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Summer strategies to reduce whole-herd heat stress

Preston Morris for Progressive Dairy Published on 11 June 2020
Cattle at the feedbunk

For many, summer weather is here, and along with it are high temperatures, high humidity and heat stress.

Dairy animals are one of the most susceptible domestic species to heat stress, and the impact is daunting, often decreasing performance, production and profitability. Producers will likely see increases in health costs and somatic cell counts (SCCs) with declines in dry matter intake (DMI), milk production, milk components, 21-day pregnancy rates and cash flow. According to a 2019 Foundation for Food and Agriculture Research study, the U.S. dairy industry experiences $1.5 billion in losses annually to heat stress.



Too often, the preparation for heat stress comes later than it should, relying on human comfort to determine cow comfort. Heat stress in cattle is caused by a combination of elevated environmental temperature and humidity. The combination is referred to as temperature-humidity index (THI). Heat stress can develop with an afternoon humidity as low as 10% to 20% and temperatures at just 75ºF to 77ºF. Mild heat stress begins at a THI of 68, moderate at THI of 72, severe at THI of 80; a THI of 90 is a dangerous level and, if prolonged for a few hours, can result in death.

Heat stress impacts more than just the milking herd. By implementing whole-herd heat stress solutions to include lactating, dry cows and youngstock, you can limit the overall health and financial impact.

Milking cows

Heat stress has the greatest financial and health impact on the lactating cow, often dramatically reducing milk production and 21-day pregnancy rates by 10% or more. Be aware of common symptoms of heat stress, such as increased respiratory rates and open-mouth breathing, which are good, quick indicators of increased body temperature. Excessive panting over time can cause metabolic disturbances in blood pH. Additionally, excessive drooling decreases the amount of bicarbonate in saliva available to utilize as a rumen buffer in the gastrointestinal (GI) system.

DMI is also impacted during higher temperatures, as well as shifts in feeding patterns. The combination of these conditions can lead to the majority of the diet being consumed at night during cooler temperatures; this results in slug feeding, which can also decrease rumen pH and the animal’s ability to buffer the rumen. This drop in pH causes changes to the microbial population, affecting fiber digestion and milkfat synthesis, both of which can lead to a drop in milkfat percentage.

While decreased DMI will also affect milk production, research shows it only accounts for 50% of the lost milk. The remaining losses can be attributed to ruminal lipopolysaccharide leaking through the GI wall and into the bloodstream. This causes an immune response which negatively affects DMI even further and decreases the amount of energy going toward milk production.


Dry cows

Like the lactating cow, the dry cow faces many hurdles during summer heat. The impact not only affects the dry cow but also the calf in utero. However, she is commonly overlooked due to her self-sufficient nature. When a dry cow experiences heat stress, the body temperature of the unborn calf increases, altering metabolism and gene expression.

Additionally, spikes in the dam’s cortisol level can signal her to go into pre-term labor which leads to smaller calves at birth, poor colostrum absorption and suppressed immune function. This can cause long-term negative impacts on the health and performance of the cow and calf for their entire life. The dam is also at increased risk of numerous post-calving disease conditions such as retained fetal membranes and metritis. Research from the University of Florida demonstrated that cooling dry cows and their fetal calves can positively impact production and body condition, which results in better performance post-calving when compared to non-cooled cows.

Managing the stress

Implementing basic heat abatement strategies throughout the adult herd can help cows manage the stress and aftermath of the summer months. First and foremost, adequate access to clean water is a must. Ensure 3 to 4 inches of water trough access per head in a pen. Fans need to be clean and should generate at least 5 mph of wind over the back of the cow. Use a wind speed meter to detect good air movement and dead areas, especially where cows congregate. Add in soakers on the feedbunk and in the holding pen to apply water to the back of the animal. In humid climates, which include much of the U.S., water should be applied as a “soak” and not a mist. Maximize equipment performance by regularly monitoring and maintaining fans and soakers.

Adjustments in feeding strategies may help reduce lost performance. Consult with your nutritionist about what ration changes might be appropriate for warmer temperatures, including the addition of products to optimize rumen health. Other strategies may include feeding later in the day to make more feed available at night when temperatures are cooler, feeding earlier in the morning or feeding smaller amounts more frequently. As always, make sure feed is pushed up often to keep feed accessible to cows and reduce sorting.

While heat abatement strategies are mission-critical to both dry and lactating cow performance, growing animals should not be overlooked.


Youngstock are also susceptible to heat stress – although generally at higher temperatures – compared to adult animals. Heat stress causes a negative impact on both health and weight gain. At high temperatures, the calf’s respiration rates increase, feed intake declines, and the calf’s immune system is suppressed. Immune system suppression can lead to an increased rate in calf pneumonia and scours, which can impair calf growth and affect the cow’s lifetime performance.


One of the first signs of heat stress in calves is dehydration. Calves may double their amount of water intake in order to combat heat and stay hydrated. Be sure to keep clean water available at all times. Increased usage of electrolytes should be considered in warmer months, especially in scouring calves.

When it comes to housing and ventilation, shade, cleanliness and air flow are top priorities. If calves are housed in hutches, it is important to realize that temperatures inside the hutch may exceed 100ºF. If possible, hutches should be elevated to allow for extra air movement and face away from the direction of the predominant sun with all vents open during the summer months. It’s important to keep consistent air flow by natural ventilation and/or fans when animals are housed indoors.

With summer temperatures on the rise, animals will be feeling the heat. Heat stress can have a devastating impact on your whole herd and not just those in the lactating pens. Between calves, dry cows and milking cows, implement a whole-herd strategy to manage the impact of heat stress and help your herd stay healthy and productive.  end mark

PHOTO: Fans need to be clean and should generate at least 5 mph of wind over the back of the cow. Use a wind speed meter to detect good air movement and dead areas, especially where cows congregate. Photo courtesy of Diamond V.

Preston Morris
  • Preston Morris

  • Technical Services Specialist
  • Diamond V
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