Horse-Breeding Compatibility

Some pairings just don’t work, and that seemingly perfect mare-stallion combination fails to result in pregnancy.

The 12-year-old mare had already successfully carried four foals to term. She was known as an easy breeder because she always settled on either the first or second cycle. The mare had conceived in previous years after having been bred with either frozen semen or cooled semen. This year, the owner wanted to breed her to a local stallion of known good fertility and get her pregnant early in the season. Nice and simple. Unfortunately, it didn’t happen that way. The previous fall, we suggested the mare be put under lights. Beginning about December 1, we had recommended that the mare be housed at night in a stall and allowed daily outdoor turnout.

The goal was to have the stall lights on timers so that the lights came on automatically at dusk (around 4:30 p.m.) and shut off at 11 p.m. This would provide 16 hours of light and allow for eight hours of darkness, and she should start cycling in about 60 days. But the owner forgot. So we had to wait for the mare to begin to cycle naturally.

What do you do after your mare does become pregnant? How do you take care of her when she has the foal? To find the answers to all of your mare and foal care questions, download AQHA's Equine Breeding Techniques and Foal Health Tips report.

She partially cooperated by having her first good heat cycle at the end of March. The mare could have waited until late April or even early May to have her first ovulation. Culture and cytology examinations were performed, and the mare was clean and ready to be bred. The stallion was collected, and the mare was inseminated. She was administered a hormone called deslorelin to induce ovulation of her large ovarian follicle. The semen looked fantastic, and she ovulated on schedule. An ultrasound examination 14 days later showed that she was open. The mare came back into heat a few days later and developed another nice large ovarian follicle. Deslorelin was administered, and the stallion was collected the following day. As with the previous cycle, the semen quality was excellent. The mare was inseminated, and she ovulated that evening, spot-on timing for optimal fertility. Disappointment showed up again 14 days later, as the mare was deemed to be not pregnant. A discussion with the owner ensued: Why was the mare not pregnant? She was the easy one of the herd. The stallion was fertile. Lots of other mares being bred to him were already pregnant. Why not this mare? In our opinion, the mare had done everything right. She was free of infection, came into heat normally, developed a large follicle, was bred with an appropriate number of progressively motile spermatozoa, ovulated on schedule and did not develop any clinically significant post-mating inflammation. The stallion was performing admirably, rating an “exceeds expectations” in his job description by achieving a high per-cycle pregnancy rate with his other mares. All but her.

To learn more about horse breeding, caring for your mare and, eventually, her foal, download AQHA's Equine Breeding Techniques and Foal Health Tips report today.

It was time for the compatibility chat. Every year, we encounter a few stallion-mare combinations that just do not result in a pregnancy. The mares involved are as normal as any other, with a history of becoming pregnant and carrying a foal to term; the stallions are fertile, and on paper, the cross seems perfect. However, the combination of mare genetics and stallion genetics either ends in no pregnancy or a pregnancy is started and then fades away. In the present case, the owner reluctantly relented to switching stallions. It was already late April, and we were two months behind schedule. This time, we shipped in semen from a different stallion and inseminated the mare. Again the mare ovulated on schedule and we waited. Nervously. We were rewarded 14 days later with an ultrasound examination that showed a normal embryonic vesicle nestled snugly at the base of one of her uterine horns. The mare was pregnant. The pressure was off. The average pregnancy rate is about 50-60 percent per cycle. Consequently, it might take two to four cycles for some mares to become pregnant. Unfortunately, some combinations of mares and stallions never produce a viable pregnancy or a live foal. At some point, a mare owner will have to make the decision that it is time for a change. Different stallion; different genetics; different karma; whatever it takes. Failure to get pregnancy doesn’t mean that the stallion is impotent or that the mare is doomed. It simply means that the combination didn’t work.