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Dep. of Crop and Soil Sci., 609 Bradfield Hall, Cornell Univ., Ithaca, NY 14853
* Corresponding author (wjc3{at}cornell.edu)
Received for publication July 1, 2002. The challenge for variable N rate management is to identify specific field areas that respond to specific N levels. We evaluated two corn (Zea mays L.) hybrids at two N rates (110–130 vs. 165–185 kg ha-1 at three sites and manure vs. manure + 55 kg ha-1 at two sites) to determine if corn responded differently to N and hybrids within fields. Spatial yield variability existed at all sites in dry years (1999 and 2001) but at only two sites in a wet year (2000). Spatial yield difference variability in response to N existed at only two of 13 site–year comparisons. Although late-spring soil NO3–N concentrations in the upper 30 cm were <25 mg kg-1 on 15 to 25% of the manured fields in the wet year, spatial yield difference variability in response to N did not exist. At a nonmanured site, spatial yield difference variability in response to N existed with temporal yield stability across dry years (r = 0.96). Surprisingly, corn responded to the higher N rate on 25% of this field where yields were least, but not where yields were greatest. Apparently, variable N rate management of corn requires more information than soil NO3–N concentrations and yield maps. Spatial yield difference variability between hybrids existed at only four of 15 site–year comparisons, despite hybrid interactions with sites. Adoption of variable hybrid selection is unlikely if hybrids that show interactions with sites do not show spatial yield difference variability within sites.
Abbreviations: dGPS, differentially corrected global positioning system • PSNT, pre-sidedress nitrogen test • Vn, nth leaf stage
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