In most parts of the country, the heat index is now well into its upward climb, and that can spell trouble for dairy producers.

“Heat stress is one of the leading causes of decreased production and fertility in dairy cattle during the summer,” says Dr. Jeffrey Keown, University of Nebraska Extension dairy specialist. “If not managed carefully, it can result in lowered milk production and reduced reproductive efficiency.”

The severity of heat stress is affected by temperature and humidity. And heat stress is a double-edged sword for producers because some of the management practices can create secondary problems if not executed properly.

“Misters are an excellent way to reduce heat stress and maintain milk production, but misters need to be used properly,” Keown warns. “It’s important to ensure that misters installed near feedbunks don’t wet the feed. Feed tends to get moldy quickly in hot conditions, and moldy feed means decreased feed intake, which has a detrimental impact on milk production. Pair that with heat stress, and it’s a double hit for producers.”

Keown also says that misters should only wet cows from midback to the head. Misting further back on the cow or applying too much water can lead to conditions ideal for mastitis development. Misters should operate on a timer, and udders should never drip with water. If possible, misters should be installed over clean, well-drained areas, such as concrete, to prevent creating muddy conditions and possibly an increase in mastitis.


“Producers also should administer a coliform mastitis vaccine approved for whole-herd vaccination to offset any additional risks of mastitis created by heat stress management techniques,” says Dr. Bruce Nosky, Manager, Merial Veterinary Services.

Along with misters, other simple solutions to beat the heat include providing shade or, in freestall barns, installing fans to keep the air moving. If barns are equipped with curtain sidewalls, Keown advises producers keep them raised when temperatures rise. Cows also need access to plenty of water.

“Creating shade over the feedbunk is a great way to maintain production during the hot summer months,” Keown says. “Cattle will linger longer at the bunk, eat more and produce more milk.”

One University of Nebraska study showed that cattle shaded and cooled by sprinklers near feedbunks ate 63 percent to 100 percent of the time as compared with uncooled cattle.

“Installing cooling systems is a one-time investment in most cases, and if a producer is losing 10 pounds of milk per cow to heat stress, it doesn’t take long to recoup the investment,” Keown says.

Managing heat stress not only improves milk production, but it also can improve summer reproductive rates. However, hot-weather breed-back can still be a challenge.

“Many producers try to avoid breeding cattle during the hottest summer months, but delaying breeding costs profits,” Nosky says. “Producers managing heat stress can still have good breeding success. However, they should use all technologies available to them to increase their success rate, including using a GnRH to treat ovarian follicular cysts and induce ovulation.”

A missed heat cycle can cost dairy producers as much as $29, and a missed pregnancy costs an average of $450.

“Summer heat can negatively affect dairy cattle production in several ways,” Nosky says. “However, producers can take a few minor precautionary steps to help cows be more comfortable as well as decrease the instances of mastitis and reproduction problems.” PD

References omitted but are available upon request at

—Excerpts from Merial VPS SHARE newsletter, June 2008