Recently a producer asked about the impact that the heat of the summer of 2012 may have had on the reduced calf crop that was discovered the following spring. In particular, they wanted to know if high summer temperatures could lead to reduced fertility in bulls. 

Several research trials have been conducted throughout the years looking at the effect of high temperatures on bull fertility. As far back as 1963, researchers exposed bulls to temperatures of 104 degrees F. and 54% humidity for an 8 period and then allowed the temperature to drop to 82 degrees F with 72% humidity for the remainder of the 24 hour period. This temperature regimen was continued for 7 days and was designed to resemble natural conditions in the subtropics. They found the high temperatures resulted in major detrimental effects on initial sperm motility, sperm concentration and total numbers of sperm per ejaculate.

In 1978, Oklahoma scientists (Meyerhoeffer, et al.) placed bulls in controlled environments of 95 degrees F. for 8 hours and 87 degrees for the remaining 16 hours while similar bulls were placed in environments of 73 degrees constantly. These treatments were applied to the bulls for 8 weeks and then all bulls were allowed to be in the 73 degree environment for another 8 weeks. During the treatment, the heat stressed bulls had average rectal temperatures of 0.9 degrees F higher than non-stressed bulls.

The percentage of motile sperm cells decreased significantly in the stressed bulls by 2 weeks of heat stress. Sperm motility did NOT return to normal values until 8 weeks after the end of the heat stress. This explains some of the reduction in fertility that is often associated with summer and early fall breedings. One cannot escape the conclusion that high ambient temperatures can result in detrimental effects on fertility by effects on both the cow and the bull.  Also remember that heat stress can also have a negative impact on female reproductive performance.  As you multiply the two reductions in reproductive soundness (male X female), it is apparent that heat stress can cause smaller and/or later calf crops.

Source: OSU Cow Calf Corner