When dairy producers think about heat stress, milk production, reproduction and feed intake often come to mind first. But research continues to show the cost can stretch beyond the cow standing in the pen today and into the next generations of the herd.
For Jimena Laporta, Ph.D., associate professor and lactation physiologist at the University of Wisconsin-Madison, heat stress during the dry period is no longer only a concern for the dam. Her work shows it can affect her daughters, granddaughters and even great-granddaughters.
“We now know that’s only part of the story, and there’s a lot more to heat stress than lactating cows,” Laporta says. “There’s more to heat stress than the summer months.”
Her research has followed calves born to dry cows exposed to heat stress during late gestation. Those daughters produced less milk, faced more health challenges and left the herd sooner than daughters from cooled dry cows.
“I think we have to start thinking about heat stress with that mindset,” Laporta says. “It’s an investment not just in our lactating cows, but also in the dry cow developing the future replacement.”
How Early Exposure Changes the Calf
Much of Laporta’s work began with controlled studies in Florida. During the final 50 days of gestation, one group of dry cows received cooling through fans and soakers, while another group experienced hot, humid summer conditions. Researchers then followed their offspring through multiple lactations.
The effects did not end after first calving.
“We’ve seen detriments in the first lactation, the second lactation and even the third lactation,” Laporta says. “When we start seeing second and third lactation differences, that’s where we start wondering what in utero heat stress is really doing to that animal.”
Late gestation is a period when fetal organs are still developing. The calf may be fully formed, but tissues continue to grow and respond to the environment inside the dam.
“If the mom is heat stressed, they are going to be heat stressed in utero,” Laporta says. “Those cells are sensing a harsh environment and changing their trajectory.”
Researchers have documented differences in several organ systems. Calves exposed to heat stress before birth are born smaller, absorb fewer nutrients, show altered liver function and have changes in mammary gland development. Laporta’s team has also found structural differences in sweat glands, suggesting those calves may regulate body temperature differently throughout life.
“They start life as a different animal with different tools,” she says.
Not Only a Southern Concern
Although many producers connect heat stress with southern climates, Laporta says dairy farms across the Midwest also need to pay attention. Midwest cows may get more nighttime relief, but stretches of high temperature and humidity can still create enough stress to affect dry cows.
“The dry cow is more resilient, but that doesn’t mean she won’t be impacted,” Laporta says.
Her team recently launched research in Wisconsin to learn whether shorter or less severe bouts of heat stress can create similar long-term effects.
Early results suggest even milder heat stress during late gestation can reduce milk production in daughters.
“It was rewarding to see even a shorter period of time in late gestation caused differences in first-lactation milk production,” Laporta says. “That period is very critical in development.”
Effects that Continue Down the Line
The research reaches beyond daughters. Laporta says granddaughters and great-granddaughters have continued to show biological differences even though they never experienced the original heat stress event.
“So far, I can tell you that heat stress insult from Florida is leading to these generational effects,” she says. “The daughters, the granddaughters and the great-granddaughters.”
Part of the explanation may begin during fetal development. While a dry cow carries her developing daughter, the germ cells that can later create future granddaughters are already present.
“Not a lot of people think about a dry cow having multiple generations inside her,” Laporta says.
Although production data for great-granddaughters remains limited, researchers continue finding changes in mammary tissue, sweat glands and molecular pathways involved in development.
“It’s fascinating,” Laporta says. “It’s complex, but it’s really interesting.”
As researchers continue studying how heat stress alters fetal development, one message is already becoming clear: cooling dry cows today may pay off in dairy cattle for years to come.
Nutrition Adds Another Tool
Cooling dry cows with fans and soakers remains the first defense against heat stress. Researchers are also looking at whether nutrition can help reduce some of the damage that occurs before a calf is born.
“When it comes to heat stress, our first line of defense is going to be heat abatement and management strategies like cooling with fans or sprinklers,” says Danielle Sherlock, ruminant innovation and customer success manager with Adisseo. “Nutrition could also play a role, and it’s kind of our second line of defense against heat stress.”
That question led Adisseo and the University of Wisconsin-Madison to study whether supplying additional methionine to heat-stressed dry cows could improve outcomes for their calves.
Methionine is an essential amino acid involved in protein synthesis, immune function and cellular growth. Researchers wanted to know whether it could support fetal development when cows face heat stress.
While heat stress changes how fetal cells develop, methionine may help regulate some of those cellular processes.
The goal is not to replace cooling systems, Laporta says, but to give producers another management option when cows face heat stress.
“We 100% need to continue cooling cows,” she says. “Whether it’s nutritionally or with mechanical cooling, we have to think about all the impacts heat stress has on the herd as a whole.”
Researchers studied 60 transition cows divided into three groups: thermoneutral cows, heat-stressed cows and heat-stressed cows receiving supplemental methionine. Cows were monitored through the transition period, and researchers continued following their heifer calves after birth.
Heat stress reduced milk fat and protein percentages, affected liver function and altered placental development. Supplementing methionine did not eliminate those effects, but it helped cows adapt during the transition period.
“What methionine seems to be doing is really helping them transition during that stressful time,” Laporta says.
Benefits for Cows and Calves
In the mammary gland, researchers found methionine increased the number of milk-producing cells and alveoli. It also improved pathways involved in protein synthesis, helping the gland maintain milk protein production despite heat stress.
“We see this rescue of milk protein percentage in those animals,” Laporta says.
The benefits reached beyond the mammary gland.
“We see improvements in antioxidant capacity not only in the liver but also in the mammary gland and in the placenta,” Laporta says. “That calf is getting some beneficial effects in utero as well.”
Sherlock says those biological changes also had economic value. Although increases in fat and protein yield were numerical rather than statistically significant, heat-stressed cows supplemented with methionine lost considerably less component revenue than unsupplemented cows.
“We’re really helping mitigate that economic loss,” Sherlock says.
Researchers also followed heifer calves born from each treatment group. As expected, calves born to heat-stressed dams were smaller, absorbed fewer immunoglobulins from colostrum and grew more slowly before weaning. Heat stress also reduced placental size and altered its structure, limiting nutrient delivery to the developing fetus.
“The placenta is setting those heifers off for a good start,” Laporta says. “The methionine rescue of placental function is a really important one that we need to take a closer look into.”
Supplementing methionine did not erase every consequence of heat stress, but researchers observed improvements in calf growth and early development.
Those benefits also appeared later in life. Previous research from Laporta’s group found daughters born to heat-stressed dry cows produced about 2,000 pounds less milk during their first lactation. In this study, daughters whose dams received methionine produced roughly 300 pounds more milk than daughters from unsupplemented heat-stressed cows.
“It’s important to see methionine is helping that calf during early life to grow better and produce slightly more milk in her first lactation,” Laporta says.
For producers, those findings point back to dry cow management. Cooling remains the foundation, but nutrition may add support when heat stress challenges the dam and the calf she is carrying.
“Whatever you’re doing to improve that dry period is going to have a two-way benefit,” Laporta says. “It benefits the cow’s next lactation and the fetus developing inside her.”
Looking Beyond Today’s Losses
Laporta says the research reinforces a message producers can overlook during hot weather: do not forget the dry cows.
“It’s not about one or the other,” she says. “It’s more of a holistic approach to heat stress.”
Cooling systems remain the base for protecting cows during high heat and humidity. Nutritional strategies may also deserve consideration as researchers continue learning how heat stress changes fetal development.
She says the work also shows the need to think beyond immediate production losses.
“We’re doing a lot of work now in newborn calves and the preweaning phase because those animals are still growing and developing,” Laporta says. “We’re excited to work in that area as well.”
For producers, the message grows clearer with each new study. Decisions made during the dry period do not just influence the next lactation. They can shape how replacement heifers grow, perform and remain in the herd years later.
As summers bring more frequent periods of heat stress, Laporta hopes producers give dry cows the same attention they already devote to the milking herd.
“Start thinking about your lactating cows, but also don’t forget about your dry cows,” she says. “Whether it’s nutrition approaches or cooling approaches, every improvement you’re making today has the potential to benefit future generations.”
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