At what point should I spread Nitrogen(Urea) on the corn field to help the cob production but not give the weeds an extra boost?
Brian

Brian - If you are going to stagger your nitrogen application, I believe you should top-dress corn at 28-day intervals.

broadcast Urea for kernal production when its about knee high - cob & kernal development is already strating internally, the plant needs that shot of N prior to tassling. if you wait till the cob is visible, very little N is used by the plant.