|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.