Corn futures plunged to a three-year low after the U.S. Department of Agriculture boosted its inventory estimate by 25 percent.
Soybeans dropped the most in six months as supplies topped analyst estimates by 11 percent.
Corn futures dropped 2.8 percent to close at $4.415 a bushel at 1:15 p.m. on the Chicago Board of Trade. Earlier, the price touched $4.4075, the lowest for a most-active contract since Sept. 1, 2010. Corn has tumbled 37 percent in 2013.
Soybean futures for November delivery fell 2.8 percent to $12.8275 a bushel, the biggest drop since March 28. Earlier, the oilseed touched $12.80, the lowest since Aug. 19. The price has dropped 9 percent this year.