Fertility Control

Porcine Zona Pellucida, or PZP, is a fertility-control vaccine given to female horses on the range through an injection via remote darting. PZP is scientifically proven, with over three decades of use, and is recommended by the National Academy of Sciences (NAS) for use in federally protected wild horse herds. This tool is a humane alternative to roundups and removals and the most promising strategy for managing wild horses in their wild habitat.


Why the need for fertility control?

mountain lionWild horses in the United States no longer have free-range over vast western rangelands. Instead, pursuant to the Wild Free-Roaming Horses and Burros Act (1971, as amended), they are confined to Herd Management Areas (HMAs) or Wild Horse Territories (WHTs) on public lands that are managed by the Bureau of Land Management (BLM) or U.S. Forest Service (USFS) for "multiple uses." Such uses include commercial industries like livestock grazing, oil and gas extraction and mining, as well as recreational pursuits, including wildlife watching, ATV use, hunting and fishing. Although mountain lions and wolves can and do prey on wild horse foals, due to hunting and government predator “control” or kill programs, these species are not present in sufficient numbers to regulate wild horse populations, except in a small number of areas.

The BLM is mandated to manage public lands for multiple uses, and is not permitted to allow the range to deteriorate. If left unmanaged, wild horse populations might eventually stabilize near what is known as their food-limited ecological carrying capacity. However, reaching this level would result in deteriorated range conditions that are ecologically unsound.

cowThe conditions that America's mustangs live in today are anything but natural. The vast majority of wild horse populations have been genetically manipulated and socially and behaviorally disrupted by a constant cycle of roundups and removals. Their ability to migrate freely to adjust to changing environmental conditions is inhibited by fences that confine them to artificial habitats known as HMAs or WHTs. They must compete on our public lands with other uses backed by powerful economic interests. Under these circumstances, the idea of natural regulation is just that: an idea that is unfortunately divorced from reality.

The AWHC's goals include securing a fairer share of resources for wild horses on our public lands and protecting predators in order to restore a more natural ecological balance on western rangelands. However, we also must also deal with modern realities, which means that wild horse population growth on our public lands must be managed in some form. The question then becomes: which form of management available today is the most humane, minimally intrusive and preserves natural behaviors?

The answer today is PZP fertility control. This proven technology provides a safe, humane, cost-efficient and effective alternative to the current wild horse management approach of roundup, removal and stockpiling of horses in government holding facilities.

So What is PZP?

Fertility ControlThe PZP vaccine, now registered with the Environmental Protection Agency (EPA) as ZonaStat-H, has a decades-long history of use in multiple wildlife species, including wild horses. The vaccine produces an immunological response that prevents fertilization. Since it does not affect hormone production, the impacts to behavior are minimal, particularly when compared to the alternatives. The vaccine is also reversible. It is being used to safely and effectively manage wild horses in the U.S. today. In the Assateague Island National Seashore, the National Park Service has used PZP to manage a wild horse population for over three decades. In a model program for western states, Friends of a Legacy (FOAL) has partnered with the BLM to control the population of wild horses in the McCullough Peaks HMA through a PZP program that achieved zero population growth in two short years. PZP has also reduced population growth in the Pryor Mountains on the Montana/Wyoming border to historic lows.

When compared to the alternatives presently employed or being considered by the federal government – traumatic roundups and mass removal of horses from their homes on the range; surgical sterilization (castration of stallions and spaying of mares) that destroys natural behaviors; experimental contraceptives that impact behavior and may have serious side effects – the PZP vaccine is clearly the superior alternative currently available for humane management of wild horse and burros herds.

How Does it Work?

PZP is given via a dart that injects at the hip site, then ejects and the dart team retrieves it. The vaccine prevents fertilization and pregnancy via an immune response that does not affect the horse’s hormonal system. As a result, the vaccine preserves the natural behaviors that distinguish wild horses from their domestic counterparts. The vaccine is safe for female horses (mares) in foal, and for mares who are nursing their young. In the first year, the mares and young horses (fillies) are treated twice: once with a primer dose, and a booster dose two weeks later. It can be used in female horses as young as 11 months.

Is PZP Expensive?

No, a dose of PZP costs $30, compared to the $2,800 it costs to warehouse a horse for just one year. This budget includes startup costs such as training and initial purchase of the dart guns. The program costs will be less in the following year. Regardless, this figure is far less than the millions that BLM spends on mass roundup, removal, and warehousing of wild horses annually. To start, the BLM could spend just 5% of the program budget on the programs it has in place to help with expanding and supporting those. Five percent of the program budget dedicated to PZP programs would make a huge difference on the bottom line - fewer horses in long-term holding means less money will be needed for the future care of those horses.

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

Is it Difficult to Locate the Horses?

No, any PZP program is customized based on the specific herd being treated, accounting for differences in herd dynamics and geography. Darting teams are assigned to a specific area and become familiar with the specifics of the herd. This enhances the ability to track horse movements, shifts in herd leadership, deaths, and births. Horses requiring booster vaccines are identified via markings and photography that is uploaded into software; facial recognition software is in development. Then each treatment is recorded in the database. The American Wild Horse Campaign’s Virginia Range Program, which manages state horses in Nevada, maintains a scientific database that is updated by photographers who are in the field on a regular basis. Horses are tracked on foot or with game cameras set up in areas visited regularly by horse bands, and drones are used to locate more remote bands. No helicopter roundups are necessary for the success of a PZP program. In lieu of helicopter use, it may be necessary to use baiting with some herds. This can often be as simple as determining when horses naturally come into an existing water source and planning to dart them at that time.

Our program on Nevada’s historic Virginia Range covers a 300,000-acre range that spans from Carson City to Reno to Fernley and Silver Springs. The program -- the largest fertility control program in the world -- shows us protocol that can be successfully implemented nearly anywhere stakeholders choose to come together to preserve our iconic wildlife.  

Learn more about the Virginia Range Fertility Program.

What’s the Main Barrier to Using PZP?

BLM simply won’t use it voluntarily. Instead, the agency has continued to rely on their failed strategy of mass roundups and removals, which has only exacerbated the very issue that they claim they are trying to solve. Removals don’t work. PZP does, and Congress should direct the agency to use it in bill language that cannot be twisted or ignored as report language has been. BLM claims it costs the agency $3,000/mare to treat but that’s based upon the use of costly round-up methods. This technique has been used on a small scale and usually not followed up with appropriately. Congress needs to direct the agency to (1) engage in cooperative efforts with 501(c)(3) organizations and volunteers, (2) purchase equipment and supplies, and (3) support training to get more PZP programs up and running.

PZP works across public lands when used properly.

This includes 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.

  • 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



Will It Be Difficult to Treat All of The Horses in the West? No, there is no need to treat every horse in the wild today. First, because wild horses are not overpopulated across the West so not every Herd Management Area (HMA) needs to be treated. Second, because only mares are treated with PZP. To implement PZP on an effective scale, the BLM could begin with PZP programs in HMAs that are prime candidates (often referred to as the ‘low hanging fruit’ where horses are more approachable and where there are volunteers ready in the area) - without further roundup operations in these areas. But in order for PZP to work, BLM must use it at an effective level of implementation (more than 0% of the program budget) and with an HMA by HMA approach (meaning each HMA needs a specific plan to accommodate the horses and the terrain in that area). Right now, Wild Horse Specialists at BLM who want to use PZP are not supported in their efforts and must instead seek outside grant funding. This system will prove its worth - BLM just needs to use it properly.

Is PZP Effective? Yes, the NAS stated that PZP is the most promising method for managing wild horse populations. Numerous studies have also demonstrated the effectiveness of PZP. The efficacy rate is 95-97%, and research has found that a mare who has been treated for 5-7 years will self-boost and no longer require additional treatments. Observations on the range shows evidence that some mares even self-boost after only the first year. Like in humans, each mare’s physiology is different.

Is PZP a pesticide? The PZP vaccine is not a pesticide. It is an extremely well-vetted immunocontraceptive vaccine that has been used for 40 years in many wildlife species including wild horses. The Food and Drug Administration (FDA) turned registration of the vaccine over to the EPA, since it was developed for use in the environment on a wildlife species (wild horses). Since the EPA has no category for "wildlife contraception," it processed PZP under the “pesticide” category as the only available process for registering the vaccine for use. The label is purely bureaucratic; it has no scientific or practical meaning. The PZP vaccine is produced by the non-profit Science and Conservation Center, which pioneered its use in wildlife and provides it to the BLM and other entities at a financial loss.

Additional resources on PZP for fertility control.