(Reuters) - The U.S. beef industry's dependence on the muscle-building drug Zilmax began unraveling here, on a sweltering summer day, in the dusty cattle pens outside a Tyson Foods Inc slaughterhouse in southeastern Washington state. As cattle trailers that had traveled up to four hours in 95-degree heat began to unload, 15 heifers and steers hobbled down the ramps on August 5, barely able to walk. The reason: The animals had lost their hooves, according to U.S. Department of Agriculture documents reviewed by Reuters. The documents show the 15 animals were destroyed. The next day, the hottest day of the...