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Research report: Fertility control slows wild horse population growth


By Bob Conrad & Tatum Gogna

A recently published research article shows fertility rates among Virginia Range feral-estray horses are slowing thanks to a darting program targeting wild mares. The range’s wild horses are considered “feral estray” because they are under the jurisdiction of the state, unlike the rest of Nevada’s wild horses, which fall under the federal government’s control.

The study’s authors include a faculty member at the University of Pretoria, Martin Shulman, and American Wild Horse Conservation (AWHC) personnel. They found that darting mares with the pZP vaccine since 2019 has slowed horse population growth on the Virginia Range. 

“This study showed that sustained, multi-dose administration of a native pZP vaccine was able to significantly reduce growth rates of even a large and extensively distributed population inhabiting challenging terrain,” the study concludes. “Population coverage had reached 72.5% within four years, which resulted in an almost 60% reduction in foaling. This was possible despite the additional complication of mares missing vaccinations due to their movement outside the approved darting area.” 

In a recent report to the Nevada Board of Agriculture, Allison Hinkle, the Virginia Range program coordinator with AWHC, shared the news about the effectiveness of the Virginia Range fertility control program. 

“The reduction in births paired with a high full mortality rate primarily due to predation has resulted in more deaths than births and thus negative population growth,” she said. “They have been able to confirm the population within the City of Reno rangeland interface has significantly declined.”

The research found that “foaling rates approximately halved in 2021 and by 2022 were further reduced by almost 60%. This was paired with a high foal mortality rate, peaking at 63% in 2022.”

Horses are rounded up via a helicopter operation by the BLM in the central Nevada Pancake Complex in January 2022. Wild horse conservation advocates say fertility control is an effective alternative to helicopter roundups for managing and reducing feral-estray and wild horse populations. Image: BLM

AWHC representatives said the study’s findings show “further evidence of the feasibility of humane fertility control as a viable alternative to helicopter roundups and removals for wild horse management on Western public lands.”

“Our hope is that this study provides impetus to the growing calls for reform of the federal wild horse management program, which relies on costly helicopter roundups, the unsustainable removal of wild horses, and confinement of these animals in tax-funded holding facilities,” said Nicole Hayes, AWHC’s conservation scientist. 

A spokesperson for the Nevada Department of Agriculture said the agency had yet to review the study.

“The NDA has plans to conduct a population census this year (2024), and we would be happy to provide more information after that,” Ciara Ressel said. “What we can say is [that] scientifically proven methods of fertility control are a key component of a multifaceted approach needed to manage the Virginia Range, and the American Wild Horse Conservation has been one of our partners in working to protect the Virginia Range’s ecosystem from overgrazing by feral and estray horses.”

Tatum Gogna, a University of Nevada, Reno student minoring in agricultural and environmental communication, contributed to this report.

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