Wheat faced pressure from the advancing US harvest while soybeans are drifting as the fallout from the China trade dispute is assessed and ahead of the USDA world supply and demand forecasts on Aug. 12.
The Chicago Board of Trade's most-active corn contract was down 0.7pc at $4.11-3/4 a bushel by 1105 GMT.
Wheat fell 0.7pc to $4.91 a bushel and soybeans rose 0.1pc to $8.70 a bushel.
The USDA's weekly crop report late on Monday said that 57pc of US corn was in good-to-excellent condition, matching analyst expectations but below the 71pc at this time last year.
"Corn is weaker today after the USDA gave a reasonable picture of the US crop with no unpleasant surprises," said Matt Ammermann, commodity risk manager with INTL FCStone.
The USDA said 73pc of the US spring wheat crop is in good-to-excellent condition, above analyst forecasts of 72pc.
It said the US winter wheat harvest was 82pc complete, up from 75pc last week but below analyst forecasts of 84pc.
"The US winter wheat harvest is being gathered at a good pace and there is currently no need for concern," Ammermann said. "US new-crop wheat is also arriving on a world market with plentiful supplies.
"Egypt is holding a wheat purchase tender on Tuesday and this could also remind the market there is a lot of cheap competition to the US, especially from the Black Sea region."
The soybean market faces continued headwinds from renewed trade tensions between the United States and China.
China's Commerce Ministry on Tuesday said that Chinese companies have stopped buying US agricultural products and it will not rule out potential import tariffs on US farm products.
"China is considering imposing further tariffs on US agricultural products bought after Aug. 3," Commerzbank said in a note. "After state companies were already instructed not to buy any more US agriculturals, private companies have also stopped ordering US soybeans."
But some in the market were not expecting more Chinese purchases anyway.
It is clear US markets want their China demand back, but there is frustration that few of China's former promises to buy have been delivered upon, Ammermann said.