The effects of heat stress on reproductive performance of beef cows has been discussed by many animal scientists in a variety of ways. After reviewing the scientific literature available up to 1979, one scientist (Christenson, R.K. 1980, J. Anim. Sci. 51: Suppl II: 53.) wrote that the most serious seasonal variation in reproductive performance was associated with high ambient temperatures and humidity. He further pointed out that pregnancy rates and subsequent calving rates of 10 to 25% were common cows bred in July through September.
Typical Oklahoma summer weather can fit the description of potential heat stress, where many days in a row can exceed 95 degrees and night time lows are often close to 80 degrees. Many hours of the day can be quite hot and cause the slightest rise in body temperature of cattle. Research conducted several years ago at OSU illustrated the possible impact of heat stress of beef cows on their reproductive capability. These cows were exposed to bulls as one group (while in a thermo-neutral environment) and one week later exposed to the environmental treatments listed below in Table 1.
They found that heat stress of beef cows from day 8 through 16 affected the weights of the conceptus (embryo, fluids, and membranes) and the increased body temperature may have formed an unfavorable environment for embryo survival. As noted in table 1, the percentage of pregnancies maintained throughout the week of severe heat stress was considerably reduced.
Florida scientists studying dairy cows reported that for high conception rates the temperature at insemination and the day after insemination was critical to success. They stated that the optimal temperature range was between 50 degrees F and 73 degrees F. Declines in conception occurred when temperatures rose above this range.
Extremely hot days and warm nights in the Southern Plains will cause core body temperatures of range cows to elevate. This data suggests that producers should make every effort to establish their breeding seasons when the temperatures are more in a thermal neutral range. Also remember that bull fertility is affected by heat stress. Fall calving (with breeding seasons beginning in late November and ending in January) allow for fertilization and early embryonic survival when heat stress is not a factor.