Managing herbicide-resistant weeds
By David G. Hallauer
Meadowlark District Extension Agent
Chemical weed control isn't a perfect science. Many things factor in to whether a given herbicide will work to perfection. Most of the time it does--sometimes it does not.
Without question, some weeds have developed herbicide resistance, often confirmed by a lengthy process where "resistant" plants are subjected to increasing herbicide rates over a period of time. That helps take out the variables caused by the fact that most herbicide failures are the results of unfavorable environmental conditions, inadequate spray coverage, oversized plants at application, or inadequate herbicide rates.
Weeds that make the Kansas list (from Dallas Peterson, KSU weed scientist) include: Field Bindweed (Synthetic Auxins--2,4-D); Kochia (atrazine/glyphosate/chlorsulfuron); Downy brome (ALS); Russian thistle (chlorsulfuron); Palmer amaranth (atrazine/imazethapyr)/Downy brome/Redroot pigweed (atrazine); Waterhemp (imazethapyr/thifensulfuron/atrazine); Common sunflower (imazethapyr); Shattercane (nicosulfuron/primisulfuron); Common cocklebur (chlorimuron/imazaquin); Waterhemp (acifluorfen/formesafen/glyphosate); Marestail (glyphosate); Bushy wallflower (several ALS); Giant ragweed (glyphosate); Common ragweed (glyphosate); and Cheat/Japanese brome (imazamox/propoxycarbazone/pyrosxulam/sulfosulfuron).
Resistance generally occurs in scattered areas of the state and not in all fields. In other words, inclusion of a weed in the "resistant" list doesn't mean you have resistance on your farm, just that you need to beware of the potential. How can we avoid resistance?
Weeds differ in their ability to avoid resistance. Species that readily cross pollinate or hybridize with related species will tend to become more resistant. Likewise, those with high seed production and a short seed life will do the same.
Some herbicides also have more tendancy toward resistance. Peterson notes glyphosate, atrazine, and the ALS class of herbicides as prime examples. Why? They have one or more of the following: very specific site of action, long residual effects in the soil, or a high degree of selectivity.
Last but not least, your management can further the resistance of weeds to herbicides. To avoid, rotate and/or tankmix herbicides with different sites of action, and do so within and across years. In other words 100 percent reliance on one active ingredient is not a best management practice. You might also consider a different crop rotation if resistance develops. For those without a cereal crop in the rotation, consider adding wheat to help "clean up" a problem field. Cultivation continues to be an option, just be wary of erodible sites. Maybe the biggest is the use of the proper herbicide rate at the proper time. Skimping on product or waiting just a little longer will do nothing but add to the potential for herbicide resistance.
For chemical weed control information, check out the 2012 Chemical Weed Control Guide available through your Extension office.