Ramp calibration strip technology for determining midseason nitrogen rates in corn and wheat

Midseason fertilizer N recommendations in corn (Zea mays L.) and wheat (Triticum aestivum L.) are not consistent from one region to the next. Preplant soil testing, yield goals, economic optimums, chlorophyll meters, and optical sensor-based yield prediction models are limited regionally. The objective of this paper is to introduce an applied approach for applying preplant N fertilizer in automated gradients used for determining midseason N rates based on plant response. This approach assumes that midseason biomass estimated using normalized difference vegetation index (NDVI) sensor readings is directly related to corn and wheat grain yield, and that delaying applied N until midseason (eight-leaf stage in corn and Feekes 5 in winter wheat) can result in near-maximum yields. The ramped calibration strip (RCS) applicator applies 16 different incremental N rates (3-to 6-m intervals), over 45 to 90 m (number of rates, intervals, and distances can be adjusted depending on the crop). Because the RCS is superimposed on the farmer practice, producers can examine plant responsiveness over the range of rates to determine the optimum topdress N rate. The point where midseason growth differences no longer exist is the topdress N rate. Recording distance is required as you walk the RCS since distance is associated with an incremental N rate. Where adequate but not excessive preplant N is available, the ramp interpolated rate provides an applied method to determine how much midseason N should be applied to achieve the maximum yields based on growth response evidenced within the RCS.