Wheat was little changed, hovering close to its lowest since Jan. 24 on ample global supplies.
"We have had a good rally in beans so it might be a bit of profit-taking," said Adam Davis, a senior commodity analyst at Merricks Capital in Melbourne. "Funds are piling into soybeans and selling corn which is going to dictate price action in beans and corn."
Chicago Board of Trade corn for March delivery fell 0.4 percent to $6.24-1/2 a bushel by 0436 GMT, after touching a low of $6.22-1/2 a bushel, the lowest since January 24.
March soy slid 0.7 percent to $12.52-1/2 a bushel while March wheat rose 0.2 percent to $6.27-1/2 a bushel.
Wednesday's gains in soybeans came as a Chinese trade delegation signed agreements with US grain companies to buy 8.62 million tonnes of soy.
Although Sino-US soy deals -- made during a visit by Vice President Xi Jinping to the United States -- have been dubbed by the industry as a political gesture, they come at a time of rising Chinese demand.
The Chinese delegation was expected to sign more purchase agreements in Los Angeles on Thursday, which would bring the total amount to a record of more than 12 million tonnes.
US grain and oilseed futures were also being driven by global markets, which fell as another delay in cementing a crucial bailout for stricken Greece underscored how far Europe is from resolving a debt crisis that threatens the stability of the financial system.
A three-hour teleconference between euro zone finance ministers failed to resolve all the issues surrounding a second aid package for Athens, putting off any decision on the matter until Monday at the earliest.
The dollar index, which measures its strength against a basket of currencies, rose 0.2 percent, making US-dollar priced commodities expensive for overseas buyers.
Chicago soybeans were also boosted on Wednesday after the US Department of Agriculture confirmed sales of 116,000 tonnes of US soybeans to China.
It was the third straight daily announcement of sizable US soybean sales, with the previous two sales earmarked for "unknown destinations" -- which could include China.
The three-day soy sales total was 519,000 tonnes, with 331,000 tonnes targeted for delivery in the current marketing year that began Sept 1.
Corn futures have been losing ground to soybeans this week on inter-market spreads as traders fret that US farmers might not plant enough soy acres this spring to meet global demand, especially if South American crops sustain further downgrades.
Traders are still digesting the baseline agricultural projections that the US Agriculture Department released this week, which forecast the most US corn seedings since 1944 and a near-doubling of US corn inventories by the end of the 2012/13 marketing year. USDA is expected to update the forecasts next week at its annual outlook forum.