Soybeans were little changed, consolidating after a near three-week low on Tuesday when the oilseed market also faced pressure from the ongoing US harvest.
Wheat slipped as weather forecasts pointed to some rain relief for dry wheat belts in the US Plains and Black Sea region. But prices were underpinned by doubts about southern hemisphere crop conditions.
The most active corn futures on the Chicago Board of Trade were down 0.2 percent at $3.48-3/4 a bushel by 1240 GMT. It earlier touched a low of $3.48-1/4, its weakest since Sept. 20.
"Despite earlier concerns, the season has shaped up pretty well and the harvest is advancing at a steady rate," said Phin Ziebell, agribusiness economist at National Australia Bank.
The US Department of Agriculture on Monday said the corn harvest was 17 percent complete, and it rated 63 percent of the crop as good-to-excellent, an improvement from 61 percent a week earlier.
Commodity brokerage INTL FCStone late on Monday raised its forecast of the US corn yield to 169.2 bushels per acre, from 166.9 a month earlier.
INTL FCStone also raised its US soybean yield forecast, to 49.9 bpa from 49.8 previously.
The most active CBOT soybean futures were unchanged on the day at $9.55-1/4 a bushel. CBOT wheat were down 0.6 percent at $4.45-1/4 a bushel.
Traders were continuing to assess crop weather conditions in major producing countries.
In South America, some rain in Brazil in recent days could help soybean planting but further showers in Argentina could prevent some fields from drying out after recent downpours.
Wheat traders were also trying to get a clearer picture of weather damage to wheat crops in Argentina and Australia ahead of harvesting.
"Eastern Australia rains early and late next week (will) ease wheat/canola stress, but yield losses to date are irreversible," the Commodity Weather Group said.
Wheat prices remained capped by export competition created by a record Russian harvest this year, as illustrated by a latest Egyptian purchase of Russian wheat on Tuesday.