Transcript for Wild horses rounded up by helicopter in program sparking controversy

Reporter: A wild mustang charging across an open plain. A simple of the untamed majesty of nature. But the predator chasing these horses is anything but natural. You can see the terror and the fear. Reporter: This is a helicopter roundup commissioned by the U.S. Bureau of land management, or blm. Sometimes these horses get stampeded for miles and miles. We feel that the American people has a right to know what's happening out here in the wild west. Reporter: It's a controversial practice where horses are herded off public lands and sometimes permanently put into holding facilities in an effort to control their population. But now the government is considering allowing the slaughter of these animals for the first time in nearly 50 years. Putting the lives of thousands of horses at stake. Most of America's estimated 75,000 wild horses live on public lands. Vast expanses that the government controls. But to find the mustangs you need to know where to look. Jim schneppel knows this range and its horses well. We are going to go out and see hopefully the big herd of horses on the north end of Davis mountain. He works with a non-profit desperately trying to find a humane and effective way to control the horse population. Because although this land seems limitless the blm says the resources here can only allot for the survival of so many. They've set what they call appropriate management levels, aml. It ranges between about 120 to 180 horses. And we think that's far too low. Reporter: My god, they're so beautiful. Being able to be out in the wild with this herd of wild horses, it's really unlike anything I've ever experienced before. Anybody who loves horses can tell you there's this natural connection you can develop with them. Reporter: But horses aren't the only animals on the range. Bureau of land management controls 1/8 of the country's land mass but leases over 60% of it to ranchers. Because their livestock rely on the same resources the wild horses do, some ranchers want the horses pushed off the land. What are the options after roundup for these horses? What they'll initially do is go to one of the local holding facilities, those that are unlikely to get adopt or get passed over typically they'll get put into long-term holding. Blm has contracts with ranchers, pays them so much per day to keep the horses. Just keep them there. Reporter: There are over 45,000 wild horses currently in holding areas, costing taxpayers about $50 million annually. An expense the U.S. Department of the interior sought to address in its 2018 budget. By lifting regulations that prevent slaughtering wild horses. Blm, the very agency in charge of protecting them, is asking congress for permission to kill them. They've stockpiled wild horses in holding pens. Now we have a bunch of wild horses that the taxpayers are paying for in holding facilities and their solution is kill them. Reporter: But the blm maintains the activists and the agency both want what's best for the animals and denies that any of their practices are cruel. Just as with any type of species, they have to be managed just so that they don't become overpopulated and diseased. And our gol is to always have healthy horses on healthy range lands. Reporter: But the key battle for wild horses won't be fought in the west. Their fate will be decided here in Washington, where congress will decide if the interior department's budget allowing for slaughter will be enacted. 84% of Donald Trump's voters oppose the slaughter of wild horses. And a very narrow band of people are for it because they profit from it. Those opposed to wild horses continuing to be free say it costs as much as a billion dollars over ten years. Does that number seem reasonable to you? No, it doesn't. They're estimating that you have to remove all these horses and put them into holding and pay for them, which is simply not the case. This is our dart gun. This is called the jm standard. Reporter: Advocates also say the government hasn't invested enough in targeting alternate solutions to population control. Oh! Wow. Reporter: Jim hopes darting mares are birth control will mean far fewer foals. That's a serious hypodermic needle, though. Yeah. Reporter: With more horses left to live free on the range for much less than it costs to keep them corralled. The dart's about two bucks and the medicine goes, in it's about 26 bucks. Reporter: And for the horses that are rounded up, a lucky few make it to places like this where people like Ellie price are dedicated to turning these wild mustangs into companion animals. Once they've been channeled and trained they're usually kind of on the quiet side, actually. Reporter: She cares for over 200 at her California ranch. I don't think people have any idea that a wild mustang can be ridden like this. Probably not. These horses are gentle now. So I don't even think of them as wild mustangs anymore. But this horse was born in the wild. Reporter: The process of training a wild horse is not usually an easy one. He doesn't like me to be in this eye. Reporter: She says he's trying but you see his whole body shakes. It makes you think of a small child shaking. Reporter: Trainers work painstakingly to acclimate these horses to a human's touch. It's important he sees it in both sides. So it doesn't terrify him. I can drop the pad. Nancy was able to get that saddle on for the very first time and he did not flinch. He's doing everything she asks him to do. What I hope people will take away from seeing our horses is that they have value. But the big picture is that wild horses need to stay in the wild on public lands. Reporter: What makes you smile when you look at these horses out here? They are basically our American spirit. The living remnants of a long past. These horses were here before our ancestors ever got here. So how lucky are we that we can like you're in the wild west. And we can still see wild horses. Reporter: For "Nightline" I'm Gloria Riviera on the inaki

This transcript has been automatically generated and may not be 100% accurate.