One result of modern agriculture and the reliance on herbicides is the emergence of weed populations which are resistant to products designed to control them. All natural weed populations, regardless of the application of any weed killer, likely contain individual plants (biotypes) which are resistant to herbicides. Repeated use of a herbicide will expose the weed population to a “selection pressure,” which may lead to an increase in the number of surviving resistant individuals in the population. Consequently, the resistant weed population may increase to the point that adequate weed control cannot be achieved by that herbicide.