There was additional pressure on soybean and corn markets stemming from benign weather across much of the US Midwest growing region.
The most active soybean contract on the Chicago Board of Trade was down 0.7pc at $8.62-1/4 a bushel by 0954 GMT after finishing last week down more than 3pc.
"The US-China trade war, but also picture-perfect weather in the US, have traders thinking that yields may make up for any lower acres," said Ole Houe, director of advisory services at brokerage IKON Commodities.
"And we have the USDA report out next week which has traders just squaring off positions and heading to the sidelines," Houe said. "There is too much uncertainty and no one really knows how the USDA will deal with the loss of planted acres."
US President Donald Trump last week said he would impose an additional 10pc tariff on $300 billion worth of Chinese imports starting Sept. 1, citing insufficient progress in trade talks between the world's two largest economies.
"China has responded to the new US punitive tariffs by instructing its own state companies to stop buying any more US agriculturals," Commerzbank said.
"This will probably also put an end to the exemptions from the 25pc punitive tariffs levied on US agricultural imports that had been granted to a few private companies," Commerzbank added.
On the technical front, the CBOT soybean November contract may test a support at $8.56-1/2 per bushel, with a good chance of breaking below this level and falling to $8.43, according to Wang Tao, Reuters analyst for commodities technicals.
The most active CBOT corn contract was down 1.5 percent at $4.03-1/2 a bushel.
The US corn crop may have gotten its latest-ever start this spring, but speculators' bullish bets have lost steam in recent weeks as the weather has not been outwardly threatening for crop development.
Large speculators cut their net long position in CBOT corn futures in the week to July 30, regulatory data released on Friday showed.
The Commodity Futures Trading Commission's weekly commitments of traders report also showed that non-commercial traders, a category that includes hedge funds, trimmed their net short position in CBOT wheat and increased their net short position in soybeans.
CBOT's most active wheat contract fell 1.9 percent to $4.81-1/2 a bushel, while December milling wheat on Paris-based Euronext was down 0.6 percent at 177.50 euros a tonne.