Navigating Herbicide Options

Navigating through numerous herbicide options can be challenging. Understanding the weeds it can control, correct dosage, and any adjuvants needed is crucial. Understanding the herbicide's mode of action is also vital for successful weed management.

Understanding "Mode of Action"

"Mode of Action" is the way a herbicide controls plants. It often refers to the biological process or enzyme the herbicide disrupts, impacting normal plant growth. 11 herbicide modes of action are frequently used, each unique in its function. Some modes involve various chemical families that differ slightly but control plants similarly.

Classification of Herbicides

Herbicides are also sorted by their "site of action" - the biochemical site affected by the herbicide, giving a more accurate activity description. "Site of action" and "mode of action" are generally used interchangeably.

Importance of Knowing the Mode of Action

Understanding each herbicide's mode of action is critical for choosing the right herbicide, diagnosing injury, and creating effective weed management. Over-dependence on a single herbicide ingredient can lead to resistant weeds, making herbicides less effective. Rotation of herbicide modes of action, alongside other weed control methods, helps prevent or delay this resistance.

The Challenge of Cross Resistance in Weed Control

Cross resistance occurs when weeds resist multiple herbicides within a single mode. To prevent herbicide-resistant weeds, rotation of active ingredients and modes of action is necessary.

Multiple Resistance in Weeds

Multiple resistance" occurs when weeds resist herbicides from two or more modes of action.

Determining a Herbicide’s Mode of Action

The herbicide’s mode of action can be found on the herbicide label, typically denoted by a particular group number. If uncertain, reach out to your local county extension educator.

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What are the Different Modes of Action? What are their Characteristics?

The following is a short description of the 11 most commonly used herbicide modes of action in production. The list of herbicides in the accompanying table (found on the inside pages) is not exhaustive and does not account for herbicide premixes that contain two or more active ingredients. If you have questions regarding mode of action, consult the individual product label and support literature from the manufacturer or contact your county agricultural Extension educator for more information.

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ACCase Inhibitors (Group 1)

Inhibitors of the ACCase enzyme in plants are used strictly for grass control. As a result, they are used primarily in broadleaf crops or fallow situations, but there are also some products labeled for use in grass crops to control specific grass weeds. These herbicides are commonly referred to by the nicknames of their chemical families, “FOPs,” “DIMs,” and “DENs.”

ALS Inhibitors (Branched-Chain Amino Acid Inhibitors) (Group 2)

ALS inhibitors, or branched-chain amino acid inhibitors, comprise the largest mode of action. Many herbicides in this mode of action fall into two chemical families: imidazolinones (or “IMIs”) or sulfonylureas (or “SUs”), but there are three other chemical families within the ALS inhibitors. Cross resistance, or herbicide-resistance to multiple chemical families within a single mode of action, is common with ALS inhibitors.

Root Growth Inhibitors (Group 3)

Herbicides in this mode of action inhibit cell division, which stops roots from extending and are distinctive because of the yellow color of their formulations. They are applied preplant incorporated or preemergence in a wide range of agronomic crops, vegetables, turf, and ornamentals for control of grasses and small-seeded broadleaf weeds.

Growth Regulators (Group 4)

This mode of action, also known as synthetic auxins, includes many commonly used plant hormone-type herbicides in wheat, corn, sorghum, and pasture settings. These herbicides are generally selective for broadleaf control in grass crops; however, there are some uses for preplant and in-season weed control in broadleaf crops.

Photosynthesis Inhibitors—Photosystem II (Groups 5, 6, and 7)

These herbicides inhibit Photosystem II, part of the photosynthesis pathway, and are used in a variety of crops for control of grass and broadleaf weeds. Because of their extensive use for several decades, some weeds have developed resistance to these herbicides, particularly atrazine and metribuzin.

Shoot Growth Inhibitors (Groups 8 and 15)

Herbicides in this mode of action are soil-applied herbicides and control weeds that have not emerged from the soil surface. These herbicides generally control grass weeds and small-seeded broadleaf weeds.

Aromatic Amino Acid Inhibitors (Group 9)

The only herbicide included in this mode of action is glyphosate. There are many generic glyphosate and glyphosate-containing products available. Depending on the product, glyphosate can be formulated as ammonium, diammonium, dimethylammonium, isopropylamine, and/or potassium salts. Despite the different salt formulations available, it is important to know that the type of salt formulation does not affect weed control, but rather it indicates the way a particular glyphosate product is formulated. Glyphosate is a generally a non-selective herbicide and will severely injure or kill any living plant tissue that it comes in contact with. However, it can be used selectively in glyphosate-resistant crops. Like the ALS inhibitors, glyphosate controls susceptible plants by inhibiting amino acid synthesis; however, glyphosate and ALS inhibitors control susceptible plants in completely different ways and should not be considered to be the same mode of action.

Glutamine Synthesis Inhibitors (Group 10)

The only herbicide included in this mode of action is glufosinate. Glufosinate can be used as a non-selective burndown treatment or as an over-the-top postemergence application in Liberty Link® crops (glufosinate resistant).

Pigment Synthesis Inhibitors (Groups 12, 13, 27)

These herbicides are also called “bleachers” because of the characteristic white plant tissue that develops in susceptible plants after application. Several of the pigment synthesis inhibitors (mesotrione, isoxaflutole) are also referred to as HPPD-inhibitors, based on their site of action.

PPO Inhibitors (Groups 14)

PPO inhibitors may also be referred to as cell membrane disruptors and are usually “burner”-type herbicides. Some PPO-inhibitors can be applied preemergence, but most are used for postemergence weed control.

Photosynthesis Inhibitors—Photosystem I (Group 22)

Photosystem I inhibitors include paraquat and diquat and are used for non-selective weed control and crop desiccation prior to harvest. These herbicides are also referred to as “cell membrane disruptors” because of their contact activity.