Are increasing maximum Return-to-Nitrogen values related to nitrogen source?

Fabian Fernandez

The University of Minnesota conduct field studies across Minnesota to refine the economic optimum nitrogen (N)rate (EONR). These studies are combined into the Maximum Return to N (MRTN) calculator (http://cnrc.agron.iastate.edu/).

This is a free tool that farmers can trust to help guide their N rates. Over the last several years, we have observed that MRTN rates for spring applications continue to increase and there is an increase in the frequency of linear responses in corn grain yield to N rate instead of the typical quadratic plateau response.

Possible reasons for this are: 1) greater N loss due to overall wetter spring conditions (last couple of years have been the exception), 2) depletion of organic N (less native supply capacity), 3) greater immobilization from more corn residue or 4) the fact that most recent spring application studies are done with urea instead of anhydrous ammonia.

The increase in the number of urea data in the MRTN database reflects the fact that since 2009, anhydrous ammonia tonnage sales have declined and urea has become the predominant N source in Minnesota. However, urea is overall more susceptible to loss than anhydrous ammonia. While the data clearly shows that N needs have increased, environmental groups often view this negatively. Understanding why these changes are occurring as well as finding ways to improve N management is important for corn farmers. Additionally, fall N fertilization is a valuable tool, but with less anhydrous ammonia availability, there is a critical need to find viable alternatives to using urea-based fertilizers in the fall.

The objectives of this three-year study are to 1) evaluate whether the increase in MRTN is related to the increased use of urea over anhydrous ammonia for spring applications, and 2) explore alternatives to fall applications using polymer coated urea and subsurface banding.