It is estimated that the annual cost of heat stress to the dairy industry is approximately $900 million. Based on these estimates, it should come as no surprise that investments in on-farm cooling systems are highly prevalent. However, cooling systems alone may not be sufficient to mitigate the economic impacts of heat stress.
During heat stress, animals may divert energy and nutrients away from production purposes and use them instead for heat dissipation or for inflammatory responses to immune challenges. After heat stress, animals are not quick to switch their nutrient utilization back to production mode, which may cause a reduction in production performance for weeks to come.
Heat stress management and leaky gut in dairy cattle
We know that upon the onset of heat stress, dairy cattle experience a marked reduction in dry matter intake (DMI), which has been hypothesized as a mechanism to help reduce metabolic heat production. This reduction in feed intake, however, only partially explains why animals under heat stress have poorer performance.
Other changes are happening inside the animal to combat heat stress, such as changes in blood flow and oxygen and energy availability.
When animals are under heat stress, blood flow is diverted from visceral tissue – tissue that lines the blood vessels, stomach, digestive tract and other internal organs – to the skin, which aids in heat dissipation. This reduction in blood flow to the visceral tissue causes a reduction in the amount of oxygen and energy available to the epithelial layer of cells lining the intestinal tract. This causes the tight junctions that hold these epithelial cells together to weaken, allowing pathogens and toxins to translocate into the bloodstream. This condition is called leaky gut.
Leaky gut can have immediate effects on your dairy cattle, but it also plays a role with delayed effects after the stress is gone. Once the bacteria, pathogens or their toxins pass between cells, the immune system recognizes them and triggers an immune response to destroy and remove the invaders. This inflammatory process and immune system activation consumes significant amounts of nutrients, pulling nutrients and energy away from other key functions within the animal such as growth, production and reproduction.
Heat stress leads to lameness in dairy cattle
Reduced feed intake and leaky gut during a period of heat stress can ultimately lead to lameness in cattle even after the temperature has gone down.
The chronic inflammation that occurs due to leaky gut causes a reduction in keratinocyte proliferation that makes up the foot horn tissue. This reduction in keratinocyte number results in the formation of lower-quality hoof horn tissue throughout the summer, a little at a time, and will eventually lead to hoof problems months later.
In addition, during periods of heat stress, dairy and beef cattle spend more time standing in order to dissipate heat beneath them. This increased standing time can result in additional hoof problems, such as sole ulcers.
Trace mineral nutrition and heat stress management
Dairy producers can help mitigate the negative effects of heat stress by feeding a more nutrient-dense diet in the summer to compensate for the reduction in feed intake. Producers can consider increasing the fat/energy content of the diet to increase the number of calories the animal is consuming.
Feeding performance trace minerals during the summer can also play a positive role in mitigating the negative effects from heat stress. Research shows that feeding zinc from performance trace minerals plays a role in maintaining intestinal integrity by strengthening tight junctions during challenges and reducing the incidence of leaky gut and related intestinal inflammation. The best responses to feeding zinc occur when zinc is introduced roughly two months prior to the stressor.
Chromium from performance trace minerals can increase thermal tolerance as well, as it helps reduce the level of cortisol or corticosterone – involved in regulation of energy, immune reactions and stress responses – in animals. Elevated levels of cortisol or corticosterone cause animals to expend energy and, ultimately, reduce feed intake. Lowering cortisol and corticosterone levels by feeding diets containing chromium from performance trace minerals can help make animals more willing to eat during heat stress events. Additionally, chromium plays an important role in insulin sensitivity, which may help cows utilize glucose better during times of stress.
Oxidative stress occurs when free radicals and antioxidants are out of balance. Zinc, copper, manganese and selenium all work as antioxidants by removing free radicals and protecting cell membranes from oxidative stress.
Conclusions
Heat stress is a widespread issue dairy producers must manage. It’s critical to have a plan in place prior to the onset of hot weather to prevent the long-term effects heat stress can have on an animal. Cooling systems alone may not be enough to mitigate the effects of heat stress. By incorporating performance trace minerals into a dairy cattle nutrition program, animals are better able to reduce the impact of leaky gut during heat stress conditions. 
PHOTO: During heat stress, animals may divert energy and nutrients away from production. Photo courtesy of Zinpro.
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Adam Geiger
- Research Nutritionist
- Zinpro Corporation
- Email Adam Geiger
