The agency finished the roundup Thursday with 143 horses removed from Blawn Wash, Reid said.
The BLM uses a contractor whose pilots fly helicopters over the area until they find a small band of horses. A chopper then "herds" the running horses into a corral temporarily set up on the range.
The horses are loaded into trucks and taken to a second temporary corral, in this case, on a nearby ranch. That is where the filly died.
Reid said the young horse was the first yearling placed in her corral that morning, and was alone and pacing. Workers were determining the genders of other horses and not watching the filly when they heard her slam into the corral panel.
Anne Novak, executive director of the organization Protect Mustangs, criticized the BLM for the loss of the horses.
The yearling "was obviously terrified by the whole ordeal," Novak said in an emailed statement. "Once they are terrified, the risk of injury is high. The BLM needs to train their staff to understand wild horse behavior so tragedies like this will never happen again."
She also said the BLM was wrong to kill the 7-year-old mare whose previous injury had healed.
"The BLM should have made an effort to give her the best veterinary care possible. Horses heal and this mare had already recovered from an injury in the wild," Novak said. "I'm sure someone would have adopted her to help her get well."
Reid said there was nothing the BLM could have done differently to save the filly. "She was not being pressured. She was not being moved," she said. "It's just an unfortunate circumstance."
The veterinarian at the roundup judged the sorrel mare to be in pain because there was severe swelling and her leg was slanted inward at a bad angle, she said. BLM rules require suffering animals to be put down.
"It's very unfortunate that someone would make a statement [while] not having all the information," Reid said of the criticism.
The Blawn Wash includes a large section of state lands that were seeded over the decades with grass for livestock and are a magnet for the wild horses. The BLM's management plan calls for no horses to be there, although they have ranged there for more than 100 years.
The 141 horses were trucked to the Central Utah Correctional Facility at Gunnison, where they'll be examined, vaccinated and prepared for adoption. The prison inmates may keep some for training.
Even with 143 horses removed, Gus Warr, the agency's manager for wild horses and burros in Utah, figured more than 100 would remain in the Blawn Wash area. Utah has nearly 4,000 wild horses, more than double the number the BLM has set as the upper limit.
Ranchers in the region sued the BLM, and county commissioners in Iron and Beaver counties threatened their own roundups if the agency did not reduce the numbers of wild horses.
Besides the Blawn Wash roundup, the BLM has trapped 25 horses and intends to trap 25 more when they go for water on private land in Iron County. The agency also plans to remove 30 from along State Road 21 in Beaver County, near Nevada, on Friday.
The plan was to remove only 10, but the Utah BLM recently got approval from Washington to remove 20 more from the Sulphur herd. Warr saw 140 horses near the highway on Sunday, Reid said.
Those 30 horses, coveted because of their Spanish lineage, will be put up for adoption via the Internet in the fall, Reid said.