Serious horse producers are concerned about their mares' reproductive performance and ability to mother strong, healthy foals. Economic survival often hinges on mares foaling early in the year, rebreeding quickly and nursing a growing foal that develops soundly. Generally speaking, the mare is a seasonally polyestrous breeder with heat periods beginning in the spring and continuing, if the mare is not bred, into late summer. Some mares will show signs of heat at regular intervals throughout the year, but only a few will ovulate during the late fall and winter months. When the mare settles into her breeding season, the estrous cycle, i.e., the interval from one ovulation to the next, is about 22 days in length. Each mare is an individual and the length of the cycle can vary between mares.