Will Variable-Rate Nitrogen Fertilization Using Corn Canopy Reflectance Sensing Deliver Environmental Benefits?

Agronomy Journa l • Volume 102 , I s sue 1 • 2010 85 Published in Agron. J. 102:85–95 (2010) Published online 13 Nov. 2009 doi:10.2134/agronj2009.0115 Copyright © 2010 by the American Society of Agronomy, 677 South Segoe Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. N fertilizer is critical in crop production and is oft en applied uniformly at a single rate over whole fi elds. However, because of a variety of factors, soil N levels and crop N needs vary spatially and temporally between fi elds (Bundy and Andraski, 1995; Mamo et al., 2003; Schmitt and Randall, 1994) and within the same fi eld (Mamo et al., 2003; Scharf et al., 2005). As a result of this variability of both N supply and crop N need, uniform application rates applied at fi eld-average need will inevitably lead to underfertilization of some areas of a fi eld, while other areas will receive excessive N fertilization (Shanahan et al., 2008). In humid environments, N fertilizer not taken up by the crop represents economic loss for producers as well as the potential for environmental impact, with nitrate contamination of ground (Power and Schepers, 1989) and surface waters (Turner and Rabalais, 1991), and N2O emissions as a greenhouse gas (Moiser, 2008). One strategy for addressing this variability is in-season, sitespecifi c N fertilization that is at the Noptimal (N rate at which optimal yield is fi rst achieved) or the economically optimal nitrogen rate (EONR, the N rate at which the value of yield increase is equal to the cost of additional N fertilizer) (Scharf et al., 2005). Th e EONR is usually marginally less than Noptimal, but can vary based on the shape of the yield response to N additions and the prices used for corn and N fertilizer (Neeteson and Wadman, 1987). When fertilization is less than EONR, profi t is compromised; and when fertilization exceeds EONR, profi t and potentially the environment are compromised. Hong et al. (2007) evaluated residual soil nitrate levels relative to the diff erence from EONR for three major soils in Missouri and found that applying N rates in excess of EONR resulted in elevated residual nitrate levels. Combined across sites of this study, N application at EONR reduced postharvest residual soil nitrate by 12 kg N ha−1 compared with Nproducer rates. Many producers see the risk of reduced yield with underapplication of N outweighing the costs of unused applied N (Scharf et al., 2005), and therefore use insurance N applications to guard against reduced yield. ABSTRACT Within-fi eld variability of corn (Zea mays L.) N need calls for development of precision fertilizer application strategies. One approach many are investigating is in-season canopy refl ectance sensing. Justifi cation for this strategy partly rests with the premise it will improve N use and reduce N loss from fi elds. Th e objective of this study was to determine the potential environmental benefi ts using corn canopy refl ectance sensing for N fertilization. On 16 fi eld-scale sites, multiple blocks of randomized N rate plots (0–235 kg N ha–1) traversing fi elds were side-dressed between the V7 and V11 growth stages. Sensor measurements were obtained from these and adjacent N-rich reference strips at side-dressing. Environmental indicators were examined at the determined optimal nitrogen rate (Noptimal) and the nitrogen rate the producer used (Nproducer). A partial nitrogen mass balance (PNB) on response blocks within fi elds highlighted how variable Noptimal likely resulted in multiple and diff erent N loss pathways. For many fi elds, Noptimal was less than Nproducer, and the observed trends were as expected: higher yield effi ciency (YE), higher nitrogen fertilizer recovery effi ciency (NFRE), lower unaccounted for N, and less postharvest inorganic N. For a measurement examining canopy sensor-based N applications, N savings of 10 to 50 kg N ha–1 would be expected, but savings varied by refl ectance readings, soil type, and fertilizer and grain prices. In some situations sensor-based N would be greater than Nproducer. Given that sensor information can be processed into an N rate that approximates Noptimal, the results support sensor-based N applications have potential for environmental benefi ts.