FARGO — The low genetic diversity of the wild horses at Theodore Roosevelt National Park makes the herd susceptible to inbreeding problems — a risk some worry is exacerbated by the way the park removes horses to prevent overgrazing.

Last month, 14 young horses were taken from the park, a routine practice to keep the herd, which now numbers about 135, from outgrowing the range capacity in the park’s south unit.

All 14 horses have found new homes following an online auction, according to park officials and horse supporters who monitor the herd.

But some supporters of the horse herd — which has hundreds of thousands of followers on Facebook and other social media sites — are asking for changes in the removal practices to help alleviate potential inbreeding problems and other concerns.

Horses selected for removal are shot with a tranquilizer dart, then loaded onto a sled and pulled to a stock trailer, where they are revived, loaded into a stock trailer and taken to the park’s corral, where they remain until picked up by their new owners.

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Horse supporters say that method is much more humane than the former method, which used helicopters to herd the horses, which were sold in large numbers at an auction barn — where in the past they sometimes were bought for slaughter.

The problem, however, is the horses targeted for removal are members of bands that roam close to roads, making them easier to cull from the herd, said Chris Kman, a follower of the horses who is spearheading calls for changes in the removal policy.

“It’s seriously hurting (breeding) lines in the park,” Kman said. “We keep taking the same ones out and leaving the same ones in. It’s like they’re controlling the numbers and not the horses.”

Kman and her husband operate a store called “Chasing Horses” in Medora that sells memorabilia featuring photos of the park horses as well as a Facebook page of the same name.

Geneticists have studied the horses at Theodore Roosevelt National Park and found that the herd lacks genetic diversity to a degree that it raises concerns.

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“The study was confirmed that the herd has low genetic diversity, which should be taken into consideration in future herd management,” Gus Cothran, a horse genetics specialist and one of the authors of the 2018 study, said in a news release issued by Texas A&M, where he conducts research.

Cothran and his co-authors, who include Blake McCann, the park’s chief resource manager, concluded that the low genetic diversity hasn’t yet caused what geneticists call an “inbreeding depression,” a vulnerability where all the members of a population are very similar to each other.

That can allow certain diseases or conditions to take root, decreasing the offspring’s ability to survive, he said.

“They’re at a level of genetic variability that should raise concern,” Cothran added. “Their genetic variability is low enough that the possibility of inbreeding is real.”

To address the inbreeding vulnerability, Cothran and his co-authors recommended introducing outside horses, especially mares, to increase the herd’s genetic diversity.

A birth defect in horses called “lethal white,” which can occur in offspring when both the stallion and mare carry the gene, is common in the park, a sign genetic problems already are occurring, Kman said.

The incurable disease is fatal, causing the foal to die a painful death within 72 hours, she said.

So far this year, the herd has had 34 live births, with 25 foals surviving. Those that died included foals born with lethal white. In other cases, Kman said, park officials have said the foals “failed to thrive.”

The danger of inbreeding also has been exacerbated by a prolonged birth-control study, Kman said.

For 11 years, researchers from Colorado State University have been testing a drug to prevent mares from becoming pregnant. The researchers have shown the drug is effective, but still it requires periodic booster doses.

In recent years, however, the drug has not been administered as researchers work to determine an effective booster dose schedule and to see whether prolonged use of the drug leads to permanent sterilization.

The herd’s genetics are skewed by preventing certain mares from giving birth, but allowing others to reproduce, Kman said.

“That’s causing a lot of the issues, especially with the inbreeding,” she said.

Other birth-control drugs already have proven effective with other public wild horse herds, she said, reducing or eliminating the need to remove young horses.

Park officials plan to update the management plan for the horse herd. The current plan, written in 1978, considers the appropriate size of the herd to be 50 to 90 horses, much smaller than the current 135, down from almost 150.

If the herd were maintained at those smaller sizes, that would only exacerbate the potential for inbreeding, Kman said. Cothran and the other study authors recommend maintaining a herd of 120.

A new horse management plan will require preparation, so is likely a few years into the future, McCann previously told The Forum. Once started, a new management plan must follow a timetable set by statute and regulation.

Park officials were not immediately available to respond to Kman’s points on Friday.

Public comments will be sought to help shape the new horse management plan, McCann has said.

“We’re looking for ways to move forward,” he said in the earlier interview.