The Bureau of Land Management has signed off on the muster in the Calico complex of northern Nevada, covering five herd management areas.
It plans to pull 2500 to 2700 horses off the range, leaving 600 to 900 in the area. The bureau also plans to use fertility control to slow population growth and reduce the need for future musters.
However, a legal challenge has been mounted against the muster, which will be heard tomorrow (US time) by US District Judge Paul Friedman in Washington DC.
The action has been filed by In Defense of Animals and wildlife ecologist Craig Downer.
The bureau, in its decision, said that since the most recent gather in 2005, forage and water supplies have become limited due to extreme drought conditions in the area.
It said the horses' winter range is limited and inadequate resources exist to support the current population.
"Without a gather, the horses' body conditions - which are already compromised - could further deteriorate, resulting in a situation later this winter that would require emergency horse removals to prevent horse fatalities," it said in its decision.
Winnemucca District, Black Rock Field Office district manager Gene Seidlitz said: "The current wild horse population in these herd management areas is more than three times what the range can handle.
"We need to gather and remove about 2500 excess wild horses in the five herd management areas to achieve a thriving natural ecological balance on the land and address the serious damage to the environment caused by this overpopulation. "By keeping the wild horse population within the appropriate management levels, we can prevent further deterioration of delicate Lahontan cutthroat trout streams, riparian zones, wildlife habitat, wilderness values and other important resources in these area."
The bureau will use helicopters to capture the wild horses.
It said the use of helicopters was proven to be a safe, effective, and practical means for gathering excess wild horses from the public lands, and large scale geographic areas such as the Calico Mountains Complex.
Horses removed from the range will be offered for adoption to qualified individuals. Unadopted horses will be placed in long-term pastures where they will be humanely cared for and treated, and will retain their "wild" status and protection under the 1971 law.
The five herd management areas that comprise the Calico Mountains Complex are the Black Rock Range East, Black Rock Range West, Calico Mountains, Granite Range, and Warm Springs Canyon. The gather is expected to continue into late February or early March 2010.
The bureau plans to begin the muster before the New Year.
The bureau manages 253 million acres of land.