US soybean futures turned lower, retreating from early advances tied to signs of thawing trade tension between Washington and top global soy buyer China.
Corn futures followed soybeans down in subdued trade ahead of Tuesday's New Year's Day holiday.
As of 12:40 p.m. CST (1840 GMT), Chicago Board of Trade March wheat futures were down 8 cents at $5.03-1/2 per bushel after touching $5.03, the contract's lowest since Jan. 24, and the lowest for a most-active wheat contract since Nov. 16.
CBOT March soybeans were down 3-1/4 cents at $8.92-1/4 a bushel. March corn was down 1 cent at $3.74-1/2 a bushel.
Fundamental news was lacking, allowing brokers and commodity funds to focus on squaring positions on the last trading day of the month, quarter and year.
"Today is a holiday trade environment, meaning it is all sputters, all risk-off for the year-end. The funds are balancing right now, and selling corn and wheat, and a little soybeans," said Don Roose, president of Iowa-based US Commodities.
Despite Monday's slip, CBOT wheat was poised to end 2018 up about 18 percent, its biggest annual rise since 2012, after a sharp fall in production in major exporters Russia, the European Union and Australia, making the grain one of the biggest gainers in commodity markets this year.
Corn has also risen this year as tightening wheat supply fueled additional demand from livestock feed makers, offsetting concerns about disruption to US shipments to China.
CBOT soybeans, however, were on track for a 7 percent yearly decline after a plunge in exports to China, coupled with big harvests in the United States and Brazil, weighed on prices.
A resumption in Chinese imports of US soybeans this month as part of a negotiated truce between the world's two largest economies has helped Chicago prices rebound from a decade low recorded in September.
CBOT soybean futures firmed on Monday in early moves after what US President Donald Trump over the weekend described as a "very good call" with China's President Xi Jinping.