The objectives of this research were to (i) compare active and passive crop canopy sensors’ sidedress variable rate nitrogen (VRN) derived from different vegetation indices (VI) and (ii) assess VRN recommendation accuracy of active and passive sensors as compared to the agronomic optimum N rate (AONR) in irrigated corn. This study is comprised of six site-years (SY), conducted in 2015, 2016 and 2017 on different soil types (silt loam, loam and sandy loam) and with a range of preplant-applied nitrogen (N) rates (PANR, 0 to 309 kg N ha-1). Crop reflectance data was acquired using four different sensors: RapidScan (handheld, active) and Tetracam, MicaSense RedEdge or Parrot Sequoia (unmanned aerial system-mounted, passive). Sensors were utilized at the V12 growth stage. For all sensors, NDVI and NDRE were calculated. In order to determine VRN, VI data from a plot was divided by that from an N-sufficient reference, generating a sufficiency index value, which then was input in the adapted Holland-Schepers algorithm for sidedress N rate determination. When using NDRE, active sensor had a significantly lower RSNR than passive sensor at multiple PANRs at 4 out of 6 SYs, when passive sensor recommended from 5 to 40 kg/ha more N than the active sensor. When using NDVI, active sensor had a significantly higher RSNR than passive sensor at multiple PANRs at 3 out of 4 SYs, ranging from 2 to 17 kg/ha more N recommended as compared to the passive sensor. VRN generated from passive NDRE was able to accurately approach AONR in 2 out of 6 SYs. Active and passive sensors have the ability to assess N stress and recommend VRN, with passive sensors recommending higher rates more frequently. The use of NDRE from either sensors generated VRN that better approached AONR as compared to NDVI at the V12 growth stage. The determination of N deficiency and sidedress VRN depends on i) the degree of stress at time of sensing as it impacts both the reference value and the remediation extent, and ii) environmental conditions from time of sensing/VRN to harvest.
