Beyond cow comfort, it’s important to reduce the effects of heat stress in a dairy herd. Heat stress causes direct financial losses – and not just from reduced milk production.
“It affects reproduction substantially,” said Jon Mills, herd manager at Morrisville State College Dairy Complex. “Their internal heat rises so when they’re bred, they’re less likely to conceive. These all affect the finances of the dairy.”
Fewer conceptions over the life of an animal means fewer lactations. Mild heat stress can reduce milk production by 10%; severe heat stress can decrease it by more than 25%. And this is not only due to reduced feed intake.
Research published in September 2024 by Cornell researcher Jingyue “Ellie” Duan, assistant professor of animal science, and her team indicates that the reduced milk products in cows experiencing heat stress is only 30% – 50% a result of reduced feed intake. The researchers believe that the remaining reasons could be that heat stress affects cow metabolism and inflammatory responses, ultimately reducing milk production.
When faced with the threats of heat stress, producing milk – which is not a life-sustaining activity – goes on the back burner.
Duan’s research team also learned that the genes responsible for milk lipid synthesis and cellular differentiation suffer suppression during heat stress incidences.
High heat can stress animals’ immune systems too. Mills said this can mean they’re more susceptible to illnesses such as mastitis and respiratory disease. Cows removed from the lactating herd and placed in quarantine aren’t producing salable milk and will require rounds of medication.
Properly running fans can help reduce risk of heat stress in dairy cattle. Photo by Deborah J. Sergeant
The timing of a cow’s lactation can matter when it comes to heat stress.
“Cows’ lactation peaks at 60 to 90 days,” Mills said. “If she won’t peak during the hot months, she won’t make as much for that lactation.”
It doesn’t take extreme weather events such as multiple 90º F days to cause heat stress. Ideal temperatures for dairy cows is 50º to 60º. Temperatures that can cause heat stress are lower than you may think. Mild heat stress in cows can begin at temperatures as low as 65º to 72º with 50% humidity, according to New York State Climate Impacts Assessment, a peer-reviewed technical report by the New York State Energy Research and Development Authority. Humidity vs. “dry heat” contributes more to the risk of heat stress.
Mills said to look for evidence of heat stress by watching for signs like panting, drops in production, less estrus and – though it seems counterintuitive – bunching near water.
He encourages farmers to reduce the risk of heat stress through barns with good ventilation, high sidewalls, fans with a continuous 5-6 mph breeze and soakers.
“Fans and soakers create an evaporative cooling for cows,” Mills said. “It cools them down substantially. Make sure your fans are operating correctly.”
In addition, providing plenty of space in holding areas will help reduce risk. Provide adequate clean, fresh water from multiple access points so that every member of the herd can have a chance for hydration. Younger, smaller animals may be turned away by the presence of larger, more dominant animals.
Pastured cows need plenty of shady areas where they can rest during rumination.
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