Heat stress costs dairy and beef producers hundreds of millions each year. However, implementing proactive measures before an extreme heat event can reduce immediate and long-term impacts of heat stress in ruminants.
Jessica Fox, veterinarian and director of veterinary services and biosecurity for Ralco, noted, “Mitigating impacts of heat stress begins before an extreme heat event. The impacts producers see are only a small portion of what is going on inside a ruminant during an extreme heat event.”
Fox explained that by the time ruminants show external signs of heat stress -- e.g., going off feed, labored breathing, panting, increased water intake, decreased activity or sweating -- heat stress has already begun to wreak havoc on vital internal systems. “Heat stress triggers a cascade of events that impact a bovine’s production ability [and] make it susceptible to disease and, in extreme circumstances, death,” Fox said.
North Dakota State University (NDSU) extension livestock systems specialist Karl Hoppe agreed, adding, “The combination of heat and humidity creates stress on livestock because respiration is the predominate route for cooling. Once cattle start to pant, some heat stress has occurred.”
Keeping the internal body temperature at normal is possible with panting and adequate shade and water. Sometimes, cattle will stand in ponds to cool off on a hot day. Bison also will wade into water to cool off.
“Healthy cattle can handle some heat stress,” NDSU Extension veterinarian and livestock stewardship specialist Gerald Stokka said. “However, too much heat, along with high humidity, can result in excessive heat stress, leading to death. Cattle can get relief from heat stress when the nights are cool. Night cooling for beef cattle occurs when the nighttime temperature falls below 72°F.”
Producers also should be aware of the increased risk of “summer pneumonia” in suckling calves following heat stress, the NDSU specialists said. An increased respiratory rate, along with the stress of heat, may overwhelm the natural defense mechanisms of the lungs. The risk is even greater in calves that may be compromised due to inadequate intake and absorption of immunity (colostrum) from the dam.
Calves that develop pneumonia may not be discovered until too late in the course of the disease. Early signs may be calves off by themselves, dams with full udders, a drooped ear and rapid respiration. Adult cattle and yearlings that have experienced respiratory disease early in life or postweaning may have decreased lung capacity and will be at a greater risk of heat stress, the specialists said.
“Cattle that have damaged lungs due to pneumonia may die on the first moderately warm day,” Stokka said.
He urged producers to consult with a veterinarian for confirmation of the heat stress diagnosis and for treatment options.
Night cooling allows the animal to cool off and get relief from heat stress. When night cooling doesn’t happen, cattle have a difficult time handling the heat stress the next day.
“Usually, healthy cattle won’t die from heat stress on the first hot day,” Hoppe noted. “Cumulative days of heat stress without night cooling physiologically challenge cattle. After three days, some cattle can’t handle the heat and humidity and die.”
Brahma and brahma-crossbred cattle are more tolerant of elevated heat and humidity. Bos indicus breeds (Brahma crosses) often are included in the cow herd breeding program in southern states.
For northern cattle, reducing heat stress includes providing adequate amounts of drinking water and access to the water; provide at least 2 in. of water per trough space per head, Hoppe and Stokka said.
“Ensuring abundant access to cool, fresh water is the single, most important step beef and dairy producers can take,” Fox added.
Because heat events can tax automatic water capacity, Hill suggests putting out extra free-standing tanks prior to a heat event. “Not only does this ensure all have access to water, but it keeps the herd from bunching up around water tanks, exaggerating the issue,” Ralco ruminant nutritionist and brand manager Dr. Jeff Hill added.
Providing shade also will help cattle reduce elevated body temperatures. Cattle will seek windy locations, wet places to stand or ponds to wade in to help cool off, NDSU said.
Cattle with dark hides tend to show heat stress and have a higher internal body temperature than cattle with lighter-colored hide.
For feedlot cattle, moving feeding time to later afternoon or evening will help reduce heat stress, the NDSU specialists said. Several hours after consuming a meal, the fermentation and digestion of feed creates heat. By feeding later in the day, the heat produced from digestion will develop during the night and not add to the daytime heat stress.
Usually, the fatter the cattle, the more difficulty in handling heat stress, Stokka said.
Heat stress can be forecast and is based on temperature, wind speed, humidity and solar radiation. The National Weather Service and the U.S. Department of Agriculture have a heat stress forecast website at https://tinyurl.com/HeatStressForecaster.
When heat stress is anticipated, cooling the ground may help. This can be done by putting water on the pen surface and/or adding bedding to change the pen’s surface color, which normally is black. Wetting the bedding also may help.
Using sprinklers or fire hoses to cool cattle that already are panting isn’t a good practice, Hoppe said, noting that spraying water onto the cattle raises the humidity and can increase heat stress. However, when cattle are in severe heat stress, soaking the animals with water may be necessary for their survival.
Heat also affects feed quality, according to Ralco.
“Feed rations heat up in the sun and begin to breakdown due to mold, yeast and bacteria growth. Not only are cattle appetites impacted during a heat event, but the feed itself is less appetizing,” Hill said.
Fox and Hill noted that many of the natural defense mechanisms triggered when heat stress occurs cause damage to a bovine’s intestinal tract, potentially leading to leaky gut syndrome. Caused by multiple factors, leaky gut describes a breakdown of the intestinal wall. This leads to bacteria, pathogens and other intestinal contents leaking through the intestine into the bloodstream.
“The majority of the bovine animal’s immune system is found in their gut. When gut health is jeopardized, the immune system takes a major hit,” Fox said.
In an attempt to cool down during an extreme heat event, blood is pulled away from the intestine to the bovine’s outer extremities, further weakening the protective lining of the gut wall, leading to leaky gut.
Another way heat stress negatively affects ruminants' immune system and causes leaky gut is through the production of free radicals. “Heat stress causes cattle to create an excessive number of free radicals, leading to oxidative stress,” Fox said.
Free radicals, Fox explained, are a normal result of bovine’s metabolism and utilized by the immune system to fight pathogens. However, when too many are produced, they damage the intestinal track and cause an overreaction of the immune system. This creates inflammation, which affects overall performance, meat and milk quality.