Submitted to: Precision Agriculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/27/2009
Publication Date: 8/21/2009
Citation: Williams, J.D., Kitchen, N.R., Scharf, P.C., Stevens, W.E. 2009. Within-field Corn Nitrogen Response Related to Aerial Photograph Color. Precision Agriculture. 11:291-305. Interpretive Summary: Research and education programs have helped make Midwest farmers be aware of the fact that cropping practices have helped cause an increase in nitrogen in the Mississippi River. Nitrogen fertilizer costs have also spiked in recent years in response to elevated oil and gas prices. These points have motivated farmers to improve nitrogen management practices. We conducted this research to learn if using photographs taken from an airplane of farmer’s fields could detect corn nitrogen need during the early to mid part of the growing season. From this we tested the idea of developing a variable-rate nitrogen fertilizer plan. We found that the optimal nitrogen rate varied tremendously within fields, between 40 and 200 lbs per acre. Farmers in the area of this research commonly blanket apply between 160 and 225 lbs nitrogen per acre. If these fields are representative of other corn fields, the results suggest significant over-application of nitrogen fertilizer. Optimal nitrogen rate only weakly correlated with reflectance values extracted from the aerial photographs. However, the amount of grain increase with N fertilization corresponded well with reflectance readings. This relationship could be used to base a site-specific N fertilizer application. This information may also be especially helpful for situations where pre-plant N has been lost off fields because of early-season rainfall, such as was seen in the U.S. Midwest in 2008. These results will be useful to farmers when considering site-specific N management strategies that increase fertilizer use efficiency. In the end, higher nutrient use efficiency translates into better economic returns for producers and less N loss off fields into lakes and streams.
Technical Abstract: Precision agriculture management of nitrogen (N) using aerial imagery of corn [Zea mays L.] canopy color has been a proposed strategy to understand crop N health and base within-season N fertilizer application rates. The objective of this study was to evaluate at field scale the relationship between canopy color and yield response to N. Six field-scale N response trials were conducted in 2000 and 2001, with alluvial, claypan, and deep loess soil types represented each year. Aerial images were taken using 35-mm slide film from greater than or less than 1100 m altitude at the mid and late vegetative corn growth stages, digitized, and processed to extract green and red digital values. Color values of N control (0 kg N/ha) and sufficient N (280 kg N/ha applied at planting) treatments were used to calculate relative ratio of unfertilized to fertilized and relative difference color values. Other N fertilizer treatments included side-dress applications in 56 kg N/ha increments. Economic optimal N rate (EONR) was poorly (r2 less than or greater than 0.34) or not related with color indices at both growth stages. For most sites, delta yield related to color indices (r2 less than or greater than 0.67) at the late vegetative growth stage, with the best relationship found with green relative difference. The findings indicate potential for color indices from aerial photos to be used for predicting delta yield from which a site-specific N rate could be approximated.