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Relationship Between Corn Yield, Expressed as a Percentage of Maximum, and the N Percentage in the Grain. II. Diagnostic Use¹

The purpose of this study was to develop a procedure for determining the amount of N fertilizer needed by corn (Zea mays L.) for maximum yield and economic optimum yield based on the relationship between the relative yield of corn, expressed as a percentage of maximum yield, and the N percentage in the grain. The three major steps in the procedure are: (a) determining the maximum potential yield and yield increase from the relative yield — percent N relationship and the present yield, (b) determining the N-requirement index (NRI) for different initial relative yields, and (c) calculating the total N requirement.

Two procedures were developed for estimating the NRI, the N required per unit of yield increase, from different initial relative yield levels to maximum or economic optimum yields. One involved simple computations using the treatment means from 13 site-years of experimental data. The other utilized the individual regressions of yield on N fertilizer from the same experiments to calculate the NRI's to the same marginal productivity point on each site-year's regression. Good agreement was obtained between the two procedures.

The average NRI's were found to vary from approximately 7.5 kg/q at 90% of maximum yield to 3.5 kg/q at 45% of maximum yield. Evidence was obtained showing the desirability of using NRI values for comparing the efficiency of N use under different environmental and management conditions.

Factors that may affect the relative yield — percent N and the NRI — initial relative yield relationships are discussed briefly. Also, the need for further research, expecially on the effect of different hybrids, is suggested.

Key Words: Critical percent N • N sufficiency • Potential yield • Economic optimum yields • N-requirement index (NRI) • Initial relative yield • Corn hybrids • N fertilizer recommendations

² Professor emeritus, associate professor, and graduate student, respectively.

Received for publication April 17, 1976.