NEW YORK—Corn prices sank Monday as investors took profits from last week’s record-setting rally and bet that forecasts for dry weather in the Midwest will allow farmers to speed planting.

Other commodities traded mostly lower, with crude oil retreating after hitting a new record above $126 and gold also falling.

The outlook for warm, drier weather after weeks of heavy rain in the U.S. corn belt is expected to ease planting delays that have driven up corn prices to unprecedented levels. Wet weather in Midwestern states had left fields too soggy for planting, putting far...