BLM outlines fertility program for wild mares; NEPA ruling limits field darting

The Bureau of Land Management on Thursday described its current use of fertility-control tools for wild mares and the operational limits the agency faces after a recent court decision. Mike Tortell, assistant field manager with the BLM’s range, wildlife and wild horse program, told the Emery County Public Lands Council the agency uses two approved fertility products — PZP (porcine zona pellucida) and gonacon — delivered either by darting in the field or by capture, treatment and release.

"We have essentially two agents that are approved right now," Tortell said, naming PZP and gonacon and explaining differences in durability and delivery. He said gonacon can offer up to five years of effectiveness in some cases and that PZP typically lasts about a year, so operational preference is shifting toward longer-lasting adjuvants where approved.

Tortell summarized the program’s record-keeping and identification requirements for treated animals: animals processed in facilities are photographed, freeze-branded or hip-branded and tracked with treatment dates, vaccine lot numbers and follow-up notes. He described modified protocols that now require a second dose for animals released from holding facilities so the treatment provides multiyear suppression.

The agency reported counts and treatment scope for local herd management units. "We did a flight in March, and direct count for Muddy Creek was 344," Tortell said; he noted a possible double count that would lower the figure to about 336 while Washington office statisticians review the survey data. He added the allocation for that herd unit is 75–125 animals, placing current counts above the target range. Tortell said six mares have been treated to date in the Muddy Creek unit — less than 1 percent of the herd — and that program activity has been limited by legal and procedural constraints.

"As of right now, with a court decision to come down last fall, we can't dart until we get NEPA back in place," he said, referring to requirements under the National Environmental Policy Act. The court action and the need to prepare defensible NEPA documentation mean planned darting or gather-and-treat operations are paused until the agency completes the required procedures and readies for potential litigation challenges.

Council members pressed for effectiveness data and operational details. Tortell said delivery accuracy varies and field darting is difficult; he estimated a field hit rate around 50 percent or less depending on distance and animal behavior. He described several case examples from 2018–2019 showing how some mares required multiple attempts and how field identification (photo or brand) affects whether animals can be retreated in follow-up visits.

On goals, Tortell said the agency’s objective is to reduce herds toward the lower end of the Appropriate Management Level and then apply capture-treat-release protocols that allow treated animals to be released and observed rather than removed for adoption. He emphasized the program is targeted and depends on site access, availability of staff and the outcome of NEPA and potential legal challenges.

The BLM presentation prompted questions about program scale and alternatives; Tortell said other methods (implants, gelding) have been trialed but have limitations, and that research continues on new adjuvants and delivery systems. He offered to bring efficacy studies and controlled-data summaries to a future meeting on request.

Next steps: the BLM said it will await NEPA completion before resuming field darting and will provide follow-up materials and efficacy references to the council.