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a Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27695-7619
b Dep. of Crop Science, Vernon James Res. and Ext. Center, 207 Research Rd., Plymouth, NC 27962
c Dep. of Crop Science, North Carolina State Univ., Raleigh, NC 27695-7620
* Corresponding author (jeff_white{at}ncsu.edu)
Received for publication December 20, 2004. Fast and accurate methods of determining in-season corn (Zea mays L.) N requirements are needed to provide more precise and economical management and potentially decrease groundwater N contamination. The objectives of this study were (i) to determine if there is a response to late-season N applied to corn at pretassel (VT) under irrigated and nonirrigated conditions, and (ii) to develop a methodology for predicting in-season N requirement for corn at the VT stage using aerial color infrared (CIR) photography. Field studies were conducted for 3 yr over a wide range of soil conditions and water regimes in the North Carolina Coastal Plain. Different fertilizer N rates were applied (i) at planting (NPL) to create a range of N supply, corn color, and near-infrared (NIR) radiance; and (ii) at VT (NVT) to measure yield response to NVT. Aerial CIR photographs were obtained for each site at VT before N application. Significant grain yield responses to NPL and NVT were observed. Economic optimum NVT rates ranged from 0 to 224 kg ha–1 with a mean of 104 kg ha–1. Better prediction of economic optimum NVT rates was obtained with spectral band combinations rather than individual bands, and improved when calculated relative to high-N reference strips measured at VT. The best predictor of economic optimum NVT (R2 = 0.67) was a linear-plateau model based on corn color and NIR radiance expressed using the Green Difference Vegetation Index (GDVI) relative to high-N reference strips (Relative GDVI, RGDVI).
Abbreviations: AOI, areas of interest • B, blue • CIR, color infrared • DGPS, differential global positioning system • DN, digital number • DVI, Difference Vegetation Index • G, green • GDVI, Green Difference Vegetation Index • GNDVI, Green Normalized Difference Vegetation Index • GOSAVI, Green Optimized Soil Adjusted Vegetation Index • GRVI, Green Ratio Vegetation Index • GSAVI, Green Soil Adjusted Vegetation Index • NCDA, North Carolina Department of Agriculture • NDVI, Normalized Difference Vegetation Index • NIR, near-infrared • Norm G, normalized green • Norm NIR, normalized NIR • Norm R, normalized red • NPL, nitrogen applied at planting • NRI, Nitrogen Reflectance Index • NVT, nitrogen applied at VT • OSAVI, Optimized Soil Adjusted Vegetation Index • R, red • RDVI, Relative Difference Vegetation Index • Rel G, relative green • Rel NIR, relative near-infrared • Rel R, relative red • RGDVI, Relative Green Difference Vegetation Index • RGNDVI, Relative Green Normalized Difference Vegetation Index • RGOSAVI, Relative Green Optimized Soil Adjusted Vegetation Index • RGRVI, Relative Green Ratio Vegetation Index • RGSAVI, Relative Green Soil Adjusted Vegetation Index • RMS, root mean square • RNDVI, Relative Normalized Difference Vegetation Index • ROSAVI, Relative Optimized Soil Adjusted Vegetation Index • RRVI, Relative Ratio Vegetation Index • RSAVI, Relative Soil Adjusted Vegetation Index • RVI, Ratio Vegetation Index • SAVI, Soil Adjusted Vegetation Index • UAN, urea–ammonium nitrate solution • VT, pretassel
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