Stimulating Mares to Cycle and Ovulate Early
  by Robert N. Oglesby DVM

Depending upon the breed of horse and the goal of the breeder, foals born earlier in the year are often times more desirable. The reason for this is that several breeds, including the Thoroughbred, recognize January 1st as the first birthday of all the foals born the year before. This means that a foal born February 2nd is considered to be one year old at the same time as a foal born on June 27th. This gives the foal born earlier a potential advantage when the two are required to race or show against each other when young.

Therefore, our goal in many instances is to get the mare bred early in the year since their gestation period is about 343 days. This presents a problem because most mares go through a winter anestrus, during which they do not cycle, followed by a transitional phase, during which estrus and ovulation aren't always synchronized. Left to Mother Nature, most foals would be born in the late spring or early summer, as it takes until then for mares to be regularly cycling and ready for breeding.

In this article the specifics of artificial lighting, altrenogest (Regumate) and sulpiride to induce mares to cycle early in the year is discussed. Recent advances have greatly shortened the time required to get a mare to ovulate in the dead of the winter.

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