Reproductive performance of commercial broodmares after induction of ovulation with HCG or Ovuplant (TM) (deslorelin)

Soon after Ovuplant (TM), the sustained-release implant containing the gona dotropin releasing hormone (GnRH) agonist deslorelin, was approved for comm ercial use in the United States for induction of ovulation in mares, anecdo tal field observations were reported that some Ovuplant (TM) -treated mares that did not become pregnant experienced a delayed return to estrus and pr olonged inter-ovulatory interval. Although those observations have been sub sequently confirmed, further data on how mares respond to Ovuplant (TM) com pared to human chorionic gonadotropin (hCG) during the post-treatment perio d is needed. The objective of this study was to further evaluate the clinic al use of Ovuplant (TM) by comparing the reproductive performance of commer cial broodmares treated with hCG or Ovuplant (TM). This retrospective study was completed by examining the 1999 reproductive r ecords of 106 mares treated with hCG during 134 estrous cycles and 117 mare s treated with Ovuplant (TM) during 151 estrous cycles. There were no diffe rences (P > 0.10) in follicle size at the time of treatment (39.4 +/- 0.5 v s. 38.9 +/- 0.5 mm), interval from treatment to ovulation (2.2 +/- 0.1 vs. 2.2 +/- 0.1 days), proportion of mares that failed to ovulate after treatme nt (3.0 vs. 4.6 %), or per-cycle pregnancy rate (47.7 vs. 51.4 %) between h CG-and Ovuplant (TM) -treated mares, respectively. The interval from ovulat ion to return to estrus (25.8 +/- 1.3 vs. 15.5 +/- 0.6 days) and the inter- ovulatory interval (30.4 +/- 1.5 vs. 20.8 +/- 0.6 days) were longer (P <0.0 01) for Ovuplant (TM) -compared to hCG-treated mares, and the proportion of non-pregnant mares that failed to return to estrus within 30 days after ov ulation (31.4 vs. 1.5 %) was higher (P <0.001) for Ovuplant (TM) -compared to hCG-treated mares, respectively. For Ovuplant (TM) -treated mares, follicle size at the time of treatment te nded (P <0.1) to be smaller for mares that failed to return to estrus withi n 30 days compared to mares that returned to estrus within 30 days (37.1 +/ - 1.1 vs. 40.1 +/- 0.6 nim, respectively). Also, the average date of ovulat ion during the calendar year was later (P < 0.05) for Ovuplant (TM) -treate d mares that failed to return to estrus within 30 days compared to those th at returned to estrus within 30 days (May 15 +/- 4 vs. April 30 +/- 4 days) . The results of this study confirm previous reports that although the ovulat ory response and fertility were not different for hCG- and Ovuplant (TM) -t reated mares, mares treated with Ovuplant (TM) that did not become pregnant had a significantly delayed return to estrus and prolonged inter-ovulatory interval. Based on recently published information, it appears this effect is due to Ovuplant (TM) -induced down-regulation of the pituitary gland, wh ich suppresses subsequent follicular growth and development. This study als o demonstrated that follicle size and/or season may influence the probabili ty that Ovuplant (TM) -treated mares would experience a delayed return to e strus/ovulation; therefore, further work is needed to determine whether the se or other factors are related to this specific outcome following Ovuplant (TM) -treatment.