Determining nitrogen (N) fertilizer rate for corn is a critical part of crop management. Major factors to consider for N rate include:

• likely yield at the end of the season,
• content of soil organic matter (SOM),
• N credits from previous season (leftover of N fertilizers, legume crop residues, irrigation water, etc), and
• N from manures if applied.

Every major corn-producing state in the US has developed its own guideline for N rate determination, often in the form of an integrated formula. Taking Nebraska for example, the following formula has been used for many years for N rate for corn:

N rate (lb/ac) = [35 + (1.2 x EY)] – [(8 x NO₃) + (0.14 x EY x SOM) + N credits]

• EY is the expected yield or yield goal in bu/ac,
• NO₃ is the NO₃-N concentration in ppm in top 3-4 feet of soil from samples taken in the fall or spring,
• SOM is percent SOM content, and
• N credits in lb/ac is the estimated overall N contribution from sources other than SOM.

This guideline resulted from field research over many years and across major crop production areas. It well represents average cases across space and time, but may not work well for fields or seasons that deviate significantly from average cases. Two of the challenges of using such a guide are yield predictions for the coming season and the amount of N credit. Both vary with crop management and weather. Another hurdle is the required soil testing for NO₃-N, which is labor and time consuming. Because of the guesswork involved with using such a calculation, it is less quantitative and its success depends much on field experience.

Researchers at the University of Nebraska–Lincoln have developed an alternative decision-support tool for N fertilizer rate recommendations for corn. It is called Maize-N, a simulation model that runs in a regular Windows-based personal PC (Figure 1). Maize-N considers the same factors that affect N fertilizer rate for a crop as used by the conventional guide, but it does so in a more quantitative and field- and season-specific manner with little dependence on field experience and soil testing (Figure 2). As such, users can run the program 1) for individual fields to reflect the differences in crop management and soil of the fields, and 2) at the time when N rate must be determined, for instance when sidedressing several weeks after emergence.

Maize-N provides season-specific recommendations for N rate by using real-time weather data. SOM content across the Corn Belt is typically around 3-5%. As a result, SOM can release 100 to 160 lb/ac of N that is readily available to the crop. Because SOM mineralization depends greatly on temperature, which varies from one season to another, using current season real-time weather data would lead to more accurate estimation of N contribution from SOM. In addition, the likely yield of the current season is predicted using the Hybrid-Maize model based on crop management information from users and combining real-time weather data and the long-term weather record for the field location.

For using Maize-N, users need to provide up-to-date weather data for the field location along with a minimum of five years of past weather record. For Nebraska producers, all weather data can be obtained from the High Plains Regional Climate Center at https://hprcc.unl.edu/. In addition, users need to provide last year’s crop data for the field, readily known crop information for this season, and basic soil properties every producer knows (Figure 1). For the output, Maize-N produces not only the economically optimal N rate (EONR) for the field, but also a series of estimates that lead to the EONR, including the likely yield, total crop N uptake requirement, how much crop N comes from each of the sources, including SOM, N leftover from last year’s fertilizers, manures if applied, crop resides, irrigation water, etc (Figure 3). In addition Maize-N also displays the course of N released from SOM, crop N uptake, and even potential N leaching (Figure 4).

Field trials showed that the Maize-N is not only easy to use, but also superior to the current one-formula type N rate recommendation method. Maize-N also provides more insight into the soil and crop system in terms of crop yield, N demand, and N supplies from various sources in response to crop management, soil properties and up-to-date weather. Moreover, soil testing for NO₃ is optional when using Maize-N. The Maize-N software is available at https://marketplace.unl.edu/nutechmarketplace/software/maize-n.html

References

Setiyono, Yang, et al. 2011. Maize-N: A Decision Tool for Nitrogen Management in Maize.  Agronomy Journal. 103, 1-8.

Thompson et al. 2015. Model and Sensor-Based Recommendation Approaches for In-Season Nitrogen Management in Corn. J 107, 2020-2030.

Figure 1. Screenshot of the front page of the Maize-N model.

Figure 2. Maize-N decision making process.

Figure 4. Maize-N predictions of N release from SOM, crop N uptake, etc. in soil during the season.