BLM officials say they are upholding the law by keeping the range healthy for wildlife and cattle, as well as horse populations, as is the agency's mandate. The 1971 law makes sale of horses to meat slaughterhouses illegal.
"The law mandates the protection, management and control" of wild horses and burros, according to a BLM statement.
But Ginger Kathrens, of the Cloud Foundation, said the BLM is "managing these animals to extinction."
The agency rounds up about 10,000 mustangs each year, she said, but is able to only adopt out about 2,800. That has left the BLM caring for more than 47,000 animals that are crammed into holding pens.
"Instead of finding ways to manage horses on the land, the BLM is stuck managing them off the range," she said.
Some animals suffer and even die from injuries suffered in roundups, she said. Beyond that the practice is expensive. In 2011, according to Kathrens, taxpayers spent $48.2 million warehousing wild horses and burros. The total cost of the BLM's wild horse and burro program is $75.7 million annually, she said.
The large number of animals in detention has led to what Kathrens calls "kill buyers," people who "adopt" wild horses for shipment to Canadian and Mexican slaughterhouses.
According to Denver Post columnist Teresa Keegan, wild horse advocates worry that a Colorado rancher who has purchased 1,700 horses from the BLM since 2009, is sending them to slaughter.
Monday, Lisa Reid, spokeswoman for BLM's Utah wild horse and burro program, dismissed that as "rumor."
It is a matter of record, however, that two Utah men were indicted last year on felony counts for allegedly trying to buy horses from a government-sponsored sale with intentions to resell the animals for slaughter in Mexico.
The agency is holding large numbers of horses for adoption, Reid conceded. She noted that not all of them are housed in cramped pens.
And she said the agency is working at finding a solution for what the BLM describes as an overpopulation problem. The agency is using fertility control and considering other options for keeping the population of mustangs in 10 Western states to a designated population of 26,500.
Currently, the BLM estimates there are 37,300 free-roaming horses and burros in the West.
But Kathrens and others say the agency does not have good numbers and that dwindling herds leave small gene pools that lead to unhealthy animals.
"They'll tell you they have 37,000 horses on the range. Then they'll remove 5,000. And then they'll tell you they have 37,000 animals on the range."