The Irish food safety watchdog said Tuesday that it had discovered traces of horse and pig DNA in burger products sold by some of the country's biggest supermarkets. Tesco that authorities said was made of roughly 30 percent horse. Tesco, the country's biggest supermarket chain, took out full-page newspaper ads today to apologize for an unwanted ingredient in some of its hamburgers: horsemeat. Ten million burgers have been taken off shop shelves after the revelation that beef products from three companies in Ireland and Britain contained horse DNA. Most had only small traces, but one type of burger sold by Tesco was 29 percent horse. The contrite grocer told customers that "we and our supplier have let you down and we apologize."