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Heat stress in dairy cattle

Quick facts

  • Heat stress occurs when cows have more heat than they can get rid of and leads to more stress, lower milk production and a higher rate of diseases.
  • Temperature and humidity levels determine when a cow may start feeling heat stressed.
  • Shade and proper ventilation are critical to lowering the incidence of heat stress.
  • Providing clean water is important especially during warm weather.
  • Monitor ventilation systems to ensure that they are running properly.

What is heat stress?

Cows generate heat by digesting feed and producing milk. They absorb solar heat when out in the sun.

Heat stress occurs when cows generate and absorb more heat than they can easily get rid of by respiration, sweating and air blowing by them (wind or fans).

Heat stress leads to increased respiration rates, body temperatures, sweating and time standing.

Why is heat stress an important issue?

Heat stress costs you money.

  • Heat-stressed lactating cows have reduced dry matter intake, milk production and pregnancy rates.
  • Heat stress also leads to increased lameness, disease incidence, days open and death rates.
  • Heat-stressed late gestation cows have shorter gestation periods, calves with lower birth weights, reduced milk production and impaired immune function.
  • Heifer calves from heat-stressed cows produce less milk for the first 30 weeks of lactation after freshening.

When do cows start to become heat-stressed?

Cows begin to experience heat stress at much lower temperatures than humans. In general, mild heat stress starts around 72°F with 50% humidity.

High-producing cows eat more and generate more heat. They can begin to experience heat stress in well-ventilated barns at air temperatures as low as 65°F.

The temperature-humidity index (THI) takes into account both temperature and humidity to estimate the level of heat stress cows will experience based on environmental conditions.

Heat stress levels and effects on the body

Heat stress level Temperature humidity index (THI) Respiration (breaths per minute) Body temperature (degrees Fahrenheit)
No heat stress Less than 68 40-60 101.5-102.5
Mild 68-71 60-75 102.5-103
Mild to moderate 72-79 75-85 103-104
Moderate to severe 80-90 85-100 104-105
Severe 90-99 100-104 Over 105

How do you know if a cow is heat-stressed?

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Preventing heat stress

Many dairy farmers use multiple practices to help their cows manage their body temperature in hot, humid and sunny weather.

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Common cooling system problems

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Joe Armstrong, DVM, Extension cattle production systems educator and Kevin Janni, Extension bioproducts and biosystems engineer

Reviewed in 2020

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