Herbicide-resistant weeds in Kansas, US

Date:06-21-2013

Herbicide-resistant weed populations have become a problem in almost every crop in Kansas over the past 20 years. Populations of certain weeds have even become resistant to more than one class of herbicide, making them especially challenging to control.

Confirming herbicide resistance in a weed population is actually a long, difficult process. It may seem as if there are more herbicide-resistant weeds than those officially on the list, but that’s because of the time it takes to do all the necessary testing to confirm resistance.

Just because a herbicide application did not control weeds in a given field does not mean the weeds have become resistant to that herbicide, of course. How can you determine whether the weeds are resistant to the herbicide used?

First of all, most herbicide failure is a result of unfavorable environmental conditions, inadequate spray coverage, oversized weed plants at the time of application or inadequate herbicide rates.

However, herbicide-resistant weeds are a very real cause of poor control in some cases. One of the factors that would indicate the possibility of herbicide resistance is the pattern of weed survival. If the surviving plants (escapes) are in middle of a group of dead plants, you might begin to suspect herbicide resistance. Also, has the same herbicide mode of action been used for several years in a row on that field? If so, that also might make you suspect herbicide resistance. Finally, if the weed control failure involves a weed that is known to have developed resistance to the herbicide that was used, that’s a strong indicator that herbicide resistance could be the problem.

The table below is the most current list of resistant weeds confirmed to exist in Kansas, in chronological order. Most herbicide-resistant weed populations occur only in isolated or scattered areas of the state, and not on all fields. So just because a weed is listed as resistant to a certain class of herbicide doesn’t mean all populations across the state will have that resistance. But resistant populations of weeds do tend to become more widespread with time once they have developed.

History of Confirmed Resistant Weeds in Kansas

Weed Species     Herbicide Mode of Action     Year

Field Bindweed, Synthetic Auxins (2,4-D), 1964
Kochia, PSII (atrazine), 1976
Downy brome, PSII (atrazine), 1977
Kochia, ALS (chlorsulfuron), 1987 
Russian thistle, ALS (chlorsulfuron), 1988
Palmer amaranth, PSII (atrazine), 1991
Palmer amaranth, ALS (imazethapyr), 1995  
Redroot pigweed, PSII (Atrazine), 1995
Waterhemp, ALS (imazethapyr, thifensulfuron), 1995
Waterhemp, PSII (atrazine), 1995
Common sunflower, PSII (atrazine), 1996 
Shattercane, ALS (imazethapyr), 1996
Common cocklebur, ALS (nicosulfuron, primisulfuron), 1997
Waterhemp, ALS (nicosulfuron, primisulfuron), 2001
Marestail (horseweed), ALS (chlorimuron, imazaquin), 2005
Bushy wallflower/Flixweed, PPO (acifluorfen, fomesafen), 2005/07
Waterhemp and Giant ragweed, ESPS (glyphosate),2006
Common ragweed and Kochia, ALS (several), ESPS (glyphosate), 2007
Cheat and Japanese brome, ESPS (glyphosate), ALS (imazamox, propoxycarbazone, pyroxsulam, sulfosulfuron), 2007

Some weeds seem to develop herbicide resistance more readily than others. The table above clearly shows this, as many weeds are listed as resistant to several different herbicide modes of action.

Weed characteristics that favor resistance include:  species with broad genetic diversity (species that readily cross pollinate and species that hybridize with related species) and species with rapid turnover rate (high seed production and short seed life).

Likewise, some herbicides are more likely to have problems with weed resistance. Glyphosate, atrazine, and the ALS class of herbicides are prime examples. Other herbicides have very few or no problems with weed resistance. Herbicide characteristics that favor resistance include: (1) herbicides with a very specific site of action (2) herbicides that have long residual effects in the soil (3) herbicides with a high degree of selectivity.

Certain management practices will tend to favor the development of herbicide-resistant weeds. For example, if the producer relies on herbicides alone to control weeds, that will favor resistance. Another practice is the use of herbicides multiple times during the growing season, or the same herbicide or mode of action on the same field for consecutive seasons. Probably the greatest contributing factor for the development of herbicide-resistant weed biotypes is the frequent and exclusive use of one herbicide, or a single herbicide mode of action, over time. 

The best way to avoid the development of herbicide-resistant weeds is to: (1) rotate and/or tankmix herbicides with different sites of action, within and across years (2) use crop rotations (3) include other control tactics (cultivation, prevention, crop competition, cultural practices) (4) use the proper herbicide rate at the proper time.