CHICAGO, March 16 (Reuters) -- Striking JBS employees took to the streets of Greeley, Colorado, before sunrise on Monday to picket against the world's largest meat company, in a rare labor stoppage at a U.S. meatpacking plant and a sign of upheaval in the beef industry. The union representing about 3,800 employees at the beef processing facility said it was the first time U.S. meatpacking workers had gone on strike in four decades. They launched a two-week strike and will remain on the picket lines until JBS negotiates fairly with workers, the union said.