Wheat was flat, with abundant global supply again pressuring prices, but with some bargain-buying support after sharp falls on Monday.
Chicago Board of Trade December corn was down 0.3 percent at $3.43-3/4 a bushel at 1127 GMT after rising 0.5 percent on Monday.
January soybeans fell 0.2 percent to $9.88 a bushel after ending little changed on Monday, and December wheat fell 0.06 percent to $4.22-1/4 a bushel after dropping 1.4 percent on Monday.
The US Department of Agriculture (USDA) said on Monday the US soybean harvest was 96 percent complete and the corn harvest was 90 percent finished. Both figures were 1 percentage point below analyst estimates, but robust progress has been made on the week in both crops.
"The US corn and soybean harvests are approaching their end very rapidly now as the USDA said on Monday and there do not seem to be too many problems being faced," said Graydon Chong, senior commodity analyst with Rabobank. "This is a weakening factor today with large US supplies coming onto the world market."
"But the downside for corn is limited by the large short position held by investment funds in corn."
"Soybeans continue to be weakened by India's decision to impose import tariffs on vegetable oils which will reduce demand."
Global vegetable oil markets tumbled this week after India's announcement on Friday that it was hiking import taxes on crude palm oil to 30 percent from 15 percent.
"Going forward, soybean market attention is focused strongly on weather in South American production regions, especially Brazil and Argentina, and prices are likely to react to changes there," Chong said.
Wheat is seeing bargain-buying support after falls on Monday and on some dollar weakness but remained burdened by intense competition to US supplies in world markets.
"Russian and other Black Sea exports continue at a very rapid pace," Chong said. "The Australian harvest is gathering speed and some indications are that results are slightly better than previously expected."