NOTE: CBOT grain markets will be closed on Thursday for the Independence Day holiday in the United States.
WHEAT - Down 2 to 5 cents per bushel
Weakness in the CBOT corn market was seen spilling over into wheat futures, traders said.
US farmers are harvesting soft red winter wheat and hard red winter wheat, bringing new supplies into the marketplace.
Russian farmers are facing unfavorably hot and dry weather, prompting consultancy IKAR to cut its forecasts for the country's wheat harvest and exports.
Crop losses in Russia could help lift demand for US grain, according to analysts.
Saudi Arabia's state grain buyer SAGO said it bought 730,000 tonnes of wheat at its latest international purchasing tender.
CBOT September soft red winter wheat last traded down 7 cents at $5.20-1/4 a bushel. K.C. September hard red winter wheat was last down 5-1/2 cents at $4.56, and MGEX September spring wheat was down 5-1/4 cents at $5.49.
CORN - Down 1 to 4 cents per bushel
The corn market was under pressure after the US Department of Agriculture, in a report on Friday, pegged US planted acres well above trade expectations.
Traders remain uncertain about how many acres were actually planted, after historic flooding in the US Midwest this spring.
Weather conditions look "favorably warm this week" for corn and soybean crops in the US Midwest, according to Commodity Weather Group.
CBOT September corn last traded down 3 cents at $4.21-3/4 a bushel.
SOYBEANS - Up 1 to 4 cents per bushel
Soybean futures reached a one-year high of $9.34-1/4 a bushel.
Washington and Beijing agreed over the weekend to restart talks to resolve a trade war that has slowed exports of US soybeans to China, the world's top buyer of the oilseed.
The USDA on Friday estimated US farmers planted fewer acres of soybeans than analysts expected.
A monthly USDA oilseed crushings report due at 2 p.m. CDT (1900 GMT) Monday is expected to show that 4.929 million short tons of soybeans were processed in May.
CBOT August soybeans last traded up 4-1/2 cents at $9.09 per bushel.