Wild mustangs, those quintessential symbols of the American West for hundreds of years, are facing their most deadly predator yet: the pen-pushing civil servants of the US Bureau of Land Management.

Growing pressure on the horses' traditional habitat has left officials contemplating a programme of mass slaughter to reduce the number of mustangs held captive in government-run pens. More than 33,000 of the animals, almost as many as the number still in the wild, have been rounded up and taken off increasingly barren public land in recent years, to reduce pressure on grazing required by the cattle-ranching industry.

But the increasing cost of keeping them fed and watered has left the Bureau of Land Management (BLM) contemplating a programme of what it calls "euthanasia" – sending healthy horses to slaughterhouses where they are likely to be turned into steak for export to France.

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The proposal sparked outrage from conservationists when it was outlined in a recent Government Accountability Office report, with welfare groups accusing the BLM of holding an unnecessarily high number of mustangs in captivity in order to appease the politically powerful ranching lobby.

"They say there are too many horses left on the range, and that they need to gather them all the time," said Jerry Reynoldson of the Wild Horse Adoption Association. "But there are only 30,000 left in the wild, and they're spread over 10 states. In Nevada alone, the BLM controls 47 million acres. But the ranchers control economic forces and pay lobbyists in Washington, so they give the horses short shrift and convince the authorities that they need to be taken off the land."

Campaigners say slaughter would mark an ignominious end for a creature that arrived in North America with the Spanish conquistadores in the 16th century, and which once existed in such numbers that maps of Texas from the 1700s marked many areas as simply: "vast herds of wild horses". Deanne Stillman, author of a new book, Mustang: The Saga of the Wild Horse in the American West, said: "Horses blazed our trails, they fought our wars, they are the greatest icons of freedom. The word 'euthanasia' suggests that the BLM will be putting these horses out of their misery. But they are not in misery in the first place. It's the most cynical thing I've ever heard."

Today, 33,100 mustangs are left in the wild, roaming in 199 herds. Because the animals have few natural predators left and do not provide sport for hunters, government officials say a quota must be taken into captivity each year. Some of the captive horses are adopted by members of the public, but supply has outstripped demand in recent years, and the BLM says drastic action is needed to reduce the number in captivity, which cost $21m (£14m) a year to feed. "We don't want to do euthanasia, but we are up against the wall on our budget," a spokesman told USA Today.

The only hope for many horses may lie with Madeleine Pickens, wife of the Texan oil billionaire T Boone Pickens, who recently announced that she was trying to establish a million-acre refuge where all captive mustangs can be released. Her plan will see private land turned into a rural theme park where Americans can interact with the mustangs. Its announcement persuaded the BLM to grant captive horses a stay of execution until the New Year.

"We will take all the excess horses," Mrs Pickens explained, "and put them somewhere where families can see them and live among them, and camp out in teepees and have bonfires and look up at the stars and get to know this incredible aspect of our heritage."