Ovulation in Mares

Mares begin to come into estrus early in the spring, with the average estrous cycle being about 20 or 21 days. They will be receptive to breeding for about five days at the end of the cycle.

To have the best chance of getting a mare in foal, she should be bred within the 12-hour period just before ovulation. Owners who want to schedule breeding can have a veterinarian examine the mare by ultrasound or palpation to check the progress of follicles as they form and mature on the mare’s ovaries. By tracking changes in the size and shape of a follicle, the veterinarian can make a fairly accurate prediction of when the mare will ovulate (release the ovum from the follicle), allowing the owner to book a live cover or order shipped semen for artificial insemination.

A developing follicle increases in diameter until one or two days before ovulation. When the follicle nears maturity, it becomes softer and changes from a spherical to a conical shape. These changes take place 12 to 24 hours before ovulation, giving the examining veterinarian information that can be used to pinpoint the optimum time for breeding

Sperm from a natural breeding tend to stay active within the mare for up to 72 hours, while those from artificial insemination will usually be active for no more than 24 hours.