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GRAND JUNCTION, Colo. — The federal government is working to control the population of wild horses in Colorado. Hundreds of them across the region are creating serious challenges for the land, livestock and the horses themselves according to Bureau of Land Management (BLM) findings.

The goal for horse advocates is to control the population without harming the horses. There are different ways to get the job done.

In Colorado, the BLM uses darts full of a birth control vaccine called PZP to prevent births for an entire year. Just outside Grand Junction, 69-year-old Marty Felix is assisting in those efforts at the Little Book Cliffs Wild Horse Range.

“I’ve known [these horses] since birth,” said Marty. “I’ve known all of these horses since birth.”

Marty, a retired school teacher and founding member of “Friends of the Mustangs,” has been visiting the herds near Grand Junction for more than three decades. She knows all their names, ages, history and more.

Her knowledge is very valuable to the BLM and its birth control darting efforts.

Every year, BLM specialist Jim Dollerschell, Marty and dozens of others study detailed notes before taking aim at a selection of horses across more than 36,000 acres—a range that should support no more than 150 horses. There are currently 157 horses at the Little Book Cliffs Wild Horse Range.

When Jim, Marty or another qualified darter finds a horse that needs PZP, a shot is taken within 50 yards.

The PZP injected into the horse lasts for a year preventing only future pregnancies. Studies show the vaccine does not harm an already pregnant horse or her unborn foal.

Population control prevents overgrazing and helps female horses live years longer by limiting the toll of pregnancies, according to Marty. Before PZP, the average female on the range gave birth 37 times. That number has now dropped to 17.

“We had to create a new age group for mares,” said Marty. “20 and over.”

PZP also saves money by preventing about 20 births per year. The prevention accounts for horses that may have otherwise been sent to holding facilities, costing taxpayers an average of $50,000 for one horse over its lifetime, according to government statistics.

“Every baby that we prevent, that’s one fewer baby I have to worry about,” said Marty.

Birth control also limits the likelihood of euthanasia considerations–something the BLM has recently rejected. But no matter any success, some critics say the darters should focus more on adoptions and let nature take its course.

“If nature takes its course, then we’re in big trouble because we’ll have too many horses on the range,” said Marty.

Volunteers with the BLM have been darting for more than a decade. They get together every February to decide which mares to target. They have a window of several months after the snow leaves and before horses start breeding in the spring. The closer volunteers can dart prior to breeding season is the best, according to the BLM.

There are many challenges to darting wild horses, and it’s only getting more challenging as the smart animals wise up to the darting process and try not to get tagged.

“It really gets frustrating when you do spend that three hours with that mare, and you don’t get her,” said Jim.

For now, darting is working in Colorado. The BLM said the range near Grand Junction is unique. Birth control doesn’t work everywhere.  Ranges elsewhere hope for round-ups and/or solid adoption rates. For Marty, the goal is for horses to be born on the range, to live on the range and die on the range.

“When one of my old buddies disappears, it kills me,” said Marty.

There are more than 100 members of “Friends of the Mustangs.”

Not many of the group’s members are from younger generations. There’s now an effort underway to attract younger members to make sure horses will continue to have advocates for years to come.