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Utah BLM tries contraception to rein in wild horse population

Federal agency has seen early success in its efforts with Tooele County herds.



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Preventing pregnancy » It was 1971, and the ink was barely dry on Congress’ Wild Free-Roaming Horses and Burros Act when Jay Kirkpatrick, a young wildlife physiologist at Montana State University-Billings, had two unexpected visitors.

At a glance

Birth control for horses

Porcine zona pellucida, PZP, is the most widely used form of birth control for wild horses.

PZP is a protein derived from pigs’ eggs. When it’s injected into a mare’s muscles, it stimulates her immune system to produce antibodies that block fertilization.

The form approved by the EPA has to be “boosted” with another shot every year, and researchers are hoping to prove the effectiveness of a longer-lasting version, PZP-22.

The EPA last year also approved the use of GonaCon in wild horses and burros, although it has mostly been used in deer. The vaccine stimulates the production of antibodies that work to reduce the release of sex hormones, decreasing sexual activity in males and females.

Controlling Utah’s wild herds

The Bureau of Land Management is using porcine zona pellucida (PZP), a form of birth control for wild horses, to control the population of two herds in Tooele County.

The Onaqui herd has 260 to 270 horses, but should have no more than 210, according to BLM goals. The Cedar Mountain herd has close to 600, but should have no more than 390.

The Humane Society of the United States is studying the effectiveness of PZP in the Cedar Mountain herd. After vaccinating 39 mares with a longer-lasting form, PZP-22, in 2008 and again in 2012, only two mares had foals in 2013, a decrease from 21 the year before. Eleven of the 83 mares treated for the first time in 2012 had foals the next year, down from 50 foals the year before.

The BLM gave 57 mares in the Onaqui herd the vaccine in 2012, and plans to “dart” as many of the estimated 130 Onaqui mares with PZP again next winter.

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"Two cowboys showed up at the door of my office … and asked, ‘Can you make horses stop reproducing?’" Kirkpatrick recalls.

The cowboys, he says, worked for the BLM and were newly charged with managing the wild horses in Montana’s Pryor Mountains. They predicted that as soon as the appetite for mustang adoptions was satisfied, the BLM would have a glut of horses.

"Within 10 years, that was proven accurate," says Kirkpatrick, whose Science and Conservation Center would go on to develop PZP to control births in horses, deer, elephants and a host of zoo animals. The center still makes the vaccine from pig eggs obtained from slaughterhouses.

By 1994, the National Park Service was satisfied enough with Kirkpatrick’s research to begin using PZP to control the wild horse population on Assateague Island in Maryland. A year later, the herd stopped growing and 20 years later, it reached its targeted size — 100 horses.

The Assateague mares, Kirkpatrick says, are much healthier because they’re not constantly foaling. They live three times longer than before, he says.

But the BLM largely ignored the fertility control, choosing instead to treat the horses like livestock.

Except for BLM specialists managing a handful of herds in Montana, Wyoming and Colorado with PZP, the BLM doesn’t use fertility control in a focused, consistent way, he says.

Kirkpatrick doesn’t think it’s going to change.


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"The higher levels of BLM are still sitting around, wringing their hands," says Kirkpatrick, whose center is working more with Indian tribes now to control their burgeoning wild horse herds.

Suzanne Roy, director of the American Wild Horse Preservation Campaign, also is dubious.

She points to the BLM’s promise of "fundamental reforms" of the wild horse and burro program in 2011. To much fanfare, then-BLM Director Bob Abbey said the agency would begin using fertility control on 2,000 mares each year.

That never happened.

According to the BLM, it gave PZP treatments to 1,051 mares in fiscal year 2012, and to just 509 mares last year.

Griffin says it would need to treat 5,000 mares a year to have an effect on horse populations in all 179 herd management areas. Each herd would need the vaccine at least every four years.

"They are never going to get off this treadmill until they start using fertility control," Roy says.

Contraception challenges » Warr, in charge of the wild horse and burro program in Utah, says it’s not that easy.

The Onaqui herd is the only one of Utah’s 19 wild horse herds where darting is feasible, he says.

The rest scatter when a person gets within even a half mile.

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