Those are among the few realistic alternatives given a crippling combination of congressional budget cuts, spiraling costs, lingering drought, a record 49,000 mustangs in long- and short-term holding, and an on-range population that doubles every four years and is expected to surpass 60,000 in 2015, Guilfoyle said.
It's "nearing the point of financial insolvency due to undesirable trends in every aspect of the program," she said in the August memo to Greg Shoop, assistant director for renewable resources and planning.
"Drastic changes in course are mandatory to remain financially solvent and reverse trends" undermining the Bureau of Land Management's goals, she wrote. "Considering the circumstances, on-range management goals may not be achieved for another 20 years."
Labeled "Internal Working Document," the Texas-based Wild Horse Freedom Federation obtained the memo under the Freedom of Information Act and first posted it on its website Wednesday. The Associated Press also independently obtained a copy Wednesday but couldn't confirm its authenticity until Friday.
Bureau of Land Management spokesman Tom Gorey said it was a "preliminary discussion document" produced in "recognition of the tight fiscal climate" and based on projections Congress would cut more from the agency's budget than it ended up doing last fall.
"It explores a range of interim measures that could be implemented until more sustainable actions are available, such as the development of longer-acting, effective contraceptives and the reduction of holding costs," Gorey said in an email to the AP.
The three-page memo offers a candid look at the political and public pressure fueling the controversy that has raged for decades but intensified since the number of animals in holding first exceeded those on the range two years ago. Last fiscal year, holding costs topped $46 million — 61 percent of the horse program's overall budget.
Guilfoyle noted "heightened and increasing interest" in policy changes among conservationists, ranchers, horse activists and Congress. She said things "we can't do" include "remove animals," "new research" and "reduce or curb on-range population growth."
"Funding and space prohibit the removal of any animals in the near future. Euthanasia of near-death animals is the only responsible alternative," she said.
While Guilfoyle's assessment was unusually frank, it echoes the General Accounting Office's conclusion in a 2008 report to Congress that there was no end in sight to rising holding costs. A year later, then-Interior Secretary Ken Salazar warned "the current path of the wild horse and burro program is not sustainable."
Last June, Arizona Rep. Raul Grijalva, the ranking Democrat on a public lands subcommittee, and 29 colleagues including Rep. Dina Titus, D-Nev., told Interior Secretary Sally Jewell the program's soaring costs had created an "untenable situation" for both the mustangs and taxpayers.
The Bureau of Land Management suspended roundups shortly after Guilfoyle's memo and has not formally scheduled any this year. But it awarded two helicopter contracts in January that could be used for gathers.
The Nevada Farm Bureau Federation and Nevada Association Counties argue roundups are necessary to cull overpopulated herds competing with livestock for forage. They filed a lawsuit in December trying to force the Bureau of Land Management to sell some older horses deemed unadoptable without the usual prohibition on resale for slaughter — an idea the agency has opposed.
Ginger Kathrens, executive director of the Colorado-based Cloud Foundation, said euthanizing or sterilizing mustangs would violate the 1971 Wild Free-Roaming Horses and Burros Act. The foundation is dedicated to the preservation of wild horses on public lands.
"It would be unconscionable to even consider use of these strategies," she said.