Mares are termed “seasonally polyoestrus” which means that they have a well-defined breeding season with multiple heat (oestrus) cycles, as well as a non-breeding season, or anoestrus, during winter when daylight hours are shortest.
Each cycle, or oestrus, lasts an average of 21 days; though in the height of summer the cycle can shorten to 18 days. During each oestrus cycle, the mare will be unreceptive for 16 days (known as dioestrus), and then accept the stallion for the last 4-5 days. This is when she is “on heat”.
Ovulation occurs approximately 24 hours before the end of the oestrus cycle. This is the best time to breed the mare either by natural cover or artificial insemination techniques. Veterinarians can check the mare’s ovaries with an ultrasound scan to determine when she is likely to ovulate, and therefore improve the chance of pregnancy.
If the mare conceives, her pregnancy will then last 11 months (length of pregnancy can be anywhere from 310 to 365 days). If she does not conceive, then she will have another oestrus cycle. Veterinarians can perform ultrasound scans to confirm pregnancy at 15 days and 45 days after either natural cover or artificial insemination.