Corn futures also hit a three-month low, turning down as wheat sagged and storms brought crop-friendly rains to the Midwest. But soybean futures clung to modest gains, supported by declining US crop ratings and uncertain yield prospects.
Chicago Board of Trade December wheat settled down 6 cents at $4.66-1/2 per bushel after dipping to $4.63-1/2, the contract's lowest level since May 14.
CBOT December corn ended down 5-3/4 cents at $3.68-3/4 a bushel after touching $3.68-1/4, its lowest since May 13. CBOT November soybeans finished 2 cents higher at $8.68-1/2 a bushel.
Wheat led the way down as traders considered global stockpiles. The US Department of Agriculture this month trimmed its projection of 2019/20 world wheat ending stocks to 285.4 million tonnes, down from 286.46 million in July, but still record-large.
"There is a lot of wheat in the world. You look at the Ukraine crop and the EU crop, it's fairly large. And (there are) no major issues down in South America," said Terry Reilly, senior analyst with Futures International in Chicago.
Along with the setback in wheat, corn fell and soybeans pared gains as storms crossed Illinois and parts of Missouri, Minnesota and Wisconsin, bringing beneficial moisture to late-developing crops.
After a wet spring, the latest weekly US Drought Monitor showed abnormal dryness across 53pc of Iowa and 44pc of Illinois, the top two producers of corn and soybeans, by Aug. 13.
The USDA's weekly crop progress report late on Monday rated 56pc of the US corn crop and 54pc of the soybeans in good-to-excellent condition, each down 1 percentage point from a week ago. Analysts had expected no change.
CBOT soy and corn futures drew early support from Monday's crop ratings and news that the annual Pro Farmer Midwest Crop Tour found below-average yield prospects in its first day of scouting.
Corn yields in Ohio were projected sharply lower than a year ago at 154.35 bushels per acre, the tour said on Monday.
In South Dakota, the tour estimated corn yields at 154.08 bushels per acre, down from 178.01 bpa last year and the tour's three-year average of 158.59 bpa.
"(The) tour is finding lower yields and there is a good chance that the US corn crop is lower than the current USDA estimate," said Ole Houe, director of advisory services at brokerage IKON Commodities.