Corn Nitrogen Recommendations Based on Yield Response Data

Nitrogen recommendations based on yield goals require reliable estimates of the factors affecting corn (Zea mays L.) N needs. If these parameters are unknown or erroneously estimated, N recommendations may be inappropriate from economic and environmental perspectives. This study was conducted to develop corn N recommendations using soil-specific data from 117 corn N response experiments and to compare these recommendations with those based on yield goals. Soil-specific recommendations for six major soil regions in Wisconsin were developed using a response function based on combined data from all experiments conducted on a specific soil, and from response surface models containing N rate and residual nitrate-N variables. Yield goal-based recommendations were generated with a widely used mass balance approach. Recommendations based on response data provide a constant N rate across a wide range of actual yields consistent with the observation that economic optimum N rates on a specific soil do not change with yield. Where yield goals are based on a multiyear average of attained yields on a given soil, N recommendations are the same as those derived from soil-specific response data. Where yield goals are much higher or lower than the average of attained yields, however, inappropriate N recommendations are obtained. Although recommendations based on response data require an adequate database for each soil, grouping of soils with similar N response characteristics is possible and minimizes the need for N response data from individual soils. Our research shows that optimum N rates are less variable than yields, indicating that a relatively small database for a representative soil should provide enough information to make sound N fertilizer recommendations. Economic returns from N use calculated for a typical range of corn/N price ratios (16.7 to 10.0 lb N/bu) show that it is usually not profitable to increase or decrease the soil-specific N rate recommendation. A base N rate derived from yield response data with annual adjustments for soil nitrate-N has the advantage of providing a very site-specific N recommendation while avoiding excessive or unprofitable N rates due to selection of unrealistic yield goals. Research QuestionExpected yield or yield goal is the primary management input used to develop corn N recommendations in most of the USA. Although many N response studies have been conducted on the major soils used for corn production, current yield goal-based N recommendations do not reflect the typical pattern of corn response to applied N. Since N response data provide soil-specific information on corn N needs, recommendations based on these data could help to minimize excess N applications due to selection of unrealistic yield goals and ensure profitable use of fertilizer N. The objectives of this work were to develop corn N recommendations based on soil specific data from numerous N response experiments and to compare these recommendations with those based on yield goal. Literature SummaryInappropriate N applications can occur if corn yield goals are not consistent with the actual yield potential of a specific production environment. Several procedures based on multiyear yield averages have been suggested to obtain reliable estimates of the yield potential of specific sites. A mass balance equation including components for the major factors affecting corn N requirement has historically provided the basis for yield goal-based N recommendations. This approach requires reliable estimates of the parameters affecting corn N needs such as internal N requirement, N contributions from soil organic matter and inorganic N, and estimated efficiencies of the various N sources. Frequently, these values are unknown or erroneously estimated for specific sites. Corn N recommendations based on the results of N response experiments have the potential advantage of providing soil specific information consistent with observed corn N response patterns. Study DescriptionData from 117 corn N response experiments on soils representing the six major soil regions used for corn production in Wisconsin were used to develop and compare N recommendations based on soil-specific response data and on yield goals. Soil-specific recommendations were developed using a response function based on combined data from all experiments conducted on a given soil and from response surface models containing N rate and residual nitrate-N variables. Yield goal-based recommendations were generated using a mass balance approach that provided the basis for most currently used yield goal-based recommendation systems. Economic optimum N rates were calculated from the rate of yield change due to applied N and fertilizer N/corn price ratios. Applied QuestionsWhat are the advantages of N recommendations based on N response data? Recommendations based on response data are consistent with the observation that optimum N rates for corn on a specific soil do not vary greatly with yield (Fig. 1). These recommendations help to minimize use of N rates that are too high or too low due to selection of an inappropriate yield goal. How do N recommendations based on N response data compare with those based on yield goals?A comparison of the two recommendation systems (Fig. 1) shows that recommendations based on yield goal do not reflect the typical pattern of corn response to applied N. This can lead to errors in identifying the optimum N rate, especially if very high or very low yield goals are selected. If, however, yield goals are based on a multiyear average yield at a specific site, N recommendations from the two systems will be very similar. Recommendations based on response data have the advantage that information on average yields or other factors affecting corn N needs are not needed for each production unit. However, N response data are required for the major soils used for corn production. Our results show that grouping of soils with similar N response characteristics is feasible, thus minimizing the need for N response data from each soil. Do N recommendations based on response data optimize economic returns at various fertilizer N/corn price ratios?Economic returns from N use calculated for N/corn price ratios representing the likely range of these values show that it is usually not profitable to increase or decrease the N rate recommendation based on response data. Application of 30 lb N/acre more or less than the recommended rate usually did not increase economic return irrespective of the N/corn price ratio. Fig. 1Comparison of corn N recommendations based on yield goal or N response data with observed yield response to applied N on a Piano silt loam from 1986 to 1988.