A horse’s breeding season occurs in March through October. Mares are most receptive to mating during the late spring and summer and can only produce one foal a year. They can begin breeding at the age of 18 months.
Stallions can begin breeding when they are two years old and are capable of producing up to 200 foals a year.
The estrous cycle determines when a mare is sexually receptive toward a stallion and usually lasts from 19 to 22 days. She goes into heat and give off a scent that attracts a mate. Prolonged or irregular estrous cycles may have various causes, one being the length of exposure to daylight. The longer days in late spring, where there can be 15 or 16 hours of daylight, stimulate a mare’s ovarian activity, creating longer estrous cycles. Once pregnancy occurs, the average gestation period is approximately 340 days.