Corn prices had been supported by worries about the slow planting, which was lagging due to cold and rain. Corn for July delivery ended Tuesday, down 1.5 percent at $6.40 per bushel.
(Read More: U.S. Corn Soars on Planting Worries, Firm Cash Market )
This year, farmers have been racing to get the crop into the ground before May 15, considered to be the prime time to plant. A late crop is expected to yield less per acre, and farmers believe they lose a bushel per acre for every day it is planted after that date.
A later planting means the crop will pollinate later in July than normal. That could be a plus since means the pollination process, which determines yield, would come after the hottest part of the summer, the first week of July.
Since 1980, there were only two years off to a slower start than this one. One was 1993, when 27 percent of the crop was in the ground, and the other was 1984 when the planting was at 26 percent.
Modern farming technology has made a huge difference and is a big reason why the crop could still be bumper-sized. The USDA forecasts that the domestic corn crop would yield 158 bushels per acre, down from a forecast it made in February due to the slow start to planting.
(Read More: Record world crops on horizon, USDA says)
—By CNBC's Patti Domm Follow her on Twitter @pattidomm