Wheat fell as the market faced pressure from winter crop harvest gathering pace in top northern hemisphere exporters.
The front month corn contract on the Chicago Board of Trade was down 0.65pc at $4.59 a bushel at 1144 GMT, soybeans lost 0.4pc to $900 a bushel and wheat gave up 0.5pc at $5.33-1/4 a bushel.
French consultancy Agritel said the return of warmer weather in the Midwest gave some hope to operators.
"This warm weather will allow corn to grow, soybean plantings to advance and winter wheat harvest to progress," it said in a note.
Updated forecasts suggest farmers may have a chance to plant more acres of soybeans this week ahead of a next wave of storms.
The US Department of Agriculture (USDA) said on Monday 96pc of corn and 77pc of soybeans have been planted.
At the same point last year, all US corn acres and 93pc of soybeans were planted.
The USDA also rated 56pc of the US corn crop in good-to-excellent condition, down from 59pc last week and behind market expectations of 59pc.
Some 54pc of US soybeans were seen in good-to-excellent condition, below expectations of 59pc.
The USDA said 61pc of US winter wheat was in good-to-excellent condition, down from 64pc last week.
The agency is due to issue its acreage report on Friday.
"The USDA's acreage report, released on Friday, is looming ever larger over the market," said Tobin Gorey, director of agricultural strategy, Commonwealth Bank of Australia.
"And the genuine 'news' in Monday's condition report was very modest compared with what is potentially in Friday's acreage report."
CBOT July corn may break a support at $4.42-3/4 per bushel and fall to the next support at $4.36-1/4, as suggested by a retracement analysis, according to Wang Tao, a Reuters analyst for commodities technicals.
These supports are identified as the 38.2pc and the 50pc retracements on the uptrend from $4.08-1/4 to $4.64-1/4.
The drop from $4.64-1/4 consists of three waves. So far, only two have completed.
Wheat market prices were weighed down by expectations of dry weather in the US Plains and the Midwest, which will boost the harvest of the hard red winter wheat crop.
Conditions for Russia's spring wheat harvest are generally good or satisfactory with only a few areas affected by a heatwave that has hit some parts of the country, state weather forecaster Hydrometcentre said on Tuesday.
Agritel on Wednesday estimated this year's wheat crop in Russia at 81.7 million tonnes, up 13pc compared with 2018, and above the analyst's initial estimate of 79.2 million tonnes published in April.
In Ukraine, the ministry expects 2019, grain harvest to exceed last year's record crop of 70 million tonnes.