(Adds details on specific stocks, updates prices)
* TSX ends down 63.5 points, or 0.39 percent, at 16,349.44
* Half of the TSX's 10 main groups move lower
* Index notches a 0.9 percent gain over shortened week
TORONTO, Jan 5 (Reuters) - Canada's main stock index fell on Friday, weighed by losses among natural resource stocks as commodity prices pulled back, muting a strong weekly gain for the index.
* The Toronto Stock Exchange's S&P/TSX composite index ended down 63.5 points, or 0.39 percent, at 16,349.44. It nevertheless notched a 0.9 percent gain over the course of the holiday-shortened week.
* Half the index's 10 main groups finished in negative territory, with decliners outnumbering advancers by almost 2-to-1 overall.
* The energy group fell 2.3 percent, as oil prices retreated from highs last seen in 2015. Canadian Natural Resources Ltd fell 2.5 percent to C$45.36 and Suncor Energy Inc was off 1.5 percent at C$46.93.
* The materials group, which includes precious and base metals miners and fertilizer companies, lost 0.9 percent, with Barrick Gold Corp down 1.3 percent at C$18.55 as gold prices also dipped after a recent rally.
* The two natural resource sectors combine to account for more than 30 percent of the index's weight.
* Brookfield Business Partners LP gained 5 percent to C$47.35 after saying it planned to buy Westinghouse Electric Co from Toshiba Corp for $4.6 billion. Its parent company, Brookfield Asset Management Inc, slipped 0.8 percent to C$54.16.
* Marijuana producers fell early before turning higher in heavy volumes. They had been rising sharply over the last week but were hit hard on Thursday when the U.S. federal government rescinded a policy that had eased enforcement of federal marijuana laws.
* Canopy Growth Co finished up 5.2 percent at C$34.00 and Aphria Inc gained 3.6 percent to C$19.16. Aurora Cannabis Inc rose 2.4 percent to C$13.35 after announcing its second investment in as many days. (Reporting by Alastair Sharp; Editing by Nick Zieminski)