I like summer. I enjoy those sunny, 80oF plus days. A dairy cow has a completely different perspective. Most of our dairy cows prefer cooler temperatures. At a temperature-humidity index (THI) of 68, a dairy cow will begin to show a decrease in milk production as a result of mild heat stress. This THI of 68 can be reached with an air temperature of 72oF and 45% humidity, 75oF and 20% humidity, or 80oF and 0% humidity. Researchers at the University of Arizona have found that cows with high milk production may be even more sensitive to heat stress. This is due to the fact that a cow’s metabolic heat output is increased as milk production increases. In a paper entitled “Quantifying Heat Stress and Its Impact on Metabolism and Performance”, the authors state that “…milk yield losses become significant when minimum THI on any given day is 65 or greater.” Their research showed that when the minimum THI was between 65 and 73, milk yield loss was almost 5 lb/cow/day.
Beyond the decrease in milk production, there are other detrimental impacts of heat stress on dairy cattle. According to an article posted on the eXtension dairy web site (http://www.extension.org/dairy_cattle), decreased rates of estrus expression, conception, and early embryo survival can be documented at THI levels as low as 55 to 60. Dairy cows that are heat stressed spend more time standing as compared to cows that are not experiencing heat stress. Increased standing time means less time lying down, which negatively impacts milk production, and in addition, there is speculation that increased standing time may contribute to higher incidences of lameness. When dairy cows experience heat stress during their dry period, fetal growth can be reduced and milk production in the next lactation can be decreased by 1000 to 2000 lb.
There are management practices that can help to reduce heat stress for dairy cattle and minimize the detrimental effects of heat stress. Some of those practices include:
- Provide shade.
- Increase ventilation and cool the surrounding air through the use of fans or a combination of fans and sprinklers. Sprinklers should be used as humidity levels increase. Sprinkler nozzles should deliver 0.5 gal/minute at 20 to 40 psi pressure. The goal is to wet the hair coat of the cows without water running down to and draining off the udder.
- According to a Virginia Tech publication, fans should be spaced 30 feet apart for a 36 inch fan and 40 feet apart for a 48 inch fan. Fans should be at a 12 foot height and positioned at a 20 degree angle.
- Keep time in the holding pen to a minimum for cows before they enter the milking parlor and include fans and sprinklers in the holding pen.
- Make sure the ration includes high quality forages that can help to decrease the heat of fermentation. Take a look at the mineral content of the ration; it may need to be adjusted to compensate for sweating that occurs when cows are heat stressed. There may be other products, such as yeast cultures, that could be added to the ration that may benefit cows under heat stress, especially high producing cows in early lactation.
More information about these practices is available in the eXtension article entitled “Dairy Feeding and Management Considerations During Heat Stress” that is available on line at: http://go.osu.edu/managedairyheatstress.
The overall impact of heat stress on a dairy cow is determined not only by the THI but also the duration of the heat stress. Summer brings us extended periods of warm (some would say hot!) temperatures and increasing humidity. Make sure you have a strategy to manage heat stress for your dairy cows.