Federal authorities in Utah will use a long-term birth-control drug delivered by dart guns to control horse numbers in the Onaqui Mountain Herd Management Area.
The Bureau of Land Management (BLM) had notified the proposal through formal channels and subsequently signed off on the plan on May 1.
The department intends to use the drug, Porcine Zona Pellucida (PZP) on selected mares within the herd management area through to 2020, with the aim of keeping the adult herd numbers between 121 and 210.
The drug would be administered through darting by trained BLM personnel or collaborating partners only.
All mares targeted for treatment would be clearly identifiable through photographs to enable darters and managers to positively identify the animals during the project, and at the time of any subsequent gathers, the BLM said.
If it is determined that a mare or mares cannot be approached within darting range on foot, then baiting (not trapping) will be used to treat the mares.
Baiting will be with salt, mineral, or weed free hay in areas that horses use in their normal movements throughout the herd management area.
The BLM says the expectations for the fertility control program include the short-term goal of bringing herd growth rates to less than 7 percent. The long-term goal is to reduce the need for gathers and removals, without jeopardizing the genetic health of the herd.
The use of PZP has divided wild horse advocacy groups. Some see it as a sensible way to control wild horse populations, while others are concerned about health effects and the drug’s influence on herd dynamics.
PZP has been shown to prevent pregnancy in mares for one year or more, after which horses must receive the drug again.
There are 19 formally designated wild horse herds in Utah.
The Onaqui Herd Management Area is 40 miles southwest of Salt Lake City, on 43,880 acres of federal, state, and privately owned land.
Wild horses have occupied the Onaqul Mountains since the late 1800s. Most of the horses are descendants of horses that escaped from local ranches.
The dominant colors within the herd area are brown and bay. Other colors found include sorrel, roan, buckskin, black, palomino, and grey.