Issue No. 14, Article 5/June 26, 2009
Nitrogen Applications Over the Top
The season's prolonged wet conditions are leaving growers with no other alternative than sidedress applications for nitrogen (N). Many are now facing the decision of when and how to apply N.
How late can I apply N? Corn takes up large amounts of N during approximately the V8 to VT (tasseling) development stages. Nitrogen uptake is mostly done shortly after pollination. So applying N before the V8 development stage is probably the best time. Research has shown that if applications are done around V6, it is rare to see yield loss due to N stress. Of course, if N was applied preplant or at planting, a delay in application of supplemental N is not likely to cause plant N stress. In cases where no N was applied, or the N supply is very low, make it a priority to try to apply early (preferably before V6) to avoid loss of yield potential.
Can I apply N "over the top"? Injection into the soil and dribbling between rows are the best ways to sidedress N because these applications can reduce volatilization of urea and protect the crop from foliar damage. If ammonia is used for the application, it is important to wait until soil conditions will allow the knife track to close properly. When injecting and dribbling are not viable options, broadcast application is another possibility. Applying dry products, such as ammonium nitrate and urea, over the top of the crop can result in foliar damage, in the form of small lesions, when granules fall into the whorl or leaf axil of the corn plant. Also, as the leaf emerges from the whorl, the margin might be white due to excess N in the leaf. Typically, though, this damage is merely an aesthetic concern and rarely translates into yield reduction. Also, urea is subject to volatilization if rain does not fall within 3 to 4 days after application. As much as 30% of the urea can volatilize if there is no rainfall within approximately 10 days after the application. For "over the top" applications, urea granules will have the least impact on leaf burn compared to UAN or dry products such as ammonium nitrate or ammonium sulfate. To minimize adhesion of dry products to the leaves, it is best to apply when the foliage is dry.
If UAN solution is broadcast over corn, this too can cause foliage damage. If the application is done when plants are small (about 6 inches), the damage will not likely result in yield loss. Even when plants are bigger (V4), the foliage damage caused by a rate as high as 90 to 100 pounds N per acre typically does not cause significant yield reduction.
One way to reduce damage from UAN is to apply in advance of rain. If rain falls within a few hours after application, it will wash the fertilizer off the foliage; it will also reduce the potential for volatilization of urea. If a broadcast application of UAN is the only option available, try to do it as soon as possible, because the smaller the plant, the less the potential for foliar damage. However, if the plant is bigger and more N is needed, the yield benefit from the additional N will likely outweigh the leaf burn caused by the application.
Research from Minnesota has shown yield reduction when a rate of more than 60 pounds N per acre was applied at V8. When N applications are needed later than V8, to avoid extensive foliage damage it is very important to fit the high-clearance equipment with drop hoses so that UAN is applied directly to the soil surface without touching the crop canopy. If you plan to include herbicide with your UAN application, make sure you read the herbicide label to make sure such an application is allowed. Also, be aware that including herbicide with the UAN solution can intensify leaf burning. In Minnesota, adding 2 pounds atrazine per acre at a rate of more than 90 pounds N per acre at V3 development stage caused severe leaf burning. Applying 2 pounds atrazine per acre at 60 pounds N per acre causes similar leaf burning as applying 120 pounds N per acre with UAN alone.--Fabián G. Fernández