The Bureau of Land Management has pulled the plug on a public-private partnership in Nevada. Its focus was shrinking the size of a wild horse herd by using a fertility drug prevalent in the West.

The dispute in Nevada’s Pine Nut Mountains is over a fertility drug called PZP. It’s used to control the population of wild horses. In 2014, the Bureau of Land Management or BLM approved a pilot project, partnering with the American Wild Horse Preservation Campaign to control the population of a herd in Nevada. But, advocacy group Friends of Animals threatened to sue if PZP was used.

Assistant Legal Director of Animals for the Wildlife Law Program, Jennifer Best, says PZP fundamentally changes the way horses live.

“Wild horses usually travel in bands and what they found is that with the use of PZP, it makes the bands less stable,” she explained.

The BLM suspended the use of the drug on the Nevada herd Monday. Best says PZP is prevalent across the western United States, including Nevada and Arizona’s Salt River Valley.

The Associated Press contributed to this report.