Hormones and Horses: The Mare's Estrous Cycle

Female horses, known as mares, have hormonal fluctuations, much like humans and other animals do. These changes can affect a mare's behavior and temperament, and they most certainly impact her availability for breeding to a stallion.

Mares are seasonally polyestrus creatures. This means they 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.

Most mares will have 21- to 22-day equine estrous cycles. This is the duration of time between ovulation. A mare's estrous cycles are usually affected by the increased 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, the summer and the early fall months, when daylight lingers longer.

"In Estrus"

A mare will remain in estrus (in season) for three to five days each time, as part of the overall equine 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 equine ovum has been released from an ovary, so that it may await fertilization for conception.

During this time, a mare will usually be willing to stand for breeding to a stallion. At other times, she will most certainly refuse.

Mares will indicate 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, a mare who is in-season may squat and urinate in place, signaling her readiness for breeding. However, if she pins her ears and squeals, the mare is probably not yet in season.

Ain't Misbehaving

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 mare may neigh and demonstrate a difficulty in standing quietly for grooming or training during her estrus phase.

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

Window of Opportunity

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

Breeding a mare to a stallion during this brief opportunity is ideal for producing a foal, either a filly or a colt, after eleven months of equine gestation.

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