Wet corn is piling up in the U.S. Midwest, boosting premiums for dry grain and raising costs for exporters who already have been undercut in global markets by cheaper Argentine supplies. The slowest U.S. corn harvest in three years was beset by rains and cool weather during the past two months that prevented later-planted crops from drying naturally in the fields. Corn supplies going into the harvest were the largest since 1988 and space in on-farm storage and dryers is at a premium.