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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.

First Published Jun 15 2014 01:01 am • Last Updated Jun 18 2014 10:47 pm

Simpson Mountains » A herd of wild horses grazes amid sagebrush and knee-high clumps of grass along the path Pony Express riders rode through southern Tooele County.

Foals lie beneath their mothers, stallions pester the mares and every little while, a quarrel erupts and studs rear up, slam their front hooves down and try to bite each other.

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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Accustomed to visitors with cameras, these 100 mustangs do not run when humans come near.

Their ease with interlopers makes this herd a perfect candidate for a kind of birth control that involves shooting the mares in the rump with a dart gun.

And that’s exactly what the Bureau of Land Management in Utah proposes to do next winter with the estimated 130 mares of the Onaqui herd, which ranges over 323 square miles between the Onaqui Mountains east of Dugway Proving Ground and the Simpson Mountains to the south of the Army base.

If approved after an environmental assessment, the fertility control effort will be the second one undertaken in the herd, which, like a sister herd 40 miles to the northwest in the Cedar Mountains, were vaccinated against pregnancy in the winter of 2012.

These efforts are important, even if they are a tiny piece of solving what has become a huge problem for the federal land agency: herds that double in size every four years.

Utah has 4,300 wild horses this year, double the number the BLM believes there should be on the range, says Gus Warr, who takes the lead on horses and burros for the BLM in Utah. "We could easily be at 5,000-plus animals next year," he says.

The problem is mirrored in nine other Western states. Altogether, there are 49,000 wild horses and burros on ranges that should have less than 27,000, according to the BLM, which is charged with managing public land for recreation, resource development, wildlife and livestock as well as for horses.


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‘Use the tools you have’ » The old way of thinning the herds — rounding up horses, and trucking those that no one adopts to long-term pastures in the Midwest — is fast fading as an option.

The BLM spent 64 percent of its $72 million budget last year taking care of 50,000 pastured wild horses, and a study commissioned by the agency found those removals are only making the situation worse.

Horse populations grow faster when there’s less competition for forage and water, according to the report, released a year ago by a research panel at the National Academy of Sciences.

"They are up against a wall. They are running out of options," says Stephanie Boyles Griffin, senior director of innovative wildlife management for the Humane Society of the United States.

The animal welfare organization has teamed with the BLM since 2008 to study fertility control in the Cedar Mountain herd as well as the Sand Wash herd in Colorado, and it is slowing population growth. The Annenberg Foundation is helping fund the research.

Only 5 percent of the Cedar Mesa mares that were first vaccinated in 2008 and given a booster in 2012 had foals last year, Griffin says.

The NAS report strongly urged the BLM to embrace fertility control — and give up removals — and pointed to the benefits of porcine zona pellucida (PZP), which is being used on the Onaqui and Cedar Mountain herds.

The BLM has been slow to adopt contraception — though, at times, it has promised to do so — in part because it has been holding out for one that will work for five or more years.

The Environmental Protection Agency in 2012 approved the most common vaccine that uses PZP, Zonastat-H, but mares have to be vaccinated every year for it to remain highly effective. A timed-release pellet version of PZP, which is only available for field trials, shows promise in controlling births for three years, albeit at a diminishing rate.

Both can be administered by dart gun, or by shots when mares are temporarily captured.

"I tell them, ‘Do not let perfection be the enemy of progress. Use the tools you have to the very best of your ability right now,’" says Griffin.

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