It seems that the Bureau of Land Management had the right to change the terms of the contract...
"Congress enacted the FLPMA, which instructs the Secretary of the Interior to manage through the BLM the public lands under the principles of multiple use and sustained yield.
43 U.S.C. § 1732(a)
"Multiple use"requires managing the public lands and their resources so that they "best meet the present and future needs of the American people," and taking into account the long-term needs of future generations for renewable and nonrenewable resources, including recreation, timber,wildlife and fish and scientific values.
43 U.S.C. § 1702(c)
. "Sustained yield" is defined as "the achievement and maintenance in perpetuity of a high-level annual or regular periodic output of the various renewable resources of the public lands consistent with multiple use."
Id.§ 1702(h)"
"The BLM has authority under the FLPMA to place restrictions on grazing when the forage declines to a level that would defeat the goals of multiple use and sustained yield."
http://www.scribd.com/doc/218116757/...st-LEXIS-23835