Federal officials claim mustang numbers are growing so rapidly that there could be mass starvation due to a shortage of forage in the drought-plagued region.
Having rejected a mass cull, Ken Salazar, the US Interior Secretary, favours a plan to move thousands of wild horses and burros, or feral donkeys, to the Midwest and eastern United States.
In a hearing this week near Reno, Nevada, members of the US Bureau of Land Management clashed with dozens of wild horse advocates who say the proposal is cruel and unnecessary.
The roundup - which would use helicopters to drive the mustangs towards and cowboys wielding lassos - has been attacked by a string of Hollywood stars including the singer Sheryl Crow, actors Ed Harris and Lily Tomlin, and the comedian Bill Maher.
The government's opponents claim officials have inflated mustang numbers so they can open up public land in the 10 affected states to development.
They argue that the mustangs are an integral part of the west's ecosystem and that using helicopters to round them up - before they can be lassoed - could traumatise, injure and even kill the animals.
At the Reno hearing, campaigners asked the bureau's national horse and burro advisory board to impose a moratorium on roundups pending an independent audit of mustang numbers.
Crow, who owns mustangs, said she had spoken to Mr Salazar about the roundup plans.
"One of the first things he said was something must be done because the horses are starving. We don't believe it," she said.
Mustangs have been a deeply controversial issue in the American West for decades.
Ranchers have long seen them as a menace to their land and were allowed to kill them until 1971 when it was made illegal.
Prompted by a public outcry, Congress granted mustangs federal protection as "living symbols of the historical and pioneer spirit of the West".
Federal officials say there are nearly 37,000 mustangs and burros on public land in the west, half of them in Nevada and 10,000 more than they feel is sustainable.
A further 34,000 live in government corrals and pastures which are nearly full.
Left unchecked, mustang herds can double in size every four years. The government has tried using a contraceptive vaccine but injecting individual mares proved too expensive.
Robin Lohse, the board's chairman, said it would make a recommendation next year but at least two of his colleagues expressed support for Mr Salazar's plan, saying it was cost-effective and efficient.
"We are concerned about the numbers. Time is not on our side," said Mr Lohse.
Terri Farley, the author of a popular series of children's books about mustangs, claimed the Salazar plan could lead to them becoming extinct in the wild.
"Children will grow up to believe wild horses were like unicorns, existing only in stories," she said.