After the corn has been picked for a while much of whatever ended up on the ground has been eaten while what is left has likely spoiled or soured after rains. Corn that is standing and still in the shucks will withstand the rains, so I mow strips in my corn patches ever week or so during the season to keep fresh corn on the ground. If it rains a good bit after I've mowed, frequently they will quit hitting the corn that has been on the ground a while.