An optimum nitrogen rate for corn is critical for yield, farm profitability and groundwater quality. Key factors to consider for crop N rate include predicted yield for the season, residual N from the previous season, and the possible N contribution from decomposition of soil organic matter (SOM).

While every state across the U.S. Corn Belt has developed its own single formula for N rate recommendations, those formulas are not very sensitive to weather variation from one year to another. On the other hand, N contribution from SOM decomposition is sensitive to weather variation, especially temperature.

Figure 1. Mean air temperature during Oct. 1 to April 30 for 2016-17 and 2017-18 at Mead, Neb. The long-term mean is for 36 years from 1982 to 2017.

For the period of October 2017 to April 2018, eastern Nebraska has experienced a colder mean temperature than the previous year, as well as the long-term norm (Figure 1). As a result, N contribution from SOM decomposition would be lower (Figure 2), assuming everything else is similar for the rest of the growing season. This would likely call for an increase in the N rate for corn for the 2018 growing season if the application decision was made using the conventional N rate scheme.

Figure 2. N released from soil organic matter decomposition during September through April 2016-17 and 2017-18 in a soil with SOM content of 3% at Mead, Neb., using the Maize-N model simulation. The long-term mean is for 36 years from 1982 to 2017.

The Maize-N computer program developed by researchers in University of Nebraska is a decision support tool that can help producers, crop advisers, Extension educators and others set and fine-tune corn N rates based on in-season weather conditions and field-specific soil properties (Figure 3). Maize-N considers the same factors that affect N fertilizer rate for a crop as the conventional schemes, but it does so in a more quantitative, seasonal and field-specific manner. So, users can run the program for individual fields and at the time when the N rate must be finalized, like sidedressing several weeks after emergence.

Figure 3. Screenshot of the front page of the Maize-N program.

For instance, based on the weather data as of April 30, Maize-N suggested an optimum N rate of 139 pounds per acre for a typical field at Mead, Neb., which is 14 pounds per acre more than the optimum N rate for same field but last year.

The Maize-N program provides season-specific recommendation for N rate by using real-time weather data. SOM content across the Corn Belt is typically around 2% to 4 %. As a result, SOM can release 70 to 140 pounds per acre of N per year that is readily available to crops. Because of the dependence of SOM decomposition on temperature, using real-time weather data can lead to more reliable N contribution estimation from SOM.

In addition, the likely current season yield is also predicted by the model based on crop management information from users and the combination of real-time weather data with the long-term weather record for the location of the field.

Maize-N users must provide up-to-date weather data for the field location along with a minimum of five years of past weather records. For Nebraska producers, all weather data can be obtained from the High Plains Regional Climate Center. All other input information required by the model is readily available from every producer. Maize-N provides the economically optimal N rate (EONR) for the field, as well as an array of other information that leads to EONR, including the likely yield, total crop N uptake demand and N from each source in the soil. In addition, Maize-N also displays the timeline for N release from SOM, crop N uptake and even potential N leaching.

Field trials show that 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 online.

For more information, suggestions and comments, contact Haishun Yang at [email protected] or 402-472-6372.

Yang is an associate professor of Agronomy and Horticulture at the University of Nebraska-Lincoln. Elmore is a Nebraska Extension cropping systems agronomist.