"This downward trend has been led by wheat," said Ted Seifried of Zaner Group. "These crop conditions are absolutely bearish."
Soybean futures turned higher, rallying from early declines, supported by a decline in weekly USDA crop condition ratings.
Chicago Board of Trade September corn futures were down 7-3/4 cents, or 1.8pc, at $4.31-3/4 a bushel at 11:21 a.m. (1521 GMT).
CBOT September wheat was down 11-1/2 cents or 2.3pc to $4.99-1/2 a bushel after hitting $4.99, falling below psychological support at the $5 mark for the first time since June 10.
August soybean futures were up 3 cents or 0.3pc to $8.82 a bushel.
The USDA said in its weekly crop report on Monday that 57pc of the US corn crop was in good to excellent condition, up from 56pc last week and matching analysts' expectations.
"There was no bullish news in this report," said Seifried.
The USDA said 64pc of US winter wheat was in good to excellent condition, up from 63pc last week and above expectations of 63pc. Some 47pc of the US winter wheat harvest was completed, above trade expectations for 45pc.
However, the USDA said 53pc of US soybeans were in good to excellent shape, down from 54pc last week and bucking analyst expectations for an improvement.
"Soybeans are trying to find some independent strength," said Seifried.
US grain futures rose on Monday after forecasts for hot and dry weather in the US Midwest in the coming days, raising concern that late-planted corn and soybean crops may be stressed. But some forecasts on Tuesday included some rain later in the week.
"We're not exactly sure what to make of this weather," said Seifried. "The dry air could help, but it could also distress the crops as well."
Wheat futures faced additional pressure from stiff competition for export business, given ample global supplies.
Russia's 2019 grain crop may be 2pc to 5pc bigger than last year's, said Roman Vilfand, head of research at Russian weather forecaster Hydrometcentre.
Egypt's main state wheat buyer purchased 240,000 tonnes of Romanian and Ukrainian wheat at an international tender. No US wheat was offered.