Working with the Delaware Soybean Board and the United Soybean Board, a herbicide classification chart is available at:
http://extension.udel.edu/ag/weed-science/weed-management-guides/
This chart groups herbicides by mode of action and site group numbers that are printed on containers and labels. The chart is designed to help diversify herbicide use by rotating effective herbicides with different sites of action to delay the development of herbicide resistance. The chart provides the number of resistant weed species in the US to each site of action, which highlight those herbicides for which we already have wide-spread resistance.