Mares can get a bad rap for recalcitrant estrous behavior, but hormones and tumors could also be at play

Anormal mare should behave just like any other horse. So says Paula Hitzler, longtime farm manager at the Michigan State University Horse Teaching and Research Center, in Lansing.

Over her 35 years at the center, home to one of the nation’s oldest Arabian horse breeding programs, Hitzler has seen herds of as many as 125 horses. When The Horse spoke with Hitzler in early 2024, the herd size was 68. Though just 15 of those horses were broodmares, most of the horses kept at the farm are mares because colts are typically sold after they’re started under saddle.

“In my opinion, (with a) normal mare, you should not know by working with her if she’s a mare or a gelding,” Hitzler says. “She should just be a solid citizen.”

Even in estrus it’s rare for otherwise pleasant, healthy mares to display sour temperament, Hitzler says.

“I don’t think that very many mares are ‘mareish,’ ” Hitzler says. Behaviors such as being grouchy, showing signs of heat around people, or urinating while being groomed—“I don’t see very much of that.”

Rather than ignoring or blowing off grouchy behavior as a normal side effect of heat cycles, Hitzler sees “bad” behavior as a potential warning sign that something has gone awry in the mare’s body; maybe the mare’s hormones are out of whack, or maybe the mare has an ovarian tumor. And, of course, mares can experience other ailments, such as injuries, gastric ulcers, or infections. Authors of a 2023 case series on behavioral disorders in mares with ovarian disorders reported that veterinarians must exclude nonreproductive causes of unwanted behavior and reduced performance, such as urinary tract disorders (vaginitis, pneumovaginitis, cystitis, and urolithiasis) and low-grade musculoskeletal pain. Because owners mentioned poor rideability, vets must also rule out rider factors¹.

What Is ‘Normal’ Behavior for Mares During Estrus?

Healthy breeding-age mares spend 15 to 17 days out-of-heat and five to seven days in heat, spring to fall (some sources say 14 to 16 days). Signs that a mare is in heat include receptivity to a stallion (standing when he’s around, for example), frequent urination, raising the tail, and “winking” the vulva.

Whether you find that behavior annoying or problematic depends partly on your perspective and priorities