Fertility Control

 A BLM Wild Horse and Burro Specialist loading fertility control vaccine PZP.
A BLM Wild Horse and Burro Specialist working with fertility control vaccine PZP.

As part of its efforts to manage the population growth of wild horses on public rangelands, the Bureau of Land Management has supported the development of an effective contraceptive agent for wild horses since 1978. Over the years, attempts at different approaches -- such as hormone implants, chemical vasectomies, and intrauterine devices -- were tried, but abandoned as ineffective or impractical at that time.

Currently the most promising agents are porcine zona pellucida (PZP) vaccines that were developed starting in the 1990s. The BLM currently uses PZP in two formulations. The most effective is a one-year liquid vaccine that must be re-administered annually. The BLM administers this vaccine, known as ZonaStat-H,  through ground-darting programs in several Herd Management Areas where the wild horses are approachable. However, ground darting is generally not practical for BLM because it is difficult to approach most wild horses closely enough on vast and open Western rangelands.

The BLM also uses a longer-lasting pelleted PZP agent known as PZP-22, which is effective for approximately 22 months. The pelleted vaccine is typically hand-injected after the mares have been gathered from the range. This method of treatment means that during gathers, more mares need to be captured (for treatment and release) than would actually be removed from the range. While this is usually possible, it can be difficult to capture a large enough fraction of a herd’s population so that significant numbers can be treated and released in order to slow population growth. Once enough horses have been captured to bring the population down to AML, catching the small number of remaining horses becomes challenging because they are scattered over larger areas and many have become more evasive.

Since 2004, the BLM has administered PZP-22 to thousands of mares on the range, but significant reductions in the rate of population increase on a national level have not yet been apparent.

The BLM is committed to both increasing its use of current fertility control vaccines and pursuing the development of the next-generation of wild horse fertility control agents and methods. The BLM will also continue to treat herds where practical, but the impacts from reduced reproduction rates may not be realized in the immediate future until more effective methods are available.


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Research to Improve Fertility Control Tools and Methods

In its 2013 report to the BLM, the National Academy of Sciences (NAS) found that no highly effective, easily delivered and affordable fertility-control methods were currently available for use on wild horses and burros. The most promising vaccine, PZP, is limited in the duration of its effectiveness (1-2 years). In response, the BLM released a solicitation for research projects to develop new or improve existing population growth suppression methods. For eight of the promising, university-led research projects that were submitted, the BLM has asked for some proposal revisions as part of the process that would lead to the research being funded. These following eight research projects were reviewed and recommended by an NAS panel of experts and are consistent with recommendations made to the BLM by its Wild Horse and Burro Advisory Board.

  • A one-year project that will aim to develop a minimally invasive surgical sterilization method for wild horse mares that requires no incisions.
  • Two projects that aim during a two-year period to develop different surgical approaches for tubal ligation in mares.
  • A six-month project that will determine whether an existing accepted surgical sterilization procedure commonly used for domestic mares can be safely conducted on wild horses.
  • A two-year project will focus on further study of Gonocon, an approved and labeled contraceptive vaccine for equids.
  • A two-year study to develop a new, permanent contraceptive vaccine for wild horse mares.
  • A four-year project that will attempt to develop a new delivery vehicle for porcine zona pellucida (PZP) – a temporary contraceptive currently used in some wild horse herds – that would increase the duration of the vaccine’s effectiveness.
  • A three-year project for the development of an injectable agent that would inactivate hormones and decrease female and male gonad viability.

The BLM also partners with the U.S. Geological Survey for further fertility control and wild horse and burro management research efforts. To learn more about the USGS partnership, click here.