Most mares do not cycle all year long. In the middle latitudes of the northern hemisphere the average mare's first cycle of the year is in March and the last one in October. Between October and March the mare's reproductive organs shut down (anestrus) and most mares do not display estral behavior (heat). The beginning and end of the natural breeding season are capped by a transitional period of prolonged, mild estrus behavior that is a bit irregular.

A complete estral cycle is around 21 days long. It consists of

• Estrus and behavioral changes that last from three to seven days
• Diestrus and normal behavior lasting approximately 14 to 18 days.

If the mare does not become pregnant, she will cycle again. If she becomes pregnant, she will not come back into heat until after giving birth.

Typical behavior for a mare in heat is to become a little more interested in the horses around her and perhaps be a little more friendly. If that were the only problem things would be fine however some mares will persistently stop, squat, and squirt urine while they are in heat and the problem worsens when she comes into contact with other strange horses. Mares do vary tremendously in their range of normal estral behavior from mare to mare. On top of that, some individual mare’s behavior will differ depending on the circumstances.

Owners frequently report that their mares become more aggressive when they are in heat. Often this is not found to be related to being in heat but just general mareish behavior. There is little doubt intact mares are a bit more standoffish than geldings and are more protective of the space around them. Some maidens that have not been around a lot of other horses seem more nervous when in heat. They are generally difficult to get to stand to breed. When considering abnormal cycling and unusual behavior or aggression of tumors on the ovaries. These common tumors of mares are a known cause of problems...more.