Hot weather does more than reduce the amount of milk cows produce; it also lowers the milk’s nutritional quality by decreasing its fat and protein content, according to a new study published in the journal Environmental Research Letters.
This is a fresh angle on how climate change hits dairy farms. The study shows that money lost from weaker milk quality is roughly as large as money lost from lower milk volume. Put those two losses together, and the total damage nearly doubles. In the United States, farmers get paid based on how much fat and protein their milk contains, so this quality drop hits their income directly.
“The heat-driven thinning of these valuable milk components is happening unnoticed,” said Ariel Ortiz-Bobea, the study’s senior author and an associate professor at Cornell’s Dyson School of Applied Economics and Management. He explained that once you factor in the quality loss, the total economic damage sits in the same range as the yield loss alone.
How the Study Worked
The research team looked at records from about 6.5 million cows between 2007 and 2016. They matched this milk data with local weather readings, mapped down to areas just 2.5 miles wide, across 43 states.
Jeisson Prieto, a doctoral student and the paper’s lead author, worked with co-authors Christopher Wolf, Kristan Reed, and Ziyi Lin. Prieto spent six months developing a method to handle the extensive dataset, which included 120 million milk records from the U.S. Council on Dairy Cattle Breeding. He used computing power from the Cornell Center for Social Sciences to run his models.
The team confirmed something researchers already suspected: milk volume drops sharply once heat and humidity pass a certain point. However, milk quality presents a contrasting narrative. Fat and protein levels start slipping at much milder temperatures, and the decline grows as it gets hotter.
“On a day in the 60s or 70s, yield stays normal,” Ortiz-Bobea said. “But the milk is already starting to get diluted.” Unlike the volume drop, which mainly happens during summer heat waves, this quality decline happens year-round.
The Cost to Farmers
The researchers measured the financial impact using a temperature-humidity index. A 10-point rise on that index led to a 1.2% drop in milk volume. But it caused a 2.8% drop in total revenue over a year. In total, the industry has incurred approximately $1.65 billion in losses. Dairy makes up roughly one-fifth of all U.S. animal-based farm products.
“It’s another headwind for dairy farmers,” Ortiz-Bobea said. He noted that low milk prices are already squeezing farmers, and added pressure like these conditions tends to push smaller operations out of business. That, in turn, reshapes rural communities both economically and physically.
Cows Aren’t Adapting Much
The study found little sign that cows are becoming more tolerant of heat over time. Age, farm size, and region made almost no difference in how animals responded to hot conditions. The one real adjustment has been geographic: farmers have shifted dairy operations toward cooler northern states.
“This is why so many cows are raised in places like New York and Wisconsin,” Prieto said. He added that heat still matters everywhere, and its effects on both quantity and quality could grow more serious in coming years.
Looking Ahead
Ortiz-Bobea hopes the findings push the dairy industry to widen its research priorities. Much of the past innovation has centered on boosting output alone, he said, even as sensitivity to heat may be quietly increasing along with it.
The team wants access to more detailed, cow-by-cow data going forward. With daily records for individual animals, they hope to spot which cows naturally handle heat better. That could reveal whether there’s a trade-off between high output and heat tolerance, insights they hope can guide breeding and management decisions across the industry.
The study was funded by the USDA’s National Institute of Food and Agriculture and the National Science Foundation’s AI Research Institutes program.
