Topdressing Corn

DR. ANGELA MCCLURE

JACKSON, TENN.
   Although most early planted upland corn looks pretty good for the year, I have had a number of discussions about streaky, yellow corn and applying more nitrogen. Fields that have large ponded or waterlogged areas that stay wet for 2 or more days at a time, run the risk of denitrification N loss of N already applied. There are formulas to estimate N loss amounts based on source used and interval between application and wet conditions but most folks assume 20 to 40 units lost N and add that back in the layby amount.
   • Apply layby nitrogen in the most efficient way to minimize loss. With smaller corn, injecting UAN behind a coulter down into the soil can’t be beat. There is no leaf burn and a stabilizer is not necessary using this system. For larger corn, a topdress of granular urea with Agrotain will burn corn less than ammonium nitrate (AN) or liquid UAN. Cosmetic leaf burn looks terrible, especially at high N rates, but with abundant rains, the crop recovers more quickly. Use a stabilizer with urea or else time the application right before a rain or irrigation. For broadcast AN or urea, apply when corn leaves are DRY during mid to late morning and afternoon. Spreading at night or early in the AM when there is dew on plants usually means fines and particles stick easier and may result in more burn. Spraying UAN on corn is a practice some use. It causes leaf burn (some have had luck timing it before a rain), in larger corn it is difficult to get liquids through a canopy to soil surface vs a granular product (again, time it before a rain) and some data suggests it ties up on surface residue more easily in no-till. A more efficient method may be to use drop hoses and dribble UAN between rows.
   • Fertigation: For corn we recommend starting with 50 lbs N/A pre-plant, accounting for any residual N from previous manure application or legume crop. Begin fertigation about 20 days after crop emergence. Fertigate to have at least 60 percent of the planned total N out by V6 and the remainder prior to tasseling. Ideally N should be applied at 20-30 lbs N/A per irrigation event. This small amount will reduce corrosion of irrigation equipment and risk of clogging. However, in a wet year if we over water a field in order to fertigate, continually ponded or waterlogged portions of the field can actually result in nitrogen losses due to denitrification. In wet years, apply higher rates of UAN (ex. 40 to 60 lbs N/A) in less frequent waterings. ∆
   DR. ANGELA MCCLURE: Extension Corn and Soybean Specialist, University of Tennessee