Corn and wheat futures also were weak, with corn weighed down by the US Agriculture Department's forecast for record US yields this fall. Wheat futures faced pressure from USDA's bigger-than-expected estimate of the Russian crop, which further dimmed prospects for US supplies on the export market.
Expectations for a record US soybean harvest added to the bearish tone.
"We are getting closer to harvest," said Bill Gentry, a broker at Risk Management Commodities. "Upward momentum is hard to maintain when you get the combines running. No matter how you slice it there is going to be more than enough (supply)."
At 10:34 a.m. CDT (1534 GMT), CBOT November soybean futures were down 2-1/4 cents at $8.37-3/4 a bushel.
Soybeans initially firmed on news confirmed by White House economic adviser Larry Kudlow that the Trump administration had invited Chinese officials to restart trade talks. Chinese Foreign Ministry spokesman Geng Shuang told reporters that China welcomed the invitation, and the two countries were discussing the details.
But Trump said on Twitter that a Wall Street Journal story on Wednesday about the invitation from Treasury Secretary Steven Mnuchin amid rising US political pressure on Trump to ease up on trade fights "has it wrong."
Soybean futures subsequently dropped to a session low of $8.35-1/4.
CBOT December soft red winter wheat futures were off 3-1/4 cents at $5.03-1/2 a bushel.
The US Department of Agriculture, in its monthly supply and demand report, increased its forecast for the Russian wheat harvest to 71 million tonnes, up from a previous estimate of 68 million tonnes. Many traders have expected lower output due to a European drought that has stoked fears of export curbs.
"Higher Russian wheat output is a bit of a surprise given the dry weather they had during the growing period," said Phin Ziebell, an agribusiness economist with National Australia Bank.
CBOT December corn was 1-1/2 cents lower at $3.51 a bushel.