FARGO — Theodore Roosevelt National Park has rounded up more than half of its herd of wild horses to collect DNA samples, health screenings and to give some mares a birth-control drug.

As of the morning of Monday, Oct. 14, a helicopter crew had rounded up 144 of the 198 horses roaming the park’s south unit, and park officials reportedly expect to gather the remainder Monday.

Citing safety concerns, park officials have not allowed the public or press to view the roundup but are allowing small groups to view the horses in a holding pasture and corrals from a distance until they are released back into the park.

During the roundup, park rangers are taking blood and hair samples to provide DNA information that will guide “a long-term management strategy to create a genetically viable, and healthy population at the park,” park officials said in a statement .

Some horses will be fitted with tracking collars so the park can “better understand their range and use of the park environment.”

Once all genetic samples and other information have been collected, and birth-control treatments are given to some mares, horses will be released by band.

“Some of the stallions are fighting,” said Chris Kman, of Chasing Horses Wild Horse Advocates. “The horses are stressed.”

Normally, each band of horses, consisting of a stallion and a bunch of mares and colts, sticks to their own territory.

Volunteers, including from the group North Dakota Badlands Horse, are helping handle the horses in the corrals in the south unit.

A small number of mares that did not successfully respond to earlier treatments with GonaCon, a contraceptive drug, will be given another dose. The park has said the extra dose is needed to ensure a few mares are not “skewing the population’s genetics by their breakthrough reproduction.”

Some other mares may receive a scheduled booster. The park released no further details on the GonaCon treatments, including how many doses each mare has been given or the treatment schedule.

Horse advocates are upset that the park is continuing widespread use of GonaCon. Researchers found 79% of mares that received two hand-injected doses, possible only during roundups, had not regained fertility seven years after their second treatment.

Kman and others have been asking the park to release detailed information about the birth-control treatments. They are concerned the treatments are impairing the herd’s genetic viability.

“We don’t know who they’re hand-injecting with GonaCon,” Kman said. “We don’t know how many,” or the frequency, and which mares are being allowed to go off the drug, she said. “We don’t know. It’s just ridiculous.”

There is a “growing movement” of horse supporters who are saying they will boycott the park as a result, she said.

Nancy Finley, the park’s acting superintendent, has said some mares are capable of becoming pregnant after the second dose has worn off but said she could not provide details. She said the “population is not at risk” from the GonaCon treatments.

“We’re just trying to create a stable, slow-growth population,” she previously told The Forum. “We don’t want to end up with a situation where we have a lot of surplus animals.”

No further roundups or horse removals are planned until after park officials have analyzed the information, which will guide management decisions.