Chicago Board of Trade corn futures ended modestly lower on Wednesday, retreating after the lead December contract hit its highest in nearly two months, as traders squared positions a day ahead of a monthly US government crop report.

CBOT December corn settled down 1-1/2 cents at $3.94-14 per bushel after reaching $3.97-1/4, its highest since August 12. Market underpinned by worries about a winter storm in the northern US Plains that is forecast to bring heavy snow to the Dakotas and portions of the Upper Midwest, further delaying a harvest that is already behind the normal pace.

Ahead of Thursday's monthly supply/demand reports from US Department of Agriculture, analysts surveyed by Reuters on average expected the government to lower its estimates of US 2019/20 corn yield, production and ending stocks. Ahead of the USDA's weekly export sales report, also due on Thursday, traders expected the government to show US corn export sales in the week to Oct. 3 at 500,000 to 800,000 tonnes.

The US Energy Information Administration said weekly US output of corn-based ethanol rose to 963,000 million barrels per day while stocks fell to 21.22 million barrels.