Traditionally, we have monitored the effect of the environment on dairy cattle using the temperature-humidity index (THI) to give an approximation of heat stress on animals. This method follows an equation that uses the dry bulb temperature and relative humidity to give us a number that best reflects the heat load placed on a cow.
Humidity is crucial because the higher its level, the less efficient evaporative cooling becomes. And since cows are evaporative coolers, heat puts more stress on the cow as the humidity grows higher.
If the THI is over 72°F (or 68°F for high-producing cows), the cows may start to feel the effects of the heat, which can vary from animal to animal. These effects include increased respiratory rates, decreased production, decreased dry matter intakes, increased risk of acidosis, increased risk of laminitis, decreased reproductive efficiency and more. Panting generally doesn’t start until the cow has experienced prolonged heat buildup, so it is important that producers know when heat stress may become an issue and then be prepared to manage it.
The problem with the THI is that it does not take into consideration some other critical factors that also affect the perceived heat stress. For instance, it does not account for solar radiation, standing in the sun versus shade, or a cloudy day. This is why shade is so vital. Wind and airflow, which play huge roles in perceived heat stress, also help with evaporation. Breed of cow, level of production and stage of gestation all play parts, as well. Some breeds are more heat-tolerant than others, and the high-producing cows and those later in gestation will also be more sensitive to the heat.
Another factor to consider is heat accumulation. Heat can build up over time in both the cow and in the environment. The ground acts as a heat sink and releases the heat back into the animal. Dark ground holds and absorbs more heat, while light ground reflects it. Heat accumulation levels depend on nighttime cooling, heat spell duration and the environmental accumulation. Nighttime cooling essentially sets up the next day. If the temperature drops below 68°F for longer than six hours overnight, most of the heat will be released back into the environment. But if not, it will become part of the cow’s heat load the next day.
A better indicator is on the horizon that will more accurately account for heat stress given these factors: dairy heat load index (DHLI) and the accumulated heat load (AHL). The DHLI, originally developed in Australia, not only uses temperature and relative humidity – it also takes into account the solar radiation using a black globe temperature and wind speeds. In addition, it factors in the breed, production and health of the cows. Australian dairy producers can use an online tool to determine DHLI, and there is interest in a version being available to U.S. dairy producers in the near future.
The DHLI has been shown to be a reliable indicator of physiological stress. Its guidelines are:
- When the DHLI exceeds 64, cows are likely to begin experiencing moderate heat stress and reproduction may be affected.
- When the DHLI exceeds 69, cows are likely to experience moderate heat stress, feed intake can decrease and the milk production of high-production cows may be affected.
- When the DHLI exceeds 74, cows are under high heat stress, and milk production and reproduction are likely further affected.
- When the DHLI exceeds 79, significant losses in milk production are likely and health can deteriorate.
- Upward and downward adjustments are possible. Upward adjustments occur when cattle have access to shade (plus 3 to 7), and downward adjustments occur when cattle show clinical signs of disease (minus 5).
So what can we do to help when we know the heat is coming? We can make sure there is adequate shade that allows for at least 25 square feet per head to reduce crowding. Take advantage of the wind and use fans. Also try using sprinklers with the fans, especially in holding pens and at the feedbunks. Make sure fresh water is available and at enough locations to minimize crowding. Work with the nutritionist to adjust rations – increase the fat and straw, reduce the starch and increase potassium and fat-soluble vitamins.
The most important thing to do is to watch. Watch the weather forecast, watch dry matter intakes and watch the cows for signs of heat stress in the morning and throughout the day. Also, try to avoid any extra handling, processing or shipping during these heated times. 
References omitted but are available upon request. Click here to email an editor.
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Stephen Foulke
- Veterinarian
- Large Animal Veterinary Services
- Merial Veterinary Services
