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The terms “mode of action” and “site of action” are often used interchangeably when referring to herbicide classifications, but they’re not exactly the same. Understanding the difference between a herbicide’s mode of action and its site of action can help you build a successful weed control program while also managing herbicide resistance.
In this post, we will explore:
Think of the site of action as the “where” and the mode of action as the “how” herbicides work. Herbicides are classified into site of action groups based on the particular enzyme or metabolic process they disrupt in a weed.
Multiple site-of-action herbicide groups may fall into broader mode-of-action groups, which describe how the herbicides disrupt weed growth and development. For example, Group 2 ALS inhibitors and Group 9 EPSP synthase inhibitors disrupt different weed enzymes, but they fall under the same mode of action group, amino acid synthesis inhibitors.
Understanding a herbicide’s mode of action can help you choose the most effective product for managing a weed based on its unique biology and life cycle. It can also help diagnose crop injuries following a herbicide application. However, understanding the herbicide mode of action is most important for managing herbicide resistance and stewarding effective chemistries so they have a longer lifespan.
When your weed management program includes herbicides with multiple modes of action, you can:
Mitigate risk
Achieve complete weed control more consistently
Delay weed resistance in your fields
You may have heard that you should rotate herbicide modes of action to reduce the risk of developing herbicide-resistant weed populations. The science behind herbicide resistance can be complex, but here’s a simplified explanation.
Imagine you have a field that contains waterhemp. Basic biology tells us that some of the waterhemp weeds in your field have innate genetic tolerance to Herbicide A. When you spray Herbicide A, most of the waterhemp plants will die, but those with natural tolerance to the herbicide will not. In turn, those more tolerant weeds will reproduce and spread their genes, building up the population of resistant weeds. Herbicide A eventually becomes ineffective on a majority of the weed population because it’s been relied upon too heavily for control.
Now, consider the same field where you use multiple herbicide modes of action to manage weeds. You’ve added Herbicide B and Herbicide C to your tank mix. Those waterhemp weeds with natural resistance to Herbicide A’s mode of action are controlled using the alternative modes of action from Herbicides B and C. The result is more consistent weed control, reducing the ability of herbicide-tolerant weeds to reproduce.
For more information on herbicide groups and a classification chart, click here.
Here are nine common herbicide modes of action groups used in agricultural production. Use the information below to help build a season-long weed management program that includes herbicides with multiple modes of action.
There is currently only one lipid synthesis inhibitor herbicide group, the ACCase inhibitors, which includes three chemical families, commonly referred to as:
FOPs
DIMs
DENs
Active ingredients with this mode of action include:
Clethodim
Fenoxaprop
Pinoxaden
ACCase inhibitors disrupt the ACCase enzyme in plants, leading to cell death due to membrane dysfunction.
Lipid synthesis inhibitors, also known as graminicides, are used exclusively for grass control. These herbicides are applied primarily to broadleaf crops or fallow fields for selective, postemergence control of annual and perennial grass weeds, but some products may be labeled for use in grass crops to control specific grass weeds.
ACCase inhibitors (Group 1)
Amino acid synthesis inhibitor herbicides include:
ALS inhibitors
EPSP synthase inhibitors
While both herbicide classes disrupt amino acid synthesis, they do it in completely different ways; thus, they may be considered different modes of action for resistance management purposes.
Common active ingredients with this mode of action include:
Glyphosate
Thifensulfuron
Imazethapyr
Amino acid synthesis inhibitors disrupt the production of critical amino acids required for proper plant growth and development.
Amino acid synthesis inhibitors are some of the most widely used herbicides in agricultural production. They are active on annual and perennial broadleaf and grass weeds and offer varying degrees of selectivity. However, due to their wide use, more weeds are becoming tolerant or resistant to amino acid synthesis inhibitor herbicides.
ALS inhibitors (Group 2)
Has more active ingredients than any other herbicide mode of action
Comprised of five chemical families, including imidazolinones (IMIs) and sulfonylureas (SUs)
May control annual and perennial broadleaf or grass weeds; varying selectivity based on the herbicide
Can be soil or foliar applied
More than 50 different weed species in the United States are resistant to Group 2 herbicides
EPSP synthase inhibitor (Group 9)
Glyphosate is the only active ingredient in this site of action group
Nonselective, contact, postemergence herbicide for grass and broadleaf weed control
Offers excellent control of perennial weeds and is also effective on annual weeds
Can be applied to genetically modified glyphosate-resistant crops, including corn, soybean, cotton, and canola
Weed injury symptoms are slow to develop; complete weed kill can take up to 14 days
More than 15 weed species in the United States are resistant to glyphosate
Growth regulator herbicides include synthetic auxins and auxin transport inhibitors. Chemical families in this group include:
Arylpicolinate
Benzoic acid
Carboxylic acid
Phenoxy
Semicarbazone
Common active ingredients with this mode of action include:
Dicamba
MCPA
2,4-D
Growth regulators disrupt plant hormone balance, cell division, protein synthesis, and respiration, causing several weed growth abnormalities.
Growth regulator herbicides are mainly used for selective broadleaf weed control in grass crops, but they can also be used pre-plant and in-season to control weeds in broadleaf crops. Herbicides with this mode of action can translocate through the xylem and phloem cells to areas of new weed growth.
Plant growth regulator herbicides can cause severe damage to susceptible crops, so it's important to follow label application instructions and take measures to limit herbicide drift and volatilization. Injured crops may have twisted stems or cupped, crinkled, puckered, strap-shaped, stunted, or otherwise malformed leaves. Even small traces of remnant herbicide in spray tanks or lines can cause damage to susceptible crops, so it’s important to thoroughly clean equipment before spraying susceptible crops.
Synthetic auxins (Group 4)
Auxin transport inhibitor (Group 19)
Photosynthesis inhibitor herbicides include photosystem II inhibitors in two different site of action groups. So, while all herbicides in Groups 5 and 6 are photosynthesis inhibitors, each group has a unique herbicide binding site.
Common active ingredients with this mode of action include:
Atrazine
Metribuzin
Bromoxynil
Photosynthesis inhibitor herbicides bind to specific sites within the photosystem II complex in plant chloroplasts to inhibit photosynthesis. Without efficient photosynthesis, weeds may slowly starve due to a lack of sugar production or, in some cases, die quickly from the accumulation of secondary toxic substances.
Photosynthesis inhibitors are used in various crops to control grass and broadleaf weeds. Herbicides in the triazine, triazinone, urea, and uracil families are assimilated into weeds via the roots or foliage and tend to have long soil persistence. Herbicides in the benzothiadiazole family are foliar-applied contact herbicides that require sufficient spray coverage for effective weed control.
Photosynthesis inhibitor herbicides like metribuzin and atrazine (Group 5) have been used extensively for many decades, leading to increased herbicide resistance. More than 25 weeds are now resistant to Group 5 chemistries.
Photosystem II inhibitors (Groups 5 and 6 — each group has a different herbicide binding site)
Nitrogen metabolism inhibitors include just one site of action herbicide group, Group 10. Glufosinate is the only Group 10 active ingredient.
Nitrogen metabolism inhibitors disrupt the metabolic processes that convert ammonia into other nitrogen compounds in a weed. This results in a lethal accumulation of ammonia that destroys plant cells and inhibits photosynthesis reactions.
Glufosinate herbicides are used for non-selective burndown of grass and broadleaf weeds in a broad spectrum of crops. Glufosinate may also be used in-season for postemergence weed control in glufosinate-tolerant crops, including soybeans, canola, corn and cotton.
Glufosinate is a contact herbicide, so adequate spray coverage is required for effective weed control. Herbicide performance is optimized when applications are made on bright, sunny days with warm temperatures and high relative humidity.
Glutamine synthetase inhibitor (Group 10)
Pigment inhibitors include herbicides in site of action Groups 12, 13, and 27. While all herbicides with this mode of action interfere with pigment production, they may act at different sites within the weed, depending on their chemical family.
Common pigment inhibitor active ingredients include HPPD inhibitors like:
Mesotrione
Isoxaflutole
Tembotrione
Herbicides with a pigment inhibitor mode of action interfere with chlorophyll production in the weed, disrupting photosynthesis. Green pigment in the leaf tissue is destroyed, giving affected weeds a bleached or yellow appearance.
Pigment inhibitors can be used for preemergence or postemergence grass and broadleaf weed control. Herbicides labeled for preemergence applications are taken up by weeds through roots and translocated to plant leaves via the xylem cells. Susceptible weeds emerge bleached and die relatively quickly.
Pigment inhibitors labeled for postemergence application are most effective on small weeds. They are absorbed through the foliage and move toward leaf margins. Some postemergence pigment inhibitors remain active in the soil to provide extended residual weed control later into the growing season.
Waterhemp (Amaranthus tuberculatus) and Palmer amaranth (Amaranthus palmeri) populations with HPPD-inhibitor resistance (Group 27) have been identified in several agronomic production systems in North America.
Pigment inhibitor herbicides may damage susceptible non-target crops, so using best application practices to limit off-target movement is important.
Phytoene desaturase (PDS) inhibitor (Group 12)
DOXP synthase inhibitor (Group 13)
HPPD inhibitors (Group 27)
Cell membrane disruptors include herbicides in site of action Groups 14 and 22. Common active ingredients with this mode of action include paraquat and PPO inhibitors like:
Saflufenacil
Flumioxazin
Fomesafen
Cell membrane disruptor herbicides are activated by sunlight to form toxic oxygen compounds that destroy plant tissue by rupturing cell membranes. Herbicides with this mode of action tend to work quickly, with noticeable weed injury within an hour of application on bright, sunny days.
Cell membrane disruptors are generally contact herbicides excellent for:
Burndown control of emerged weeds
Postemergence control of annual weeds
Crop desiccation before harvest
Cell membrane disruptor herbicides may be selective or non-selective based on their chemical family. For example, paraquat is a non-selective herbicide often used for burndown of grass and broadleaf weeds, while saflufenacil is a selective broadleaf herbicide that requires an effective grass herbicide in the tank mix for broad-spectrum weed control. While most PPO inhibitors are used for postemergence broadleaf weed control, several have preemergence soil activity.
PPO inhibitors (Group 14)
Photosystem I electron diverter (Group 22)
Seedling root growth inhibitors include Group 3 microtubule inhibitor herbicides. Common active ingredients with this mode of action include:
Trifluralin
Pronamide
Pendimethalin
Seedling root growth inhibitors act on germinating weeds to inhibit cell division, which limits root extension and lateral formation. These herbicides are active early in a weed’s life cycle, from germination until seedling emergence. Seedling root growth inhibitors are not readily translocated through the weed and have no foliar activity. Susceptible weeds may appear stunted, nutrient deficient, or wilted due to their poorly developed root systems.
Seedling root growth inhibitors are typically applied preplant incorporated or preemergence to soil to control small-seeded grass and broadleaf weeds in a variety of agronomic crops, vegetables, turf, and ornamentals. Large-seeded weeds typically escape control with normal herbicide doses because their roots grow through herbicide-treated soil zones quickly.
Microtubule inhibitors (Group 3)
Seedling shoot growth inhibitors include herbicides in site of action Groups 15 and 29. Common active ingredients with this mode of action include:
Acetochlor
S-metolachlor
Pyroxasulfone
Thiobencarb
Seedling shoot growth inhibitors interfere with fatty acid and lipid biosynthesis in a weed, and research suggests they can affect multiple sites within a plant:
Thiocarbamate herbicides are primarily absorbed from the soil solution through the emerging shoot and growing point.
Chloroacetamide herbicides like acetochlor are absorbed through roots (broadleaf species) and emerging shoots (grass species).
Seedling shoot growth inhibitor herbicides may be translocated through the weed’s xylem cells.
Seedling shoot growth inhibitors are applied to the soil to control unemerged grass and small-seeded broadleaf weeds. Some herbicides with this mode of action may provide residual weed control that can take pressure off postemergence herbicide applications.
Long-chain fatty acid inhibitors (Group 15)
Cellulose biosynthesis inhibitors (Group 29)
FBN has a comprehensive herbicide portfolio that makes it easy to rotate multiple modes of action to proactively manage weed pressures and prevent weed resistance. With 24/7 digital shopping access, direct-to-farm delivery, transparent pricing and savings opportunities, and detailed label information for each product, FBN offers the information and products you need to build an effective herbicide strategy this season.
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