Deep inside the Theodore Roosevelt National Park in North Dakota, a herd of wild Mustangs are doing what wild horses do–grazing, resting, playing, pooping and reproducing.

And Colorado State University researchers are hoping that last one is starting to slow down.

“Theodore Roosevelt National Park is a unique park in that is it totally enclosed by fence,” said Terry M. Nett, a professor of the Animal Reproduction & Biotechnology Laboratory at Colorado State University. “So there’s no place for the animals to go, and as their numbers increase, they’ll start causing problems such as forage damage, and then they’ll start dying of starvation.”

To prevent that, park officials became interested in limiting the Mustang herd’s numbers, Terry said, and wanted to try the same gonadotropin-releasing hormone (Gnrh) contraceptive vaccine that proved to be successful on the elk herd in Colorado’s Rocky Mountain National Park.

While the vaccine can be administered by dart, Terry said, researchers instead opted for the wild horse herd to be rounded up and treated, in order to make sure all the Mustangs got the full vaccine at the same time.

So 110 of the wild horses were rounded up, and that number was culled to 67 by selling part of the wild horse herd for domestic use. Of the remaining wild horse herd, half the mares were treated with the vaccine, and half were treated with a placebo.

Now for the next four years, researchers will be observing the herd and collecting poop samples left by identified mares in order to run pregnancy tests on them.

“We’ll check their fecals to see if any are pregnant or lost a foal,” Terry said.

And who gets that fun job? Some Colorado State University students, as well as some volunteers who have been observing, photographing and identifying the herd for years, he said.

The study, which began in October 2010, will run through October 2014, Terry said. It’s co-sponsored by the Colorado Horse Development Authority (CHDA), as well as the Morris Animal Foundation.

“It worked great on the elk–the vaccine had a 90 percent success rate after the first year,” Terry said. “It’s too early to tell if it’s going to be effective on horses, but observations after the first year are that the vaccine is 50 to 60 percent successful so far.”