When summer temperatures soar, some producers turn to bulls to help them get the cows bred.

However, research conducted at Virginia Tech shows that heat stress negatively affects semen production in bulls, and that, too, can undermine your summer breeding program, explains Mel DeJarnette, reproductive specialist with Select Sires.

Researchers collected semen at three-day intervals for a two-week period to establish a baseline for semen production in each bull. Next, they mimicked the effects of mild heat stress by covering the scrotum of each bull with a sock made of insulated batting for 48 hours. Then, they collected semen at three-day intervals for the next eight weeks. Here's what they found:

  • Semen quality - as measured by sperm motility, sperm membrane integrity and sperm morphology - began to decline about nine days after the heat stress insult.
  • Maximum decline in semen quality was observed at 18 days after the heat stress insult.
  • Normal semen quality was not regained until 35 to 40 days after the heat stress had been removed.

Since it takes about 63 days for the production of sperm cells, even short-term heat stress can affect the semen quality of herd bulls for as much as 1.5 months. In addition to decreases in semen quality, extended periods of heat stress can negatively affect semen quantity and can decrease the bull's libido, says DeJarnette.

One possible solution is artificial insemination with a timed breeding program. Although AI will not eliminate all of your summer breeding problems, at least you can be sure that semen quality is not the limiting factor.