Editor’s note: The following answer is provided by Donna M. Amaral-Phillips, extension dairy specialist at the University of Kentucky.
Q: What are the important dairy feeding and management considerations during heat stress?
A: Heat stress results in decreased milk production, reproductive performance, and immune function in both milking and dry dairy cows. Both environmental temperature and humidity impact the amount of heat stress that dairy cows undergo. Recent research has shown that milking dairy cows start to decrease milk production when the temperature-humidity index (THI) exceeds 68 (i.e., temperature of 72°F with 45 percent relative humidity, or 80°F with no humidity) and not 72 as shown in previous research with lower-producing dairy cows. The detrimental effects on the estrus expression, conception rates, and early embryo survivability occur before declines in milk production are observed and may occur at a temperature-humidity index as low as 55 to 60. Generally, the maximum declines in milk production as a result of heat stress are not seen until 36 to 48 hours after the initial heat stress event. Older dairy cows seem to be more severely affected compared to younger cows, and not all cows respond to heat stress in a similar manner.
To maintain normal metabolism, a cow’s core body temperature needs to remain relatively constant. In addition, core body temperature must be slightly higher than the ambient temperature to allow heat to be transferred to the external environment. Heat is generated from the digestion of feeds and nutrient metabolism. When dairy cows are subjected to increased environmental temperature and/or humidity outside their thermal neutral zone, the cow’s environment must be cooled to allow this heat exchange between the cow and her environment to occur and to prevent, or at least minimize, increases in a cow’s core body temperature. By providing dairy cows shade, increased ventilation, and cooling of the surrounding air by fans alone or in combination with sprinklers, dairy cows are better able to minimize the detrimental effects of heat stress on milk production, reproduction, and their immune system.
Some key points to remember include:
- Fans over free-stalls, in the housing area, and over feed bunks should be automatically programmed to turn on when the temperature and humidity reach a THI of 68 (i.e., temperature of 72°F with 45 percent relative humidity, or 80°F with no humidity).
- In more humid climates, fans should be used in combination with sprinklers (nozzles need to deliver 0.5 gallon/minute of water, 20 to 40 pounds/square inch of pressure [psi]) which will wet the hair coat of cows. Sprinklers should generally be on for 1 to 3 minutes, then off for the remainder of a 15-minute cycle. The length of time sprinklers run increases with increasing temperature. Fans should run continuously. (Janni, University of Minnesota Engineer, Evaporative systems for cooling dairy cows)
- Fans and sprinklers (in humid environments) should be used in the holding pen to cool cows waiting to be milked, and time in the holding pen should be kept to a minimum.
- Adequate number of fans should be spaced at about 12 feet high along the length of the free-stall barn. The recommended distance between fans is 30 feet for 36-inch fans and 40 feet for 48-inch fans.
- Check fans to make sure they are angled correctly (20-degree angle) and are operating properly. Fans also should be cleaned regularly.
- Minimize cow movement, and work dairy cows and heifers during the coolest part of the day.
- If facilities housing far-off and close-up dry cows do not allow for cooling, an hour in the holding pen with fans and sprinklers operating will help cool dry cows.