When it comes to judging heat stress for livestock, the biggest miscalculation that many farmers make is that they go by the outside ambient temperature instead of the combination of temperature and humidity, explains Maurice Eastridge, an OhioStateUniversity extension dairy specialist. They often forget that it is often warmer and more humid in the housing area where the cows reside.

Cattle can exhibit mild heat stress (a combination of temperature and humidity known as the THI index) with temperatures as low as 75 degrees Fahrenheit and a relative humidity of 65 percent. An animal’s response to heat stress is to eat less. For each pound of dry matter not consumed, 2 pounds of milk can be lost. Heat stress also can increase a cow’s susceptibility to diseases, specifically mastitis and digestive disorders, and create fertility problems as it is difficult to get heat-stressed cows pregnant, or keep them pregnant.

“The heat can impact any animal, but dairy cattle are more susceptible because of their high metabolism. Dairy cattle are already generating heat for milk production and additional environmental heat can just make them more stressful,” said Eastridge. “Heat stress can impact health, fertility, and milk production, all of which means money lost from a farmer’s pocket.”

Eastridge recommends that livestock producers use the following practices to help minimize heat stress on their dairy cattle:

  • Keep the housing area well ventilated, either by opening the sides for air exchange and/or installing fans.
  • Use sprinkler systems to help regulate body temperature. “Place sprinklers over the feeding area to encourage cattle to go to the bunk to eat,” said Eastridge. “A combination of sprinklers and fans is the best option for farmers in Ohio. Sprinklers without fans tend to increase the humidity and create additional stresses.”
  • Keep cattle in holding pens for no more than an hour, as the close proximity of the animals to each other increases heat stress. Make sure holding pens have adequate ventilation, including the use of fans with or without sprinklers.
  • Provide an adequate clean water supply — 1 linear foot for every 20 cows. Keep the water source close to the housing area and provide water to cattle immediately after they exit milking parlors.
  • Design a feeding strategy that suits hot conditions. Cut back on wet feed, increase feed with high nutrient, fat, and mineral content, and include high quality forage.
  • Don’t overgraze in pastures. Typically, the taller the grass, the cooler the pasture will be.
  • Consider feeding more at night rather than in the morning to shift heat fermentation to a cooler part of the day. The heat of digestion can place additional stress on the animal.
  • Switch to frequent feedings since high moisture products, like silage, spoils more rapidly in hot weather. This will help control illnesses.
  • Work cattle early in the morning to decrease the risk of heat stress.
  • Don’t over-exercise or transport livestock during the hottest part of the day.

For more ideas on heat abatement, please see the Dairy Herd Management Cow Comfort and Heat Abatement Resource Center, at: http://www.dairyherd.com/cowcomfort.asp?ts=cc&pgID=750&ed_id=6105

Ohio StateUniversity