Jan 24 (Reuters) - Canada's main stock index fell at the open on Tuesday, as energy and consumer discretionary stocks dragged the index lower while investors eyed U.S. business activity data to gauge the impact of the Federal Reserve's aggressive monetary policy.

At 9:31 a.m. ET (1431 GMT), the Toronto Stock Exchange's S&P/TSX composite index was down 90.37 points, or 0.44%, at 20,541.21. (Reporting by Shashwat Chauhan in Bengaluru; Editing by Krishna Chandra Eluri)