Corn edged lower in a technical-selling and profit-taking setback after hitting a one-week high, while wheat firmed on signs of improving US export demand.
A firmer US dollar, which makes US exports more expensive for buyers holding other currencies, created some headwinds for grains.
Largely favorable harvest weather in the US Midwest also weighed on prices as most farmers are expected to finish gathering their bumper corn and soybean crops without major disruptions. Persistent rains earlier this month had delayed harvesting in some areas and damaged some crops, particularly soybeans.
"There is a large soybean crop out there," said Don Roose, president of Iowa-based US Commodities. "Even if soybean yields are smaller, we have more than enough of a cushion there. We still have a monster supply."
Chicago Board of Trade November soybeans ended down 6 cents at $8.39 a bushel after earlier hitting a low of $8.37-1/4, the lowest since Sept. 25.
December corn fell 1 cent to $3.66-3/4 a bushel, while CBOT December wheat gained 2 cents to $5.07-1/4 a bushel.
The US Department of Agriculture (USDA) was due to release its weekly harvest progress report later on Monday.
The US soybean harvest was expected to be about 70 percent finished, still behind average but a jump from 53 percent complete a week earlier, according to the average estimate of analysts polled by Reuters.
The corn harvest was expected to be 64 percent complete, in line with the average pace.
A lagging US soybean export pace remained a concern as China, the world's top buyer of the oilseed, continues to largely shun US shipments amid a trade dispute with the United States.
The USDA on Monday said private exporters reported sales of 120,000 tonnes of soybeans to undisclosed buyers, and the agency reported stronger-than-expected soybean export inspections last week. But the pace of sales and shipments remains well behind last year so many traders expect soybean supplies to remain very large.
Meanwhile, farmers in top soy exporter Brazil are likely to harvest another record soybean crop next season amid good planting weather, according to a Reuters poll.