Dairy cows perform best when the environmental temperature is between 40 and 75 degrees Fahrenheit. Once the outside temperature gets above 77 degrees, a cow has to work (expend energy) to keep cool. As the temperature rises, she has to work harder to keep cool. High producers have the most trouble because they are already working hard just to make milk!

When the temperature exceeds 90 degrees, feed intake drops, followed by milk production. Heat-stressed cows have a harder time controlling metabolic problems and fighting off disease. Heat stress on dry cows in the last trimester will lead to more metabolic problems after calving and a reduction in milk produced in the subsequent lactation. Reproduction suffers due to poor expression of estrus, reduced activity on the ovaries, and increased embryonic death.

The temperature-humidity index, or THI, is commonly used to determine a threshold for management action. The THI is calculated by multiplying the temperature and relative humidity. Dairy cattle begin to show signs of heat stress when the THI exceeds 72. For example, a daily temperature of 95 degrees and a 90 percent relative humidity yields a THI of 85.5. Severe heat stress can accompany the temperatures we saw over the 4th of July weekend. Cows pant with an open mouth and body temperatures become elevated. Unless there is an opportunity for night cooling, the body temperatures may still be elevated on the following morning.

The most practical methods to reduce heat stress are shade, ventilation, and cooling. Areas where cows tend to congregate should be managed to reduce heat exposure. Holding areas, feed bunks, and loafing areas are key targets. Access to adequate, clean water cannot be overemphasized.

Cows dissipate heat through the their lungs (respiration/panting) and by sweating. When the THI is elevated, water consumption will increase by as much as 50 percent. If access to ample water is inadequate or heat stress becomes severe, cows will divert body water normally used for milk production to the metabolic processes of heat dissipation.

Water intake may increase by 5 to 6 gallons on summer days due to temperature effects alone. Michigan State workers (Beede et al., 1992) have shown that cows consume about 3 lb. water/lb. dry matter intake (DMI) with temperatures between 0 and 41 degrees. When summer temperatures increased, cows reached 7 lb. water/lb. DMI at high temperatures. Some high-producing cows were capable of consuming 50 gal. water/day.

To encourage more water consumption:

• Put waterers in the shade.
• Provide access to water immediately after milking.
• Install at least two water locations per group/pen or at least 1 watering station per 20 cows (and this may not be enough).
• Make sure the water supply delivers at least 3 to 5 gallons/minute (cows can consume 6 gallons/hour.).
• Maintain a minimum of 3 in. water depth.
• Keep water tanks clean. (Use a brush and a chlorine solution to disinfect surfaces and cut down on algae growth.)

For more information on management of heat stress in dairy cattle visit http://traill.outreach.uiuc.edu/ and type "heat stress" in the Search Engine. Another great resource covering engineering and design issues related to heat stress management can be found at http://www.mrec.org/ventilation_&_cooling_text.htm.