Chicago Board of Trade soft red winter wheat futures were weaker, weighed by weak demand on the export market for US supplies.
All three commodities were on track to post weekly losses, with corn and wheat set to notch their biggest weekly declines since the summer of 2016.
Gains in corn and soybeans were capped by forecasts for cool weather and some crop-boosting moisture in the coming days.
At 10:41 a.m. CDT (1541 GMT), Chicago Board of Trade December corn futures were up 3 cents at $3.80-3/4 a bushel. CBOT November soybeans were 1/4 cent higher at $9.60-3/4 a bushel.
"Weather forecasters' predictions are largely being realised with more rain and cooler temperatures," said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia in Sydney. "The forecasters also expect more of the same at different times for the next week or so. August, the danger period for soybean crops, is looking benign for now."
For the week, corn futures have dropped 2.0 percent and soybean futures have dropped 5.3 percent, which would be the biggest weekly loss since the week ended July 23, 2016.
"Weather has been favorable across the Midwest and we can expect the weaker tone in the soybean market to continue," CHS Hedging market analyst Ray Remmert said in a note to clients.
Market attention is turning towards the USDA's monthly supply and demand report on Aug. 10 in which it will update its corn yield projection including first data from field surveys.
But the full impact of a hot and dry summer in the US Midwest is unlikely to show up in the government's next estimate of the US corn crop as it typically makes just small adjustments to its harvest outlook during August.
CBOT September soft red winter wheat futures were down 1-1/4 cents at $4.56-1/2 a bushel.
CBOT wheat has fallen for four straight weeks, shedding 14.6 percent of its value during that time.
US wheat faced stiff competition on the export market with rising expectations for harvest production in Russia, a leading wheat exporter, adding to the global stockpile.