On‐Farm Evaluations to Calibrate Tools for Estimating Late‐Season Nitrogen Status of Corn

Abstract

Properly calibrated diagnostic tools are needed to evaluate the performance of different N management practices for corn (Zea mays L.). Until now, mostly controlled studies were used for such calibrations. We utilizedon‐farm evaluation studies to verify current and identify new N status categories using the corn stalk nitrate test (CSNT) andlate‐season digital aerial imagery of the corn canopy. From 2007 through 2010, producers conducted 125 trials across Iowa. Each trial had treatments of a producer's normal N rate alternated with a rate that was about one‐third lower or higher. Categorical yield response (YR), expressed as profitable and unprofitable, was related to green reflectance (GR), relative green reflectance (RGR), or CSNT sampled within nine areas in each trial. Multilevel binary logistic regressions were used to estimate the probability of receiving profitable YR for a range of CSNT, RGR, and GR values. Among the three diagnostics, RGR performed slightly better but required applying at least two N rates within producers' fields. For CSNT, the identified optimal category was almost the same as that currently recommended in Iowa (700–2000 mg NO3–N kg−1), even when corn and N prices deviated from their long‐term averages by 30%. Due to the uncertainty in N availability, however, the critical CSNT value for fall manure treatments was about 3000 mg NO3–N kg−1 higher than that for fall anhydrous NH3, spring urea–NH4NO3, or sidedress N. On‐farm studies can be used to calibratelate‐season N diagnostic tools for evaluating management practices that differ in rates, forms, and timing of N applications.