Optical canopy sensing tools may improve corn (Zea mays L.) nitrogen (N) management, but their usefulness in far northern latitudes remains unclear. For this reason, the utility of SPAD, GreenSeeker normalized difference vegetation index (GS-NDVI), RapidSCAN normalized difference vegetation index (RS-NDVI), and RapidSCAN normalized difference red edge (RS-NDRE) were evaluated to predict corn grain yield, plant N accumulation, and plant N deficiency in 12 site-years throughout Minnesota. Six to seven N rates (35−45 kg urea-N ha−1 increment) were pre-plant applied. Canopy sensing measurements and aboveground plant N accumulation were obtained at V4, V8, V12, and R1 stages. Regardless of the tool, low predictive power of grain yield, plant N accumulation, and N deficiency occurred at V4, likely because of low crop N demand and sufficient N supply. At V8, sensors provided good estimations of grain yield (R2 =.75−.85) but underestimated the agronomic optimum nitrogen rate (AONR) by 33, 94, 102, and 46 kg N ha−1 with the SPAD, GS-NDVI, RS-NDVI, and RS-NDRE, respectively. At V12 RS-NDRE measurements provided the most accurate estimations of grain yield (R2=.92) and AONR [R2=.84 and N rate differential from agronomic optimum nitrogen rate (dAONR) at −2 kg N ha−1]. At R1 SPAD also provided good estimations of grain yield and N deficiency. The mismatch between the best timings for predicting N fertilizer requirements (V12 and R1) and the best timings for sidedressing (V4−V8) highlight that sensing tools may have limited utility to improve the standard maximum return to N approach in the Upper Midwest.
Bibliographical noteFunding Information:
This study was funded by the Minnesota Corn Growers Association. The University of Minnesota also provided financial support in the form of student research assistantship.
This study was funded by the Minnesota Corn Growers Association. The University of Minnesota also provided financial support in the form of student research assistantship. We thank Univeristy of Minnesota Field Crew personnel, Thor Sellie, Andrew Scobbie, Nicholas Severson, Darby Martin, Erik Joerres, and the many students who helped with this project. We also extend our gratitude to the growers who allowed us to conduct research trials on their farms.
© 2020 The Authors. Agronomy Journal © 2020 American Society of Agronomy
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