Fertility & C rop N utrition Corn Hybrid Growth Stage In fl uence on Crop Re fl ectance Sensing

Published in Agron. J. 104:158–164 (2012) Posted online 12 Dec 2011 doi:10.2134/agronj2011.0213 Copyright © 2012 by the American Society of Agronomy, 5585 Guilford 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. A number of recent studies have assessed the use of active-light crop canopy sensors to help guide corn N fertilizer management decisions (Raun et al., 2005b; Teal et al., 2006; Dellinger et al., 2008; Solari et al., 2008; Schmidt et al., 2009; Samborski et al., 2009; Sawyer and Barker, 2010; Kitchen et al., 2010). Th ese crop refl ectance sensors make use of light emitting diodes (LEDs) to project light at prescribed wavelengths onto plants and then use photodiodes to sense the energy of the refl ected light (Stone et al., 1996). Active sensors are designed to be insensitive to ambient light, detecting only the refl ected energy of the light emitted by the sensor. In many of these studies refl ectance measurements have been eff ectively related to metrics of N management, such as crop biomass, plant color, yield potential, and economic optimal N rate. Common to most of these studies has been the employment of an N suffi cient reference where measurements from corn known to be nonlimited in N are compared with measurements from plants suspected of being N defi cient. Th is comparison, either as a suffi ciency index or response index, has been described as essential in making eff ective N management decisions using these crop sensors (Shanahan et al., 2008). Th e value of using an N-suffi cient reference for monitoring and responding to crop N needs is in helping to normalize confounding management and environmental factors (Blackmer and Schepers, 1995; Blackmer et al., 1996; Shanahan et al., 2003; Samborski et al., 2009). Guidelines for using activelight crop canopy sensing recommend producers establish an N-suffi cient area or fi eld-long N-suffi cient strip for each fi eld where sensing will be used to conduct in-season N fertilizer applications (Missouri USDA-NRCS, 2009; Shanahan, 2010). Whenever growing conditions vary within fi elds as a result of factors such as variable planting date, multiple hybrids, or changing soil type, a separate N-suffi cient reference for each unique condition has been encouraged. Th e basis for hybrid-specifi c guidelines comes from earlier work where an N suffi cient reference was found important for each hybrid when using a leaf chlorophyll sensor or other passive refl ectance crop sensing (Bausch and Duke, 1996; Blackmer et al., 1996; Scharf et al., 2006). Comparable studies evaluating the eff ect of corn hybrids on active-light crop sensing is lacking. Current guidelines for active-light crop sensors also lack information on the normal ranges of canopy refl ectance readings for N-suffi cient reference (Missouri USDA-NRCS, 2009; Shanahan, 2010), to guard against situations when the crop has been negatively aff ected (e.g., poor crop germination, disproportionate weed growth compared to the rest of the fi eld). Since a refl ectance value for N-suffi cient corn is a key component in determining N fertilizer recommendations, having values that represent a normal range of N-suffi cient ABSTRACT Active-light crop canopy sensing for corn (Zea mays L.) N fertilizer rate decisions typically include measurements of N-suffi cient plants as a reference. When producers use multiple hybrids in one fi eld, the question is raised of whether an N-suffi cient reference is needed for each hybrid. Th e objective of this research was to assess the impact of suffi ciently N-fertilized, similar-maturing corn hybrids on crop-refl ectance measurements. Eleven similar-maturing hybrids were selected from three sites in 2008 and eight hybrids from two sites in 2009. When the corn was about 10 cm tall and on 3 to 5 d intervals canopy refl ectance, leaf chlorophyll, and plant height measurements were obtained. Results were classed into two growth periods based on crop height: 20 to 70 cm and 71 to 120 cm. In three of the four growing periods assessed corn hybrid had no signifi cant eff ect on refl ectance. In 2008, refl ectance for corn 71 to 120 cm tall was aff ected by hybrid; however the eff ect was minor leading to an average N rate recommendation diff erence of 10 kg ha–1. More pronounced diff erences occurred among hybrids for leaf chlorophyll measurements, resulting in diff erences in N fertilizer recommendations of 55 kg ha–1. Refl ectance diff erences among similar maturing hybrids would have minimal impact on N fertilizer recommendations. Models were also developed to represent typical upper and lower values for various vegetative indices as a function of corn height. Th ese models can help guard against using questionable data when assessing N-suffi cient corn.