Environmental Influences on Reproduction
In general, we are usually not concerned about heat or cold influences on conception rate in beef cattle. However, heat stress can impact on fertility. Heat stress is known to affect oocyte viability and fertilization rate. Also, the breeding period closely coincides with the period of peak milk production, which is associated with greater metabolic heat load and will exacerbate any effects of climatic heat load. Thus, the 10 days prior to conception to approximately 20 days after are critical for optimum ovum development and embryo survival.
An analysis of 10 years of reproductive data from a herd of crossbred cows in southeast Nebraska found that conception rates are clearly lower during summers when it's hot during the breeding season than during cooler summers. In normal years, for each 1 degree Fahrenheit that temperatures are above normal, conception rates tend to be reduced about 1 percent. The heat stress has the potential to be greater in years when the combined effects of temperature and humidity are considered.
Hot, humid weather during the first 30 days of the typical 60- to 75-day breeding season can potentially have the greatest economic impact even if the cows get bred later. Research elsewhere has shown that for every reproductive cycle or 21 days that a cow fails to breed, per cow profits decrease 10 percent on average. Also, the first 45 days of a typical breeding season are significant because it's normally the coolest time when the majority of cows get bred. Those cows may breed later but if it's hotter than normal, you're reducing the probability they are going to conceive during subsequent cycles.
The study showed that if hot weather persists throughout the first 60 days of breeding, the impact on reproduction continues. From this research it was also found that minimum temperature and average temperature-humidity (THI = [Temperature - .55 * (1-%RH/100) * (Temperature-58)]) had the highest correlation to cows being pregnant. Average daily minimum temperatures between 55 and 60 produced optimum pregnancy rates, while an average daily THI between 64 and 68 was optimum for cows to conceive. Pregnancy rates declined approximately 1.5 to 2% for each unit change or increase in average THI or minimum temperature from these optimum levels. Across the state of Nebraska, average July THI for Beatrice, Norfolk, North Platte, and Scottsbluff are 73.2, 70.3, 70.0 and 68.7, respectively. Average June THIs are 3 to 4 units lower, while August THIs are approximately 1 unit lower than July THIs. Thus, heat and humidity-related reproductive problems in beef cows probably are most severe in the east because of generally higher humidity, but this research has implications statewide, particularly if adverse weather conditions prevail.
Producers can take several steps to minimize heat and humidity stress on their cow herds, including:
- Minimize cattle activity and movement during breeding season, especially when it's hot. Physical activity can raise a cow's body temperature 1 to 2 degrees.
- If you must move or work cattle, do it very early in the morning when it's cooler, to allow cattle 2 to 3 hours to cool down after working or moving.
- Provide plenty of clean water as it's one of the most useful tools for cooling cattle.
- Insure water access and availability is not limited in mob-grazing and intensive pasture rotation systems.
- Provide shade or a place for cattle to cool off when possible.
- Control flies to discourage bunching and physical activity associated with cattle fighting flies. Biting flies, in particular, will significantly exacerbate heat stress.
- Make sure bulls are kept cool as well as cows. In an abnormally hot and humid year, conception rates could easily be reduced by as much as 6 to 8 percent just due to weather effects. Only toward the very end of the breeding season do effects of weather patterns begin to lessen. By August, cattle begin to adapt to hot weather plus nights are longer, which allows more time for cattle to cool.
Dr. Terry Mader
Emeriti, Professor of Animal Science
Northeast Research and Extension Center
University of Nebraska–Lincoln