Reno, Nev. • Animal rights activists are suing to block what they say is an unprecedented federal plan to capture thousands of wild horses over 10 years in Nevada without the legally required environmental reviews intended to protect the mustangs and U.S. rangeland.

Friends of Animals accuse the U.S. Bureau of Land Management violating the National Environmental Policy Act and other laws by approving the removal of nearly 10,000 mustangs across an area near the Nevada-Utah line almost twice as big as the state of Delaware.

The “roundup decision is unprecedented in size and scope,” according to the suit filed Thursday in U.S. District Court in Reno.

It would allow BLM to “continually roundup, remove, drug and castrate wild horses for 10 years after the initial roundup,” the suit said.

The agency has used fertility control in isolated cases before but has not previously adopted castration as a way to help keep the size of the herds in check.

Friends of Animals President Priscilla Feral said that in addition to placing male horses at risk of hemorrhaging and infection, those that survive castration “will be robbed of their natural behaviors, putting them at a disadvantage on the range in terms of survival.”

“This is the definition of animal cruelty,” Feral said. “These are wild animals, not domesticated dogs and cats.”

Michael Harris, director of the group’s Wildlife Law Program in Colorado, said the 10-year Nevada plan would allow BLM to roundup mustangs without public notice or comment, and without site-specific analysis of each individual gather proposed in seven different herd management areas with an estimated 9,525 horses across 4,900 square miles (12,690 sq. kilometers) of federal rangeland southeast of Elko between U.S. Interstate 80 and U.S. Highway 50.

Wild horses far exceed U.S. government population goals, and officials say the free-roaming horses that number about 60,000 in 10 western states can face starvation. Captured horses are offered for adoption, but 46,000 are being held at government corrals and pastures costing taxpayers $50 million annually.

Harris said the roundups are based on outdated population targets adopted in management plans that haven’t been updated in a decade, and in one case not for 25 years. The suit points to a 2013 study by the National Science Academy’s National Research Council, which found little scientific basis for establishing what BLM considers to be appropriate, ecologically based caps.

BLM spokesman Greg Deimel said Monday agency officials cannot comment on pending litigation.

Jill Silvey, district manager for the BLM in Elko, wrote in the formal decision she signed in December authorizing the gathers that they are needed to protect the rangeland from overgrazing in an area that has 11 times more mustangs than the land can sustain.

“Native vegetative communities in parts of the complexes have already crossed critical ecological thresholds that could prevent or significantly slow their recovery,” she said. “This resource degradation and potential for irreversible ecological damage will continue without immediate action to remove excess wild horses and to bring the wild horse population back to (appropriate management levels.)”

Jennifer Best, Friends of Animals’ assistant legal director, said BLM recently withdrew a 5-year-plan in Colorado after the group sued to block a similar strategy involving fertility control and castration.