ALTHOUGH a percentage of mares cycle all year round, most mares remain in anoestrus (a period of time where the mare does not cycle or show any signs of follicular activity) throughout winter, they experience a phase of transition during spring and have regular ovulatory cycles throughout the summer in response to the increase in day length.

The beginning of the natural equine breeding season in the northern hemisphere is considered to be sometime in April or May depending on proximity to the equator. The thoroughbred industry’s breeding season officially begins on February 14th in the northern hemisphere, much earlier than the natural breeding season. Therefore many methods to advance normal cyclicity in the mare have been investigated, some of which are discussed below.