In nature, mares have a high fertility/ conception rate since they are continually with a stallion who teases and breeds them at the best times. But few domestic horses are bred under natural conditions in pasture harems, and broodmare managers must detect when each mare should be bred. A successful breeding program depends on being able to determine when each mare is in heat and likely to conceive, whether she’s being hand bred by natural cover or by artificial insemination.

Some mares are so obvious in their heat cycles that even if they hear a stallion’s vocalizations they will show heat, says Carlos Pinto, MedVet, PhD, Dipl. ACT (theriogenology, or reproduction), an assistant professor at North Carolina State University. “Some older broodmares may show heat in their stalls without a stallion on the premises,” says Pinto. “Some owners can tell when the mare comes in heat just because of her behavioral change, but some need closer contact for teasing.

“In our teaching herd, about 20% of mares can be identified in estrus without being teased, whereas even with a stallion, about 10 to 15% may not display signs of heat unless teased at close quarters,” he says