A view of the rugged Calico Mountain wilderness area. © BLM
The Bureau of Land Management has been seeking public feedback on its proposal to remove about 80 per cent of the wild horses that live in the so-called Calico Complex, a swathe of land that includes five herd management areas.
To date, it has received more than 7000 submissions.
However, yesterday (NZ time), the group In Defense of Animals and ecologist Craig Downer filed a suit in the federal US District Court for the District of Columbia, to stop the roundup - by far the largest of any roundup planned by the bureau for this financial year.
"This suit aims to halt the inherent cruelty of the bureau's wild horse roundups, which traumatise, injure and kill horses, subvert the will of Congress and are entirely illegal," said William Spriggs, a partner at Buchanan, Ingersoll & Rooney and lead counsel on the law suit.
The firm is representing the group and Mr Downer on a pro bono basis (done without compensation for the public good).
The suit alleges that the bureau plans to use helicopters to chase as many as 2738 of the estimated 3095 Calico horses into holding pens violates the Wild Free Roaming Horse and Burro Act, passed unanimously in 1971.
The act designated America's wild horses and burros as "living symbols of the historic and pioneer spirit of the West", specifying they "shall be protected from capture, branding, harassment, or death ... [and that] to accomplish this they are to be considered in the area where presently found, as an integral part of the natural system of public lands".
Spriggs said: "Americans strongly support protecting wild horses on their natural ranges in the West.
"We hope to stop the cruel roundups and mass stockpiling of wild horses and burros in government holding facilities while the Obama Administration crafts a new policy that protects these animals and upholds the will of Congress and the public's desire to preserve this important part of our national heritage."
Since 1971, the bureau has removed more than 270,000 horses from their Western home ranges and taken away nearly 20 million acres of wild horse habitat on public lands that were protected by Congress to sustain their populations.
Wild horse advocates argue that the removal strategy is based on the unsupportable claim that Western ranges cannot sustain wild horses and burros. They say that horses comprise a tiny fraction of animals grazing the range. An estimated 8 million livestock, but only 37,000 horses and burros, graze on public lands.
Thirty-two thousand wild horses removed from the range are currently held in government holding facilities, and the bureau has plans to round up 12,000 more this financial year.
Wild horse advocates have also voiced their opposition to proposals announced by Interior Secretary Ken Salazar that involves relocating herds further east on to more productive lands. The so-called Salazar plan also involved more extensive use of long-term contraceptive measures.