March 4 (Reuters) - Canada’s main stock opened lower on Thursday, weighed down by weakness in materials stocks on concerns around rising Treasury yields, while data showed that the domestic labor productivity fell in the fourth quarter. * At 9:31 a.m. ET (1431 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 50.68 points, or 0.28%, at 18,269.99.