Breeding racing mares as early in the season as possible offers a financial incentive to breeders. Doing so means foals born early in the season, and the physical maturity of larger, early foals is in often in greater demand at yearling sales and in 2-year-old races than their later-born counterparts. Early in the breeding season, however, mares are transitioning from winter anestrus to normal ovulatory estrous cycles. During this phase, mares can experience on average 60 to 80 days of erratic estrous behavior and ovulation failure.

In a study conducted over four consecutive breeding seasons involving 227 transitional-phase Thoroughbred mares, researchers in New Zealand examined the effect of an intravaginal progesterone-releasing device (CueMare) on expediting conception early in the season, as well as overall conception rates by the end of the breeding season.

Randomly paired mares were either treated with the progesterone-releasing device or were assigned to an untreated control group. Mares treated with the device were examined after seven days, and the device was removed in those with ovarian follicles of 35 mm or greater. The device remained in place and the mares were re-examined on day ten if the follicle did not reach the required size after seven days.

When mares in either group exhibited estrous behavior in conjunction with ovarian follicles 35mm or greater, researchers administered 1,667 IU human chorionic gonadotropin (hCG) and the mares were subsequently serviced by live cover within 24-36 hours. Ultrasounds performed at 24-hour intervalsdetermine ovulation, followed by further ultrasound at 14 to 16 days after service to determin