As most of America is struggling through one of the coldest winters in recent memory, dairy producers must start planning for the annual summertime problem of heat stress. While we long for warmer temperatures, heat stress can result in significant losses on the dairy.

The mercury need not rise into the triple digits, as heat stress can occur when the average daily temperature rises above 70°F. The ideal temperature for dairy cattle is between 25°F and 65°F. When the temperature exceeds 80°F, dry matter intake, milk yield and reproductive performance can decline. High temperatures can reach 90°F as early as April in places like California’s Central Valley and as late as June in upstate Vermont. However, no matter what your latitude, the heat is coming.

It’s not just the heat, it’s the humidity. Humidity is also a concern when it comes to heat stress. A temperature of 70°F and 100 percent humidity can start to cause stress on the herd and some type of cooling should be applied. As the temperature reaches 100°F and 50 percent humidity, the herd has reached a danger zone. Humidity is lethal at 100°F and 90 percent humidity.

Signs of heat stress: Both visual and measurable

When the temperature and humidity increase, heat-stressed cows display a variety of symptoms, both visual and measurable. Signs of heat stress include:

  • Seeking out shade and often not leaving to eat or drink
  • Increased water intake
  • Reduced feed intake
  • Standing rather than lying down
  • Increased respiration rate
  • Increased body temperature
  • Increased saliva production
  • Open-mouth panting
  • Decreased milk production
  • Reproductive challenges

During times of heat stress, feed intake can be reduced by 8 to 12 percent. The reduction in feed intake reduces volatile fatty acid production in the rumen, resulting in decreased production. This can also lead to acidosis and reduced fat content in milk. Lameness can also come into play from acidosis and the increased output of bicarbonate.


Heat stress can result in the prolonged care of sick cows. The “heat disease” can result in difficult births, heat exhaustion, fatty liver in fresh cows, mastitis and adverse reactions to vaccinations, which can ultimately lead to abortions and death.


The good news regarding heat stress is that producers have many options to choose from in combating the problem. The four main methods are water, shade, cooling and feed management.

Water: Water is probably the most important element for cows to take in during times of heat stress. Producers should increase both the accessibility and the amount of available water, make sure the water is cool and never let the water become stagnant. Cleaning out water troughs daily and keeping the water and the feed in a cooled area will help to increase H2O consumption.

Shade: If all cows had access to pasture, perhaps trees would be a way to offer shade, but in today’s dairy industry, that is not possible or practical. Providing an artificial shade area through the use of shade cloth or a naturally ventilated structure with open sidewalls are some methods for keeping the herd away from solar radiation. When building a shaded area, it is important to keep the height at 12 feet or higher for maximum effectiveness. Another key step in shade support is to place the shaded area over the feeding area, as this will help to encourage feed intake. Finally, make sure obstructions of ventilation, such as stacked hay, are not limiting air flow across pens and corrals.

Cooling: When it comes to the cooling method, producers have many options to consider. Each area of the dairy facility must be examined when looking at cow cooling. To increase the ventilation or air exchange rate, producers should clean existing fans and install more fans and inlets. Distribution of inlet location and adjustment should also be considered, as this can help with the ventilation effectiveness. Air flow can be increased by opening up the sides of the barn.

Air flow isn’t the only way to achieve coolness for your cows. Sprinkling systems can be used along feedbunks and in holding areas to wet the cows regularly. Droplet size should be examined. It should not be a fine mist, but rather enough to wet the hair and skin. The key to sprinkler systems is to keep it on a schedule. Excessive sprinkling does not help with heat stress and can create wet bedding, which can lead to mastitis.

Feed management: The final option producers can turn to during heat stress situations is feed management. Some nutritional tips during times of heat stress are:

  • Feed total mixed rations.
  • Increase the number of feedings.
  • Feed during cooler times of the day.
  • Keep feed fresh.
  • Use high-quality forage.
  • Provide adequate fiber.
  • Make mineral changes such as sodium and potassium.
  • Avoid secondary fermentation in feedbunk.

Fermentation by nature produces heat. Inefficient fermentation can cause increased temperatures, but it also uses excess energy to displace excess heat. Diets high in fermentable carbohydrates are best described as “hot rations” because of the excess heat generated during fermentation.

A natural solution for heat stress is the use of live yeast cultures. Yeast cultures have been shown to stimulate dry matter intake, helping maintain performance and rumen health during heat stress. Research on the use of yeast cultures has shown to lower rectal temperatures of dairy cows during heat-stressed situations.

The yeast cell does not need to live long or multiply in the rumen. Due to its anaerobic conditions, the rumen is too hostile of an environment for yeast cells to maintain life for an extended period of time. This means they will not multiply and will eventually die, but this hostile environment to the yeast cell is what makes it do what it does to benefit the rumen. Once yeast culture is removed from the diet, the number of yeast cells found in rumen fluid will quickly diminish. That is why yeast culture is effective in promoting feed efficiency. The yeast cell will utilize all of the oxygen and excess lactate found in the rumen that is either toxic to rumen microflora, or causes acidic conditions that suppress the fermentation efficiency of fiber. Think of the yeast cell as a martyr of such; its true benefit to the dairy cow is in its death.

2010 will be an especially trying time for producers that are feeding the 2009 corn crop. After a cool, wet season, the harvest came in with elevated levels of mycotoxins, particularly fusarium toxins. These mycotoxins will cause additional stress on the animal, thereby taxing an already overworked immune system. Producers must pay attention to the quality of the feed in order to ensure that mycotoxins don’t pile on to an already stressful situation, making the care and attention to keeping cows comfortable even more critical. PD

Randy Cragoe