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REMOTE SENSING

Active Sensor Reflectance Measurements of Corn Nitrogen Status and Yield Potential

^a Monsanto, Pergamino, Buenos Aires, Argentina
^b USDA-ARS, Lincoln, NE 68583
^c Dep. of Agronomy & Hortic., Univ. of Nebraska, Lincoln, NE 68583
^d School of Nat. Resources, Univ. of Nebraska, Lincoln, NE 68583. Mention of commercial products and organizations in this article is solely to provide specific information. It does not constitute endorsement by USDA-ARS over other products and organizations not mentioned. The USDA-ARS is an equal opportunity/affirmative action employer and all agency services are available without discrimination

^* Corresponding author (John.Shanahan{at}ars.usda.gov).

Active sensor reflectance assessments of corn (Zea mays L.) canopy N status are advocated to direct variable N applications and improve N use efficiency (NUE). Our goals were to determine: (i) growth stage and (ii) sensor vegetation index with greatest sensitivity in assessing N status and grain yield. Variable crop N was generated by supplying N at different amounts and times in three field studies. Chlorophyll meter (CM) and sensor data were gathered at two vegetative (V11 and V15) and two reproductive (R1 and R3) growth stages, using the Crop Circle sensor that measures reflectance in visible (590 nm) and near infrared (NIR) (880 nm) bands. Sensor data were converted to the normalized difference vegetation index (NDVI₅₉₀) and chlorophyll index (CI₅₉₀) values. Grain yields were also determined. Sensor indices were more highly correlated with CM readings for vegetative vs. reproductive growth (r² of 0.85 vs. 0.55). The CM vs. CI₅₉₀ slope was over twice the NDVI₅₉₀ slope value, indicating CI₅₉₀ was more sensitive than NDVI₅₉₀ in assessing canopy greenness. Indices did not differ in ability to distinguish yield variation. Results indicate sensor CI₅₉₀ values collected during vegetative growth are best suited to direct variable N applications.

Abbreviations: CI, chlorophyll index • CM, chlorophyll meter • GDD, growing degree days • MSEA, Management Systems Evaluation Area • NDVI, normalized difference vegetation index • NIR, near infrared • NUE, nitrogen use efficiency • SEq, sensitivity equivalent • SI, sufficiency index • TSAVI, transformed soil-adjusted vegetative index • VIS, visible

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Received for publication July 12, 2007.