Senator Mary Landrieu
Writing on Thehill.com, Landrieu, who has been pushing for reforms, said the Bureau of Land Management had justified the roundups by citing a rapidly growing horse population and the need to prevent exhaustion of the land's resources.
"The bureau has been stating this for more than a decade now, but has yet to implement a sound management plan," she said.
She noted that when the National Wild Horse and Burro Programme was created in 1971, 54 million acres were available for wild horses and burros. That had since been halved.
"And in every year since the bureau was entrusted with this programme, populations have exceeded the agency's appropriate management level by thousands of animals, reaching over 60,000 at one point.
"Enough is enough. It is time to find a more effective management plan for these horses that does not result in the careless death of so many," she said, referring to deaths that occurred during roundups.
Landrieu said more than 32,000 wild horses were now held in captivity.
"Not only is this not a safe or desirable solution for the animals, but also it is costing the American taxpayers more than $US30 million a year.
"There are an estimated 37,000 mustangs and burros that live in our Western states.
"We now have nearly that amount in custody, and the BLM plans to remove an additional 12,000 wild horses from the ranges at a cost of millions to the American taxpayer."
Landrieu said the bureau's actions were contrary to the act's original intent to manage the wild horses and burros in their natural state and to protect them from capture and harassment.
"I have repeatedly called for an end to these roundups until a more humane and cost-effective solution has been put in place.
"The Obama Administration should be ashamed that this is happening under its watch."
Wild mustangs, she said, were living symbols of the United State's history and pioneering spirit.
"Anyone who has had the privilege of watching a wild herd graze freely and calmly understands what majestic animals these wild mustangs truly are.
"It is hard to fathom that hundreds of our wild horses have died at the hands of the federal agency entrusted to protect them."
Landrieu vowed to continue to work to find a better legislative solution. "Letting the death toll of America's mustangs continue to rise is simply not an option."