March 18 (Reuters) - Canada’s main stock index opened lower on Thursday, as energy and materials stocks tracked weaker commodities, while data showed the nation shed jobs for the second straight month in February. * At 9:30 a.m. ET (13:30 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 53.97 points, or 0.28%, at 18,929.13. (Reporting by Amal S in Bengaluru; Editing by Shailesh Kuber)