BLM authorizes two wild horse fertility control vaccine trials

Long-lasting fertility control vaccines could help reduce growth rates in wild horse populations


Bureau of Land Management

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The Bureau of Land Management is authorizing two studies of promising new fertility control vaccines that could potentially help address the overpopulation of wild horses on public lands by slowing their annual growth rates. The research projects, which will take place at a corral facility in Carson City, Nevada, are part of the BLM’s broader efforts to manage healthy wild horses on healthy public lands.

“Wild horses are incredible animals, but they can reproduce at a very high rate on public lands, which creates a host of challenges in arid environments,” said BLM Director Tracy Stone-Manning. “The development of humane, safe, and long-lasting fertility control vaccines is critically important as we continue to ramp up our efforts to protect these herds from the effects of wild horse overpopulation, drought, and climate change.”

Wild horses on public lands are protected and managed by the BLM and U.S. Forest Service. Without intervention by management officials, wild horse herds on public lands increase rapidly, doubling every 4-5 years. As of March 1, 2022 (the most recently published annual estimates), there were 82,384 wild horses and burros on public rangelands. This is nearly three times the appropriate management level deemed suitable to maintain a thriving ecological balance. Many of these herds live in arid environments with little water or forage. Constant overpopulation can stress critical ecosystems to the brink, causing severe damage to riparian and rangeland resources that can take decades to recover, if they recover at all. Moreover, chronic wild horse overpopulation can lead to the inhumane death of horses from thirst or starvation, and the destruction of habitat important to other wildlife, such as elk, deer, and sage grouse.

For decades, the BLM has used fertility control vaccines to help manage wild horse herd growth on public lands. However, the most common fertility control vaccines for wild horses in use today require more than one treatment to remain effective and are often not effective beyond one or two years. A single-dose vaccine that can last multiple years could provide several benefits for the populations of wild horses that the BLM manages, including requiring fewer gathers for retreatment or reducing instances of permanent removal.

One study led by scientists associated with Cornell University College of Veterinary Medicine and Northwest Wildlife Conservation Research, a small non-profit research organization, will test whether a form of porcine zona pellucida (PZP) vaccine known as ‘SpayVac’ lasts longer when injected in the neck muscle or the flank. The other study, led by the U.S. Department of Agriculture Animal and Plant Health Inspection Service’s National Wildlife Research Center, will test which formulations of the Oocyte Growth Factor vaccine cause long-lasting contraception from a single dose.

To test how well the vaccines prevent pregnancy, groups of vaccinated mares will live in a pen with a stallion. Researchers will monitor the mares’ responses to the vaccines and compare them against a control group. The health and welfare of all the animals will be monitored by researchers and other personnel, with veterinary care always available if needed. The approved projects will also have animal welfare oversight from independent animal care and use committees of the research institutions involved in the studies.

Details about the decision are on BLM’s eplanning website. The studies were analyzed in an environmental assessment that was released for public comment in 2022. The BLM also analyzed but is not authorizing at this time a third study that would have tested the effects of an intrauterine device. If the BLM authorizes that study in the future, it will do so through a separate decision. The BLM’s responses to public comments about all three studies are available on BLM’s eplanning website.

In addition to supporting the development of better fertility control tools, the BLM continues to take action to curb wild horse and burro overpopulation to protect animal and land health. The BLM set new records last year for the number of animals gathered, removed for private care, and treated/released with fertility control. Additionally, the BLM continues to ramp up its efforts to find good homes for excess animals; the agency placed nearly 7,800 animals into private care just last year.

Find more information about the BLM’s Wild Horse and Burro program at

The BLM manages more than 245 million acres of public land located primarily in 12 western states, including Alaska, on behalf of the American people. The BLM also administers 700 million acres of sub-surface mineral estate throughout the nation. Our mission is to sustain the health, diversity, and productivity of America’s public lands for the use and enjoyment of present and future generations.