Corn gave back nearly all the gains from the previous session and wheat also retreated.
The most active soybean futures on the Chicago Board of Trade were down 0.2% at $9.10-3/4 a bushel by 0934 GMT, having earlier reached $9.14-1/2 a bushel - the highest since July 1. Soybeans firmed 0.9% on Wednesday.
Analysts said the market was nervously awaiting the U.S. Department of Agriculture's latest world supply and demand estimates.
"The last few USDA reports have been counter-intuitive. People do not want to be caught on the wrong side of any moves and so are very wary of taking positions," said Phin Ziebell, an agribusiness economist at National Australia Bank.
Analysts expect the USDA to lower its soybean yield estimate to 48.6 bushels per acre (bpa), down from 49.5 bpa in June.
The expected downgrade will come just days after the USDA said the condition of North American soybeans declined.
The USDA said 53% of the oilseed crop was good to excellent, down from 54% a week earlier.
The most active CBOT wheat futures were down 0.35% at $5.03 a bushel while December wheat on Paris-based Euronext fell 0.1 percent to 178 euros a tonne.
Dealers said a generally favourable crop outlook in the European Union continued to weigh on prices.
"Harvesting has begun in the EU's leading wheat countries. A significantly larger crop looks likely than in 2018, which was hard hit by drought," Commerzbank said in a market note.
"The ongoing harvest is depressing the price, as is the expectation of high competition on the export markets. This is because a good crop is also expected in the Black Sea region, even if the expectations for the Russian crop had to be corrected downwards in recent weeks."
The most active CBOT corn futures were down 0.5% at $4.37-1/4 a bushel.
As with soybeans, the USDA is expected to downgrade its U.S. 2019 corn yield to 165.0 bpa, from 166.0 bpa in June.
Corn, however, was under more pressure as fears about the outlook for crops was brightened by forecasts calling for hot, dry weather.
The warmer weather may benefit crops by drying out fields and accelerating plant growth.