It's preemptive Right:
4. Having or marked by the power to preempt or take precedence
----
It was a Right to public grazing that homesteaders received as an incentive to settle the West. Acknowledgment of these Rights can be found in the Library of Congress during the discussion concerning the selling of land for the railroad.
(Sorry for the cut & paste - the Library disables active links)
http://memory.loc.gov/cgi-bin/ampage?collId=llsb&fileName=040/llsb040.db&recNum=818
40th Congress, 2nd Session, S190, December 13, 1867 (page3, Line12)
Provided, this privilege shall not extend to lands upon which there many be rightful claims under the preemption and homestead laws
These Rights preempt, (or prevent) any government law or organization from affecting the use of the public land as long as the landholder meets his end of the bargain.
I don’t know if it is exactly like homesteading. The land we own was homesteaded, the first entry on the deed is US Government to [homesteader].
I think grazing rights are more like a legal easement, the right to use the land for grazing purposes. The people that held preemptive rights or grazing rights never got a actual deed to the land. If they had got the actual deed like homesteaders did, Bundy’s wouldn’t be having the problems they are having now.