Jan. 29 (Bloomberg) -- Commodities headed for the biggest monthly drop in 13 months on concern that demand may wane as governments seek to control economic growth.The Standard & Poor’s GSCI Index of 24 raw materials is down 5.9 percent this month, the most since December 2008, led by slides of 16 percent for zinc and 14 percent for lead. Copper has lost 7.3 percent this month, also the most in 13 months, and crude oil is down 6.4 percent, the first decline since July. Sugar, feeder cattle and platinum climbed.
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