Time: In summer, the bison on Thunderheart Ranch opt for the feathery shade of a mesquite tree as temperatures reach 100. This land, just a handful of miles from the Mexican border, was once known as The Wild Horse Desert, lawless, rough brush country where, in a good year, 21 inches of rain fell and in a bad year, less than a dozen descended from the clouds. "My grandfather used to say we get two 10-inch rains and never get the other inch," says Hugh Fitzsimmons, owner of the 13,000-acre ranch. Fitzsimmons...