Losses were limited, however, by hopes that trade talks between Washington and Beijing will open the door for more US soyabean exports to China following sizable sales to state-owned firms on Monday.
Corn futures also eased after hitting fresh three-week highs earlier in the day, while wheat advanced on hopes for a pickup in export demand amid higher prices in rival exporting countries.
China and the United States will continue trade talks in Beijing for an unscheduled third day as officials from the world's two largest economies looked to resolve their bitter trade dispute that has slashed China's imports of US soya.
In a positive sign on Tuesday, China approved five genetically modified crops for import, the first in about 18 months, which could boost its overseas grains purchases and ease pressure from the United States to open its markets to more farm goods.
The move followed a wave of US soyabean purchases by Chinese state firms on Monday.
The market also remained focused on poor weather in South America, where hot and dry conditions have prompted some forecasters to begin trimming forecasts for Brazilian crops.
Chicago Board of Trade March soyabeans were up 6-1/4 cents at $9.18 a bushel at 11:41 a.m. CST (1741 GMT) after failing to break through chart resistance at its 200-day moving average for a second day.
March corn fell 2 cents to $3.80-1/4 a bushel. CBOT March wheat rose 5 cents to $5.21-3/4 a bushel, a 2-1/2 week high.
Growing hopes of more US export sales underpinned wheat.
There is a major purchase tender in the market from Algeria but this is likely to be largely French business.