In performance horses, there is always a question about exercise during breeding season, so research has been undertaken to answer this question. In the study, one group of mares were exercised at a long trot and canter for 30 minutes from 1-3 p.m. when the temperature was above 86 degrees Fahrenheit and humidity greater than 50 percent, while the control group of mares was not exercised. Results indicated that exercised mares developed smaller follicles and fewer embryos were recovered from exercised mares compared to non-exercised mares. Also, the exercised mares that did have embryos had poorer quality embryos than the non-exercised mares.
Another study had a control group with no exercise, a group with daily exercise until ovulation occurred, and another group that was exercised the entire time. As in the previous study, fewer embryos were recovered from mares with any exercise. Heart rates were increased in exercised mares after exercise as were cortisol levels, and ovarian blood flow also increased, but the increased blood flow did not increase embryo recovery rates. It is believed that increased cortisol with exercise was the reason for the decreased recovery rates. However, cortisol also increases with transportation of mares but it has been shown that transportation does not have an effect on ovulation or pregnancy rates in mares. So if you are planning on breeding your mare, transporting her for breeding seems to be okay but exercising her in the heat can decrease the pregnancy rate.