Jason Norsworthy (University of Arkansas) explains the importance of using multiple herbicide modes of action (2:59).
Diversifying herbicide modes of action
Relying heavily on the same herbicide mode of action from year to year causes unwanted selection of herbicide resistant weed populations. This is because the resistant weeds survive repeated applications of the same herbicide every year, while susceptible weeds die off. By diversifying the number of effective herbicide modes of action in a tank and from one application to the next, the producer is limiting the opportunities for weeds to develop resistance.