Weeds: Mode of Action

Upon proper identification of the weed species in the field, selection of the correct herbicide is important. There are certain herbicides that work very effectively against monocots and other herbicides for dicots. When looking at various herbicide products available it is important to know the “mode of action” in which that herbicide works.

The “mode of action” is the biological process or enzyme that the herbicide interrupts, affecting normal plant growth and development. For example 2, 4-D is a growth regulator mode of action herbicide that affects auxin growth. This product causing bending and twisting of leaves and stems which is evident almost immediately. Delayed symptoms development include root formation on dicot stems; misshapen leaves, stems, and flowers; and abnormal roots.

Knowing and understanding each herbicide’s mode of action is an important step in proper herbicide selection but is also important in diagnosing herbicide injury and designing a successful weed management program. To develop a successful weed management program, it is important not to rely on a single herbicide or herbicide(s) with the similar mode of action to control weeds. Over-reliance on that single mode of action places heavy selection pressure on weed populations and result in the evolution of herbicide-resistant weed(s). Over time, the resistant individuals will multiply and become the dominant weed in the field, resulting in that herbicide mode of action no longer effective. Thus, rotating herbicide modes of action to reduce selection pressure is critical to maintain the selection of herbicides available to manage various weed species. To assist in determining mode of actions refer to the
“Guide of Weed, Disease, and Insect Management in Nebraska”
(extension circular 130) that is reprinted every year. In this publication all herbicides are listed in their appropriated mode of action.

Below is a short description of the 11 most commonly used herbicide modes of action. Table 1 is a list of herbicides associated with each group of mode of action, however this is not an exhaustive list and does not account for herbicide premixes that contain two or more active ingredients. Always consult the individual product label for more information regarding mode of action of the product as well as application rates, adjuvants to mix, and application timings. Specific information regarding various modes of action is listed. Refer to Table 1 for specific chemical family, active ingredients and product examples found with each mode of action.

ACCase Inhibitors (Group 1)

  • Used strictly for grass control, also known as graminicides
  • Inhibits the ACCase enzyme in plant
  • Nicknames of chemical families in this group are “FOPs,” “DIMs,” and “DENs”
  • Example herbicides: Select Max, Assure II

ALS Inhibitors (Group 2)

  • Branched-chain amino acid inhibitor
  • Comprise of the largest mode of action herbicide
  • Five chemical families in this group, includingimidazoliones “IMI’s” andsulfonylureas “SUs.”
  • Example herbicides: Pursuit, Raptor, Septor

Root Growth Inhibitors (Group 3)

  • Applied preplant or preemergence to control grasses and small-seeded broadleaf weeds
  • Inhibit cell division which stops roots from extending
  • Example herbicides: Prowl H2O, Treflan

Growth Regulators (Group 4)

  • Generally selective for broadleaf weed control in grass crops, however some uses for preplant and in-season weed control in broadleaf crops
  • Also known as synthetic auxins
  • Example herbicides: 2,4-D, Clarity

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

  • Used to control grass and broadleaf weeds
  • Inhibit Photosystem II which isapart of the photosynthesis pathway
  • Example herbicides: Atrazine, Tricor

Shoot Growth Inhibitors (Groups 8 and 15)

  • Generally control grass weeds and small-seeded broadleaf weeds
  • Soil-applied herbicides and control weeds that have not emerged from the soil surface
  • Example herbicides: Warrant, Outlook

Aromatic Amino Acid Inhibitors (Group 9)

  • Only herbicide in this class is glyphosate
  • Non-selective herbicide (kills grass and broadleaves)or as an over-the-top post-emergence application in Roundup Ready crops
  • Example herbicides: Roundup, Touchdown

Glutamine Synthesis Inhibitors (Group 10)

  • Only herbicide in this class is glufosinate
  • Non-selective burndown treatment or as an over-the-top postemergence application in LiberyLink crops.
  • Example herbicides: Liberty

Pigment Synthesis Inhibitors (Groups 13, 27)

  • Prevent chlorphyll production thus giving plants a “bleached” appearance
  • Also referred to as HPPD-inhibitors
  • Example herbicides: Callisto, Laudis

Cell Membrane Inhibitors (Group 14)

  • Destroy cell membranes
  • Activity is delayed in the absence of light
  • Also referred to as PPO-inhibitors
  • Known as “burner”-type herbicides
  • Example herbicides: Marvel, Cobra

Photosynthesis Inhibitors -Photosystem I (Group 22)

  • Cell membrane disruptors
  • Non-selective weed control and crop desiccation prior to harvest
  • Example herbicides: Gramoxone, Reward

Symptoms that are observed with various herbicide injuries are also associated herbicide mode of action. This is because of the specific area of plant growth and development is affected by different herbicides. Refer to Table 1 for information regarding symptoms observed with various modes of action on soybeans.


  Go In Depth

Get an in depth look at Mode of Action:

Herbicide Mode and Site of Action

  Quick Fact:

The University of Wisconsin has a great on-line resource:

herbicide injury diagnostic key
Nebraska Soybean Checkoff

Nebraska Soybean Board graciously provided the funding for the Soybean Management Guide.

Course authored by:

Amy Timmerman, Extension Educator, Aaron Nygren, Extension Educator, Brandy VanDeWalle, Extension Educator, Loren Giesler, Extension Plant Pathologist, Ron Seymour, Extension Educator, Keith Glewen, Extension Educator, Charles Shapiro, Extension Soil Scientist, Amit Jhala, Extension Weed Scientist, Don Treptow, Graduate Student