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Why does a cow suffer from heat stress?

Excessive heat during the summer months is one of the most significant stressors that affects the productivity of dairy cows, yet it is also one of the most predictable pressures to manage. During periods of heat stress, the herd as a whole will display a range of behavioral and physiological change such as an increased respiration rate, higher body temperature, increased water intake and more time spent standing in an effort to lower the body temperature. Feed intake will also decrease during periods of hot weather. Consequently heat stress can significantly impact the herd’s productivity as well as the overall well-being and health of individual cows, all of which will be to the detriment of the farm’s profitability.

11-06-2020, updated on 26-06-2020
cow and tree

Why does especially a high yielding cow suffer from heat stress?

Cows produce a lot of heat during ruminal fermentation, digestion and metabolism of nutrients. This heat must be dissipated so that internal body temperature remains about constant. There are several mechanisms that the cow uses to dissipate heat, including conduction (direct contact with cold objects), evaporative cooling (sweating, panting) and convection (heat dissipates into the surrounding moving air). The efficiency of all these mechanisms gets compromised under high external temperature and humidity, and consequently internal body temperature starts to rise. Climate conditions have become a stressor, and the cow will respond with several adaptations (increasing respiration rate, decreasing movement and intake, etc.) which may ultimately compromise cow health and production. The more milk cows make, the greater the metabolic rate, the more heat they produce, and the more heat they must dissipate; that’s why high yielding dairy cows are particularly sensitive to heat stress.


Recognizing heat stress in cows

What is the threshold for heat stress to impact on dairy cow productivity? The most common way to predict heat stress is to calculate the Temperature Humidity Index (THI), which accounts for the combined effects of environmental temperature and relative humidity. Dairy cows begin to show signs of heat stress at a THI of 68 or more. An example, a THI of 68 can be encountered when the air temperature is 25ºC and the relative humidity is 10%, or at a temperature of 22ºC and a humidity of 50%. Any increase in either temperature or humidity will cause the impact of heat stress to worsen.

The table below illustrates the impact of temperature and humidity on the severity of heat stress.


Source: National Animal Diseases Information Services

The consequences of heat stress

Heat stress can impact on dairy cow productivity in in several ways including reducing feed intake and milk production, and affecting health and reproductive performance. For more information about how heat stress affects cow performance and how to prevent it, please read more here.


OmniGen and heat stress

Research has shown that heat stressed cows fed OmniGen had a lower body temperature and respiration rate, and maintained a higher feed intake and milk production.

Learn more about how OmniGen can help your cows during heat stress by requesting a farm consult.



Paolo Bozzi
Paolo Bozzi
Paolo Bozzi is a Dairy Technical Specialist, based in Italy. With almost 10 years of experience in the field as farm vet, he switched to working as technical advisor and nutritionist for some of the main feed and additives companies. With joining Phibro Animal Health Corporation five years ago, his main goal is now to support farmers and feed industries with expertise and technology to reduce health events and antibiotic use on farms and to help them achieve the best profitability.

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