Submitted to: North Central Extension Industry Soil Fertility Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 11/6/2006
Publication Date: 11/6/2006
Citation: Roberts, D.R., Kitchen, N.R., Scharf, P.C., Sudduth, K.A. 2006. An environmental assessment of sensor-based variable-rate nitrogen management in corn. North Central Extension Industry Soil Fertility Conference Proceedings. Interpretive Summary: Fertilizers and herbicides are commonly used in modern agriculture to help meet world-wide food needs. Farmers need to balance their use of these agrichemicals so that food is produced profitably and the environment is safeguarded. A good example of this need for balance is with nitrogen fertilizer rate. Nitrogen is relatively inexpensive when compared to the value of the resulting yield increase, yet unused nitrogen from excess fertilization may lead to impaired ground water and streams. Most farmers apply nitrogen to fields uniformly, at a single rate. However, soil nitrogen levels and crop nitrogen needs vary within fields, justifying the need for variable-rate nitrogen management to reduce situations of excess nitrogen fertilization. One management strategy explored in recent years is the use of crop canopy light reflectance sensors. These sensors can be used "on-the-go" with fertilizer application equipment when corn is about knee-high, providing a measure of plant biomass and color, and indicating overall crop nitrogen health. From this information a nitrogen fertilizer recommendation can be made. This study assessed the environmental implications of using the reflectance sensor approach on producers' cornfields in Missouri. Optimal nitrogen rate was related to the reflectance sensor measurements, but not when droughty conditions persisted during the growing season. Our ability to accurately predict the optimal nitrogen rate with the sensors improved when we included data from another sensor that indicates soil texture. Our findings show that if optimal nitrogen rate could be accurately determined with these sensors, the amount of unused nitrogen in the soil at the end of the growing season would be less than with uniform applications for many fields. The likely consequence of this strategy would be less nitrogen loss from surface runoff and leaching. Farmers will directly benefit from these technologies by having lower nitrogen fertilizer costs. Further, if such technologies reduce nitrogen applications to fields, then the general public will benefit because nitrogen losses to lakes and rivers will be reduced and the environment will be improved.
Technical Abstract: In order to address the problem of nitrate contamination of surface and ground waters, various methods have been used to try to account for spatial variability of N within agricultural fields. One approach to account for this variability and thereby reduce nitrate pollution is in-season site-specific N application according to economic optimal N rate (EONR). Recently, active crop canopy sensors have been tested for mid-season, on-the-go N fertilizer application in corn. This 2004 and 2005 study was conducted on 12 Missouri producer corn fields to (1) evaluate the relationship between EONR and active canopy sensor readings, and (2) evaluate the relationship between environmental measurements and EONR. Measurements included EONR, yield efficiency (YE), N fertilizer recovery efficiency (NFRE), and post-harvest soil inorganic N levels. In 2004, EONR was significantly related to active crop canopy sensor indices, but with regression model coefficients of determination (r2) less than or greater than 0.35 for all sensor indices evaluated. As N rate approached EONR, both YE and NFRE declined, while post-harvest inorganic N levels increased. A relationship between EONR and the indices could not be established for 2005 data, primarily because of droughty conditions. These preliminary results show promise for using active-light reflectance sensors to achieve EONR and reduce N loss off fields.