Improved growing conditions in the US Midwest also weighed on soybeans and corn before weekly government crop ratings later on Monday as well as monthly supply and demand forecasts on Thursday.
Sweltering heat across the Midwest farm belt is expected to ease by next week, when more of the corn crop will be entering its critical pollination stage. Crop boosting rains are also forecast for portions of the Midwest later this week.
"A lot of the market's focus today is on the changing weather forecast and the idea that the intense heat will not hit a good portion of corn pollination in the weeks ahead," said Rich Nelson, chief strategist with Allendale Inc.
"Falling temperatures are going to help ease pollination stress," he said.
Chicago Board of Trade (CBOT) August soybeans
fell 26-1/2 cents, or 3 percent, to $8.51 a bushel by 12:08 p.m. CDT (1708 GMT), returning more than half of Friday's 4.5-percent gain.
CBOT September corn
dropped 8-3/4 cents, or 2.4 percent, to $3.51-1/2 a bushel.
Analysts polled ahead of the US Department of Agriculture's (USDA) weekly crop conditions report expected the agency to trim corn and soybean conditions in the latest week, although overall ratings should remain near historic highs.
Grain traders are also positioning ahead of the USDA's monthly supply and demand reports due on Thursday. The USDA is expected to begin cutting US soy exports and increasing projected stockpiles amid a trade war with top importer China.
Expectations that Beijing's extra 25 percent tariff on US soybeans will shift more sales to Brazil, the world's top exporter, has contributed to a five-year high in the spread between US and Brazilian prices.