Keeping cows in their thermoneutral zone – 41 and 68 degrees – can minimize the impact of seasonal changes on milk production, reproduction, feed efficiency and income-over-feed cost, said John Smith, a dairy science Extension specialist in the Department of Animal Sciences and Industry at Kansas State University. Speaking at the Cornell Fall Dairy Conference last year, Smith pointed out the negative and well-documented impacts of heat stress to a dairy’s bottom line. “When daily milk production is reduced 2 to 12 pounds per day per cow, the gross income loss related to heat stress ranges from $32.40 to $324 per cow,” Smith said.
Heat stress not only lowers milk production during summer months but also reduces the potential for future milk production. “For every pound of peak milk production that is lost, an additional 250 pounds of production will be lost over the entire lactation,” Smith said.
What’s going on with cows during heat stress? When housed in an environment beyond their thermoneutral zone, they adapt by altering their behavior and physiology, Smith said. “These adaptations are necessary to maintain a stable core body temperature, but they effect nutrient utilization and profitability on dairy farms.”
When temperatures exceed the upper limit of lactating dairy cows’ thermoneutral zone – estimated to be 70 to 80 degrees – cows decrease feed intake, sweat and pant, Smith said. “These mechanisms increase cows’ energy costs, resulting in up to 35% more feed necessary for maintenance. When dry matter decreases during heat stress, milk production also decreases.”
Reproduction suffers greatly under heat stress. “Even short-term rises in body temperature can result in a 25 to 40% drop in conception rate,” Smith said. “An increase of .9 degrees in body temperature causes a decline in conception rate of 13%.”
Controlled environment
What options do dairies have to mitigate heat stress? Fans, misters or soakers, and tunnel ventilation are some of the typical mechanical solutions. Smith talked about another option – low-profile cross-ventilated (LPCV) freestall barns.
With fans along one side of a LPCV freestall barn and an evaporative cooling system on the facing wall, these barns can “reduce the variation in the core body temperature of cows by providing a stable environment,” Smith said. This is key to lessening the impact of heat stress.
“These facilities allow producers to have some control over the cow’s environment during all seasons of the year and maintain an environment closer to the thermoneutral zone of the dairy cow in both summer and winter, resulting in more stable core body temperatures,” he said.