As thermometers start to boil, dairy producers often turn their attention to the cows who are paying the bills: the lactating herd. But research shows that dry cows and bred heifers are just as vulnerable to heat stress, even if the signs are less visible.
While it’s easy to overlook this group of animals during the hustle and bustle of summer, doing so can quietly set off a chain reaction. When a dry cow or bred heifer experiences heat stress, her unborn calf does too.
Carly Becker, a former dairy extension educator for Pennsylvania State University, and Bethany Dado-Senn, a calf and heifer technical specialist at VitaPlus, weigh in with practical advice and insights on why cooling dry cows matters more than ever.
Lifelong Consequences for Calves
Numerous studies have shown that the effects of in-utero heat stress can be severe, starting at birth and continuing throughout a calf’s life.
“Calves born from dams under heat stress are basically doomed from the start,” Dado-Senn says. “They’re born smaller, have smaller mammary glands and are more likely to leave the herd due to either fertility or health issues. If they are fortunate enough to make it to the lactating herd, they’re going to make less milk across three lactations.”
That means the heat of one summer can ripple through a herd’s productivity for years.
Becker explains that these calves often face reduced placental function, are typically born about 8.8 lbs. lighter and struggle with compromised immunity due to failed colostrum transfer.
“Late gestation is a time when the fetus gains 60% of its birth weight,” she says. “If the dam is under heat stress, that calf is growing in a nutrient-restricted environment. Plus, these calves are often born early, which can further lower their chances of thriving.”
Unfortunately, the setbacks don’t stop there. In-utero heat-stressed calves have been shown to consume less feed, require more tube feedings and experience delayed growth well into their first year — all of which increase the cost to raise them and delay first calving.
The long-term impact can even extend into future generations.
“We know that this is a transgenerational problem as well,” Dado-Senn says. “The granddaughters born from calves who experienced heat stress in utero will also make less milk and have impaired mammary development. This severe stress can perpetuate across three to six lactation cycles and really get us off on the wrong foot.”
The Case for Cooling Dry Cows and Bred Heifers
While the lactating herd often gets the fans, misters and extra attention, dry cows and bred heifers need cooling just as much, especially those in the final weeks of gestation. Shade, ventilation and water access are baseline necessities, but sprinklers or soakers and tunnel ventilation systems in dry pens can pay big dividends.
“The dry period is a critical 45- to 60-day window,” Becker explains. “It’s when the cow is regenerating her mammary tissue, maintaining body condition and growing her calf. Yet too often, dry cows are left in less-ventilated barns or out in lots with little to no cooling.”
That oversight comes with a cost. Research shows heat-stressed dry cows produce 6.6 to 16.5 lbs. less milk per day in the following lactation and often face more health challenges after calving.
To help combat heat stress in dry cows and bred heifers, consider the following strategies:
- Install and maintain fans or tunnel ventilation in dry pens and holding areas to keep air moving.
- Use sprinklers or soakers on timers during peak heat hours to help cows dissipate heat effectively.
- Provide ample shade, especially for animals housed outdoors. Natural tree lines, shade cloths or roofed shelters can offer relief.
- Ensure unrestricted access to clean, fresh water. As temperatures rise, so does a cow’s water demand — sometimes doubling intake on the hottest days.
- Manage stocking density to reduce crowding, which increases heat load and reduces airflow around animals.
- Feed during the cooler parts of the day, like early morning or late evening, to encourage dry matter intake and minimize heat-related feed refusals.
Nutrition Plays a Role, Too
In addition to physical cooling strategies, nutrition can offer another line of defense against the effects of in-utero heat stress. According to Dado-Senn, one nutritional tool gaining traction is the use of rumen-protected methionine during late gestation. Methionine, an essential amino acid, is often the first limiting nutrient in dairy rations, especially during periods of heat stress when cows’ metabolic demands shift.
“Methionine supports the cow’s metabolism, reproductive function and immune health, but it also has direct benefits for the unborn calf,” Dado-Senn says. “Calves born to dams fed rumen-protected methionine often exhibit better growth, improved feed efficiency and enhanced energy metabolism. We also see developmental advantages like stronger intestinal growth, more favorable DNA methylation patterns and improved resilience to stress after birth.”
This can be particularly valuable when heat stress cannot be fully avoided. “For example,” she adds, “if a dam experiences heat stress but is supplemented with methionine, her calf may still be born with better thermal regulation and stronger postnatal growth.”
Rumen-protected choline is another promising nutritional intervention with similar benefits.
“When supplemented during late gestation, choline has been shown to improve calf growth, feed efficiency, and circulating metabolites,” Dado-Senn says. “In one study that included both beef and dairy calves, increased maternal choline intake, particularly in beef bull calves, was associated with higher weaning weights and better feed efficiency.”
While no nutritional strategy can replace the need for adequate cooling, these tools offer added insurance to help ensure calves are born with a stronger start.
Help Eliminate Heat Stress’s Silent Toll
Though heat stress during the dry period can often go unnoticed, its hidden costs run deep. Calves developing in heat-stressed dams typical arrive smaller and weaker, carrying challenges with them from birth that slow growth, delay breeding and reduce milk production over their lifetime.
Cooling with fans, shade and fresh water are easy ways to help minimize discomfort, yet nutrition can play a powerful role as well.
As Becker emphasizes, “Investing in dry cow and bred heifer cooling isn’t just about comfort. It’s about protecting your future herd and setting calves up for success.”
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