The move is being made because the refuge was created for pronghorn antelope and other native wildlife, and horses and burros have a negative effect on habitat, said Joan Jewett, spokeswoman for the U.S. Fish and Wildlife Service in Portland, Ore.
"They trample the habitat and overgraze and disturb the water sources," she told The Associated Press. "We're required by law to manage our refuges in accordance with the purposes for which they were established, and Sheldon was primarily for pronghorn antelope."
Horse advocacy groups sharply criticized the refuge's comprehensive conservation plan, which will guide its management over the next 15 years. It was publicly released late last month.
They say horses and burros lived in the area long before the refuge was created in 1931, and the animals actually heal the land and help prevent wildfires through grazing.
"We are extremely disappointed that the federal government has chosen to eradicate wild horses and burros from the lands where their ancestors have lived for more than a century and a half," Suzanne Roy, director of the American Wild Horse Preservation Campaign, said in a statement.
An aerial survey in July showed the 575,000-acre refuge along the Oregon border is home to at least 2,508 antelope, 973 mustangs and 182 wild burros, said Aaron Collins, a park ranger at Sheldon.
"We're recording the highest numbers of pronghorn antelope since we began counting them in 1950," he said.
Federal officials began the planning process on the refuge's management plan in 2008, and received several thousand comments from individuals, organizations and government agencies during it, Collins said.
The final plan will be signed sometime after Sept. 24 by the regional director of the Fish and Wildlife Agency, he added.
Under federal law, only horses and burros removed from lands managed by the Bureau of Land Management and the Forest Service are protected from slaughterhouses if they can't be adopted.
"Rounding up indigenous wild horses is wrong — especially when they can be sold to the meat buyers at auctions," said Anne Novak of California-based Protect Mustangs. "These horses are vulnerable to ending up going to slaughter ... The Sheldon plan to wipe out wild horses is nuts and goes against the public's wishes."
Activists said the final management plan rejected a more humane alternative to phase out horses and burros over 15 years using fertility control, an option that would have allowed unadoptable animals to live out their lives at the refuge.