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Fluid Journal : Fluid Journal 2002-2004
then additional N is required. Using the sensor values, a response index (RI) is calculated for that specific field environment. When the applicator crosses a field, the boom has sensors mounted every two feet approximately 32 to 48 inches above the crop canopy. The sensors measure normalized difference vegeta- tive index (NDVI) and project the potential yield of each 4.5 ft2 area. Using the RI of a specific field, the system projects what the yield potential is with additional N and applies that amount of N to meet the yield potential. Pitfalls Avoided By using the optical sensing variable rate application system, all four previous assumptions associated with traditional N management can be avoided. Because potential yield is predicted within the growing season, not based on a 5-year average, one can identify the year-to-year variability in yield and manage N accordingly. By cutting back on preplant N rates and including an NRS within the field, N contributed by the soil is recognized and accounted for when determining the amount of supplemental N neces- sary to achieve maximum yield. TD=topdress 1971 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 Year -40 -20 0 20 40 60 80 100 120 100lbN Return on N, $/A 40lbN+TD GainfromN+TD Average = $10.94/A/yr Splitting N applications between preplant and topdress avoids creating a large pool of N that may be subject to losses if all N is applied at once before seed is ever placed in the ground. Using a system that operates at such a fine resolution, spatial variability is addressed and each 4.5 ft2 is treated independently to maximize fertilizer efficiency and economic return. Dr. Robert Mullen is agronomist for NTech Industries, Inc, Stillwater, Oklahoma; Dr. Bill Raun and Dr. Gordon Johnson are professors at Oklahoma State University. Figure 1. Return from application of 100 lbs/A of N versus 40 lbs/ A preplant and using GreenSeeker technology to apply topdress, if this technology had been available over 31 years at Lahoma, Oklahoma. 2 Fluid Journal Late Spring 2003
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