Wheat futures plunged nearly 4 percent to fresh contract lows on technical selling and concerns about stiff global competition in the export market. Corn followed wheat lower.
US President Donald Trump on Monday said he was optimistic a final trade deal with China could be reached but offered few details.
Trump said Sunday he would delay an increase in US tariffs on Chinese goods thanks to "productive" trade talks and that he and Chinese President Xi Jinping would meet to seal a deal if progress continued.
China committed to buy an additional 10 million tonnes of US soyabeans in a meeting in the Oval Office on Friday, US Agriculture Secretary Sonny Perdue said on Twitter.
Chicago Board of Trade (CBOT) May soyabeans were up 3 cents at $9.26-3/4 a bushel by 11:43 a.m. CST (1743 GMT) after reaching a peak of $9.34-1/2, the highest since Feb. 7. CBOT May corn fell 3-1/2 cents to $3.81 a bushel, breaching chart support at its 20-, 50- and 100-day moving averages.
CBOT May wheat dropped 16-3/4 cents to $4.75 a bushel. The 3.4-percent drop was the steepest for a most-active contract since July 11. All contracts posted new life-of-contract lows.
Wheat was pressured by a lagging US export pace amid competition from lower-cost suppliers such as Ukraine and Russia. The US Department of Agriculture on Monday said 693,964 tonnes of US wheat was inspected for export last week, above market expectations. But season-to-date inspections are about 1.4 million tonnes behind a year ago.