Weed resistance to WSSA Group 2 herbicides is a global challenge that all growers, including those of major crops in Kansas, must be aware of. This crucial issue directly impacts their crop management strategies and overall productivity.
For over four decades, WSSA Group 2 herbicides have been a tool for growers worldwide1. These herbicides, which target the enzyme called Acetolactate Synthase (ALS), have been used in all major crops to control a wide range of weed species. For instance, the active ingredient chlorsulfuron, found in products like Glean and Finesse, was the first sulfonylurea herbicide registered for use in the United States in 1982, specifically for the control of broadleaf and some grasses in cereal crops2.
Unfortunately, these herbicides are well-known for their ability to select resistant weeds. This resistance can lead to significant control failures, leading to important yield losses and annual revenue loss. Previous research showed that resistance to Group 2 herbicides was reported to have occurred with as few as three applications of these herbicides3, and rigid ryegrass populations that received four or more previous applications of these herbicides and tested for herbicide resistance were all resistant to these herbicides4.
More weed species are resistant to these Group 2 herbicides than to any other herbicide group. The frequent cases of herbicide-resistant weeds to Group 2 herbicides can be attributed to their intense use, the strong selection pressure they apply, and the 'resistance mechanism.' They were repeatedly used as the main strategy for weed control in many cropping systems.
Group 2 herbicides are also heavily used in Kansas cropping systems (e.g., winter wheat, grain sorghum, sunflower) and include herbicides such as Amber, Ally Xtra, Affinity SG, Affinity Broadspec, Beyond, Express, Finesse, Glean, Harmony, ImiFlex, Peak, Permit, Zest, etc. An expanded list of Group 2 herbicides used in Kansas can be found at https://bookstore.ksre.ksu.edu/pubs/chemweedguide.pdf.
Herbicide-resistant crop trait technologies are a valuable tool for 'over-the-top applications,' which means applying herbicides directly to the crop foliage without damaging the crop due to the presence of the traits introduced. Wheat, grain sorghum, and sunflower have trait technologies allowing over-the-top applications of specific Group 2 herbicides (Table 1).
Table 1. Crops with ALS-inhibiting herbicide resistance traits and the herbicides that those traits provide resistance.
|
Crop |
Trait |
Products (active ingredients) |
|
Wheat |
Clearfield (1- and 2-gene) |
Beyond Xtra (Imazamox) |
|
Grain sorghum |
Igrowth |
Imiflex (Imazamox) |
|
|
Inzen |
Zest (Nicosulfuron) |
|
Sunflower |
Clearfield |
Beyond (Imazamox) |
|
|
ExpressSun |
Express (Tribenuron) |
Weeds with resistance to ALS herbicides in Kansas
Five years after chlorsulfuron registration, the first sulfonylurea-resistant prickly lettuce was reported. Also, earlier research reported that sulfonylurea-resistant kochia populations were collected in 1987 from wheat fields near Liberal, KS2.
A list of weeds that have evolved resistance to Group 2 herbicides in Kansas is provided in Tables 2a and 2b5. Marestail, flixweed, and bushy wallflower were reported resistant to seven different Group 2 herbicides. Each of them also has cross-resistance, meaning they are resistant to Group 2 herbicides belonging to more than one family. For example, marestail is resistant to Varro (thiencarbazone-methyl) from the Triazolinone family and to the Sulfonylurea family. The Sulfonylurea family has herbicides with active ingredient names ending by (with the suffix) uron, such as chlorsulfuron. Palmer amaranth is resistant to four different products but is also cross-resistant to Sulfonylureas and Imidazolinones, called the “imi’s”, which have active ingredients with the prefix ima (imazamox and imazethapyr).
From a chemical perspective, one of the most effective ways to delay herbicide resistance is to select a program with herbicides from multiple groups or sites of action.
The pivotal role of crop rotation in weed management cannot be overstated. Its absence leads to a less diverse weed community, reducing the effectiveness of herbicide options for weed control. The risks of resistance are significantly lower in systems where crop rotations are frequently practiced, underscoring the importance of this practice in maintaining a better weed control program.
It's important to note that rotations of Group 2 herbicides, from a chemical family to other Group 2 herbicides of different chemical families, provide the same selection pressure and are ineffective in slowing the evolution of resistance. Herbicide-resistant crop rotation without the diversity of herbicide sites of action (e.g., ALS-resistant winter wheat followed by ALS-resistant grain sorghum) will have no effect on reducing the evolution of herbicide-resistant weeds. Instead, rotating different herbicide-resistance crop traits (in the same or a different crop) increases herbicide MOA diversity within cropping systems, such as rotating Clearfield winter wheat crops with Double Team grain sorghum, which can enhance herbicide MOA diversity within the cropping system, thereby reducing the risk of herbicide resistance.
Table 2a. Weeds and herbicide resistance to WSSA Group 2 herbicides in Kansas. Resistance is indicated by cells with yes/green shading.
|
Brand (chemical) |
Palmer1 |
Waterhemp |
J. brome2 |
Cheat |
Marestail |
Flixweed |
|
Accent Q (Nicosulfuron) |
|
|
|
|
|
|
|
Ally (Metsulfuron-methyl) |
|
|
|
|
Yes |
Yes |
|
Amber (Triasulfuron) |
|
|
|
|
|
Yes |
|
Autumn (Iodosulfuron-methyl) |
|
|
|
|
Yes |
|
|
Beacon (Primisulfuron) |
|
|
|
|
|
|
|
Beyond Xtra (Imazamox) |
Yes |
|
Yes |
Yes |
|
Yes |
|
Classic (Chlorimuron-ethyl) |
|
Yes |
|
|
|
|
|
Express (Tribenuron-methyl) |
|
|
|
|
Yes |
Yes |
|
Glean (Chlorsulfuron) |
Yes |
|
|
|
Yes |
Yes |
|
Harmony (Thifensulfuron-methyl) |
Yes |
Yes |
|
|
Yes |
|
|
Maverick (Sulfosulfuron) |
|
|
Yes |
Yes |
|
Yes |
|
Olympus (Propoxycarbazone) |
|
|
Yes |
Yes |
|
|
|
PowerFlex HL (Pyroxsulam) |
|
|
Yes |
Yes |
|
Yes |
|
Pursuit (Imazethapyr) |
Yes |
Yes |
|
|
|
|
|
Resolve (Rimsulfuron) |
|
|
|
|
Yes |
|
|
Varro (Thiencarbazone-methyl) |
|
|
|
|
Yes |
|
1Palmer amaranth; 2Japanese brome
Table 2b. Weeds and herbicide resistance to WSSA Group 2 herbicides in Kansas. Resistance is indicated by cells with yes/green shading.
|
Brand (chemical) |
Wallflower3 |
Sunflower4 |
Kochia |
Henbit |
Shattercane |
Cocklebur5 |
|
Accent Q (Nicosulfuron) |
|
|
|
|
Yes |
|
|
Ally (Metsulfuron-methyl) |
Yes |
|
|
|
|
|
|
Amber (Triasulfuron) |
Yes |
|
|
|
|
|
|
Autumn (Iodosulfuron-methyl) |
|
|
|
|
|
|
|
Beacon (Primisulfuron) |
|
|
|
|
Yes |
|
|
Beyond Xtra (Imazamox) |
Yes |
|
|
|
Yes |
|
|
Classic (Chlorimuron-ethyl) |
|
|
|
|
|
Yes |
|
Express (Tribenuron-methyl) |
Yes |
|
|
|
|
|
|
Glean (Chlorsulfuron) |
Yes |
|
Yes |
Yes |
|
|
|
Harmony (Thifensulfuron-methyl) |
|
|
|
|
|
|
|
Maverick (Sulfosulfuron) |
Yes |
|
|
|
|
|
|
Olympus (Propoxycarbazone) |
Yes |
|
|
Yes |
|
|
|
PowerFlex HL (Pyroxsulam) |
|
|
|
|
|
|
|
Pursuit (Imazethapyr) |
|
Yes |
|
|
|
Yes |
|
Resolve (Rimsulfuron) |
|
|
|
|
|
|
|
Varro (Thiencarbazone-methyl) |
|
|
|
|
|
|
3Bushy wallflower; 4Common sunflower; 5Common cocklebur
How do we rotate herbicides or choose herbicide premixes targeting multiple sites of action?
The herbicide classification chart was developed by weed scientists from the Midwest as a tool to help select herbicides with different sites of action.
The pdf version of the herbicide chart can be downloaded from here:
The left side of the chart shows the site-of-action groups of herbicides and their meanings. It also includes active ingredients and products that target only one site of action.
An example of familiar herbicide products on the far-right column of this left side of the chart is Liberty, Rely, and Noventa. These products have the active ingredient glufosinate and belong to the WSSA Group 10 herbicide group. This group number, displayed in the far-left column, is associated with the site of action of the herbicide. This site-of-action group number is written on the label of several herbicides. The Group 2 herbicides, the acetolactate synthase-inhibiting herbicides group, is the largest group of herbicides.
The right side of the chart has herbicide premixes, products with multiple active ingredients, and targeting different sites of action. The top product, Acuron, has four different active ingredients and targets three different sites of action.
Familiarizing oneself with this chart can become very helpful when selecting herbicide programs that allow a rotation of sites of action or choosing premixes with different active ingredients targeting various sites of action.
Thinking beyond herbicides
It is also highly recommended that we think beyond herbicides and use multiple integrated weed management strategies to preserve these chemistries. Cultural practices that improve crop competitiveness and suppress weeds, such as cultivar selection, narrow row spacing, high seeding rates, and planting date adjustments, can help. Also, the use of mechanical practices, when appropriate, will help. Additionally, the integration of cover crops for weed suppression and the management of weed seed at harvest to reduce weed seedbanks will help prolong the effectiveness of these chemistries. An additional resource can be found at https://bookstore.ksre.ksu.edu/pubs/MF3448.pdf
References
1Tranel & Wright (2002)
2Thompson et al. (1994)
3Powles et al. (1997)
4Gill (1995)
5Heap (2024)
The use of trade names is for clarity to readers and does not imply endorsement of a particular product, nor does exclusion imply non-approval. Always consult the herbicide label for the most current use requirements. Users should read and follow all label instructions.
Jeremie Kouame, Weed Scientist, Agricultural Research Center – Hays
jkouame@ksu.edu