March 23 (Reuters) - Canada’s main stock index opened lower on Tuesday, as energy stocks slipped nearly 3% tracking weaker crude prices, while investors remained cautions ahead of testimonies from Fed Chair Jerome Powell and Treasury Secretary Janet Yellen. * At 9:30 a.m. ET (1330 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 36.32 points, or 0.19%, at 18,778.81. (Reporting by Amal S in Bengaluru; Editing by Maju Samuel)