GOSHUTE VALLEY, Nev.—Nevada cattle ranchers, having long battled the land's harsh elements, now find themselves up against a new force of nature: Madeleine Pickens.

Mrs. Pickens, wife of Texas billionaire T. Boone Pickens, caused an uproar when she proposed the Bureau of Land Management let her fence off more than 500,000 acres of federal land to create a sanctuary for wild horses near a 14,000-acre ranch she bought in October.

Her...