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a Dep. of Crop Sci., North Carolina State Univ., Box 7620, Raleigh, NC 27695-7620
b Dep. of Crop Sci., North Carolina State Univ., Vernon James Res. and Ext. Cent., 207 Research Rd., Plymouth, NC 27962
* Corresponding author (mflowers{at}cropserv1.cropsci.ncsu.edu)
Received for publication May 19, 2000.
There is increasing evidence that scouting of winter wheat (Triticum aestivum L.) fields to determine tiller density at Growth Stage (GS) 25 is useful in deciding if N should be applied. However, to obtain an accurate average of field tiller density, frequent and intensive measurements must be made. A solution to this problem may be remote sensing. The objectives of this study were to determine (i) if a spectral index or digital counts in the near infrared (NIR), red (R), green (G), or blue (B) wavelengths could be used to estimate GS-25 tiller density across environments and (ii) if the inclusion of within-field references would improve the estimation of GS-25 tiller density for determining N recommendations. Research was conducted at four site-years in 1998 and 1999 using two wheat varieties. At three locations, a randomized replicated strip-plot design with three seeding rates was used. The fourth location was an on-farm test with one seeding rate. Spectral indices and individual NIR, R, G, and B digital counts were tested for correlation with tiller density at each site. Tiller density at GS 25 and NIR digital counts were found to be consistently correlated (0.67 
r 0.87). The inclusion of within-field tiller density references resulted in a high correlation (r = 0.88) between relative tiller density and relative NIR digital counts across environments. Using relative NIR digital counts to predict tiller density would have resulted in the correct N recommendation 82% of the time.
Abbreviations: B, blue • DVI, difference vegetation index • G, green • GS, growth stage • NDVI, normalized difference vegetation index • NIR, near infrared • OSAVI, optimized soil-adjusted vegetation index • R, red • RVI, ratio vegetation index • SAVI, soil-adjusted vegetation index • TRS, Tidewater Research Station
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