By Scout Nelson
Summer heat affects cattle comfort, health, and reproductive performance. When temperatures stay high and humidity increases, cattle may experience heat stress. This condition can affect breeding success and may continue to impact fertility long after the hottest days have passed.
Jessica Drum, Assistant Professor & SDSU Extension Beef Reproductive Physiology Specialist, explained that heat stress can influence reproduction in both cows and bulls. These effects may not appear immediately, but they can reduce overall herd fertility and pregnancy success.
Heat stress occurs when cattle cannot cool their bodies effectively. High temperatures, humidity, low wind, and warm nights can make cooling difficult. Cattle normally perform best within their thermoneutral zone, which is generally between 32°F and 77°F. As temperatures move closer to 80°F, especially when humidity is high, the risk of heat stress increases.
Researchers often use the Temperature-Humidity Index (THI) to measure heat stress because it considers both temperature and humidity. This approach gives a better picture of the total heat load experienced by cattle.
General THI guidelines include:
- THI below 72: Little to no heat stress
- THI 72 to 78: Mild heat stress
- THI 79 to 88: Moderate to severe heat stress
- THI above 88: Severe heat stress with a higher risk of mortality
When cattle experience heat stress, they often eat less feed, breathe faster, and use more energy to control body temperature. This shift can reduce the energy available for growth, production, and reproduction.
Effects on Cow Fertility
Heat stress can affect reproduction even before visible signs of stress appear. Research shows that body temperature can rise as environmental temperatures increase, leading to reduced fertility.
Common reproductive effects in cows include:
- Reduced expression of estrus
- Lower detection of estrus during breeding
- Delayed return to normal reproductive cycles after calving
- Lower conception rates
- Increased early embryo loss
These reproductive losses result from changes in egg quality, embryo development, and follicle growth. The period immediately after fertilization is especially important. Higher body temperatures during this time can reduce embryo survival before pregnancy can be detected.
As a result, herds exposed to extreme heat during the breeding season may experience lower pregnancy rates than expected.
Heat Stress Also Affects Bulls
Bull fertility is highly sensitive to heat. Normal sperm production requires temperatures below the animal's core body temperature. Prolonged heat exposure can reduce sperm quality, lower sperm movement, and increase abnormal sperm cells.
The effects of heat stress on bulls often appear later because sperm development requires about 60 days. This means that reproductive issues caused by a heat event may not become visible until several weeks afterward.
Because of this delay, producers may notice more cows becoming pregnant later in the breeding season. In some cases, pregnancy checks may reveal a larger number of open cows if bulls experience significant fertility problems.
When evaluating pregnancy results, producers should consider whether major heat events occurred approximately two months earlier, as these events may influence bull fertility outcomes.
Management Practices to Reduce Heat Stress
Although weather cannot be controlled, producers can take practical steps to reduce heat stress and protect reproductive performance.
Important management practices include:
- Providing abundant clean drinking water
- Offering shade whenever practical
- Avoiding unnecessary cattle handling during peak heat
- Limiting stress around insemination periods
- Maintaining proper nutrition and body condition
- Monitoring breeding performance closely
Regular evaluation of herd records can help producers identify reproductive challenges early and make timely management decisions.
Pregnancy diagnosis remains an important tool for measuring the impact of heat stress on herd fertility. Early identification of open females allows producers to make informed culling, feeding, and marketing decisions before winter resources are committed.
While a higher number of open cows may be disappointing, it can also provide an opportunity to strengthen herd selection. Keeping females that successfully conceive may improve overall reproductive efficiency and herd productivity.
Producers should also consider breeding soundness examinations (BSEs) before the next breeding season, particularly for older bulls or bulls exposed to extended periods of heat stress. These examinations help identify fertility concerns before breeding begins and reduce the risk of future reproductive losses.
Photo Credit: gettyimages-jacqueline-nix
Categories: South Dakota, Livestock, Beef Cattle, Dairy Cattle