* CBOT September corn settled 11-1/2 cents lower at $4.24-1/2 per bushel. New-crop December corn sank 11-3/4 cents to $4.29-3/4.

* Some traders were unwinding spread trades in which they were long corn and short soybeans, said Jim Gerlach, president of broker A/C Trading.

* Better than expected rains in Iowa, eastern Missouri and northern and western Illinois added pressure on prices, said Dan Cekander, president of DC Analysis. Weather models also showed cooler temperatures in the Midwest next week, he said in a note.

* Some traders have worried that a US heat wave may threaten corn crops this week, after late plantings due to heavy rains this spring.

* Poor export demand also hung over the corn market, traders said.

* Weekly US corn export sales came in at a combined 333,000 tonnes, according to the US Department of Agriculture. That was below market forecasts that ranged from 350,000 tonnes to 800,000 tonne.?Reuters