USE OF CROP CANOPY REFLECTANCE SENSOR FOR IN-SEASON N MANAGEMENT OF CORN

Authors: Shanahan, John; Schepers, James; Francis, Dennis; CALDWELL, ROBERT - U OF NE/LINCOLN

Submitted to: Proceedings Great Plains Soil Fertility Conference
Publication Type: Proceedings
Publication Acceptance Date: 9/14/2003
Publication Date: 9/14/2003
Citation: Shanahan, J. F., Schepers, J.S., Francis, D.D., Caldwell. R. 2004.Use of crop canopy reflectance sensor for in-season N management of corn. In A. Schlegel (ed.) Proceedings of Great Plains Soil Fertility Conference. 10:69-74.

Interpretive Summary: Over-application of N fertilizer on corn has resulted in elevated levels of N in ground and surface waters. A major factor contributing to decreased N use efficiency and environmental contamination for traditional N management schemes is the routine pre-season application of large doses of N, well before the time when the crop can effectively utilize this N. Previous work by our group using a chlorophyll meter to demonstrated that the chlorophyll meter could be used as a research tool to maintain an adequate N supply for corn by fertilizing as needed and that yields could be maintained with reduced N rates relative to a single preplant application of N. Our findings show that it is realistic for producers to move away from the uniform early season approach to N management and toward a more reactive approach involving crop evaluation and in-season N application to coincide better with crop N uptake. Our long-term research goal is to reduce N over-applications on corn by using remote sensing to assess crop N status and to direct fertilizer only to areas needing N at times when the crop can most efficiently utilize the N (Fig. 1), and thereby improve water quality through reduced N leaching and runoff. Before we can recommend the crop-based strategy for in-season N management to farmers, we need to validate and refine the recommendations with further research. Specifically, the research objectives for this project are: 1) Build and test a prototype high clearance in-season N applicator configured with on-the-go active sensors, controller, and nozzle/valve system to deliver variable rates of liquid N fertilizer, and 2) Develop algorithms for triggering N applications based on crop-based in-season canopy reflectance along with soil-based spatial data. To develop and test a flexible N management system, we have assembled a prototype high-clearance tractor intended to operate as an in-season N applicator. Key components of this applicator consist of 1) on-the go active crop canopy sensors, 2) drop nozzles with electronic valves, delivering liquid N fertilizer, 4) Raven sprayer control System 5), commercial Field Point controller system connected via serial port to PC running commercial Lab View measurement and control software. Prescription map and active sensor modules have been written in Lab view, which are available to deliver variable applications of N fertilizer based on prescription map (ie. management zone map), active sensor readings, or a combination of both. The red and green versions of the GreenSeeker® active sensor manufactured by Ntech Industries, Inc. were tested and evaluated. These sensors operate by using LED's to generate their own source of modulated light in the red and NIR bands (red version) or green and NIR bands (green version) and determining with a detector the percent of modulated light reflected back from the crop canopy. Reflectance in the respective bands is used to compute the red version of the normalized difference vegetation index (NDVI) and green version of this same index (gNDVI), which provides a measure of variability in canopy nitrogen status. Various small plot and on farm strip trials were conducted in 2003 to evaluate various components of the high-clearance applicator and response of corn grain yields to varying rates of N applied at different growth stages and across landscape spatial variability. One of the studies conducted involved four N rates applied on two dates (V-10 and V-14 growth stages) and replicated nine times across the landscape, to evaluate N response across landscape variability. A bare soil aerial photograph was acquired, digitized, and inputted into the GIS to provide information regarding spatial variability in soil color. A soil electrical conductivity survey was also collected with an EM-38 sensor, and inputted into the GIS, providing another soil-based data

Technical Abstract: Over-application of nitrogen (N) fertilizer on corn has resulted in elevated levels of N in ground and surface waters. A major factor contributing to decreased N use efficiency and environmental contamination for traditional corn N management schemes is routine pre-season application of large doses of N before the crop can effectively utilize this N. Our long-term research goal is to reduce these over-applications by using remote sensing to direct fertilizer only to areas needing N at times when the crop can most efficiently utilize the N. We have assembled a prototype high-clearance tractor configured with red (red and NIR bands) and green (green and NIR bands) versions of the active GreenSeeker® sensors, drop nozzles with electronic valves, and variable rate controller that is intended to deliver in-season variable rates of liquid N fertilizer based on crop needs. Various small plot and on farm strip trials were conducted in 2003 to evaluate the various components of the high-clearance applicator and the response of corn grain yields to varying rates of N applied at different growth stages and across landscape spatial variability. Preliminary results indicated that the green version of the active sensor is more sensitive than the red version in detecting variation in canopy N status during the window we propose to apply N. Yield responses to N application were observed to vary across the landscape, and N responsiveness was more highly associated with variation in canopy reflectance assessed by the sensor than spatial variation in soil properties such as soil color. The objectives of our future research activities are to verify recommendations on canopy reflectance thresholds for triggering in-season N applications by field-testing sensor/applicator systems at a scale appropriate to farmers.