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 72⁰F with 45% relative humidity, or 80⁰F 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.
Dry cows are also negatively affected by heat stress. Heat-stressed dry cows produce 1,000 to 2,000 pounds less milk during the next lactation. Thus, proper management practices, facilities, and, to a lesser extent, nutrition are needed to mitigate the effects of heat stress not only in milking dairy cows but just as importantly in dry cows.
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. When dairy cows are subjected to increased environmental temperatures 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 freestalls, 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.
- 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 freestall barn. The recommended distance between fans is 30 feet for 36-inch fans and 40 feet for 48-inch fans (Gay, Virginia Tech Extension Engineer, Pub 442-763)
- 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 be with fans and sprinklers operating will help cool dry cows.
environmental modifications can help mitigate the effects of heat stress on dairy cows and should be implemented before the effects of heat stress are noticed. These modifications are needed not only for the milking herd but just as importantly for the far-off and close-up dry cows.