Heat Stress in Livestock – Recognition, Mitigation, and Prevention

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

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 also 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 impacts.

Causes of Heat Stress

Heat stress is not solely dependent upon daytime high temperatures; many factors add together to cause the body to become overwhelmed. 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 boy temperature rising above 105 indicate severe heat stress. In extreme heat stress animals can tremble and be unsteady walking, have raspy, loud breathing, and drool.

Species

Normal Temp  

            F

Resting Respirations – breaths/min

Heat Stress Resp

Low       Med        High

Beef Cattle

     98 – 102.4

          26 – 50

40-60   60-80    80-120

Dairy Cattle

 100.4 – 102.8

          26 – 50

“             “             “

Horse

     99  – 100.5

          10 – 30

 

Sheep

 100.9 – 103.8

          16 – 34

40-60    60-80   80-120

Goat

 101.3 – 103.5

          10 – 30

 

Pig

 101.5 – 103.6

          32 – 58

 

  • Normal temp and respirations per Merrick Vet Manual

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 handle livestock in the coolest part of the day, manage fly populations, provide shade, increase air flow, provide shade, adjust nutrition and provide appropriate minerals, and increase their access to cool water.
Signs of Heat Stress:
– 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

In part two of the article, we will explore practical steps producers can take to mitigate and prevent heat stress.

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