This is a highly studied area of bovine science, and the more we learn, the more we are realizing cattle are affected at much lower temperatures than originally thought. Cows can begin to experience heat stress when the temperature-humidity index (THI) rises above 70ºF. Issues seen when cattle are heat stressed include decreased reproductive performance and depressed milk yield and immune suppression. The greater energy demands of the thermoregulatory system further stress an already unstable energy balance. Depending on the severity of the heat stress, milk yield can decrease by more than 25%. Heat-stressed cows are also less likely to become pregnant; fewer than 20% of inseminations during these periods result in pregnancies – all which ultimately lead to significant economic losses.
Cattle are susceptible to heat stress due to their inherent physiology and basic metabolism. As energy demands are placed on the lactating animal, her dry matter intake is necessarily higher as well, in turn, creating more heat through increased fermentation.
A cow’s physiological adaptations to heat stress include altered blood flow (distribution toward the peripheral tissues), altered endocrine activity, differed energy metabolism, increased respiration rate, sweating, panting and drooling. Panting is important for cattle, as it helps dissipate heat because of their limited capacity to sweat. Excessive panting, however, may affect the acid-base balance and raise blood pH, leading to respiratory alkalosis. While panting, saliva is evaporating, and cattle tend to drool. Both actions cause animals to lose saliva, thereby buffering capacity is lost, leading to potential rumen acidosis.
One of the most dynamic adaptations to heat stress is behavioral. Cows will change drinking and eating patterns drastically, eating when temperatures drop and standing at the water trough for extended periods of time. They will also stand for far longer periods of time to increase surface area for heat abatement. Riding time and social interaction can be severely limited, making reproduction more of a challenge.
Initial reactions by dairy cows to acute heat stress could represent an emotional, rather than a thermoregulatory, response. They move because the sun is physically imposing. Research in areas unrelated to heat stress is beginning to show us this same idea: Cows flee from bright sunlight, not just because of heat units.
We know lameness in cattle is associated with pain. Heat stress causes increased standing, and we know increased standing causes lameness. It is not much of a stretch to say heat stress may induce conditions that cause pain. Researchers agree, stating we need further research to investigate these associations more exactly.
I would be remiss if I left heifer and dry cows out of this discussion. While lactating cows are the most susceptible to the effects of heat stress, dry cows and heifers are still subject to the negative effects. Poor reproduction, decreased intake, bunching and acidosis are all signs your heifers may be suffering from heat stress. Often facilities for non-lactating animals are not as modernized, so we may be able to have a greater effect nutritionally. To me, adequate cooling or heat-stress abatement techniques for dry and close-up cows should be a priority on all dairies. These transition animals are some of the most important linchpins on the dairy. If we have issues in the fresh pen, we can often trace them back to a problem in the close-up area.
The first step in abating heat stress is to make the cows’ environment more comfortable. We can accomplish this by providing shade, soaking, ventilation and clean, accessible water. In a typical Western dairy environment, evaporative cooling was increased from 68 grams per square meter per hour (g/m2h) before wetting to 508 g/m2h after wetting the skin surface, and the rate was further increased from 296 g/m2h before wetting to 961 g/m2h after wetting, when air velocity over the wetted skin surface was increased to 0.9 to 1.0 meter per second. In a Midwestern or Southeastern dairy environment, hot and humid, the increase was relatively modest, from 258 g/m2h before wetting to 490 g/m2h after wetting. This is an amazing effect, to think we can achieve almost 10 times the cooling by just soaking the animal. This is a tool we should all be investigating or investing in.
Secondarily, we can affect the animal through nutritional means. The primary focus of nutritional management of heat stress should be sustaining rumen health and function. Adjusting feeding schedules so they coincide with the times of the day when temperatures are the lowest should encourage cows to eat more, keeping production level. Additionally, since feed intake is depressed, increasing the density of ration by feeding more grains or fats may be advisable.
This strategy needs to be implemented with caution, however, as heat-stressed cows are already at risk for rumen acidosis. Ensuring a high-quality forage source during times of heat may help in minimizing digestive heat. The digestion of protein in the rumen has been shown to increase heat production and thus, should be limited, providing good-quality bypass protein and slow-release urea products. Buffers and minerals, such as potassium, sodium, magnesium and chlorine, should be balanced so what is lost through saliva and sweat can be replenished.
Cows experiencing heat stress are more prone to subacute ruminal acidosis because of changes in their feeding behavior, for example fewer and larger meals, slug feeding, and because of the tendency to feed a high-grain ration. Several feed additives, such as yeast, fat-soluble vitamins, B vitamins, niacin, chromium, selenium and direct-fed microbials, may improve immune and antioxidant systems, energy utilization and rumen function during heat stress.
A live yeast has been proven to stabilize rumen pH, stimulate microbial growth, enhance fiber digestion and increase animal performance, all of which are negatively impacted during heat stress. Yeast can effectively increase rumen pH by stimulating fiber digestion and lactate-utilizing bacteria, leading to regular feed consumption and more efficient rumen fermentation. Supplementing a live yeast to dairy cows year-round has resulted in higher feed intake and milk yield and protein and fat yield. These responses were even greater during the summer months.
To minimize the effects of heat stress on lactation and reproduction performance, producers must take an integrated approach that includes cooling, access to water and adjustments to ration. Correctly balancing a ration can be an effective tool for mitigating the negative effects of heat stress on rumen function and productivity.
A brief public service announcement as a veterinarian and former dairy general manager: Those of us working outdoors and with employees exposed to the elements need to be aware of the dangers. If you haven’t familiarized yourself with the human symptoms of heat stress, heat stroke and dehydration, please do so. This is a deadly issue, and catching the symptoms in an employee, or even in yourself, before it becomes dangerous may just save a life. There are helpful tutorials online. Be aware of the conditions and the health of the humans as well as the cows around you. 
PHOTO: Photo by Mike Dixon.
References omitted but are available upon request. Click here to email an editor.
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Luke Miller
- Dairy Technical Support Specialist
- Alltech
- Email Luke Miller
