Extension Notebook: Summer’s thermal heat stress manifesting in livestock

The weather this year has been interesting. At Dixon Springs Ag Center, there were several days of one or more inches of rain and even a 72-hour period where 11 inches of rain fell. Shortly thereafter the rains stopped and a moderate drought ensued. Subsequently the heat bore down with the normal oppressive humidity. Normally the heat and humidity is shrugged off, but this year there was no nighttime cooling for humans or animals.

Fast forward to fall calving. In southern Illinois reports of lightweight calves (some as small as 35 pounds), early calves, abortions, stillborn, and long gestation (20-30% of heifers had not calved by their AI calving date). Also at the fall herd pregnancy check, more cows are being called open than normal.

What could be the cause all these poor outcomes? Heat stress appears to be the primary cause. During the summer, heat stress was impacting our livestock. Heat stress is generally through of the interaction of temperature and relative humidity but also includes solar radiation and air movement. From June through August the humidity in most Illinois was muggy to oppressive to actually miserable on many days.

For example, Weather Spark — https://tinyurl.com/3dde4drm — illustrates the hourly reported temperatures at the Mt. Vernon Airport. Starting in late May, temperatures began to rise, but by June/July and through August, temperatures from 9 p.m. through 6 a.m. remained warm (75 to 85 degrees) or comfortable (65 to 75 degrees). However, the comfortable designation can be a little misleading, especially when considering livestock; temperatures on the higher end do not allow livestock to cool down overnight before the return to daytime hot temperatures.

Data provided by Illinois State Climatologist Trent Ford depicts the average temperatures and temperature departures for summer 2025. The vast majority of the state averaged more than 1 degree warmer than average; however, there are areas in the state where the temperature departures were more than 3 degrees warmer than normal. These increased temperatures and lack of cooler evenings took a toll on our livestock.

So why is it important for livestock to cool down overnight? There is an abundance of data that demonstrates the effects of heat stress on livestock — impacts on reproduction, fetal development and the livestock themselves. Maternal heat stress will trigger numerous physiological and behavioral changes aimed at decreasing core heat production (e.g. reducing feed intake) and increasing heat dissipation to the environment (standing in ponds). Additionally maternal heat stress may indirectly impact the developing fetus through intrauterine growth restriction — causing impaired development during gestation resulting in long-term negative postnatal development. Regardless of postnatal care, permanent lifelong changes occurred in utero that can never be reversed.

What kinds of perturbations can occur when livestock alter their physiology and/or behavior, or are there other heat-stressed induced effects? In males, high ambient temperatures affect reproductive hormones — reduced secretion of pituitary luteinizing hormone and testosterone, reduced sperm production, and ultimately decreased male fertility rates. Heat stress affects females at various stages of their physiological development and almost all reproductive events — secretion of hormones, altered estrous cycle, folliculogenesis, oogenesis, oocyte competence, fertilization, implantation, embryonic growth and thus pregnancy.

There a numerous articles describing the impact of thermal stress on lactating dairy cows — decreased feed intake, altered metabolism, compromised lactational performance, increased disease incidence, and impaired reproductive performance. Thus, extrapolating to late gestation beef cattle, reduced feed intake could impair metabolism which would affect lactation, immune function, and breed back. But importantly, fetal development would also be impaired due to lack/reduction of nutritional intake by the cow (not to mention the programming of the fetus at the DNA level). After birth, colostrum is vital to newborn livestock. Reduced feed intake due to thermal stress would reduce quantity and/or compromise colostrum quality (e.g. reduced immunoglobulins needed to confer immunity from the cow). Reduced feed intake due to thermal stress may also result in small calves due to lack of nutrition crossing the placenta especially during the last one-third of gestation when calves will accumulate approximately 60% of its birth weight.

It is unfortunate that fall calving is resulting in many aberrant outcomes due to prolonged thermal stress on livestock during the summer/late fall months in 2025. It is important to recognize that with increasing costs of raising/maintaining livestock, breeding soundness exams for bulls and pregnancy checks for cows and heifers should be part of normal herd management.

Teresa L. Steckler is a University of Illinois Extension specialist, commercial agriculture.