Check out U.S. Geological Survey's Web Page with GIS color-coded maps of herbicide and nitrogen fertilizer usage at: http://h2o.usgs.gov/public/pubs/bat/bat000.html This site contains the electronic version of the USGS Water-Resources Investigation Report 94-4176: "Spatial Data in GIS Format on Agriculture Chemical Use, Land Use, and Cropping Practices in the United States."