Breeding mares on their foal heat is a strategy used to maximize reproductive efficiency. Since income is generated from selling offspring, yearly foal production is critical to offset maintenance and breeding expenses incurred by the mare owner. With an average gestational length of 333 to 345 days, mares must become pregnant within one month post partum to continue producing foals each year. Mating mares on the first postpartum estrus is one method used to improve the chance of maintaining yearly foal production. Reviewing this topic for us is guest writer, Dr. Margo Macpherson with an excerpt from the chapter Breeding Mares on Foal Heat co-authored by Dr. Margo Macpherson and Dr. Terry Blanchard in the 2nd Edition of Equine Reproduction.