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Birth control for wild horses could prevent roundups to thin herd

FARGO - An experiment involving birth control for the wild horses in Theodore Roosevelt National Park in western North Dakota is about to get a boost.

Researchers have been testing a vaccine on some of the mares in the wild horse herd in the park's South Unit since 2009.

The immuno-contraceptive vaccine is intended to keep treated mares from ovulating, therefore helping to control the population of the herd.

If successful, the vaccine could provide an alternative to periodic roundups and removals of some horses to prevent overgrazing the range.

So far, the vaccine has achieved about a 50 percent success rate, far below the 90 percent success rate earlier tests demonstrated, said Dan Baker, a Colorado State University research biologist who is leading the study.

For some reason, free-roaming grazing mammals are less responsive to the vaccine than captive animals, Baker said.

One theory is that the immune systems of animals in the wild, which have to fend off more parasites and infections, are less able to generate the antibodies that the vaccine requires to work.

"You have to get the antibodies up to a certain threshold to get the horses infertile," Baker said.

With another roundup planned for September, researchers will try giving the treated mares a booster shot in the hope of strengthening the immune response to the birth control.

Typically, a second dose of a vaccine delivers more powerful and long-lasting effects, Baker said.

To evaluate the effectiveness of the booster shots, researchers will track the treated mares and control group of mares for three more years.

"That's the idea behind the second phase of the study," Baker said.

Bill Whitworth, chief of resource management at the park, is eager to see the results of the booster shots.

"After three years it's not as effective as we would have hoped," he said.

On the other hand, no adverse effects, physical or behavioral, were observed among the 28 mares that received the vaccine. Another 28 mares received injections of a saline solution to serve as a comparison group.

If the results of the second injections are promising, the park will seek public comment on the use of contraceptive vaccine as an alternative to roundups, which are costly and stressful for the horses.

"The ultimate goal is to minimize or eliminate roundups down the road," Whitworth said.

The roundups, done by helicopter to herd the horses, also involve risks. During a 2007 roundup, the helicopter crashed when hovering close to the corrals. No injuries resulted, of those onboard the helicopter or among the horses.

Use of contraceptive vaccines is being eyed as a solution to manage many animal populations, Baker said, including elk, deer, buffalo and wild boars.

The only way to determine the effectiveness of the vaccine on the park's wild horse herd is through experimentation, Baker said.

"It may not work at all," he said. "It could be a complete failure or the horses could be completely sterile."

Maggie Bauer, a technician who has been observing the mares in the park during spring and summer for the past three years, has gotten to know the herd well.

She and her colleagues have named more than 100 horses, names that fans of the horses use in tracking the animals.

One horse in particular is a sentimental favorite, a 3-year-old female Bauer named Esprit, born in March 2011. It was the first foal she saw in the study.

If that horse ends up being one of more than 100 young horses offered at a public auction on Sept. 28 in Wishek, Bauer hopes to buy her. If not, she will visit Esprit periodically in the park.

Esprit, which means joy of life, now has a foal of her own and is a member of Silver's band, one of 20 stallion bands in the South Unit.

The population of the horse herd is between 205 and 210, well above the target range of 50 to 90 or a bit more. Between 30 and 40 horses are born each year, Bauer said.

Patrick Springer

Patrick Springer first joined the reporting staff of The Forum in 1985. He can be reached by calling 701-241-5522. Have a comment to share about a story? Letters to the editor should include author’s name, address and phone number. Generally, letters should be no longer than 250 words. All letters are subject to editing. Send to