In the continuing effort to increase the fertility of horses there comes a new weapon–superovulation. Inducing a mare to ovulate multiple oocytes can translate into producing more foals from selected females. Assisted reproductive technology can help solve the problem of how to increase the number of foals from a valuable mare, or how to obtain a single foal from a subfertile mare.

Fertility in the mare begins with ovulation. The mare naturally ovulates one ovum (egg) per 21-day cycle. In a typical breeding season, February through July, this presents six to eight opportunities for ova to be fertilized. Compared to the mare’s spontaneous (unassisted) ovulation, superovulation maximizes the fertilization opportunities by inducing the development and ovulation of a greater number of follicles. This method can optimize fertility–ova mature in follicles, so a greater number of follicles means more oocytes and an increased likelihood of conceiving more foals.

With a higher probability of establishing a pregnancy, superovulation might result in the fertilization of multiple oocytes. This gives the equine practitioner a possible opportunity to collect more than one embryo for implanting into donor mares via embryo transfer.

Research at Colorado State University (CSU) has recorded multiple ovulation pregnancy rates per donor at approximately twice those of mares which ovulate a single follicle each cycle. Patrick McCue, DVM, PhD, Diplomate ACT, Department of Clinical Sciences, has reported on these efforts to breed more foals from selected females. “Development of a superovulation technique that is successful, safe, and commercially available would revolutionize the equine breeding industry,” said McCue.

Follicular Development

In the ovary, a follicle contains an ovum. Ovulation, or the event of a follicle releasing an ovum, marks the beginning of the 21-day estrous cycle. Follicular dyn