To Breed or Not to Breed — All About Foal Heat

There are two parties in the breeding world, ones that support the breeding method of foal heat and ones who do not. Horses have long gestation periods (the average gestation is 333-345 days or 11 months) and in order to have another live foal on the ground around the same time the next year she will need to be bred during her first estrous cycle after foaling, called a “foal heat”.

So, what exactly is foal heat? According to The Colorado State University Equine Production Laboratory, foal heat is the first estrus or heat period that occurs after foaling. Mares usually first come into heat by 6 to 8 days postpartum. The average interval from foaling to first ovulation is 10 days, although mares can ovulate as early as 7 to 8 days or as late as 14 to 15 days postpartum and be considered clinically normal. It’s important to note that pregnancy rates are usually higher for mares that ovulate 10 or more days after foaling than mares that ovulate on or before day 9 post foaling.

The ultimate goal with the foal heat strategy is for the mare to be rebred quickly and successfully allowing the least amount of stress on both the mare and foal currently at her side. Rebreeding the mare as quickly as possible leaves the most uninterrupted time for the mare and foal to bond. Lots of factors depend on a successful foal heat breeding, including the mare’s current postpartum health and age, and of course the chosen stallion’s fertility and performance demands (if in competition) at the time. If you’re considering rebreeding your mare this spring but aren’t sure where you stand, check out some primary advantages and disadvantages that may help you decide:


  1. Increasing the odds of the foaling date to be around the same time next year compared to mares bred on their second postpartum estrus cycle.
  2. Cumulative season pregnancy rates that are comparable between mares mated for the first time on their foal heat and mares mated for the first time on later postpartum estrous periods.
  3. Meet an in-demand stallion’s breeding window, especially if he’s on a schedule and a performance horse.


  1. It’s been noted that there are lower pregnancy rates achieved for mares mated on foal heat compared to mares mated for the first time on later postpartum estrous cycles.
  2. Potential for greater pregnancy losses in mares conceiving on foal heat matings compared to mares conceiving on later postpartum estrous periods.
  3. Stress on mare and foal on the ground, especially if live-cover involving transport is required.

Before making a final decision, evaluating the mare’s post-foaling health is critical. The first week after giving birth, the uterus is recovering and can be hostile. It’s inflamed and can potentially be full of bacteria working its way out. The quicker the uterus returns to normal, the higher chance of success of the mare being bred successfully during foal heat. Before considering a rebreeding, your vet should perform a full evaluation on your mare, including an ultrasound of the reproductive tract to ensure that she would be a good candidate.

Mare health should be accounted for first, but if all factors line up there’s no reason she can’t be rebred. Of course, it’s ultimately an owner's decision, but the veterinary results should be taken into account. By careful management of the mare and foal on the ground, the mare can be rebred successfully and deliver the next year.