OAK BROOK, Ill. (Reuters) - McDonald’s Corp said on Tuesday that it had begun serving fresh beef Quarter Pounders and other premium burgers at about 3,500 restaurants in the United States, and most of its other U.S. locations will follow suit by May. McDonald's U.S President Chris Kempczinski speaks about fresh beef expansion at a McDonald’s event in Oak Brook, Illinois, United States March 5, 2018.