Corn futures declined, losing ground to soybeans on intermarket spreads, while wheat sagged on technical selling and sluggish export prospects.
Chicago Board of Trade November soybean futures settled up 3-1/2 cents at $9.73-1/4 per bushel but pared gains toward the close. December corn ended down 3 cents at $3.83-3/4 a bushel and September wheat fell 6-1/2 cents at $4.57 a bushel.
Traders were adjusting positions ahead of key supply/demand reports due from the US Department of Agriculture on Thursday.
Soybeans firmed on weather worries, including dry patches in parts of the Midwest. The US Department of Agriculture late on Monday rated 60 percent of the US soybean crop in good-to-excellent condition, up from 59 percent a week earlier. However, ratings declined in Illinois and Iowa, the top two US soy states.
The latest weekly US Drought Monitor showed 36 percent of Iowa in moderate drought and 7 percent in severe drought, up from 1.7 percent a week earlier.
Traders have also begun to worry that cool temperatures expected in the Midwest this month could extend the growing season, leaving soybeans vulnerable to an early frost.
"That frost risk would trim the soybean yield more than it would hurt the corn. That's worth a little bit of premium," said Brian Hoops, analyst with Midwest Market Solutions.
The market also drew support from Chinese customs data showing that China's July soybean imports surged 30 percent to their highest level since at least 2010.
CBOT corn firmed in early moves but turned lower. Most of the US corn crop has completed pollination, its key phase for determining yields, so the crop was seen as less vulnerable than soy to frost damage.
"For corn, maturity isn't going to be as much of an issue," Hoops said.
Also bearish, the European Union introduced a corn import tariff for the first time in nearly three years in view of low prices linked to swelling global supplies of the feed grain.
Wheat fell in what appeared to be technically driven trade, with a firmer dollar adding pressure. The US dollar index rose to a more than a one-week high after data showed US job openings surging to a record high in June.
A strong dollar tends to make US grains less competitive on the world market.