A feasibility study of direct injection for variable-rate herbicide application

Variable-rate application of herbicides based on soil properties and crop yield potential may be a viable approach to reducing the quantities of herbicides used in crop production. Application accuracy of direct injection equipment is an important issue for variable-rate application. A simulation model was developed using SLAM II to assess the performance of a direct injection sprayer used for site-specific application of preemergence herbicides in corn. Field data from a university research farm were used as input to the model (i.e., soil texture, organic matter content, and yield potential). Several factors such as in-line mixing location, hose diameter, nozzle spacing and size, and ground speed were investigated for their effects on system performance. A complement of system parameters was identified to reduce application errors. Multiple system errors were combined using the mean square error approach. Results of the simulation were input to a GIS software to generate herbicide application rate maps and the corresponding error rate maps. Application errors for direct injection systems were as high as 40% for mistreated areas of the field, with changes in chemical concentration at the remote nozzles occurring after as much as 80 m of travel past the point of a step change of the input command to the controller. Recommendations for reducing delay times at the nozzle include moving the in-line mixing location close to the boom and using smaller line sizes to reduce line volume, without appreciably increasing pressure drop.