The impact of herbicide-resistant weeds on Kansas agriculture

Herbicides are important components of weed management programs for most Ka nsas farmers. Monocropping systems and repeated use of the same or similar herbicides in some areas of the state have resulted in the development of h erbicide-resistant weeds. The development of herbicide-resistant weed popul ations can have an immediate and a long-term effect on the cost, implementa tion, and effectiveness of weed control programs. In Kansas, resistance to triazine herbicides has been confirmed in kochia (Kochia scoparia), redroot pigweed, common waterhemp (Amaranthus rudis), Palmer amaranth (Amaranthus palmeri), and downy brome (Bromus tectorum) populations, and resistance to acetolactate synthase (ALS)-inhibiting herbicides has been confirmed in koc hia, Russian thistle (Salsola kali), common waterhemp, Palmer amaranth, com mon cocklebur (Xanthium strumarium), shattercane (Sorghum bicolor), and com mon sunflower (Helianthus annuus). The frequency and distribution of herbic ide resistance varies among species. Producers who experience herbicide res istance problems adjust their weed control program accordingly. Producers t hat have not encountered an herbicide resistance problem tend to continue w ith a successful herbicide program until it fails, The recommended manageme nt strategies for herbicide-resist ant weed populations include an integrat ed system of crop rotation, rotation of herbicide modes of action, tank-mix es of herbicides with different modes of action, and cultivation. The great est direct cost to the producer occurs during the first year of poor weed c ontrol. The first response to an herbicide failure often is to reapply the same herbicide that has worked well previously. By the time the producer re alizes that the treatment is not going to work, it usually is too late for any other remedial action. Consequently, the farmer experiences reduced cro p production from weed competition, high herbicide costs, and a tremendous increase in the seed bank. The increase in seed bank may cost the farmer th e most in the long run because the increased weed pressure often requires a n intensified control program for several years.