Jeffrey G. White1, Carl Crozier2, Ronnie W. Heiniger3, Nan Hong4, Ravi Sripada5, and Randy Weisz3. (1) Soil Science, North Carolina State University, Campus Box 7619, Raleigh, NC 27695-7619, (2) Soil Science; Vernon James Research & Extension Ctr., North Carolina State University, 207 Research Station Road, Plymouth, NC 27962, (3) Crop Science; Vernon James Research & Extension Center, North Carolina State University, 207 Research Station Road, Plymouth, NC 27962, (4) Monsanto, 800 N Lindbergh, St. Louis, MO 63141, (5) USDA-ARS Pasture Systems & Watershed Mgmt Research Unit, Building 3702, Curtin Road, University Park, PA 16802-3702
In-season, site-specific, variable-rate (SS) N management based on remote sensing (RS) (here: aerial color-infrared photography) may increase yield and fertilizer-use efficiency, thus improving profitability and groundwater quality. By addressing spatial variability in crop N requirements, it may also reduce spatial yield variability. We evaluated the spatial variability and agronomic consequences of in-season, RS-informed N management applied either on a uniform, field-average (FA) or SS basis, compared to the current uniform N-rate determination based on "Realistic Yield Expectations" (RYE) in a typical two-year southeastern U.S. coastal plain rotation: winter wheat (Triticum aestivum L.)–double-crop soybean [Glycine max (L.) Merr.]–corn (Zea mays L.). The experiment was established in an RCB design with the three N management treatments replicated ten times in 0.37-ha plots. The spatial variability of RS-determined N requirements and corn and wheat grain yield (2 seasons each) were characterized using geostatistics and mixed model analyses. Considering cleaned yield monitor data for each treatment across the field (i.e., disregarding blocks), SS tended to have the lowest range in yield compared to FA and RYE. For 2002 corn, the RS-derived treatments (FA & SS) had lower yield CV than RYE. All other years, yield CV were similar among treatments. The CV ranged from 9 to 26%. Grain yield tended to exhibit hole-effect semivariograms (characteristic of periodic phenomena) when the full spatial extent (~440 m) was considered. With consideration limited to half the maximum lag or less, yield was characterized by relatively well-defined, isotropic, spherical semivariograms. The spatial correlation range varied from 44 to 176 m and appeared to be affected by treatment, but with no consistent treatment trends within crops or among years. Nugget-to-sill ratios sometimes appeared to be affected by treatment, but with no consistent trend; they ranged from 0.27 to 0.5, indicating that yield was moderately spatially dependent. These results will be examined in the context of the spatial variability of RS-determined N requirements and SS applications to determine whether further insights regarding SS N management can be gleaned.
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