Well the cool weather is over. Many producers have told me that they were trying to catch up on hay production. It definitely was a pleasant weekend; too bad it would not last through October!
Even though we had a brief reprieve from the weather, keep in mind that it has been hot and humid. It is important to keep in mind that bulls cannot perform their job quite as well versus cooler weather. The research is there to demonstrate the effect on hot weather on semen production.
In 1963, researchers exposed bulls to temperatures of 104 degrees F. and 54% humidity for 8 hours then dropped the temperature to 82 degrees F with 72% humidity for the remainder of the 16 hours. 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, 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:illinois.edu