Mares, or mature female horses, have hormonal fluctuations, even as humans and other animals do. These hormonal changes may affect a mare’s behavior and temperament, and they most certainly impact her availability for breeding to a stud, or stallion.

As seasonally polyestrous creatures, mares undergo hormonal cycles during certain seasons of the year, but not all year long. In most climates, mares do not experience estrous cycles during the winter months.

Generally, mares will have 21- to 22-day equine estrous cycles. This is the duration of time between each ovulation. A mare’s estrous cycles are usually impacted by the number of hours of bright sunlight in a day. For this reason, in most geographical regions, a mare will cycle into season, or be in-heat, during the spring, summer and early fall months, when daylight lingers longer.


A mare will remain in estrus (in season) for three to five days each time, as part of the overall estrous cycle. Known as the follicular phase of a mare’s estrous cycle, this is the period during which a mare is fertile for breeding to a stud, or stallion (an uncastrated male horse). At this point, an ovum has been released from an ovary, so that it may await fertilization for conception.

During this fertile period, a mare will usually be willing to stand for breeding. At other times, she will most vehemently refuse.

Most mares will display their in-season condition by their behavior (lifting their tails and winking, or showing).

The surest sign of equine estrus is a mare’s acceptance of a stallion in proximity. If a mare is walked directly past a stallion in a stall or round pen, she will either invite his presence or resist it. When placed near a stallion, perhaps in front of his stall window, an in-season mare may squat and urinate in place. However, if she pins her ears and squeals, she is probably not yet in season.


During her estrus period, a mare may display a marked change in temperament. The horse may seem a bit testy or jumpy, and she may resist girth or leg pressure around her abdomen. A quiet horse may neigh and demonstrate a difficulty in standing quietly for grooming or training.

Some equestrian trainers and veterinarians recommend hormone therapy for mares who demonstrate challenging behavior during their cycles. Others vehemently oppose such measures. This is a matter of personal choice.


The optimum moment for equine conception is the last day or two of the equine estrus window. At that time, the egg has traveled into the ideal spot for fertilization and implantation.

Immediately after this point, the mare enters the diestrus portion of her estrous cycle. This portion lasts approximately 14 to 16 days. During this time, breeding becomes impractical.

VIsit Classic Equine for more information on Horse Breeding.