Herbicide Groups: How to Avoid Herbicide Resistance

Herbicide Groups: How to Avoid Herbicide Resistance

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Keith's Pro Tips

"If a weed is already resistant to a herbicide in your yard, then that product is no longer effective on that weed species. It would be helpful to start with a clean field. Till the soil to consistently disturb potential germinated weeds and invigorate nutrients in soil and decrease moisture."

Herbicide Groups: How to Avoid Herbicide Resistance

Herbicides are essential tools when it comes to managing weeds, vines, trees, and shrub vegetation. There are various types of herbicides that are available and what is applied on your property depends on the weed or foliage that are you facing.

In this section, you can sort through the various herbicide groups to learn more about each one as well as to avoid herbicide resistance by purchasing a herbicide within that group classification. 

How Herbicide Groups Work

Herbicide groups are sorted by their mode of action, or biochemical site within the weed that the active ingredient interferes within the foliage in order to elminate it. The biochemical site refers to the site of action in which the herbicide acts in the plants area or its specific enzyme to disrupt plant growth. Most herbicides block or inhibit processes that are critical for plant growth such as photosynthesis, pigment development, and cell creation.  

Some groups of herbicides work to distrupt the cellular membrane of plants, allowing its cellular contents to leak out, but not cease other metabolic functions. There are weed species or groups of foliage that are susceptable to certain herbicides because they use different groups or ingredients. 

It should also be noticed that herbicides can be grouped into two or more classes that may have the same mode of action. 

Benefits of Herbicide Groups

Herbicide resistant weeds and/or plants have several negative effects on a lawn, garden, ornemantal landscape, or agricultural terrains. Although everyone would prefer a simple solution to their infestation or situation, using a herbicide based on its group provides consistency of weed control. Foliage where alternation of herbicide groups becomes essential can be seen in kochia, cocklebur, foxtail, and ragweed as these plants are commonly resistant to glyphosate (group 9). 

By alternating herbicides with different modes of actions or groups, there is much less of a chance for the foliage to develop a resistance. 

Herbicide groups also deliver specified control of plants life cycle and reproduction rate. To be effective, herbicides must contact plants through the leaf surface or soil surrounding it in order to properly be absorbed, and move within the plants to the site of action (cell walls, cell membrane, or other sites), without being deactivated through means of mowing, weather, irrigation, or other.  

Drawbacks of Herbicide Groups

Weeds can develop resistance to certain herbicide groups if the same herbicide group is constantly used. In any weed population, there are likely to be a group of plants that are naturally resistant to specific herbicide groups, even before the product is used. When a herbicide is used, these treated weeds still have the chance to survive and spread seeds before the herbicide is reapplied.

Classifaction of Herbicide Groups

The classification of herbicides by mode of action or site of action are as follows. Knowing and understanding each herbicide group is an important step in selecting the proper product for each weed, injury to susceptable plants, and implementing a sucessful weed management program for your infestation. 

To find each herbicides group, it is described on the product label as being apart of a numbered group. These numbers refer to a specific mode of action or site of action. For example, "Group 1" herbicides refer to ACCase inhibitors. Depending on the herbicide, the group number will be listed in the general description or on the instruction portion of the herbicide label. In some situtations, the herbicide may not mention the mode of action anywhere on the product label. 

Within each group there could be single or multiple chemical families, which will vary based on the activie ingredient being used in the herbicide.

Group 1

Mode of Action: ACCase Inhibitors. 

Site of Action: Lipid Synthesis Inhibitors.

          Chemical Family: Arloxyphenoxypropionate "FOPs".

          Active Ingredient: Quizalofop, diclofop, fluazifop, or fenoxaprop. 

         Chemical Family: Cyclohexanedione "DIMs".

         Active Ingredient: Clethodim, or sethoxydim. 

        Chemical Family: Phenylpyrazoline "DENs".

        Active Ingredient: Pinoxaden. 

Group 2

Mode of Action: ALS Inhibitors.

Site of Action: Amino Acid Synthesis Inhibitors.

       Chemical Class: Imidazolinone "IMIs".

       Active Ingredient: Imazamox, imazapic, imazaquin, or imazethapyr. 

       Chemical Class: Pyrimidinylthiobenzoic Acid. Ecomazapyr 2 SL Herbicide. 

       Active Ingredient: Bispyribac, or pyrithiobac.

       Chemical Class: Sulfonylaminocarbonyltriazolinones. 

       Active Ingredient: Flucarbazone, propoxycarbazone, or thiencarbazone. 

      Chemical Class: Sulfonylurea "SUs".

      Active Ingredient: Bensulfuron, chlorimuron, chlorsulfuron, halosulfuron, iodosulfuron, mesosulfuron, metsulfuron, nicosulfuron, orthosulfamuron, primisulfuron,        prosulfuron, rimsulfuron, sulfosulfuron, thifensulfuron, triasulfuron, tribenuron, trifloxysulfuron, or triflusulfuron.  

     Examples: Martin's Nutgrass Eliminator. Negate 37WG. Sedgemaster Herbicide. Fahrenheit Herbicide. SFM Extra Pre-emergent Herbicide. SFM 75 Sulfemeteron  Herbicide.

     Chemical Class: Triazolopyrimidine.

     Active Ingredient: Cloransulam, florasulam, flumetsulam, penoxsulam, or pyroxsulam. 

     Examples: Firstrate Herbicide. 

Group 3

Mode of Action: Microtubule Inhibitors.

Site of Action: Seedling Root Growth Inhibitors.

        Chemical Class: Benzamide. 

        Active Ingredient: Pronamide.

       Chemical Class: Dinitroaniline.

       Active Ingredient: Ethalfluraline, pendimethaline, or trifluralin.

Group 4

Mode of Action: T1R1 Auxin Receptors. 

Site of Action: Growth Regulators.

        Chemical Class: Arylpicolinate.

        Active Ingredient: Halauxifen.

       Chemical Class: Benzoic Acid.

       Active Ingredient: Dicamba.

       Examples: Fahrenheit Herbicide. 

       Chemical Class: Carboxylic acid.

       Active Ingredient: Clopyralid, fluroxypyr, triclopyr, or quinclorac.

       Examples: Quinclorac 1.5 L. Triclopyr 3 Herbicide. Sonora (Clopyralid 3 Herbicide). Flagstaff Fluroxypyr Herbicide. Triclopyr 4 Brush Killer. 

       Chemical Class: Phenoxy. 

       Active Ingredient: 2,4-D, or MCPA. 

Group 5

Mode of Action: Photosystem ll Inhibitors.

Site of Action: Photosynthesis Inhibitors.

       Chemical Class: Phenylcarbamate.

       Active Ingredient: Desmedipham, or phenmedipham.

       Chemical Class: Triazine.

       Active Ingredient: Atrazine, prometryn, or simazine.

      Examples: Pramitol 25E Herbicide.

      Chemical Class: Triazinone. 

      Active Ingredient: Hexazinone, or metribuzin.

      Chemical Class: Uracil.

      Active Ingredient: Terbacil.

Group 6

Mode of Action: Photosystem ll Inhibitors. 

Site of Action: Photosynthesis Inhibitors.

       Chemical Class: Benzothiadiazole.

       Active Ingredient: Bentazon.

       Chemical Class: Nitrile.

       Active Ingredient: Bromoxynil.

Group 7

Mode of Action: Photosystem ll Inhibitors. 

Site of Action: Photosynthesis Inhibitors.

       Chemical Class: Amide.

       Active Ingredient: Propanil.

      Chemical Class: Urea.

      Active Ingredient: Diuron, fluometuron, or linuron. 

Group 8

Mode of Action: Lipid Synthesis Inhibitor.

Site of Action: Seedling Shoot Growth Inhibitors.

       Chemical Class: Thiocarbamate.

       Active Ingredient: Butylate, cycloate, EPTC, thiobencarb, or triallate. 

Group 9

Mode of Action: Amino Acid Synthesis Inhibitors.

Site of Action: EPSP Synthase Inhibitor.

       Chemical Class: Organophosphorus. 

       Active Ingredient: Glyphosate.

      Examples: Martin's Eraser A/P Herbicide. Glyphosate 5.4 Aquatic Herbicide. 

Group 10

Mode of Action: Nitrogen Metabolism Inhibitor.

Site of Action: Glutamine Synthetase Inhibitor.

       Chemical Class: Organophosphorus.

       Active Ingredient: Glufosinate.

Group 12

Mode of Action: Pigment Inhibitors. 

Site of Action: Phytoene Desaturase (PDS) Inhibitor. 

       Chemical Class: None Recognized.

       Active Ingredient: Fluridone.

Group 13

Mode of Action: Pigment Inhibitors. 

Site of Action: DOXP Synthase Inhibitor.

       Chemical Class: Isoxazolidinone.

       Active Ingredient: Clomazone.

Group 14

Mode of Action: Cell Membrane Disrupters. 

Site of Action: PPO Inhibitors.

       Chemical Class: Aryl Triazinone.

       Active Ingredient: Sulfentrazone, carfentrazone, or fluthiacet.

       Examples: Expel Turf Herbicide.

      Chemical Class: Diphenylether. 

      Active Ingredient: Acifluorfen, fomesafen, or lactofen. 

      Chemical Class: N-phenylphthalimide.

      Active Ingredient: Flumiclorac, or flumioxazin. 

      Example: Promenade WDG Herbicide. 

     Chemical Class: Pyrimidinedione.

     Active Ingredient: Saflufenacil.

Group 15

Mode of Action: Seedling Shoot Growth Inhibitors.

Site of Action: Long-chain Fatty Acid Inhibitors.

       Chemical Class: Chloroacetamide.

       Active Ingredient: Acetochlor, alachlor, s-metolachlor, or dimethenamid-p. 

       Chemical Class: Oxyacetamide.

       Active Ingredient: Flufenacet. 

       Chemical Class: Pyrazole.

       Active Ingredient: Pyroxasulfone.

Group 16

Mode of Action: Seedling Shoot Growth Inhibitors. 

Site of Action: Unknown.

       Chemical Class: Benzofurane.

       Active Ingredient: Ethofumesate.

Group 17

Mode of Action: Undefined. 

Site of Action: Nucleic Acid Inhibitor.

       Chemical Class: Arsenical.

       Active Ingredient: MSMA. 

Group 19

Mode of Action: Growth Regulators.

Site of Action: Auxin Transport Inhibitor.

       Chemical Class: Semicarbazone.   

       Active Ingredient: Diflufenzopyr.

Group 22

Mode of Action: Cell Membrane Disrupters.

Site of Action: Photosystem l Electron Diverter.

       Chemical Class: Bipyridylium.

       Active Ingredient: Diquat, or paraquat.

Group 27

Mode of Action: Pigment Inhibitors. 

Site of Action: HPPD Inhibitors.

       Chemical Class: Isoxazole. 

       Active Ingredient: Isoxaflutole.

      Chemical Class: Pyrazole.

      Active Ingredient: Pyrasulfotole.

     Chemical Class: Pyrazolone.

     Active Ingredient: Topramezone.

    Chemical Class: Triketone.

    Active Ingredient: Mesotrione, tembotrione, or bicyclopyrone.

Herbicide Resistance Management

Spraying Turf

Some populations of weeds have one or multiple levels of resistance to herbicides containing glyphosate or ALS-inhibiting herbicides. Herbicide resistance in weed species occurs over time when the same herbicide and mode of action is consistently used. The speed of resistance depends on the amount of times you are applying your herbicide. 

To avoid resistant weeds and the odds of passing these traits to the next generation you will want to follow these practices to influenze your herbicide success.

Rotate Herbicides- Integrate different herbicides for each application within the same treatment area. Avoid making more than two consecutive applications of herbicides with the same site of action to the same treatment area in the same year or the following year. 

Rotate Foliage and Crops- Depending on the foliage or crop type, each plant varies against weeds based on their life cycle or rate of growth. Rotation of crops and foliage can help prevent some weed species from further spreading. By consistently disturbing the soil weeds are denied excess moisture, shade, and sources of nutrients. 

Monitor and Keep Notes- Keep accurate notes and records of previously used herbicides and their site and mode of action, how much was applied, and how often it was applied. It is also best to note which weed species was present at the time of the application and what parts of your yard it was infesting. Make sure to also write about how well the herbicide being used controlled the weed species.

Plant Clean Seeds- Sometimes weeds can be introduced by contaminated seed, grain, machinery (unclean equipment used for weed control), or hay. Certified seeds approved by your states department of agriculture would be best to plant in your crop or landscape. 

Integrated Weed Management Practices- Thoroughly clean machinery used in fields or in areas with known or suspected weed activity before moving the machinery used to other parts of your yard or storage. Till in-between rows of plants until weeds and soil is disturbed. 

Key Takeaways

Signs of Herbicide Resistance

  • Failure to control weeds with the regular herbicide and normal application rate. Previously treated weeds are still upright and not discolored around wilted weeds. Weeds are forming into larger patches or creating a path towards other unaffected parts of your property. 

What to Do When There is Herbicide Resistance?

  • Do not reapply the herbicide used previously or with herbicides that use the same mode or site of action. Wait until the reapplication date has passed before applying another herbicide with a different mode of action. 

How to Handle Herbicide Resistance

  • Monitor treated areas for residing weeds. Clean equipment or machines used previously where there is resistant weeds to avoid spreading them to other parts of your property. Rotate plants each season to disturb weed growth. Adjust your herbicide treatment program with other products that use different modes and sites of action on a continual basis.  
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