SALEM — The U.S. Environmental Protection Agency unlawfully rejected an animal rights group’s request to consider banning a horse birth control drug, according to a federal judge.

U.S. District Judge Michael Simon has ruled the EPA violated administrative law with its “arbitrary and capricious” decision not to conduct a special review of porcine zona pellucida, or PZP, which is used to control wild horse populations on federal lands.

However, while the EPA must reconsider its denial of a petition from the Friends of Animals nonprofit, the judge will not require the agency to suspend PZP’s registration as a pesticide, which would have halted its usage.

Though the ruling doesn’t stop PZP usage, Friends of Animals considers the decision “a big win” because the judge has effectively said the EPA must take the group’s allegations seriously, said Michael Ray Harris, the nonprofit’s attorney.

“He’s telling them there’s enough evidence here that they can’t just blow it off,” Harris said. “Whenever you get an opinion like this, it means you’ve got evidence.”

The judge would not have sent the matter back to EPA for reconsideration just to waste taxpayer dollars, so the agency won’t likely want to get “hammered” in court again for disregarding new evidence about PZP, he said.

Friends of Animals ultimately wants to stop PZP usage on wild horses, preferring that federal land managers give them more space to roam rather than rely on birth control, Harris said.

Federal managers could also discourage the killing of predators, especially cougars, which would then control wild horse populations while curbing overgrazing and overbreeding, he said.

“Animals act much as we would. We’re not going to put our young in jeopardy,” Harris said. “Animals react instinctively.”

Simon’s decision affirms an earlier finding from U.S. Magistrate Judge Patricia Sullivan, who determined that EPA “impermissibly deferred” an analysis of PZP’s environmental risks to “wild horse management experts.”

Friends of Animals petitioned the EPA to conduct a special review to determine whether PZP’s pesticide registration should be canceled or revised, then filed a lawsuit against the agency in 2017 after that request was denied.

The group alleges new research has found that PZP, which is extracted from pig ovaries, is “causing undue physical, social and biological harm” to wild horses by creating a “genetic bottleneck” of reduced diversity and increasing deaths among foals because they’re born at undesirable times.

Without ruling on the merits of these accusations, the magistrate judge agreed with Friends of Animals that EPA should have responded directly to these claims instead of saying these questions were better left to wild horse experts.

“Whether a risk would be better addressed by some other expert involved in pest management, or could be addressed by an applicator’s determination of ‘where, when, and whether to use the pesticide’ is not, on its own, a factor that Congress intended EPA to rely on in deciding whether to initiate a Special Review,” Sullivan said.

Attorneys for the EPA and the Humane Society of the United States — which intervened in support of using PZP — could not be reached for comment as of press time.