Shades, misters and fans are all in place and running, but it’s still a hot summer day. You know the cows are starting to feel the effects of the heat because they aren’t eating as much. What else can you do? Protect them by changing their diet.

Heat stress in cattle appears from a combination of conditions like wind, ambient temperature and humidity that create an environment that doesn't allow them to dissipate heat. Accumulating heat load happens when the cow’s body temperature increases, indicating the animal is experiencing some degree of heat stress.

Impacts on the cow

The cow wants to release as much heat as she can. To do that, she vasodilates at the periphery of the skin to try to maximize blood flow to release as much heat as possible.

“Vasoconstriction occurs in the gastrointestinal tract. It’s also known as hypoxia and is caused by a severe restriction of blood flow to the gut or low levels of oxygen delivered to the gut,” explains Lance Baumgard, animal science professor at Iowa State University.

When cows become heat stressed, they also experience a reduced appetite. Baumgard’s heat stress research indicates that about 50 percent of reduced productivity or milk yield is a result of reduced feed intake. The other 50 percent is due to the direct physiological effects of being heat stressed, which goes back to the gut.

Along with responsibility for digesting and absorbing nutrients, another important function of the gut is to be a barrier, preventing unwanted molecules that reside inside the gut from entering the animal's body.

“Envision the gut as a tube running from the animal's mouth to its anus. Inside that tube is a variety of unwanted molecules, pathogens, etc., that are supposed to stay inside the tube and be discarded in manure,” says Baumgard. “But because of the reduced blood flow to the intestine, that tube's barrier starts to become leaky or permeable, and pathogens and other antigens have an opportunity to infiltrate into the body.”

The immune system recognizes the unwanted molecules as foreign, which initiates an intense immune response requiring a large amount of nutrients, specifically glucose and amino acids. Those nutrients are now going to be prioritized for the immune system rather than being used by the mammary gland to make milk. Consequently, milk yield goes down, and sometimes further than predicted from the reduction in feed intake.

Heat stress effects on nutrition

To combat reduced feed intake due to heat stress, one strategy is to maximize the energy density of the diet. However, it must be done with caution because cows are already prone to conditions that can lead to rumen acidosis.

“One effective strategy is to increase the fat content of the diet,” says Baumgard. “Another strategy is to feed more buffers, like bicarb, to minimize the rumen acidosis. Feeding ionophores is also an important component to safely increase the production of propionate in the rumen.”

Antioxidants' role as rumen protector

When heat stress hits, it can cause rumen acidosis. The combination of reduced blood flow to the gut and the acidic conditions in the rumen and large intestine causes several stresses within the cell lining of the GI tract, including oxidative stress.

“We want to minimize oxidative stress, and antioxidants likely play a key role in helping to maintain gut integrity and prevent the infiltration of unwanted molecules,” Baumgard says.

Oxidative stress causes an imbalance between production and removal of free radicals and peroxides. Disruptions of oxidative balance will lead to cell and tissue damage. As free radicals build up, they inhibit normal microbial protein synthesis and fiber digestion as well as reduce bio-hydrogenation of unsaturated fatty acids, which can lead to milk fat depression. All of these issues can cause a decrease in fluid milk production, milkfat production and overall cow health.

When a combination antioxidant is added to feed, it stabilizes and neutralizes the harmful free radicals, allowing the metabolic pathway to function normally both in the rumen and in the cow. Antioxidants also help repair cellular damage that occurs from peroxides and oxidative stress. Ultimately, antioxidants keep the heat-stressed cow in oxidative balance.

Cows receiving antioxidants eat more feed during heat stress. Customers speculate that the total mixed ration has longer “bunk life” or there are fewer anti-palatability compounds produced by yeasts and molds. Better feed intake during periods of heat stress means healthier cows and less feed wastage in the long run.

If you have a happy rumen, you have a happy and productive cow, and that means a happy and profitable dairyman.  

Lyle Rode is the North American technical services manager for Novus International.

PHOTO: Even when fans and misters are running, cows can still experience negative impacts of heat stress, including reduced feed intake. Dietary adjustments may help minimize these effects. Staff photo.