Cow-Calf Commentary for Iowa Cattleman Magazine
By Randie Culbertson, Cow-Calf Extension Specialist
July 2025
Heat stress and fetal programming.
Fetal programming is a hot topic that attracts attention and is an area of research that is difficult to study, particularly for cattle. Studies examining fetal programming in cattle require multiple years to characterize the progeny’s performance through harvest or, in the case of replacement females, her reproductive life. Most fetal programming research has focused on nutrient restriction of the gestating dam and how that influences the calf's performance. More recently, research has examined the effects of heat stress on pregnant cows and its potential influence on the fetus and subsequent calf performance. So far, this data has helped bring to light the actual cost of extreme heat events in the summer, especially when cattle do not have access to shade or adequate water during the events.
First, let's take a step back and discuss what fetal programming is. Fetal programming is the theory that environmental events during pregnancy can have long-lasting effects on calf performance after birth. Often, this boils down to some form of stress experienced by the cow during gestation that affects fetal development, resulting in long-term effects on the calf's performance. A drastic alteration to a maternal environment affects fetal organ and tissue formation, which ultimately results in a “fetal programming” effect.
The majority of past fetal programming research has focused on nutrient restriction of the cow during gestation by comparing the performance of calves born to cows fed inadequate diets during gestation to the calves born to cows fed diets to meet or exceed nutritional requirements. In general, results from these studies have shown that calves from nutrient-restricted dams have a decrease in lifetime performance for weaning and yearling weights, carcass quality, and feedlot performance compared to calves from dams that were not nutrient-restricted during gestation. Heifer performance for development and reproductive success is not nearly as clear-cut. Initial research indicated that heifers from nutrient-restricted dams were able to remain in the herd longer, but more recent studies have shown this may not be the case. In short, fetal programming for heifers' long-term reproductive success is still being debated.
More recently, research has begun examining the effect of extreme heat events on fetal development. Although more of this research has been conducted in dairy cattle, there has also been some preliminary work in beef cows. Heat stress vs. cold stress has been the focus, as most spring calving herds experience breeding season during the warmest time of the year. Researchers have found that breeding during heat events has demonstrated a decrease in conception rates, especially for AI, and increases in pregnancy losses. For fall calving herds, these cows will enter their second or third trimester when exposed to heat events. These effects can begin as early as the peri-conception period and persist throughout pregnancy. Significant data in all classes of cattle demonstrate that cattle experiencing heat stress have lower intakes, and if the heat event is prolonged, this affects nutritional status. For cows, a decrease in uterine blood flow when experiencing heat stress would also impact fetal development and, ultimately, calf performance. Preliminary results in long-term studies have demonstrated that the female progeny from dams experiencing heat stress during pregnancy had an increase in age at puberty and age at first calving, as well as a higher culling rate.
In pasture-based systems, maternal environmental stress is multifactorial and cumulative. The severity of heat stress would depend mainly on the duration of exposure to heat. Fetal programming is considered an adaptive strategy to prepare the fetus for survival in the environment its dam was exposed to during gestation. If maternal stress occurs during gestation, the fetus adapts for the calf’s survival in that challenging environment. Unfortunately, this adaptation may be hindering calf lifetime performance. Nutrient restriction and thermal stressors may be an additive effect for cow herds predominately maintained in pasture systems. Most research on heat stress on cattle has been conducted on dairy and feedlot cattle, with minimal work on beef cattle in pasture systems.
Although it gets hot and humid in Iowa, we don't experience the extreme and prolonged heat of the southern regions of the U.S. Even so, when we experience those hot and humid days, we should be aware of the stress experienced by cows out on pasture. Providing shade and clean water are crucial during these times, and minimizing handling during heat events. Let's be honest; no one (even yourself) wants to be out working in the heat. And in the long run, the next calf crop will thank you.
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