Brad Horn,File, Associated Press
In this July 13, 2008 file photo a livestock helicopter pilot rounds up wild horses from the Fox & Lake Herd Management Area from the range in Washoe County, Nev.

RENO, Nev. — A federal judge temporarily blocked the roundup of more than 300 wild horses in northern Nevada on Wednesday, saying the government cannot rely on a nearly five-year-old environmental analysis that critics say ignores the latest scientific evidence about the potential harm to the mustangs.

U.S. District Judge Larry Hicks granted the rare preliminary injunction and temporary restraining order preventing the Bureau of Land Management from gathering the horses in the Pine Nut Range southeast of Carson City and injecting dozens of females with a fertility drug that prevents them from reproducing for two years.

He said the federal agency appears to have violated federal law, including the National Environmental Protection Act requiring a more stringent examination of the impacts than the one BLM scientists conducted.

Horse advocates who sought the court order say there's been significant new research about the potential harms of PZP since BLM reviewed the effects in 2010. New studies confirm earlier concerns the interference with the wild herds prompts mares unable to become pregnant to leave in search of stallions in other bands of mustangs, they said.

"This is a major victory for wild horses and reflects rising concerns about rounding up and drugging wild horses with PZP," said Jennifer Barnes, a lawyer for Friends of Animals, which brought the suit with the San Francisco-based Protect Mustangs.

Hicks concluded in an eight-page order issued in Reno late Wednesday the two groups are likely to succeed proving the merits of their complaint alleging BLM broke the law by authorizing the roundup without conducting a new or supplemental environmental assessment under NEPA.

"The court finds that the public interest will be best served by enjoining the BLM's proposed gather, at least until the court has an opportunity to fully consider the merits of the plaintiff's claims," the judge wrote. He did not indicate when he might hear additional arguments or set deadlines for future filings on the dispute.

BLM argued that the previous review contemplated further roundups in the Pine Nut Range and that another formal review of the potential impacts on the herds is not required under federal law. The agency's current plans call for the roundup of all 332 mustangs, with about 200 shipped to holding facilities. The rest would be returned to the range, including some mares that would first be injected with PZP.

BLM maintains there are nearly twice as many horse in the Pine Nut Range as the high desert habitat can support without causing ecological damage, some of which could impact the imperiled sage grouse.

Overall, Nevada is home to about half the nearly 50,000 wild horses and burros roaming federal lands in 10 western states, according to BLM. The agency argues the land — much of it in the throes of multiple years of drought — can only sustain a total population of fewer than 27,000 of the animals across the 10-state region.

Horse advocates blame livestock for the degraded condition of the range BLM attributes to overgrazing by horses.

Protect Mustangs executive director Anne Novak said the court order marks a "milestone" for the horses she says have been "scapegoated for range damage and forcibly drugged with PZP in experiments for decades."

The last time a federal judge issued a ruling disrupting a Nevada roundup was in August 2012 when U.S. District Judge Howard McKibben granted a temporary restraining order that cut short by a day a gather near the Nevada-Utah line after he determined a helicopter flew too close to a horse in violation of the law. He later allowed the gather to continue, with special protections aimed at guarding against mistreatment of the animals.

Neither officials for the BLM nor Justice Department lawyers who represented the agency during a brief hearing before Hicks on Monday immediately responded to requests for comment from The Associated Press late Wednesday.