Spatially Variable Corn Yield is a Weak Predictor of Optimal Nitrogen Rate

Historically, a mass-balance approach (yield goal times a factor) has been the dominant method for making N fertilizer rate recommendations. Although several states have moved away from the mass-balance approach for N rate recommendations for corn (Zea mays L.), much of the effort that has gone into variable-rate N research has focused on combining spatial yield predictions with a mass-balance approach. Our objectives were to evaluate, at field scale, the relationship between spatially variable yield levels and economically optimal N fertilizer rates (EONR) and to evaluate the performance of yield-based N rate recommendations. Eight experiments were conducted in three major soil areas (Mississippi delta alluvial, deep loess, claypan) over 3 yr. Treatments were field-length strips of discrete N rates from 0 to 280 kg N ha⁻¹. Yield data were partitioned into 20-m increments and a quadratic-plateau function was used to describe yield response to N rate for each 20-m yield cell. The EONR varied much more widely than did plateau yield. Yield level explained on average only 15% of the variability in EONR. Averaged over the eight fields, variable application of mass-balance-based N rates based on actual yields would have increased yield by only 31 kg ha⁻¹, and profit by $2 ha⁻¹, relative to uniform mass-balance N rates based on field average yields. In comparison, variable-rate application of EONR would have increased profit by an average of $38 ha⁻¹. Of this, $14 ha⁻¹ could have been obtained by uniform application of the median optimal N rate for each field. We conclude that although we observed considerable spatial variability in optimal N rates, this was due mainly to variations in soil N supply and N uptake efficiency, rather than to variations in crop demand for N. Yield variability appears to be at best a small part of the information that must be used to make successful variable-rate N recommendations for corn.