Heat stress can reduce pregnancy rates

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 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 were reduced in 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 thermoneutral environment) and one week later exposed to the environmental treatments listed below in Table 1.

Table 1. Effects of Imposed Heat Stress on Reproduction in Beef Cows   (Biggers, 1986;OSU) 

  Control Moderate Heat Stress Severe Heat Stress
Day time temp (F.) 71 97 98
Night time temp (F.) 71 91 91
Relative humidity (%) 43 27 38
Rectal temp (F.) 102.0 102.7 103.6
Pregnancy (%) 83 64 50
Conceptus wt (g) 0.158 0.111 .073

They found that heat stress of beef cows from day 8 through day 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.  Marked declines in conception occurred when temperatures did not fall in this range.

Beef producers conducting Artificial Insemination or Embryo Transfer may want to take heed of this information.  Make certain that cows are allowed access to shade and adequate air movement, at breeding, and immediately following breeding.  Of course, adequate cool water is important anytime during the summer months.  Avoid forcing  recently inseminated cows to stand in treeless, drylot situations where relief from the Oklahoma heat is impossible.