May 5, 2006
UNIVERSITY OF MISSOURI EXTENSION
SOUTHWEST REGION NEWS SERVICE
Contact: Tony Rickard, dairy specialist
Headquartered in Barry County
Tel: (417)  847-3161
E-mail: rickardt@missouri.edu

Managing Dairy Cows During Heat Stress

The modern dairy cow could be compared to a factory, consuming large quantities of raw 
materials (feed nutrients) and producing large quantities of product (milk), according to Tony 
Rickard, dairy specialist, University of Missouri Extension.

“If the flow of raw materials, nutrients in this case, is reduced or interrupted then there can be a 
severe reduction in production,” said Rickard.

A prime example of something like that happening here in the Ozarks is heat stress. The primary 
factors are high environmental temperatures and high relative humidity. 

“It is not unusual to see a 20 to 40 percent reduction in daily production during the summer 
because of the heat,” Rickard said.

One way to measure the combined effect of temperature and humidity is to use the temperature-
humidity index (THI). When the THI exceeds 72 one can expect heat stress and reduced feed 
intake in cows.

When the relative humidity is high, the temperature that causes a THI of 72 can be quite low. 

For example, a temperature of 75 degrees and 70 percent relative humidity or 80 degrees and 25 
percent humidity would result in the same THI.

When the THI reaches 77 it is considered to be in the danger zone. In this range, cattle are at risk 
and environmental modifications are needed.

“Assuming adequate drinking water is available, the easiest and most obvious way to help heat-
stressed cows is to provide adequate shade,” said Rickard.

Direct sunlight adds a tremendous heat load to the cow. Eliminating the heat load lowers body 
temperature and respiration rates of cows and increases feed intake and milk production.

“Research has actually shown a 10 to 20 percent increase in production when only shades were 
installed,” said Rickard.

Additional cooling in the form of fans and sprinklers, in conjunction with the shade, has also 
proven very beneficial. 

“Research shows an 11percent increase in milk yield when these practices are implemented 
when compared to shading alone,” said Rickard.

However, if given the choice of either fans or sprinklers, fans would be the first choice. If cows 
were sprinkled only, without air movement, the humidity could easily build up past the critical 
THI since the heat generated by a group of cows may not be dissipated.  

“Even though all of the problems associated with heat stress cannot be totally eliminated, using 
shades, fans and sprinklers, in that order, can be very beneficial,” said Rickard.

University of Missouri Extension improves people’s lives with education and research from the 
four campuses of the University of Missouri System. All extension programs focus on the 
identified high-priority needs of people throughout the state. Each county extension center, with 
oversight by locally elected and appointed citizens, is your local link to these unbiased resources 
and programs.
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