RENO, Nev. (AP) — Under the threat of another legal battle, the U.S. Bureau of Land Management has pulled the plug on a public-private partnership in northern Nevada aimed at shrinking the size of a wild horse herd through the use of contraceptives, according to documents The Associated Press obtained on Tuesday.

BLM officials confirmed they have suspended the pilot fertility-control project southeast of Carson City pending completion of additional environmental analysis.

Unlike most conflicts over mustangs that pit protection groups against ranchers, the dispute in Nevada’s Pine Nut Mountains has divided horse advocates themselves over the appropriate use of fertility-control drugs on the range.

The federal agency approved the pilot project in 2014 working with the American Wild Horse Preservation Campaign and the Gardnerville-based Pine Nut Wild Horse Advocates to treat a herd that a federal judge in Reno has forbidden the agency from gathering.

BLM suspended the project Monday after Friends of Animals threatened to sue based on claims the drug, PZP, harms horses and violates the judge’s order, according to an internal email obtained by AP.

“Administration of PZP to these wild horses is hereby suspended, pending further review,” BLM Sierra Front Field Manager Bryant D. Smith wrote in informing his staff he had revoked the decision record for the Fish Springs Wild Horses PZP Pilot Project.

While some groups advocate fertility control as a preferred alternative to government roundups, others say scientific research suggests PZP can have long-lasting physical, behavioral and social effects on wild horses. Among other things, they say mares that cannot get pregnant choose to leave their bands, creating instability that affects the health of the entire herd.

“We are extremely happy to have killed the pilot project and to put a stop to the forced drugging of Pine Nut mares with the fertility control pesticide PZP for a second time,” said Pricilla Feral, president of the Connecticut-based Friends of Animals, an international advocacy group founded in 1957.

The BLM maintains the Pine Nut herd is seriously overpopulated, and it intended to round up more than 300 horses last year before U.S. District Judge Larry Hicks sided with Friends of Animals and blocked the effort. He ruled the BLM failed to conduct the necessary analysis required under the National Environmental Policy Act, and soon after the agency voluntarily withdrew its roundup plan.

Meanwhile, the BLM had been stepping up its efforts in concert with the American Wild Horse Preservation Campaign — a coalition of more than 60 groups nationally — and the local group to administer PZP to members of the herd that wander into nearby neighborhoods.

Michael Harris, Friends of Animals’ wildlife law program director, said arming private landowners with rifles to dart mares appears to violate the 1971 Wild Free-Roaming Horses and Burros Act, which prohibits landowners from intentionally harassing wild horses.

“It now looks as if BLM’s withdrawing its 2014 management plan involving a roundup and PZP, was somewhat disingenuous,” Harris said Tuesday. “Instead, they dusted off an even older plan — the Fish Springs PZP Pilot Program — in an attempt to drug the Pine Nut mares without complying with the court decision or NEPA.”

Deniz Bolbol, the preservation campaign’s programs director, confirmed local residents have been “instructed to temporarily halt the program because of the lawsuit.” She hopes the project will resume once the BLM completes the necessary review, but she fears some horses may end up in U.S. corrals.

“This is a lawsuit filed by people sitting in an office in Connecticut against the folks in Nevada doing the hard work on the ground to keep wild horses free on the range,” Bolbol said. “If this group wants to help wild horses they need to focus on the BLM’s current effort to conduct barbaric spaying of wild mares and the castration of stallions on the range rather than target this type of humane birth control.”

Smith said in a statement Tuesday afternoon that the BLM is “proud of our partnership” with the preservation campaign and the local group.

“It has been an attempt to control the Pine Nut herd’s population to keep the mustangs off neighborhood lawns and ultimately out of government holding pens,” Smith said. “Although we have decided to end the pilot program for now, (BLM) is currently in the process of preparing a new Environmental Analysis to guide long term wild horse and burro management in the Pine Nut Mountains.”