As amazing as they might be, officials at Theodore Roosevelt National Park are taking steps to reduce the population of wild horses at the park.

Sarah Gustin has the story.

helicopter nats Capturing the wild spirits of the badlands

helicopter They're an amazing sight, but park officials say there's just too many

1:02 (Valerie Naylor / Park Superintendent) "We can only keep so many horses in the park. They are grazers and we also have bison, elk, prairie dogs and other grazers, so we have to keep the herd at a resonable level." Since 2003 only 10 horses have been rounded-up

Right now about 165 horses roam 46-thousand acres

Park officials would like to see that number closer to 70. So 90 head are heading to auction

3:21 (Valerie Naylor / Park Superintendent) "Alot of people like to see the horses, others don't like seeing them. they are a historic demonstration herd. So there are people that feel they don't belong in the park, but many people still really love them. And it does give us a glimpse into what it was like in Theodore Roosevelt's time when wild horses roamed these areas." Today helicopters fly the skies

But Round-ups are expensive and can be traumatic for the animals. shoot nats Now birth control is being considered an option

1:35 (Valerie Naylor / Park Superintendent) "What we are doing with the contraceptive is really a scientific study. What we are trying asertain whether it will work on horses like that. it is just a study at this point, it's not something that we are using to try to manage the herd." 1/2 of the 50 mares in the study will receive the contraceptive. The other 1/2....a placebo. 5:50 (Jenny Powers / Wildlife Vet) "It doesn't tend to effect the welfare as far as they continue to eat, they continue to move, they don't become lame. We think that the side effects will be rather mimimal

Powers says the vaccine is temperary. They aren't sure how long it lasts or if it is reversible

6:12 (Jenny Powers / Wildlife Vet) "Just about with any drug nothing is going to be 100 percent effective. In other animals we have found the vaccine is about 95 percent effective the first year and then two years down the raod about 75 percent and about three years down the raod 50 percent. so as the years go on it is going to become less and less effective." Powers says the vaccine has been used in deer and elk

She says other species have returned to fertility in 2 to 4 years

She says they will be monitoring the horses closely for at least the next 5 years

North of Fryburg for KX News, I am Sarah Gustin.

All adult females older than 2 will be kept in the park.

The 90 horses heading to auction will be sold Friday in Dickinson. watch the video | save this article / add to your favorites list