As temperatures rise each summer, dairy producers often focus their heat abatement efforts on the cows currently paying the bills: the lactating herd. Fans turn on, soakers run and producers closely watch milk production for signs of heat stress.
But one of the biggest mistakes producers make is forgetting that heat stress impacts far more than the milking string.
Dry cows, bred heifers and even unborn calves can suffer long-term consequences from excessive heat exposure. And with heat stress events expected to become more frequent in the future, those impacts may only grow more costly.
Research from NASA’s Goddard Institute for Space Studies shows global temperatures have steadily risen over the last century, while additional research projects that most regions of the U.S. could experience six to eight additional heat stress days per decade through 2100.
For dairy producers, that means heat stress has gone from a short-term seasonal inconvenience to a long-term management challenge.
Historically, the dairy industry has used the temperature-humidity index (THI) to determine when cows experience heat stress, with most experts placing the threshold between 68 and 70. Researchers note many dairies now spend much of the summer above that range.
While lost milk production is often the most visible sign, experts say producers may be overlooking several other costly consequences.
Dry Cows and Bred Heifers Need Cooling, too
One of the most commonly forgotten groups during the summer months is dry cows and bred heifers.
“Calves born from dams under heat stress are basically doomed from the start,” says Bethany Dado-Senn, calf and heifer technical specialist with Vita Plus
Research continues to show that heat stress during late gestation affects both the dam and her developing calf. According to Carly Becker, former dairy extension educator with Penn State Extension, late gestation is a critical developmental window because the fetus gains roughly 60% of its birth weight during that time.
“If the dam is under heat stress, that calf is growing in a nutrient-restricted environment,” Becker says.
Studies have shown calves exposed to heat stress in utero are often born lighter, struggle with immune function and may experience reduced growth for months after birth. Researchers have also linked in-utero heat stress to smaller mammary gland development, lower feed intake and reduced lifetime milk production.
Unfortunately, the effects may not stop with one generation.
“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.”
Despite these risks, dry cows and bred heifers are often kept in barns or outdoor lots with less airflow, shade and cooling than the lactating herd.
That oversight can carry major financial consequences. Research has shown heat-stressed dry cows may produce 6.6 to 16.5 pounds less milk per day during the following lactation.
Heat Stress Impacts More than Milk Production
Milk loss is often the first thing producers notice during hot weather, but heat stress affects nearly every aspect of cow health and performance.
Research has linked heat stress to reduced dry matter intake, poorer reproductive performance and increased disease risk. Pregnancy rates commonly decline during warmer months, while cows calving during periods of heat stress face greater odds of retained placentas, ketosis, displaced abomasums and mastitis.
Other studies have reported higher somatic cell counts and increased mortality during prolonged heat waves.
Heat stress can also create welfare concerns. Cows experiencing heat stress tend to spend more time standing in an effort to dissipate heat, which may contribute to lameness issues. Researchers have also observed elevated cortisol levels in heat-stressed cows, signaling increased physiological stress.
Considering the widespread impacts on production, reproduction and health, the economic losses quickly add up. One estimate placed annual livestock industry heat stress losses at $2.3 billion annually, with the dairy industry accounting for more than half of those costs.
Researchers note the true economic impact may actually be underestimated once long-term calf performance and multigenerational losses are considered.
Cooling Strategies Matter During the Dry Period
Experts say cooling dry cows and bred heifers should no longer be considered optional.
“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.”
Simple cooling strategies can make a substantial difference, including:
- Fans and tunnel ventilation in dry cow pens
- Sprinklers or soakers during peak heat hours
- Shade for outdoor animals
- Unlimited access to clean, fresh water
- Reduced stocking density to improve airflow
- Feeding during cooler times of the day to support intake
Research has shown cooling strategies such as fans, shade and soakers improve thermoregulation and productivity not only in cows, but also in heifers and calves.
Nutrition May Provide Another Layer of Protection
While cooling infrastructure remains the foundation of heat stress management, nutrition may offer additional support during periods of unavoidable heat exposure.
According to Dado-Senn, rumen-protected methionine has gained attention for its ability to support metabolism, immune function and calf development during late gestation.
“Calves born to dams fed rumen-protected methionine often exhibit better growth, improved feed efficiency and enhanced energy metabolism,” she says.
Rumen-protected choline has also shown promise in improving calf growth and feed efficiency.
Other nutritional strategies being explored by researchers include chromium supplementation, yeast products and other feed additives designed to support cows under heat stress conditions.
Researchers are also evaluating genetic traits associated with improved thermotolerance, including the SLICK haplotype, which has been linked to lower body temperatures and improved heat tolerance in Holstein cattle.
Protecting the Future Herd
As summers continue to get hotter and heat stress events become more common, dairy producers may need to rethink how they approach heat abatement across the entire herd.
While lost milk production is often the first visible sign of heat stress, the hidden impacts can show up months or even years later through poorer reproduction, weaker calves, reduced lifetime performance and increased health challenges.
Research continues to show that cooling dry cows and bred heifers is an investment in the future herd. Providing shade, airflow, water access and proper nutrition during late gestation can help support calf growth, improve immunity and protect future milk production.
Heat stress management is no longer just about helping cows stay comfortable during the summer months. It is about protecting the long-term productivity, health and success of the herd.
