BLM outlines fertility, darting and gather plans for Muddy Creek wild horses and nearby burros

Mike Tortell, assistant field manager for the Bureau of Land Management’s range, wildlife and wild horse program, told the Emery County Public Lands Council that the BLM is using both PZP and the longer‑lasting product GonaCon to reduce reproduction in wild mare populations, and described operational limits and recent results.

Tortell said PZP (porcine zona pellucida) requires annual boosters and is administered by dart or by injection in corrals, while GonaCon is a premixed adjuvant that can be delivered in darts and can suppress fertility for multiple years. “Since then, any mares that have been released on the ground, if they have not received a previous dose, they will get a dose. They'll be held for 30 days. They'll get a second dose before they go out. That should hold them for 5 years,” Tortell said.

Tortell walked council members through the program’s record‑keeping requirements: treated animals must be identifiable by photos or freeze brands so staff can track who has been dosed and when. He described field challenges for darting — animals’ wariness, the need to be within 50 yards for reliable hits, and the logistics of retrieving spent darts — and said overall hit rates are variable, “about 50% or less,” depending on distance and conditions.

Nut graf: The BLM’s stated near‑term goal is a traditional gather to reduce the Muddy Creek herd to the unit’s low AML (appropriate management level) and then use capture‑treat‑release and targeted darting to keep mare recruitment down. Operational constraints, equipment limits and legal requirements mean progress has been slow: only a small fraction of horses in the larger herd have been treated so far.

Tortell gave recent population figures and program status. A March flight produced a direct count of 344 horses in the Muddy Creek herd (the BLM later flagged a potential double count that could reduce the number to about 336). He said the Muddy Creek allocation is 75–125 animals; the BLM aims to bring the herd down to the low end of that range in a future gather and then treat mares released back to the range.

On treatments completed to date, Tortell said the BLM’s tracking sheet shows six mares treated in the herd; he noted that treatment coverage is therefore “less than 1 percent” of the population at present. He also told the council that a court decision last fall has restricted the BLM’s ability to conduct darting until NEPA (the National Environmental Policy Act) analyses are completed or reinstated for the activity: “As of right now, with court decision to come down last fall, we can't dart until we get NEPA back in place. So once we get NEPA back in place, then we just start treating again,” he said.

Tortell described capture‑treat‑release as the preferred method when feasible. In that approach, staff corral animals, treat identifiable mares (including freeze branding if needed to ensure later recognition), and release them back to the HMA (herd management area). When animals are too hard to identify visually, the BLM freezes brands at handling to make later field identification possible.

On burros, Tortell said the Burroughs herd is much smaller than Muddy Creek and that officials documented a concerning pattern during recent gathers: an asinine herpes virus has been killing pregnant jennies under the stress of gathers and captivity. He estimated around 40–60 burros in the HMA at present and said fertility work there will require capture‑treat‑release because individual identification is difficult in that population.

Council members asked about vaccine efficacy and references; Tortell said long‑term efficacy varies by adjuvant and dosing and offered to bring controlled‑study references to a future meeting. He noted that GonaCon should last roughly five years between treatments, while PZP generally requires yearly doses (a pellet form can extend that to about 22 months).

Ending: Tortell said the BLM’s operational plan is to complete NEPA for the Muddy Creek management actions, then gather toward low AML (75 animals), treat released mares and resume targeted darting. He closed by inviting follow‑up questions and offering to return with more data and study references on efficacy and protocols.