U.S. Department of Agriculture: Animal and Plant Health Inspection Service


Date of this Version

February 2004


From the Proc. 2lth Vertebr. Pest Conf. (R M. Timm and W. P. Gorenzel, Eds.) Published at Univ. of Calif., Davis. 2004. Pp. 263-268.


Overpopulation of feral horses in several western states is an unquestioned problem. Current management strategies of removal and adoption are expensive, logistically challenging, and ineffective as a means of population control. We are testing three long-acting contraceptive approaches on feral Nevada mares. Modified reversible immunocontraceptive vaccines for gonadotropin releasing hormone (GnRH) and porcine zona pellucida (F'ZP) (SpayVac), and intrauterine contraceptive devices (IUDs), are being evaluated to determine: I) their safety and efficacy for preventing pregnancy for multiple years, 2) whether the effects are reversible, and 3) whether there are notable contraindications. Preliminary data after 1 year suggest that IUD- and PZP-treated mares continue to exhibit breeding and estrus, while GnRH-treated mares are less likely to cycle. All mares in the GnRH and PZP treatments were infertile for the breeding season. Eighty percent of the IUD-treated mares were infertile; those mares that became pregnant likely failed to retain the IUD. A notable contraindication was that uterine edema normally observed in mares in the follicular phase of the estrous cycle was commonly observed in PZP-treated mares. Because administration of each contraceptive approach is different, and each has different effects and expected duration, one approach or a combination of approaches may be best suited for specific field applications. Subsequent years of this study should establish the efficacy and safety of one or more long-acting contraceptive approaches for feral horse population control.