Broadcast urea or ammonium sulfate will burn a corn plant if the granules fall within the corn whorl. Broadcast UAN has less volatile loss than dry urea, but a urease inhibitor will help prevent that. However the best cure is for rain to move the nitrogen into the root zone. But broadcasting UAN will also cause leaf burn and impede early growth. He says research has shown cosmetic plant damage if the corn is at the V3 growth sage and the application exceeds 60 pounds of N per acre. However, higher rates will begin to depress yields if the growth stage exceeds V3.
Sawyer suggests using the late spring soil nitrate test if nitrogen is going to be sidedressed. That collects soil from the top 12 inches of soil when the corn is 6-12 inches tall and the rates of nitrogen application can be adjusted.
But what if the corn becomes too tall for sidedressing, based on continued wet conditions? High clearance equipment can be used, says Sawyer if normal sidedress knives cannot be used. But that will change your nitrogen choice away from anhydrous ammonia. You would be using UAN solution either dribbled between the rows or shallow injection with a coulter. He says, “Research in Iowa has shown corn can respond to mid- to late-vegetative growth stage N application when there is deficient N supply, but there can be loss in yield potential. Reduced yield occurs more frequently when soils are dry at and after application (applied N not getting into the root zone) and with severe N stress. Best responses occur with sufficient rainfall shortly after application to move N into the active root zone.”
Corn should be planted first, and then nitrogen applied, if it has not already been applied. If it does not delay planting, fertilize first. Sidedressing nitrogen can occur after corn is visible, but choices should be carefully selected and broadcast forms of nitrogen could end up burning the corn to the point of yield loss. Before planning to sidedress, ensure the appropriate equipment is available, along with the necessary nitrogen products.
Source: FarmGate blog