WINNEBAGO COUNTY, Ill. — A March 30 paper published by the University of Illinois (U of I) College of Agriculture, Consumer and Environmental Sciences (ACES) seems to confirm that extreme heat affects milk production quantity and, in some cases, quality. 

Drawing from data from 22 different climate models, U of I researchers projected potential U.S. milk volume losses forward to 2050. Most weather models indicated that extreme heat days will occur much more frequently, causing milk yield losses to increase as much as 30 percent by midcentury.

“If there’s interest in continuing to have a healthy and robust small dairy production presence in the U.S., that probably will require financial incentives to help farmers implement mitigation strategies, as well as investments in further research on how to manage the highest levels of heat stress,” according to U of I professor and study co-author Marin Skidmore, whose study concluded that smaller producers suffer from heat-related milk loss more than larger producers.

Winnebago County dairy farmer Brent Pollard has experienced milk loss volume from his cows as a result of heat stress. The northern Illinois farmer takes precautions to control the amount of time his cows spend in stressful environments.

“As a dairy farmer we see milk loss every year from heat stress. We do things like provide sprinklers over the feed lines and fans throughout our barn, but it still isn’t enough to offset the environmental effects on our cows. Having those heat maintenance procedures helps, but if you look at a dairy cow…it is just a big furnace,” said Pollard, who is an Illinois Farm Bureau board member and also serves on the board of the Illinois Milk Producers Association. 

While reduced milk quality can be associated with heat stress it tends to be more of an issue for southeastern U.S. dairy producers, Pollard noted, though there is likely some slight deterioration in the Midwest during summer months. “The one thing that we do see is that the amounts of protein and fat (in milk) will go down in the summer, and that is the first thing that we as dairy farmers get paid on,” he said. “It’s more of a quantity (issue) than a quality one with us.”

Taking heed of global warming forecasts, Pollard is being proactive in preparing for the coming years of milk production on his farm. He’s especially concerned about the trend of warmer evenings in Illinois and the Midwest. 

“Here in Illinois we have been experiencing some very warm summer evenings with temperatures in the barn staying in the 80s overnight. This can be very stressful to the cows when they don’t have adequate time to cool down at night, and can affect milk output. Our cows start feeling heat stress at about 65 degrees,” he said.

Pollard intends to install larger fans in his barn before the end of April, an upgrade he’s been planning for around two years. He believes it’s the little things — like increasing air circulation in the summer — that can help restrict milk volume loss to heat stress. Pollard consults with researchers at the U of I and University of Wisconsin-Madison to stay up to date on animal welfare and cow comfort. 

Pollard believes that dairy cattle in the southwest and warmer regions of the U.S. possess a greater innate ability to adjust and stay adjusted to seasonal weather patterns, while Midwest cattle often face a roller coaster of temperature and weather fluctuations. “They know what the weather is going to be, where our weather tends to see-saw up and down. That recent 80-degree day had our cows freaking out; they don’t know if it’s time to adjust to being hot or if it’s going to be cold. It kind of tricks them,” he said. 

For the study, Skidmore and her colleagues analyzed milk production data from nine U.S. Midwest states, including over 56 million cow-level production records from 18,000 dairy farms from 2012 to 2016. The researchers adjusted the milk data for protein and fat content to more accurately estimate milk quality, which determines price.

“Previous studies have focused on fluid milk yield. But in our dairy marketing system, milk is sold on components. When you calculate revenue, it’s not just about how many gallons of milk, but whether it’s high-quality milk with high protein and fat content,” Skidmore said.

The U of I study found that, on average, 1 percent of annual milk yield is lost to heat stress. This amounts to about 1.4 billion pounds of milk (adjusted for energy content) over five years for the 18,000 herds included in the study. Based on average milk prices, this is equivalent to about $245 million in lost revenue, the researchers concluded.

Looking further, the study determined that heat stress disproportionately affects smaller farms. Herds of fewer than 100 cows lost an average of 1.6 percent of annual yield. While supplying less than 20 percent of total volume output in the sample, smaller producers represented 27 percent of total heat loss damages.

The study, “Vulnerability of US dairy farms to extreme heat,” is published in Food Policy [DOI:10.1016/j.foodpol.2025.102821]. Funding was provided by a Center for the Economics of Sustainability Seed Grant.