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  • Writer's pictureWendell Koehn

Preparing for post plant Nitrogen app in Corn

“You can create something that is pure genius, but you have to get your timing right.” -Lang Leav

Growers in Missouri use a wide variety of practices and multiple sources to apply nitrogen to corn. This article main focus is on the application after corn has been planted. For this article, we will assume a pre plant nitrogen source of NH3 or some dry variant was used (i.e. Urea, AMS, or Nitrate). With the heavy rain that followed the majority of planting, I am getting asked; "How much Nitrogen is left? How will I know how much extra will need to be top dressed for full yield potential?" Some Nitrogen no doubt was lost or moved quite deep into the soil profile. The good news is most fields had nitrogen applied in the spring due to the wet winter. This will be a good thing to keep losses less than if applied last fall.

Application Timing: Iowa State University finds that maximum uptake of N occurs in periods of maximum growth. This is roughly between V9 and V18, which is around hip high if you are taller than me, to just before tassle. Applying nitrogen prior to, yet as close to this point as possible would be a best management practice (BMP). This minimizes loss from that point on and also places the nitrogen just ahead of when the corn needs easy access to it as well. In a year with frequent rain chances, we might take advantage of anytime around knee high on that we have. If, for example we have a shot around the first week of June; I would go for it even if corn is not at V9.

Application Rate: Even with the new technology of quick soil scans and tissue testing MU finds that crop color appears to be the most reliable indicator of how much N is needed. One practice that can be really helpful in this was given to me by David Hughes, a field agronomist in the state of Missouri. When applying nitrogen to your field, apply one pass with your implement with double the rate. For example, if you are applying actual 120 lb NH3 in February or March, make one pass in a field with 240 lb. This pass will have sufficient nitrogen regardless of rainfall, and will be your check strip to identify how much N is lacking on the rest of the field. Applying this test strip next to a grain bin can allow you to see a broad area from higher up. If you have a drone aircraft available or know someone who can fly one for you will work as well. When applying your post app now, you might use this same strategy. This will be your indicator as we move toward tassel to give you the confidence you have enough N on for the rest of grain fill.

Nitrogen Fertilizer Sources: Ammonium nitrate, urea, and urea-ammonium nitrate (UAN/liquid/32%) are the three main N fertilizer sources used in Missouri in a post app. All are excellent nitrogen products. Thus, the decision maker here should be focused on the logistics of application. Since prompt application is so important, any small perceived advantage of one product over another would be erased if it interferes with the timeliness or the correct placement of this app. Choosing a source that is easily found, commonly applied, and equipment that is more readily available can help guide which product is best for your farm. On all the dry products, I can't stress enough the importance of making sure the spinner is pan tested for even broadcast. If you are hiring this done make sure you have this conversation about the truck's spinner calibration.

If applying:

Ammoniam Nitrate: One of the good things about this product is it does not take much rain to activate. However, many facilities do not carry this product due to liability, and this product burns the leaves the worst of any of the products on post app.

Urea: using volatilization inhibitor on this product for surface broadcast is important. Any other product that slows the conversion rate of at this time of the year would not be a good idea (like Super U, etc)

UAN/liquid N: do not broadcast on high residue surfaces, yield penalty can result. Injection with coulters or Y drop is preferable. Dribbling solution can also be done if other two are not an option.

Some other Nitrogen BMP to consider:

-Use application practices that minimize soil compaction.

-Ask your dealer to know which hybrids are the most affected by nitrogen practices

-Have good weed control, large weeds can drain substantial amounts of N away from the crop.

Works cited in this article:

Best Management Practices for Nitrogen Fertilizer in Missouri

Peter C. Scharf Plant Sciences Extension
John A. Lory Plant Sciences Extension
In collaboration with: Judy Grundler Missouri Department of AgricultureMissouri Corn Growers AssociationMO-AG/Missouri Agribusiness AssociationDOW AgroSciencesU.S. Environmental Protection Agency

Agronomist David Hughes

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