Advocates have been concerned at what they see as the growing pace of musters from herd management areas. Many are also concerned about a proposal from Interior Secretary Ken Salazar to relocate up to 25,000 wild horses further east to new herd areas, with more aggressive use of long-term contraception.
Widespread opposition also exists to a planned muster of up to 2700 horses from an area of northern Nevada known as the Calico Complex.
The Bureau of Land Management, responsible for managing the wild horse herds, received more than 10,000 submissions on that muster alone.
A court challenge to the muster has been lodged. If unsuccessful, the muster is scheduled to begin just before New Year.
The bureau manages the wild horses under a 1971 law enacted by Congress.
Its budget for horse management is under pressure, with more than 30,000 horses held in captivity - roughly the same number they estimate still inhabit the western rangelands.