Chicago corn and soybean futures lost more ground on Monday as a rapidly advancing US harvest season and a slowdown in exports put pressure on the commodities.
Wheat edged lower although trader concerns over tight global supply curbed losses.
"There is a bit of pressure on corn and beans from the US harvest," said Ole Houe, director of advisory services at brokerage IKON Commodities in Sydney.
"There is more (downside) to come as crops are large and China is not showing interest in buying US corn yet."
The most-active corn contract on the Chicago Board of Trade (CBOT) was down 1% at $5.22 a bushel, as of 02:19 GMT. Soybeans gave up 0.7% to $12.74-3/4 a bushel and wheat lost 0.1% to $7.08 a bushel.
Favourable weather over the weekend boosted US harvest, while exports remain capped by terminals on the US Gulf Coast that continue to struggle with power outages and hurricane-led damage as the country heads into its busiest export season.
Losses in wheat prices were also capped by lower output in key Northern Hemisphere suppliers, although Australia's second consecutive bumper crop is expected to ease some of the tightness in global supplies.
Last week, Australian forecasters upgraded wheat production targets to 32.6 million tonnes for the season ending 30 June 2022. This would make the target second only to last season's record-breaking harvest of the country's most valuable crop.
There are estimates of lower output of Canada's drought-hit crop, as well as reduced outlook for French and European Union crops and expectations for a fall in winter wheat planting in Russia.
Large speculators cut their net long position in CBOT corn futures in the week ended 14 September, 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, increased their net short position in CBOT wheat and cut their net long position in soybeans.