Wild Horse Fertility Control

Fertility control is currently used to achieve and maintain the Appropriate Management Level (AML) of the Pryor Mountain population by reducing herd growth rates and recruitment. Use of fertility control began in 2001 with a primer of the immunocontraceptive, porcine zona pellucida vaccine (PZP), administered to yearling and two-year-old mares. Treatments were initially given to younger mares to allow them more time to mature and support a foal and to older mares to allow them a year or more of better physical condition on the range. In 2005 and 2006, fertility control was only applied to mares 11 years old or older to partially suppress herd growth rates in order to manage for healthy horses on healthy rangelands and manage the herd closer to the AML. Natural mortality, including unpredictable predation, was also expected to bring the population closer to but not below the AML (BLM 2006).

Initial treatments and booster vaccines of PZP are remotely administered in the field which results in fewer disturbances to the herd. PZP meets most of the requirements for an ideal contraceptive agent, including safety and efficacy. PZP appears to be completely reversible when administered annually for up to five consecutive years, possibly six or seven years (Kirkpatrick and Turner 2002). PZP does not appear to have ill effects on behavior, already pregnant mares, the survival of foals, or the subsequent fertility of the foals (Powell 1999; Kirkpatrick and Turner 2002, 2003). Treated mares are monitored for reactions. The treatment is easily administered in the field and requires a single annual booster dose to maintain infertility for a breeding season. Darting typically takes place in July. In addition to fertility control, removals and natural mortality are currently used to achieve and maintain AML.


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