Fertility control vs. surgical sterilization: Where RTF stands and why

/ Featured, Staff Blog
Ruby Tuesday was born at RTF’s Lompoc, Calif., headquarters sanctuary in January. RTF’s use of the safe, proven and humane fertility control vaccine PZP slows but does not stop reproduction while allowing family bands to remain together at the sanctuary.

By Celeste Carlisle

Return to Freedom strongly supports the implementation of fertility control and strongly opposes surgical sterilization of wild mares and burro jennies, but why?

In fact, to begin with, why support slowing reproduction, at all? 

Because under the Wild and Free-Roaming Horses and Burros Act of 1971, and with the continued support of Congress, the Bureau of Land Management and U.S. Forest Service have been granted broad powers to set population goals (called “Appropriate Management Levels”) and to take actions to keep herds from growing what the agencies deem too large on the areas set aside for them (the BLM calls these “Herd Management Areas,” the USFS “Wild Horse Territories”). 

In the absence of predators, the agencies have relied on the capture, removal and warehousing of wild horses as their primary tool to reduce numbers on the range for nearly 50 years. RTF believes that AML needs to be adjusted to reflect a more equitable share of the range for the horses and burros on Herd Management Areas. We continue to fight for the resource needs of wild horses and burros to be considered equally, not last as they have been for decades. They should be weighted at least equally against other uses, like private livestock grazing, which overlap Herd Management Areas / Wild Horse Territories as part of the federal multiple-use mandate imposed on agencies overseeing public lands.

Given the agencies’ powers to set population targets for wild horses and burros and to act to reduce those levels, and given Congress’s blessing to do so, RTF has emphasized a humane alternative that it believes can and should replace controversial and often deadly helicopter roundups: fertility control.

These are vaccines of the sort that we at RTF have been using at our American Wild Horse Sanctuary for over 20 years under the guidance of the Science and Conservation Center, with a 91-98% efficacy rate (in other words, we still have a few foals most years but the numbers are much smaller). Our sanctuary was the fourth large project in the world to use a fertility control vaccine called PZP, which has been used across a variety of species, from elephants to deer. The vaccine we use is non-hormonal and reversible. Using fertility control allows us to manage numbers responsibly while allowing stallions and mares to remain together in their family bands and retain natural behaviors. We see this as a model for how wild horses and burros can be managed on the range, where they belong. This model was implemented successfully on Assateague Island by The Science and Conservation Center in collaboration with the National Park Service and has inspired the growth of over 32 projects on and off the range in the last 15-plus years.

Wild horses are fortunate enough to have high reproductive success: 15 to 20 percent annually, according to the National Academy of Sciences. In biologic terms, this means that wild horses tend to produce offspring who then produce offspring who then produce offspring, and so on and so forth.  That’s great if you are a large mammal in a balanced ecosystem, with population controls like predators, and limitless lands and waters. Unfortunately for wild horses, those balances are way out of whack, (mainly due to human encroachment and industry), and what we have today are herds of our revered horses relegated to drying lands doing their fantastic job of reproducing their populations, with little at hand to check those populations and keep them within the agency-set carrying capacities of their ranges. And so it is that BLM and USFS have come to manage wild horses and burros through an unworkable system of gather-removals, ad infinitum. 

The 2013 National Academy of Science report (p. 266), reminds us of the futility (and population growth enhancement) that “gathers-only” management does to wild horse and burro populations: “The management strategy of removing free-ranging horses and burros from the range leaves the animals that remain on the range unaffected by density-dependent population processes. Thus, population growth is not regulated by self-limiting pressures, such as lack of water or forage, and this allows horse, and possibly burro, populations to grow at an annual rate of 15-20 percent.” (National Research Council 2013. Using Science to Improve the BLM Wild Horse and Burro Program: A Way Forward. Washington, DC: The National Academies Press.)

You can gather and remove whoever you deem “excess.” However, if the challenge at hand is reproduction, it would really be better to address that

Alas, the BLM / USFS have resisted the challenge of implementing fertility control. Through 2019, the BLM’s Wild Horse & Burro Program has never spent as much as 4 percent of its annual budget on fertility control, despite the lobbying of RTF and others. And, yet, the BLM has attempted use its base wild horse budget to pursue research into permanent sterilization.

Return to Freedom has always supported fertility control, but “fertility control” is a broad category, which “surgical sterilization” is technically underneath.  Below are various forms of fertility control available, and our thoughts behind why we support some, but not all, methods of fertility control:

PZP and PZP-22

Return to Freedom Wild Horse Sanctuary supports the use of the wildlife contraceptives Porcine Zona Pellucida (PZP and PZP-22) for wild horse management on our public lands.

PZP, or ZonaStat-H, is an elegant immuno-contraceptive vaccine.  All mammalian eggs are coated with a membrane called the “zona pellucida,” which contains proteins necessary for sperm reception.  When the PZP vaccine is injected into a mare, her immune system develops antibodies against the vaccine, and also to the zona pellucida of her own eggs.   In effect, the mare cannot become pregnant because her eggs will never “allow” for the transmission of genetic material from the sperm to the egg.  Ovulation can occur, natural reproductive behaviors can occur, but fertilization will not occur.  Since PZP is a vaccine which must be boosted, if a mare does not receive a scheduled injection, she may still become pregnant.  What this means is that in terms of population management; PZP slows, but does not altogether stop, reproduction.  Thus, it is a simple and effective tool for stabilizinga population of animals (and by strict oversight by the Humane Society of the United States, it can NOT be used to zero out wild horse populations).  

Safe, effective contraceptive techniques have the following characteristics (Kirkpatrick and Turner 1991): 1) Contraceptive effectiveness of at least 90 percent; 2) The capacity for remote delivery with minimal handling of animals; 3) Reversibility of contraceptive effects; 4) Safety for use in pregnant animals; 5) Absence of significant health side effects; 6) No passage of the contraceptive through the food chain; 7) Minimal effects upon individual social behaviors; 8) Low cost. 

It should be stated that PZP is safe, with 30 years of data, collected and analyzed by equine scientists, wildlife biologists, geneticists, animal behaviorists, and reproductive physiologists.  These are professional people who have dedicated significant portions of their lives to finding elegant and humane solutions to wild horse population management through serious scientific study, with oversight from peers and research institutions.  PZP is not an untested, hazardous material, wreaking havoc on herd behaviors and causing out-of-season births, as has been intimated by some wild horse advocates. Indeed, Return to Freedom is proud to provide our own PZP-use and population data towards the goal of a less-intrusive way to maintain healthy herds on public lands.

PZP-22 operates by the same mechanism that ZonaStat-H does, but utilizes a pelleted, slow-release primer to lengthen the efficacy of the booster dose.  If timed properly, a mare can receive a pelleted primer dose which will initiate her antibody response and offer her protection from pregnancy, and then a well-timed booster dose of ZonaStat-H could protect her for 3-4 years.


GonaCon is a hormone releasing antagonist which works by interrupting the hormone cascade that ultimately results in fertilization being successful.  We remain guarded about the use of the longer-lasting fertility control vaccine GonaCon because it may cause other behavioral changes that would affect herd dynamics. As such, RTF would like to see more studies to ensure that GonaCon meets the parameters of ethical and thoughtful wildlife fertility control. That is the case with PZP, a non-hormonal vaccine with more than three decades of research behind it that RTF has used at its sanctuary with a 91-98% efficacy rate.

Surgical Sterilization

Recently posed methods to surgically sterilize wild horses or burros on the range include a spay procedure called ovariectomy via colpotomy and gelding of males.  Return to Freedom does not support these methods of surgical sterilization for management of wild horses and burros on public lands.

Ovariectomy via colpotomy removes the ovaries of a restrained mare in a chute via an incision made high enough inside the vagina so the abdominal cavity can be accessed. A tool called an ecraseur is used to crush the ovarian pedicle and excise the ovaries.  This is an extremely painful procedure, and a wild mare or jenny cannot receive the post-surgical care necessary for a surgical spay: how can meaningful pain management or emergency medical intervention be performed on a frightened, unhandled mare in a holding facility?  Further, there are no substantive studies to evaluate long-term health of ovariectomized mares.  Advanced musculoskeletal deterioration is a concern if estrogen is removed from a mare’s hormonal system, as an ovariectomy does.

Surgical spays polarize stakeholders and lead to litigation, which delays projects.  The BLM has an opportunity here to set this management strategy aside – because it can: because other forms of proven, safe, humane reproductive growth suppression exist and their use is generally supported by the public (immuno-contraceptive vaccines).  

Gelding is, of course, the surgical removal of the stallion’s testicles. We do not advise gelding as a population management tool since there are effective and well-studied, safe and humane and reversible population growth suppression alternatives and there are not sufficient studies to understand the behavioral effects of gelding some proportion of a population.  Modeling for population effects of a certain percentage of male horses in a population being geldings is a guess, at best. 

Celeste Carlisle is RTF’s biologist and science program manager.

Take Action: Urge Congress to support the use of fertility control

Take Action: Tell Congress to oppose surgical sterilization

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