The roundup will focus on branded horses with "no apparent owner," Miller said.
Wild horses in the West are managed by the BLM through the federal Wild Free-Roaming Horses and Burros Act of 1971. The federal act prohibits public roundups of wild horses and burros, and wild horse advocates have called on the BLM to block any roundups by ranchers in Utah or Nevada.
In recent years, wild horses have been joined by a growing number of horses released by people who could no longer afford to stable and feed them. Those "feral" horses are the ones Iron County will target, Miller said.
"We may only see 50 or 60 horses this week, but maybe as many as 100 to 150 next week," Miller said.
The roundup, according to a letter sent from the Iron County Commission to the BLM, is needed to get wild horse populations within the numbers established by the federal agency.
The issue is coming to a head because drought is already threatening range conditions, and ranchers have been asked to reduce by 50 percent the number of cattle they have grazing on public land under permits.
The horses to be captured this week would be moved to private property, fed by the BLM and eventually moved to federal holding facilities for public adoption, according to Miller.
Plans for the roundup will be expanded to include wild horses, he said, when more details are worked out with the BLM.
Under the federal act, those who remove or attempt to remove wild horses or burros from public land without authority could face a fine of up to $2,000 or imprisonment for up to a year, or both.
Maliciously causing the death of the animals, harassing them or capturing one for private use also violates the act.
Miller argues the BLM is breaking federal law by not appropriately managing the herds. As the chair of the Iron County Commission, he said, he has authority under Utah State Code to arrange a roundup.