Cattle in holding pens at the Simplot feedlot located next to a slaughterhouse in Burbank, Washington on Dec. 26, 2013. Merck & Co Inc is testing lower dosages of its controversial cattle growth drug Zilmax drug in an effort to resume its sales to the $44 billion U.S. beef industry. Ross Courtney/Reuters/Landov hide caption

itoggle caption Ross Courtney/Reuters/Landov