Developing nitrogen fertilizer recommendations for corn using an active sensor

Producers often overapply N fertilizer to corn (Zea mays L.) because of the uncertainty in predicting the economic optimum nitrogen rate (EONR). Remote sensing represents a potential opportunity to reduce this uncertainty with an in-season assessment of crop N status. This study examines the relationship between EONR and reflectance from a ground-based sensor, and considers its potential for developing sidedress N recommendations for corn. Four fields with unique cropping histories were planted to corn during each of 2 yr. Three preplant whole plot treatments (control, 56 kg N ha^-1 as NH ₄NO₃, and manure) were used to create a range of N availability. Split plot treatments included seven sidedress rates (0, 22, 45, 90, 135, 180, and 280 kg N ha^-1) and one preplant rate (280 kg N ha^-1) as NH₄NO₃. The EONR for the sidedress N rates was determined for each whole plot treatment at each site. A ground-based active sensor was used at the sixth- to seventh-leaf growth stage (V6-V7) to collect reflectance data at 590 and 880 nm, which were then used to calculate the Green Normalized Difference Vegetation Index (GNDVI). The EONRs for sidedress N application for the 24 preplant treatment-site combinations ranged from 0 to 202 kg N ha^-1. The EONR was strongly related to relative GNDVI (r² = 0.84) for the control and manure preplant treatments; but unrelated when NH₄NO₃ was applied at planting (r ² = 0.20). Developing sidedress N recommendations for corn using an active sensor could be an effective N management tool in Pennsylvania.