Chicago corn futures slipped on Wednesday, retreating after Tuesday's strong gains, after a report showed U.S. crop ratings unchanged despite talk of lower yields after poor weather.

Wheat and soybeans firmed ahead of monthly U.S. Department of Agriculture (USDA) world supply and demand reports on Friday.

"Corn has been weakened by the U.S. crop conditions report late on Tuesday which showed no deterioration on the week despite concerns that last week's hot and dry weather could have eroded conditions and hurt yields in the United States," said Stefan Vogel, head of agricultural commodity markets research at Rabobank.

"Wheat and soybeans remain supported by positioning ahead of the USDA reports on Friday, but the favorable U.S. crop conditions report on Tuesday has taken some of the upward momentum seen on Tuesday out of these markets today."

Chicago Board of Trade December corn fell 0.1 percent to $3.67-3/4 a bushel at 1005 GMT after rising 1.4 percent on Tuesday.

December wheat rose 0.6 percent to $4.78 a bushel, having closed up 1.5 percent on Tuesday. Chicago wheat has been recovering after touching five-year lows on Friday.

November soybeans gained 0.2 percent to $8.81 a bushel, after rising 1.5 percent on Tuesday.

The USDA on Tuesday said 68 percent of the U.S. corn crop was in good to excellent condition, the same as the previous week.

Meanwhile, the USDA's forecasts of U.S. soybean and corn crops on Friday will be closely watched by the market.

"We are likely to see volatile markets for the rest of this week until the USDA report is issued, with some  expecting the USDA to reduce corn and potentially even soybean yield estimates," Vogel said.

Results from the early U.S. corn harvest in the Mid-South and Delta have not reached expectations.

"The early results of the U.S. corn harvest were somewhat disappointing but the initial areas harvested often tend to show poor results as the weakest fields are brought in first," Vogel said. "I think that when the U.S. harvest becomes more extensive in the U.S. Midwest in coming days and weeks that we will see a different yield picture."

"Wheat is seeing some support from reduced forecasts of the Russian harvest and reports that Russian exports have fallen this season following the Russian export tax and the late harvest."

Russian consultancy IKAR lowered its forecast for Russia's 2015 wheat crop.

Russia's wheat exports are restrained by a floating export tax.