Rounding up wild horses in Montana

The U.S. Bureau of Land Management will round up about 190 wild horses in the Pryor Mountains along the Montana-Wyoming state line on Thursday – much to the chagrin of horse advocates.

A federal judge rejected a request by two Colorado-based advocacy groups, the Cloud Foundation and Front Range Equine Rescue, to halt the action.

The wild horse advocacy groups argued that gathering the herd could end up ruining one of the most genetically pure herds of Spanish colonial horses in the country. 

VIDEO: Horse lovers gather to offer a prayer to preserve wild horses right to run free

But the Bureau of Land Management, which operates the 38,000-acre Pryor Mountain Wild Horse Range, says there are too many horses on the range. The agency's Web site says that it is necessary gather the herd of wild horses and reduce their numbers  in order to "achieve a thriving natural ecological balance" in the Pryor range.  

The roundup had been delayed while U.S. District Judge Emmet Sullivan considered an injunction, but he denied the request Wednesday.

The Pryor Mountain Wild Horse Range was created in 1968 to protect an icon of the American West – the wild mustang.

But while the herd has grown to approximately 190 horses, the Bureau of Land Management says that the ideal number of horses for the range is about 120. The BLM has culled the herd before, in 1997, 2001 and 2003.

The horse advocacy groups opposed the roundup this year because they say the action is "unprecedented in size and scope."

The BLM will use a helicopter and wranglers on horseback to drive the horses into corrals Thursday. They plan to capture the range's entire population, with 70 adult horses and their foals to be put up for adoption and sale on Sept. 26.

The remaining 120 horses would be returned to the range and freed after some of the mares are given a contraceptive vaccine.

Watch the video link above to see NBC News' Charles Hadlock report on a recent prayer vigil and group ride held by advocates from the Cloud Foundation for the mustangs in the Pryor Mountain range.

And see NBC News' Kerry Sanders report on the roundup of wild mustangs on the Today Show on Friday morning. 

Click here for more information about how to adopt one of the horses.