Although all herbicides have a common purpose, which is eliminating unwanted weeds, there are dozens of different herbicides which accomplish this in very different ways. Today, it isn’t enough for a farmer to see that the weeds have died after applying a certain product. It is critically important to understand how that product killed the weed.
HRAC and WSSA groups of herbicides
There are currently two major organizations tasked with classifying the different types of herbicides so that farmers can develop the best weed control strategy. The first is the HRAC, Herbicide Resistance Action Committee, which has letter groups for different herbicide families. The second is the WSSA, Weed Science Society of America, which uses a numeric system.
As of this writing, HRAC recognizes 16 groups of herbicides (A-I, K-P, and Z) according to mode of action with a few subgroups (C1-3, F1-3, and K1-3). Subgroups are made when herbicides have the same mode of action, but different sites of action.
WSSA recognizes 26 groups (1-15, 17-27) which more or less overlap with HRAC groups, but subgroups are assigned a separate group instead.
Group A herbicides (Group 1)
Group A herbicides have the mode of action: inhibition of acetyl CoA carboxylase or ACCase. ACCase is an enzyme that is needed for plants to create fatty acids. By blocking the activity of this enzyme, herbicides like the “FOPs” (e.g. fluazifop), “DIMs” (e.g. butroxydim) and “DEN” prevent plants from making the fatty acids they need to survive. This leads to the death of the plant.
Group B herbicides (Group 2)
HRAC’s Group B herbicides kill weeds by inhibiting a different key plant enzyme: inhibition of acetolactate synthase or ALS. ALS is used by plants to create certain amino acids. Several different herbicides fall under this group, most notably the sulfonylureas.
Group C herbicides (Groups 5, 6, 7)
All Group C herbicides work by blocking the weed’s ability to do photosynthesis. The subgroups and WSSA groups correspond to different ways of blocking this process. Since plants require photosynthesis to make their food, inhibiting photosynthesis causes the plant to die.
Group D herbicides (Group 22)
Herbicides in Group D (e.g. paraquat) also block photosynthesis, but they do it at a different step in the process.
Group E herbicides (Group 14)
This group of herbicides block an enzyme called protoporphyrinogen oxidase or PPO. Blocking this enzyme means plants aren’t able to create chlorophyll, which they need for photosynthesis.
Group F herbicides (Groups 11, 12, 13, 27)
HRAC’s Group F herbicides all work through a plant bleaching mechanism, with different subgroups and WSSA groups corresponding to which pigment is targeted.
Group G herbicides (Group 9)
These herbicides, notably glyphosate, block a plant enzyme called EPSP synthase. Plants use this enzyme to produce certain types of amino acids, without which the plant will die.
Group H herbicides (Group 10)
Group H herbicides work by blocking the plant enzyme glutamine synthetase, which is used to make the amino acid glutamine and is very important for the plant’s ability to metabolize nitrogen.
Group I herbicides (Group 18)
This group contains dihydropteroate synthase (DHP synthase) inhibitors. This enzyme is used by plants to produce folic acid, and blocking it causes the plants to die. The sulfonamide group of antibiotics works by inhibiting this same enzyme in bacteria.
Group K herbicides (Group 3, 15, 23)
The three subgroups of Group K herbicides work by preventing plant cells from being able to organize microtubules, a structure that is needed for cell division.
Group L herbicides (Group 20)
HRAC Group L herbicides work by inhibiting the plant’s ability to make cellulose, which makes up the cell wall in plants.
Group M herbicides (Group 24)
These chemicals work by disrupting the cell membrane in plant cells.
Group N herbicides (Group 8, 26)
Herbicides in Group N block the plant’s ability to make lipids, which are required for many cell processes.
Group O herbicides (Group 4)
These chemicals (e.g. halauxifen-methyl, fluroxypyr, 2,4-D) are known as synthetic auxins because they trick the plant into accepting them in the place of auxins, a type of plant hormone.
Group P herbicides (Group 19)
Group P herbicides block the plant cell’s ability to transport auxins for use in different biological processes.
Group Z herbicides (Group 17, 25, 26)
This final “Unknown” group is a catch-all for HRAC for herbicides whose mode of action has not yet been determined well enough to be placed into another group.