The Pryor Mountains can support another five to 15 head of mustangs, according to a new reckoning by the U.S. Bureau of Land Management.

A judge ordered the bureau to recalculate the appropriate management level (AML) in 2015, said Nancy Cerroni of the Pryor Mountain Wild Mustang Center in Lovell.

In July, U.S. District Judge Susan Watters noted that the bureau had stated, in a 2009 decision, that it would recalculate the appropriate management level within five years.

“The court finds that federal regulations, case law and its own representations to the public bind BLM to this commitment,” Watters said. 

In a Dec. 21 notice, the bureau announced it had “completed an analysis of monitoring information and recalculated the AML.”

“In summary, the recalculation formula indicates a maximum AML of 98 to 121 adult wild horses; therefore the report states the current AML of 90 to 120 adult wild horses would achieve a thriving natural ecological balance on the PMWHR (Pryor Mountain Wild Horse Range),” the notice from the U.S. Department of Interior/Bureau says.

Cerroni said she supports the work of the bureau’s Billings field office.

“I think the management plan is really good,” Cerroni said, saying it’s flexible, adaptive to range conditions and employs porcine zona pellucida (PZP) — a form of birth control for the horses. The bureau’s aim is a healthy population. It does not manage for specific colors in the horses. The bureau cares about the Spanish influence in the mustangs’ blood and a strong genetic pool, she said.

Cerroni’s son, Matt Dillon, has been tracking the herd’s lineage since the 1970s, she said.

“I think, quite honestly, one of the biggest obstacles to their continued survival is their gene pool,” Cerroni said.

The bureau is working with the center and Ginger Kathrens to watch genetics, Cerroni said. Kathrens is executive director of the Cloud Foundation, a Pryor wild horse advocacy group.

The herd has averaged around 160 head for years, Cerroni said.

There were 160 adult horses on the range in 2016, but there are no plans in the immediate future to gather the horses for later sale, said Jim Sparks, Billings field manager for the bureau.

Colt control

The Pryor Mountain Wild Mustang Center supports the use of PZP to prevent pregnancy in mares, but its leaders want to observe how it affects fertility when a mare is taken off the drug, Cerroni said. The bureau has been sharing its Pryor-PZP data.

The bureau is administering PZP to achieve a birth rate equal to the wild horse death rate, Sparks said.

Darting horses with PZP can be effective if the herd is small enough to distinguish individual mares, and if shooters can get close enough for a shot.

In herds ranging in the thousands, PZP is not so successful, Sparks added.

Cerroni said she believes the range can support 160 adults, because it has for years.

The population has averaged 155 adults for the last nine years, according to the bureau’s recalculation document.

Quoting Gus Cothran, Cerroni said the herd must have at least 150 adults to maintain healthy genetics. Cothran is the director of the Equine Blood Typing Research Laboratory at the University of Kentucky-Lexington.

A minimum herd size of 50 effective breeding animals with a total population size of about 150-200 animals is recommended, Cothran said in the bureau’s 2010 fertility control environmental assessment comment section.

Water & fewer foals

In 2009, the bureau undertook some water projects to encourage the horses to stay put longer in the mid-level of the Pryors to allow the upper (summer) and lower (winter) ranges to recover from grazing, Sparks said. If nutritious flora are  overgrazed, they will die off or be unable to compete with other less-beneficial plants.

The higher and lower grounds are at less than their potential, Sparks said.

The horses migrate to the upper range every summer, he said, “just like a herd of elk.”

Speak up

The public is welcome to comment on the recalculation and/or the report. Send comments to Bureau of Land Management, Billings Field Office, 5001 Southgate Drive, Billings, Montana, 59101-4669. The deadline is the close of the business day on Jan. 24.

The center will review the reports, Cerroni said.

The Pryor herd is doing OK, Sparks said. “They’re healthy.”