factors, one of which being solar radiation. During heat waves, black-hided cattle perish at a higher rate than other hide colors. Because calves rely on respiration to regulate their body temperature, respiratory function is critical. Early in the feeding phase, cattle with severe respiratory disease will have a reduced ability to manage their heat burden.
Managing Heat Stress
During heat stress, cattle’s water requirements increase. Increased respiration and perspiration cause cattle to lose water. Furthermore, drinking water is the quickest way for calves to lower their core body temperature. As a result, water consumption will be higher than normal metabolic needs. During the summer, cattle require 3 inches of linear water space per head as a rule of thumb. Prior to significant heat events, extra water tanks should be introduced to allow cattle to develop accustomed to them. To encourage cattle to drink enough water, waterers must be kept clean. The water supply should be capable of delivering 1.1 percent of the cattle’s body weight every hour. A 1000-pound animal requires approximately 1.5 gallons of water every hour.
4 to 6 hours after feeding, heat generation from feed intake peaks. As a result, heat output in morning-fed cattle will peak in the middle of the day, when ambient temperatures are also high. Cattle should get at least 70% of their feed 2 to 4 hours after the ambient temperature reaches its highest. Changing the ration has been divisive, but studies show that lowering the energy level of the diet reduces heat burden. The typical guideline is to cut the energy level of your diet by 5 to 7%.
Shade, especially for black cattle, can be crucial in determining whether or not cattle perish during extreme heat events. Each animal requires 20 to 40 square feet of shade to be effective. If the shadow structure is oriented east-west, the ground beneath it will stay cooler. If mud is a problem, however, a north-south orientation will speed up the drying process as the sun passes across the ground during the day. To provide sufficient air movement under the shade, the shade structure should be at least 8 feet tall.
Cattle can manage better with high heat by increasing air flow. The ability of cattle to control their heat burden has been linked to wind speed. Although we have little control over wind speed, feedlots can improve cattle’s capacity to be exposed to air movement. In the winter, use temporary wind breaks to allow maximum air movement in the summer. Within 150 feet of the feedlot pens, remove tall vegetation. Cattle will be exposed to more air movement because to the tall earthen mounds. Feedlots should review their operations and determine whether pens have insufficient airflow. These pens should not be used for cattle that will be slaughtered in the middle to late summer.
Controlling flies is another issue that feedlots can solve. Biting insects encourage cattle to congregate, reducing cooling. Prior to periods of high heat stress, it is worthwhile to reduce fly breeding places and use insecticides to reduce fly numbers.
Sprinklers can be used to keep cattle cool while they are under stress. Sprinklers help to cool the earth by increasing evaporative cooling. Sprinklers should thoroughly soak the animal rather than simply sprinkle the air to keep it cool. Make sure the water supply is sufficient to provide drinking water and sprinklers before constructing a system. To avoid mud and excessive humidity, sprinkle on a regular basis. Sprinklers should not be installed near feed bunks or waterers. Cattle must be exposed to sprinklers before being exposed to excessive heat. Cattle that aren’t used to being sprayed by sprinklers may try to avoid them. Sprinklers should also be utilized before livestock are in a state of high stress. When cattle are stressed, thermal shock from cold water can be fatal. Sprinklers should be used until the heat event has passed and the cattle are able to handle on their own.
Throughout the summer, feedlots must keep an eye on environmental temperatures. Cattle will be under heat stress if the Temperature-Humidity Index (THI) is more than 80. The THI can be greatly increased by hot weather following precipitation. Finally, cattle will experience significant heat stress if nocturnal temperatures exceed 70°F. Heat stress projections from the USDA-ARS and NOAA are available at http://www.ars.usda.gov/Main/docs.htm?docid=20426, which feedlots can utilize to make management decisions.
Cattle should be constantly monitored during periods of heightened heat stress to see if extra strategies are required. Feed consumption will decrease at first, and animals will become restless. Cattle will begin to slobber and their respiration rates will increase as heat stress increases. Cattle will eventually begin to form herds. Cattle under severe heat stress will breathe through their mouths with a strained effort. To avoid severe heat stress death, feedlots must monitor for heat stress and employ techniques to reduce its impact on cattle.
Grant Dewell, Ph.D.
Animal Medicine Veterinary Diagnostics and Production Veterinary Diagnostics and Production Veterinary Diagnostics and Production
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