While some breeders are content to let Mother Nature decide when a mare's body is ready for pregnancy, others take a more proactive approach. There are many reasons why an owner might seek closer control over a mare's estrous cycle, ranging from herd synchronization to a desired foaling date to putting a breeding career on hold for a performance career. And nearly as numerous as the reasons for starting or stopping an estrous cycle are the ways veterinarians and owners can accomplish the task.

At the 2014 Kentucky Breeders' Short Course, held Feb. 8 in Lexington, Ed Squires, DVM, PhD, Hon. Dipl. ACT, executive director of the Gluck Equine Research Foundation, reviewed methods owners have by which to control mares' estrous cycles.

Heat Cycle Review

Before talking about starting and stopping a mare's heat cycle, it's important to understand how the cycle works normally.

In early spring, as days get longer and temperatures rise, the mare's hypothalamus gland is stimulated and starts producing a gonadotropic releasing hormone (GnRH). GnRH stimulates the mare's pituitary gland, which secretes two hormones that affect the ovaries. The first hormone is follicle stimulating hormone (FSH), and it stimulates development of one or more follicles