Remember the first warm day of spring? Sixty-eight degrees can feel downright heavenly.
But if you’re a high-producing dairy cow, 68 degrees F is beginning to get uncomfortable. In fact, recent research shows that it’s at this threshold — not the previously thought 72 degrees F — that cows can begin to experience the effects of mild heat stress.
Since heat stress zaps $5 to $6 billion in lost production and animal performance from U.S. dairy farmers annually, new ways to combat this serious challenge are worth a second glance.
Here’s why you need to take a look at cooling cows sooner, rather than later.
Different cows, different times
For years, the dairy industry has relied on a temperature- humidity index (THI) to help determine when cows begin to experience heat stress and to calibrate cooling programs.
According to the traditional THI, cows can begin to show the effects of mild heat stress at 72 degrees F, depending on humidity.
But, cows today produce a lot more milk than they did when these tables were first introduced.
The original THI was developed from analyses of milk production and cow response to thermal stress between 1955 and 1965, says Robert Collier, professor of environmental physiology in the Animal Science Department at Arizona State University. “Cows then averaged between 35 and 50 pounds of milk per day,” he explains. “That’s much different from the cows in our study that averaged around 80 pounds of milk per day.”
Since increases in production increase a cow’s sensitivity to heat stress, it’s easy to see that it’s time to update this concept.
The THI is also criticized for its timing. Collier and his colleagues note that the time interval between stress initiation and milk yield measurement in the original THI analysis was two weeks, and there was no clear definition of the time required at a given THI to induce milk yield loss, other than two continuous weeks of heat stress. That lack of data or clear-cut parameters just doesn’t fly in today’s competitive dairy environment.
Furthermore, the original THI does not account for radiant energy and any cooling by convection.
So, researchers took all of these factors into account and followed the effects of heat stress on today’s modern dairy cow.
They found that cooling high producing cows at 68 degrees F makes cows more comfortable and more productive than following the original THI. And it makes economic sense. That is, the milk production gained (or losses limited) at this temperature compensate you for the cost of cooling.