Chicago corn futures fell nearly 3 percent on Tuesday, the biggest drop for a most-active corn contract in a month, after the US Department of Agriculture raised its 2017 corn yield forecast, bucking most trade expectations for a reduction.
Soyabean futures also fell, pressured by a US yield forecast that topped the highest in a range of trade expectations.
Wheat futures firmed, rallying from early declines, after the USDA tightened its forecast of global wheat ending stocks for the 2017/18 marketing year.
As of 12:50 p.m. CDT (1750 GMT), Chicago Board of Trade December corn was down 9-1/2 cents at $3.48 per bushel after dipping to $3.45-1/2, its lowest since Aug. 31.
CBOT November soyabeans were down 14-3/4 cents at $9.45-1/4 a bushel while December wheat was up 2-1/4 cents at $4.37 a bushel.
Corn fell after the USDA in a monthly report raised its estimate of the average US 2017 corn yield to 169.9 bushels per acre (bpa), from its August estimate of 169.5 bpa and above an average of analyst expectations for 168.2 bpa.
USDA also raised its soyabean yield estimate to 49.9 bpa, from 49.4 in August, topping a range of trade expectations.
The increases came in spite of dry conditions in portions of the Midwest crop belt.
CBOT wheat firmed on long wheat/short corn spreads and on the USDA's forecast for slightly smaller global inventories by the end of the 2017/18 marketing year. The USDA trimmed its world wheat ending stocks forecast to 263.14 million tonnes, from 264.69 million in August.