There is, perhaps, one way to turn down the heat -- something the BLM itself began back when it was first tasked with protecting wild horses and burros.

On a hot summer afternoon in 1971, a young reproductive biologist named Jay Kirkpatrick was working in his office at Montana State University when, as he recalls, "two BLM cowboys with sweaty hatbands and shit on their boots walked in."

"They said, 'Can you make horses not reproduce?' " Kirkpatrick, now 72, remembers. "Even back then, they knew it was going to be a problem."

Kirkpatrick said he thought he could. First, he looked into spaying or neutering in the field, but quickly learned that ornery horses and rugged terrain made it nearly impossible. Next he tested hormonal steroid drugs similar to human birth control pills, but discovered the large doses required could not be delivered through a dart. Plus, they changed horse behavior and lingered in the food chain.

In the late 1980s, he finally settled on Porcine Zona Pellucida, or PZP, a sticky protein that coats domestic pig eggs, allowing sperm to bind to their surface. All mammals have similar egg coatings. When PZP is injected into any mammal, its immune system flags the intruder and responds by making antibodies shaped to bind to the chemical. But the antibodies also bind to the animals' own eggs, blocking sperm for about a year.

The drug is simple to apply with a dart and easily reversible, so managers can change course if there's an epidemic or a big winter die-off. It also breaks down and doesn't linger in the food chain. Just as important, Kirkpatrick says, "It's cheap," about $25 per dose.

To make the stuff, Kirkpatrick's lab at Zoo Montana in Billings gets cast-off pig ovaries from a Midwestern butcher, grinds them to isolate the coating, then parses tiny doses into vials to be loaded into darts in the field. A few other researchers also make PZP. Kirkpatrick says the process could easily be ramped up if there was more demand.

On Maryland's Assateague Island, Kirkpatrick has successfully used the drug over 20 years to trim the local wild horse population from 175 down nearly to the goal of about 100. He has also used PZP effectively on elephants in enclosed game reserves in Africa, water buffalo in Guam, urban deer on the East Coast and scores of zoo animals -- sending the drug to wildlife managers around the world. "Anything with a hoof, it seems to work," he says.

Paradoxically, the agency that inspired his research has been hesitant to adopt the drug. The BLM has been "studying" the use of PZP in wild horses for over 20 years, according to agency documents, but has never treated much more than 1,000 of the estimated 19,000 free-roaming mares under its management in a given year, so the drug has had little big-picture impact. "It's a cultural thing," says Kirkpatrick. The BLM still has "a cowboy mentality and it will take a generational change to get over that."

But BLM spokesman Gorey counters that the obstacles to wide use of PZP are more practical than cultural. Though PZP shows promise in small studies, it's hard to apply to huge herds in wide-open spaces. "We want to pursue the PZP avenue as far as it can go. The challenge is that our horses roam over essentially 30 million acres. This is not Assateague Island."

There are very few places the BLM can get close enough to use a dart gun, he says, so the agency rounds up horses by helicopter, then injects mares by hand. For PZP to work, the agency then has to recapture each mare every year to treat them. "That is just not a functional solution." Researchers are working on a form of PZP that could last four or five years, which could make catch-and-release practical, Gorey says, but results so far have been mixed.

And because wild horses are above the target populations set by the BLM in most of the West, Gorey adds, in many cases the agency can't treat and release them without risking lawsuits or damage to the land. It must remove horses until it reaches its target. "It is a wonderful goal to treat and release and not keep adding to holding, but we are not there yet and it is a tough road ahead."

Dissatisfied with such answers, Kirkpatrick searched for another way to get the drug out to the herds. He needed people who were able to recognize individual mares in a vast landscape and get within 50 meters to dart them. Only one group fit this description: Wild horse lovers.

All across the West, people like T.J. Holmes have fallen under the spell of these animals. They visit specific horse herds, photograph and blog about their favorites. They ride with them and camp with them. After roundups, they often adopt them.

In 2001, Kirkpatrick began inviting such enthusiasts to his lab for a three-day PZP training. One of his first graduates was a retired Colorado schoolteacher named Marty Felix. After she learned how to mix the drug and shoot a dart gun in 2002, she convinced her local BLM office to let her dart the 150 wild horses of the Little Book Cliffs herd near Grand Junction. Within a few years, PZP cut the herd's offspring by nearly half, she says. "Because of what we've done, they've postponed roundups again and again. They just don't need them."

News of her success inspired other groups in Wyoming, Montana, New Mexico and Colorado. More people trained with Kirkpatrick and cajoled their local BLM offices into trying PZP. Looking for a better solution than roundups, Holmes trained with Kirkpatrick in 2010.

These volunteers now account for about 16 percent of PZP applications. Yet even with their help, the BLM has sometimes failed to meet its goals. In 2012, for example, it aimed to inject 2,000 mares, but treated only about 1,015. Next year, it plans to treat only 900, claiming that it must redirect limited resources to gathering horses threatened by the effects of this year's serious droughts and wildfires.

Altogether, volunteer horse groups are now treating five herds in four states. The herds are small -- none larger than 150 animals -- but advocates say they are seeing results. And despite the currently limited reach of their efforts, many feel lessons learned in the field could inform larger attempts to use fertility drugs in the future.

"We pushed for it because there has to be a better solution than roundups," says Karen Herman, who began treating horses in northern New Mexico in 2009. "And it has made a real difference."