Temperatures this week may top 100 degrees Fahrenheit (38 Celsius) in the southwest quarter of the Corn Belt but recent rains may blunt the impact of the heat.
Most-active December corn settled down 14-3/4 cents at $3.48-1/2 per bushel after dipping to $3.48-1/4, its lowest since July 7.
The US Department of Agriculture late Monday rated 76 percent of the US corn crop as good to excellent. Analysts had expected a slight decline in ratings.
The USDA also said the crop was more mature than usual, with 56 percent in the silking stage, ahead of the five-year average of 46 percent. The quick growth pace may limit the crop's vulnerability to weather stress, analysts said.
China's National Grain Trade Center said 15,516 tonnes of corn were sold at an auction of state reserves, out of a total of 1,966,449 tonnes of corn available at auction.