Source: Larissa Tucker, animal and food sciences extension associate 

Heat stress is a significant issue in dairy farming, affecting cow health, productivity and well-being. It occurs when cows generate and absorb more heat than they can dissipate, leading to increased stress, lower milk production and higher disease rates.  

What is Heat Stress? 

Cows generate heat through digestion and milk production. They also absorb heat from the sun. When cows produce or absorb more heat than they can dissipate through respiration, sweating, and airflow, they experience heat stress. This condition is characterized by increased respiration rates, higher body temperatures, sweating and prolonged standing. 

Heat stress has a direct impact on dairy farming economics and cow health. It reduces dry matter intake, milk production and pregnancy rates while increasing the incidence of lameness, diseases, extended days open and, in extreme cases, even death. Heat stress in late gestation cows results in shorter gestation periods, lower birth weights and impaired immune function in calves. Research has shown calves from heat-stressed cows produce less milk in their first lactation. 

When Do Cows Experience Heat Stress? 

Cows begin to experience heat stress at lower temperatures than humans. Mild heat stress starts around 72°F with 50% humidity, while high-producing cows can begin to feel heat stress in well-ventilated barns at air temperatures as low as 65°F. The Temperature-Humidity Index combines temperature and humidity to estimate heat stress levels. 

Recognizing Heat Stress 

Cows' respiration rates and body temperatures are reliable indicators of heat stress. Normal respiration rates for adult dairy cows range from 40 to 60 bpm. If more than 10% of cows have a respiration rate exceeding 100 bpm, immediate action is necessary. Similarly, body temperatures above 105°F in more than 5-10% of cows signal an emergency. Observing daily milk production can also indicate heat stress, but production declines often lag by a couple of days. 

How Do We Manage Heat Stress? 

Effective heat stress management involves providing shade, ventilation, cool water and the use of sprinklers or misters. 

• Shade: If cows are on pasture, make sure adequate shade is available, generally 40 square feet per animal. Shade cloths may be used as well as sheds. Keep cows in well-ventilated barns, especially during the hottest periods of the day. 

• Ventilation: Proper barn ventilation ensures air exchange between the barn interior and the outside. Some barns use tunnel and cross-ventilation systems. Others may use high-velocity, low-speed fans to help maintain air flow. Make sure that fans are adequately distributed so there are no “hot” spots within the barn.  

• Water: Hydration is vital. In hot weather, lactating cows may drink 25-35 gallons of water per day. Adequate water space and clean cool water sources are essential. 

• Sprinklers and misters: Sprinklers wet cows’ skin, allowing body heat to evaporate the water, while misters cool the air. Placing these in heavy traffic areas such as feed bunks, alley ways or holding pens can be effective in keeping the cows cool in crowded areas. Regular maintenance of cooling systems, such as ensuring filters and nozzles are clean, is important. 

Managing heat stress is essential for dairy farming success. By implementing shade, ventilation, and cooling systems, along with providing ample clean water, farmers can help their cows cope with high temperatures, ensuring better health and productivity. 

Contact your local (COUNTY NAME) Extension office for more resources, tips and information about avoiding heat stress in dairy cattle.   

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