Intravaginal progesterone releasing device to hasten first ovulation in mares: side effects and pregnancy rates in a commercial Thoroughbred farm

Many protocols have been evaluated to hasten first ovulation of breeding season in mares, including use of progesterone releasing devices. However, use of these devices in clinical practice is not widespread. In the Thoroughbred industry, there is immense pressure to have mares cycling in the nonphysiologic breeding season. Thoroughbred mares (n = 20) over 2 consecutive breeding seasons in a commercial stud farm underwent treatment with a PRID™ Delta (1.55 g progesterone) inserted intravaginally for 10 days. Mares (5 - 23 years) were not under artificial lights. Mares at time of insertion were deemed to be in early transition via transrectal ultrasonography, having at least 1 follicle of 2.0 cm, no uterine edema, and no previous ovulation noted for that breeding season. On removal of PRID™ Delta, mares had a preovulatory follicle of at least 3.5 cm. On day 11 (1 day after removal), mares had a uterine edema score of at least 1 (on a scale 0 - 3) and were given deslorelin acetate to induce ovulation. All mares (100%) ovulated within 3 days after removal of PRID™ Delta (day 13). Eight mares (40%) cultured positive for endometrial Streptococcus equi subspecies zooepidemicus. Remaining 12 mares (60%) were inseminated via live cover, resulting in 10 established pregnancies (50%) diagnosed on day 14 by transrectal ultrasonography. No vaginal discharge was observed in any of the mares, although all mares were noted to have some increase in vascularization of the vaginal wall with some hyperemia (n = 20). No other complications were noted in the reproductive tract of any of the mares. Discomfort upon removal of the device, including straining, mild colic-like signs, and weight shifting, were noted in some mares (6/20) all of which were transient in nature, and required no medical interventions. It is concluded that PRID™ Delta use to hasten first ovulation in mare is effective with minimal side effects and results in a fertile ovulation. Use of intravaginal progesterone releasing devices along with other methods to manipulate mare’s physiologic breeding season, such as artificial lighting regimes, can hasten time to first ovulation and reduce associated costs.

Keywords: Intravaginal progesterone, breeding management, mare, first ovulation

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.