All About Actives
INTRODUCTION TO ACTIVES AND MODES OF ACTION
With the rise of herbicide resistance, it’s important to understand herbicide active ingredients and their modes of action. It’s also important to understand why we differentiate between active ingredients and modes of action. Both are significant to consider when selecting a herbicide.
An active ingredient is the chemical agent in the herbicide formulation that has a specific effect on the weed. The active ingredient is often only a portion of what makes up the herbicide and there can be multiple active ingredients in a herbicide. Active ingredients are part of a chemical family under which there are often multiple variations of that chemical to make each active ingredient distinct.
A mode of action is the way in which the active ingredient controls the weed. It describes the biological process or enzyme in the plant that is interrupted, affecting normal plant growth and development, and ultimately leading to plant death. In some cases, the mode of action may also be a description of the injury symptoms seen on susceptible plants. Multiple active ingredients and chemical families can have the same mode of action and a herbicide can contain multiple modes of action.
Modes of action are organized by a system of numbered groups, aligning active ingredients that share the same mode of action. You can determine the mode of action of your herbicide by finding its group number, which is usually found on the product label. If the herbicide is classified in multiple groups the herbicide will contain multiple modes of action.
Resistance in a plant occurs when the plant learns to adapt and survive when faced with a threat, the threat being an attack to a biological process or enzyme. The resistance is always responding to the mode of action and not necessarily the herbicide, which is why it’s important to understand modes of action and active ingredients and how they
should be applied when selecting your herbicide(s).
Group 1
ACCASE INHIBITORS
Group 1 herbicides are considered ACCase inhibitors. They work by attaching themselves to the acetyl-CoA carboxylase (ACCase) enzyme. The ACCase enzyme helps the formation of lipids in the roots. Without these lipids, cell growth is inhibited, and susceptible plants die. ACCase inhibitors are used strictly for grass control.
Chemical families in Group 1 are divided into three groups, shortened to the fops (aryloxyphenoxy propionate), dims (cyclohexanediones), and dens (phenylpyrazolin).
Group 2
ALS/AHAS INHIBITORS
Group 2 herbicides are acetolactate synthase (ALS) or actohydroxyacid synthase (AHAS) inhibitors, also known as branched-chain amino acid inhibitors. These chemicals block the normal function of the ALS or AHAS enzymes, which are essential in synthesising amino acids (proteins) isoleucine, leucine, and valine. Without these proteins, plants starve to death.
Group 2 herbicides comprise one of the most used herbicide groups in Western Canada and are also the most susceptible to cross-resistance or resistance to multiple chemical families within a single mode of action. The majority of Group 2 active ingredients fall under two chemical families: IMIs (imidazolinones) or SUs (sulfonylureas).
Group 3
MICROTUBLE ASSEMBLY INHIBITORS
Group 3 herbicides are microtubule assembly inhibitors, otherwise known as root growth inhibitors. Group 3 herbicides are absorbed primarily by the roots of emerging seedlings and can also be absorbed by emerging shoots. These active ingredients work at the point of uptake to stop cell division and inhibit new growth.
Group 3 is limited to one chemical family: dinitrianilines.
Group 4
SYNTHETIC AUXINS
Group 4 herbicides are synthetic auxins, also known as growth regulators. Synthetic auxins disrupt plant cell growth by translocating from leaves, stems, and roots to the growing points in roots and shoots. There they simulate uncontrolled cell division and growth, affect protein synthesis, and cause malformed growths and tumors and a slow plant death.
Group 4 is one of the larger groups with four chemical families, including benzoic acids, carboxylic acids, picolinic acids, and phenoxy, and 14 active ingredients. These herbicides are generally selective for broadleaf control in grass crops; however, there are some used for pre-seed and in-season weed control in broadleaf crops.
Groups 5, 6, and 7
PHOTOSYNTHETIC INHIBITORS AT PHOTOSYSTEM II
Groups 5, 6, and 7 are all considered photosynthetic inhibitors at photosystem II. Each of these groups, however, acts at a different site on the plant, which separates these actives. These chemicals interfere with photosynthesis and disrupt plant growth, ultimately leading to plant death. These are contact herbicides that are primarily absorbed by plant foliage. They act quickly, inhibiting photosynthesis, resulting in water-soaked lesions that quickly become chlorotic and die within days of application.
Group 5 is comprised of the actives that target site A for this action. Group 6 is comprised of the actives that target site II for this action. Group 7 is comprised of actives that target site B for this action.
Group 9
EPSP SYNTHESIS INHIBITORS
Group 9 herbicides are inhibitors of the 5-enolpyruvylshimate-3-phosphate (EPSP) synthesis, which is a key amino acid in protein synthesis, allowing for Group 9 herbicides to also be called aromatic amino acid inhibitors.
The only active ingredient in Group 9 is glyphosate. Glyphosate is 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.
Group 10
GLUTAMINE SYNTHESIS INHIBITORS
Group 10 herbicides are glutamine synthesis inhibitors, stopping the activity of glutamine synthase, the enzyme that converts glutamate and ammonia to glutamine. When ammonia is left unconverted, its accumulation in the plant will destroy cells.
Glufosinate is the only active ingredients in Group 10. Glufosinate can be used as a non-selective burndown treatment or as an over-the-top postemergence application in glufosinate-resistant crops.
Group 13
DOXP INHIBITORS
Group 13 herbicides are inhibitors of deoxy-D-xyulose phosphate (DOXP) synthase. Group 13 herbicides can also be considered pigment synthesis inhibitors or “bleachers” because of the characteristic white plant tissue that develops in susceptible plants after application. DOXP synthase is needed for plant metabolism.
Group 13 has one chemical family: isoxazolidinones.
Group 14
PPO INHIBITORS
Group 14 herbicides are protoporphyrinogen oxidase (PPO) inhibitors, which is an enzyme of chlorophyll and heme biosynthesis. Group 14s can 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.
All Group 14 actives are in the aryl triazone chemical family.
Group 15 (and 8)
SHOOT GROWTH INHIBITORS
Group 15 herbicides, which now also includes what were Group 8 herbicides, are considered shoot growth inhibitors. These herbicides are absorbed primarily by shoots of emerging seedlings causing the inhibition of a very long chain fatty acid resulting in the disruption of cell membrane formation and cell growth. Herbicides in this mode of action are soil applied and control weeds that have not emerged from the soil surface causing malformation or non-emergence.
Actives in this group control grass weeds and small-seeded broadleaf weeds.
Group 22
PHOTOSYNTHESIS INHIBITORS AT PHOTOSYSTEM I
Group 22 herbicides are photosynthesis inhibitors in the photosystem I category. These herbicides can also be referred to as cell membrane disruptors as their function is to disrupt the internal cell membrane and prevent the cells from manufacturing food.
Group 22 herbicides are largely used as crop desiccants prior to harvest but can also be used for non-selective weed control.
Group 27
PHOTOSYNTHESIS INHIBITORS - PHOTOSYSTEM I
Group 27 herbicides are hydroxyphenyl pyruvate dioxygenase (HPPD) inhibitors, which inhibit plant pigment biosynthesis and photosynthesis by stopping production of the carotenoids resulting in severe bleaching of foliage which appears two to three days after herbicide application. Plant growth is prevented leading to plant death.
All Group 27 actives are in the benzopyrazole chemical family.
