PZP Fertility Control

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Porcine Zona Pellucida, or PZP, is a fertility-control vaccine given to the mares on the range through an injection or a simple dart. The vaccine prevents fertilization and pregnancy via an immune response that does not affect the animals’ hormonal system. As a result, the vaccine preserves the natural behaviors that distinguish wild horses from their domestic counterparts. It is scientifically proven with over three decades of use and is recommended by the National Academy of Sciences for use in BLM herds as an alternative to roundups and removals.

Success in the Field PZP works every place it is used properly, including in numerous BLM wild horse herds.

  • Spring Creek Basin, Colorado: PZP in use since 2012. The wild horse population has been stabilized at 62, and no horses have been removed since 2011. Bait trapping is prioritized over helicopter roundups for future removals, if any are necessary.
  • McCullough Peaks, Wyoming: PZP in use since 2012. Zero population growth achieved in 2015. No removals since 2013. Bait trapping is prioritized over helicopter roundups for future removals, if any are necessary.
  • Challis, Idaho: Use of PZP has slowed population growth rates, meaning a longer time between round- ups (5 years instead of 3 years) and the need to remove 70% fewer horses from the range. Bait trapping will be used instead of helicopters to remove just 50 horses from the range in 2017.
  • Little Book Cliffs, Colorado: PZP use has stabilized population growth rates in this herd. The last helicopter roundup was in 2009. In 2013, bait trapping was used to remove just 14 horses, who were all placed locally in adoptive homes.
  • Pryor Mountains, Montana: According to the BLM, “The Billings Field Office is excited to be on the cusp of nearly eliminating the need for wild horse removals due to the use of PZP.” (Jim Sparks, Billings Field Manager, 2013) Cost Effective Alternative to Expensive Roundups/Removals


PZP is “a more affordable option than continuing to remove horses to long-term holding facilities.” (National Academy of Sciences, 2013)

PZP use with select removals could save about $8 million over 12 years in one Herd Management Area alone. (de Seve and Griffin, Journal of Zoo and Wildlife Medicine, 2013)

  • Cost per horse/year in short-term holding:$1,829
  • Cost for long-term holding:$664
  • Cost of yearly PZP vaccination: $30.