Heat Stress in Livestock - Recognition, Mitigation, and Prevention

By Denice Rackley

Heat stress is defined as the inability to self-regulate body temperature that impacts performance and health.

Mammals regulate their internal body involuntarily. Humans and other mammals shiver to maintain warmth and sweat to cool off without thinking about it. The body handles small rises in temperatures on its own by increasing evaporative cooling. Adjusting breathing, increasing blood flow to the extremities and skin, and sweating all lower body temperature. Stress occurs when the body can no longer maintain the core temperature at an optimal level and function normally.

Impacts of Heat Stress

When livestock experience heat stress, they don’t consume as much feed and the rate of gain is decreased. One study of dairy cattle showed two pounds of milk production is lost for every pound of dry matter the cattle don’t consume when it’s hot and humid.

But, the impact of heat stress goes far beyond feed consumption. Fertility and milk production decreases, hormone levels change, and gestation shortens. Heat stress also causes physiological changes in the digestive system and the acid-base blood chemistry of livestock.

Blood flow is altered during heat stress, decreasing flow to organs, and increasing flow to skin as a means of cooling. Lambs and calves born to heat-stressed mothers are smaller because blood flow is directed away from organs. These newborns can have compromised immune systems and lower birth and weaning weights.

Studies show that heat-stressed ewes deliver lambs that are 20% smaller on average. Cows that were heat-stressed between days 100 to 174 of pregnancy have 22% smaller calves. The livers and brains of these newborns were also smaller. Heat stress contributes to an increase in fetal deaths.

Meat quality is also compromised in heat-stressed slaughter animals leading to an abnormally high percentage of dark meat that results in lower grading of carcasses.

The multitude of serious health complications that result from heat stress make it imperative that we understand when livestock are most at risk and have a plan in place to mitigate the impacts.

Causes of Heat Stress

Many factors add together to cause heat stress; it’s not solely dependent upon daytime high temperatures. Nighttime low temperatures, airflow, and humidity all influence an animal’s ability to cope with warm weather. Just like us, animals are more impacted when the humidity is high. The combination of heat and humidity makes it increasingly challenging to cool off with evaporative cooling methods.

On the Cattle Chat podcast, an interview with Dr. Brian Lubbers DVM, Kansas State University associate professor, noted that the nighttime low temperatures are critical, impacting the occurrence and severity of heat stress. “If it doesn’t cool down overnight, the cattle can’t dissipate heat as they would otherwise, so those are the days that you need to be extra vigilant in watching for signs of heat stress,” Lubber says. The overnight temperatures only have to be 70F or above to increase heat stress in cattle. 

Hair coat, color, and genetics can add to the predisposition for heat stress or contribute to heat tolerance. Livestock with lighter colored coats who shed their hair or have less wooly coats are more heat tolerant. Even though the genetic correlation is small (-0.30), selecting for heat tolerance will impact livestock’s ability to tolerate heat.

Recognizing Signs of Heat Stress

Moderate heat stress can occur with temperatures 80 – 90 F and humidity of 50 – 90 %. Rising respiration rates and body temperatures are typically the first signs of heat stress. Cattle and goats will exhibit rapid shallow birthing and sweating. Wool sheep will also have increased respirations but, it’s difficult to notice sweating in sheep.

Open-mouthed breathing, panting, drooling, tongue hanging out, neck outstretched, and temperatures rising above 105 indicate severe heat stress. In extreme heat stress, animals can tremble and be unsteady walking, have raspy, loud breathing, and drool.

As heat stress increases, respirations also increase. Cattle are in severe distress if respirations are over 150 per minute, and for sheep, that number is 200 per minute. Body temperatures continue to rise as livestock overheat, 108 – 113F can be fatal.

Managing and Preventing Heat Stress

The best options to help livestock manage heat stress are to only handle livestock in the coolest part of the day, manage fly populations, provide shade, increase air flow, provide shade, adjust nutrition and provide mineral, and increase their access to cool water.

Water

Drinking cool water is the quickest method for livestock to reduce their core temperature. Additionally, animals lose water through respiration and perspiration with warm temperatures, which increases their water requirements. According to a study of dairy cattle in Arkansas, cows drink up to 50 percent more water when the temperature/humidity index is above 80.

Livestock will be better able to cope with warm weather if water is conveniently accessible. Stock will not want to cross an open field or exit the shade during extreme heat, so strategically placing the water for easy access will encourage them to hydrate.

Feed

Eating produces heat. Dr. Greg Brickner, DVM, says, “Digesting forages releases more heat than concentrates.  The reality is that farmers feed more grain hoping to maintain productivity during heat and times of lower dry matter intake.  What then happens is that animals develop rumen acidosis and have both heat stress and acidosis to deal with and the resulting decline in productivity and health is even more pronounced.  In times of heat stress it is important to feed high quality forages.  High quality forages release less heat than poor forages and if intakes are going to be lower, then the extra energy density will be of help while maintaining rumen health.”

 Peak heat production occurs 4 to 6 hours after eating, which means livestock fed in the morning will peak mid-day when daytime temperatures are warmest, exacerbating heat stress potential.  

When fed 2 to 4 hours after daytime highs, their temperatures will rise as outdoor temperatures decrease. Many recommend providing the majority of the ration toward evening and reducing the energy content of the feed. Adjusting the feed ration to account for livestock not consuming as much feed may also be necessary.

Mineral supplementation may need to be adjusted in the summer months to account for minerals lost through increased respiration and perspiration. In particular, potassium and magnesium levels are decreased as temperatures and humidity rise. 

Shade and Air Flow 

The ability to stay out of direct sun can increase livestock’s tolerance to heat. Shade is essential. However, one tree or shaded spot will not do a group of animals much good. There needs to shade and air movement to be most effective.

“Whether it is a tree in the pasture or an artificial shade, it is important to confirm it’s big enough to allow good airflow moving underneath the shade.  If there are 30 cows all grouped together sitting under the shade tree, they will not be that much cooler than if they were all sitting out in the sun because of the lack of air movement,” notes nutritionist Phillip Lancaster. Cattle Chat 

Cattle need 20 to 40 square feet of shade per animal. An open tree grove is ideal, but portable shade structures or barns work well. Portable shade offers two advantages over permeate structures, they are less expensive and can be moved to new locations. The shade structure needs to be at least 8 ft tall to allow good air movement.

The orientation of shade structures also influences cooling. An east-west orientation will keep the ground cooler which works well with concrete flooring. However, if moisture and mud accumulate under the shade, humidity will rise, as will the fly population. A north-south orientation will keep the ground underneath a structure drier.

Airflow can be increased by fans, vents inside structures, and by using earthen mounds outside. Having a rise in elevation, even a relatively small one will increase wind speeds. 

It is also important to note that tall grass restricts airflow. Therefore, animals lying in tall grass can overheat, even if they are lying in the shade. Keep in mind that young stock are particularly vulnerable to heat stress, and laying in tall grass will only add to rising temperatures.

Misting cattle can be a good strategy when added to shade and good air movement in dry environments. However, misting adds to the humidity and worsen situations where there is already an excess of moisture.

Fly Control 

Fly control is also essential to reduce stress and reduce the instinct to bunch together. Darker environments typically reduce the fly population. A natural alternative to controlling the fly population is to incorporate fly predators in your management practices.  

Removing manure, increasing airflow, keeping the ground or flooring dry in humid conditions, providing adequate shade, and using fly control will reduce the number of flies agitating stock.

Handling of Stock

Any movement, handling, or working livestock should be accomplished in the coolest part of the day and as calmly as possible to avoid added stress. This is especially important for pigs since they do not sweat and are at an increased risk for heat stress.

Heat Stress Forecast Map

Although specifically designed for cattle, the USDA’s heat stress forecast map will give producers details that can be used to anticipate problematic weather patterns and the potential for heat stress in all livestock across the United States.

 https://www.ars.usda.gov/plains-area/clay-center-ne/marc/docs/heat-stress/main/

Preparation is Key

 Once the outside temperature, combined with humidity, rises to the point that livestock aren’t able to use shade, sweating, and mild increases in respiration for evaporative cooling to dissipate enough body heat, their health and performance suffer. Knowing the early signs of heat stress and being proactive to help animals cope with the heat may avoid the more severe impacts of overheating like weight loss, decreases in fertility, and young animals who don’t thrive. Having a management plan in place when heat and humidity rise will enable quick implementation, offer livestock relief, and could play a critical role in maintaining the health and production of your herd and flock. 

•         Poor appetite

•         Lethargy

•         Elevated rectal temperature

•          Increased respiration

•          Elevated heart rate

•          Bunching in the shade or around water tanks

•          Panting

•        Salivating excessively

•          Foaming around the mouth

•          Trembling or lack of coordination

•          Seizure

•          Death

Management adjustments to reduce heat stress

• Modify diet and feeding time to maintain feed intake

• Increase access to mineral

• Increase the amount of water available to your livestock

• Provide shade

• Select a good location for increased air circulation

• If in a dry environment – fans & misters to help cool the cattle inside

• Manage manure and environment to reduce flies

• Work livestock in the coolest part of the day

• Pay special attention to young animals

** We wish to thank Dr. Greg Brickner, DVM for reviewing this article.