SINGAPORE - Chicago corn slid on Wednesday, pressured by forecasts of a record U.S. harvest, after climbing to a six-week high earlier in the session on news of a modest downgrade in crop ratings.

Wheat edged eased on ample global supplies while soybeans were largely unchanged with a lack of Chinese buying keeping a lid on the market.

"Corn and soybean crop ratings have slipped slightly but we are still looking at very large U.S. crops," said one grains trader in Singapore. "It is hard for the market to sustain gains with such large supplies entering the market."

The most-active corn contract on the Chicago Board of Trade (CBOT) fell 0.2% to $4.22-1/4 a bushel, as of 0402 GMT, after climbing to its highest since July 22 at $4.24 a bushel.

Soybeans lost a quarter of a cent to $10.40-3/4 a bushel and wheat fell 0.1% at $5.27-3/4 a bushel.

U.S. farmers are on track to harvest a record corn crop and a bumper soybean crop, with largely favourable weather across the key growing Midwest.

However, the U.S. Department of Agriculture lowered its condition ratings for the nation's corn and soybean crops in a weekly report on Tuesday.

The agency rated 69% of the corn crop in good or excellent shape as of Sunday, down from 71% a week earlier but in line with analyst estimates. The soybean crop rating slipped to 65% good to excellent from 69% a week earlier and below analyst expectations.

Strong export demand has provided additional support to the corn market, but a lack of new-crop soybean purchases by China is keeping soybean futures under pressure.

Beijing's hosting this week of non-Western leaders, including Russian President Vladimir Putin and Indian Prime Minister Narendra Modi, has underscored its geopolitical opposition to the United States.

Commodity funds were net buyers of CBOT corn and soyoil futures contracts on Tuesday and net sellers of soybeans, soymeal and wheat, traders said.