Herbicides: Common classification methods and representative varieties

Apr 28, 2023

According to statistics, my country loses about 20 billion kilograms of grain every year due to weeds. As an economical, efficient, and reliable solution to field weeds, herbicides play a vital role in ensuring high and stable grain yields.

Common active ingredients in herbicides include 2,4-D, glyphosate, and atrazine. 2,4-D is a selective herbicide used to control broadleaf weeds in crops such as corn, wheat, and soybeans. Glyphosate is a non-selective herbicide used to kill weeds in both agricultural and non-agricultural areas. Atrazine is another common herbicide used to control weeds in corn and other crops.

Glyphosate Molecular Structure
Glyphosate Molecular Structure

Global herbicides account for approximately 40% of the total value of pesticides. Therefore, clarifying the classification of herbicides has important guiding significance for the scientific and rational application of herbicides. Herbicides are classified according to their mode of action, chemical structure and spectrum of activity.

Here are some common classification methods and representative varieties of herbicides:

According to the nature of the action

According to the nature of action, it can be divided into two categories: fungicidal herbicides and selective herbicides.

Biocidal herbicide

Some herbicides kill all kinds of weeds and crops indiscriminately, called fungicides, such as sodium pentachlorophenol, glyphosate, etc.

Nonselective herbicides are often used in areas where complete vegetation control is required, such as non-crop areas, along fence lines, cracks in the pavement, and industrial or construction sites. They are also used as a pre-planting or pre-emergence treatment to remove existing vegetation before planting a new crop or establishing a new lawn or garden.

Non-selective herbicides are potent and care should be taken to avoid spraying it on plants it is not intended to control and should be used strictly in accordance with label directions and local regulations to ensure safe and effective use.

plant spraying

Selective herbicide

Some herbicides kill some weeds but have no effect on others, and may be safe for some crops but harmful for others. Herbicides with this property are called selective herbicides.

The selectivity of herbicides is not absolute, but relative. That is to say, the choice of herbicide does not have no effect on crops and can kill all weeds, but is selective under certain objects, doses, times, methods, and conditions.

Selectivity is determined by the selectivity coefficient. The so-called coefficient refers to the ratio of the amount of a herbicide that kills (or inhibits) less than 10% of crops to the amount that kills (or inhibits) more than 90% of weeds. The larger the coefficient, the better the effect. It’s more secure. Selective herbicides can only be promoted if the selectivity coefficient is greater than 2.

spraying application of pesticide

According to Chemical Structure

Herbicides can be classified into different categories based on their structure, and herbicides in the same category usually have certain common characteristics of action. The specific classification is shown in Table 1.

Table 1 Classification of Common Herbicides by Chemical Structure

Herbicide category Representative compound
Triketone Sulcotrione, Mesotrione , etc
Pyrazoles Cypyrafluone, Bipyrazone, Tripyrasulfone, Fenpyrazone, etc
Pyridines Fluroxypyr, Halauxifen-methyl, Florpyrauxifen-benzyl, etc
Sulfonylureas Tribenuron-methyl , Bensulfuron methyl, Nicosulfuron, Pyrazosulfuron-ethyl, etc
Sulfonamides Florasulam, Penoxsulam, Pyroxsulam,  Flumetsulam, etc
Pyrimidine salicylates Pyribenzoxim, Bispyribac sodium, Pyrithiobac-sodium, Pyriftalid, etc
Imidazolinone Imazethapyr, Imazamox, etc
Aryloxyphenoxypropionate esters Fenoxaprop-P-ethyl, Quizalofop-ethyl, Clodinafop-propargyl, Cyhalofop-butyl, etc
Phenoxyalkanoic acids 2,4-D, MCPA, etc
Triazines Atrazine, Simetryn, Ametryn, etc
Amides Acetochlor, S-metolachlor, Butachlor, etc
Dinitroanilines Trifluralin, Pendimethalin, etc
Cyclohexenones Sethoxydim, Clethodim, etc
Substituted ureas Isoproturon, Chlorotoluron, Diuron, etc
Diphenylethers Fomesafen, Benzofluorfen, Oxyfluorfen, etc
Cyclic imines Oxadiazon, Flumiclorac-penty, etc
Carbamates Benthiocarb, Phenmedipham, Molinate, etc
Organophosphorus Glyphosate, Glufosinate ammonium
Bipyridine Paraquat, Diquat, etc

Herbicides with the same chemical structural class have a general commonality.

Take sulfonylureas as an example:

Their common feature is high activity and extremely low dosage
Wide range of weed control, all varieties can control broadleaf weeds, and some varieties can also control grasses or sedges
Strong selectivity and crop safety
Easy to use, most varieties can be used for soil treatment and stem and leaf treatment
Roots, stems, and leaves of plants can be absorbed and spread rapidly
The mechanism of action is to inhibit the acetolactate synthase (ALS), hindering the synthesis of branched-chain amino acids
Some varieties have longer soil residues, which will affect the next crop
Very low toxicity to humans and animals; easy to develop drug resistance

According to the mode of action

According to the mode of action, herbicides can be divided into more than 20 categories. For ease of memory, letters or Arabic numerals can also be used. The letters are the code pattern of the International Herbicide Resistance Action Committee (HRAC), and the numbers are the code pattern of the Weed Science Society of America (WSSA). The specific classification is shown in Table 2.

Table 2 Classification of Common Herbicides by Mode of Action

Mode of Action
Lipid synthesis inhibition (inh. of ACCase) 1 A
Inhibition of ALS (branched-chain amino acid synthesis) 2 B
Inhibition of photosynthesis (PSII) 5, 6, 7 C
Photosystem I (PS I)  electron diversion 22 D
Inhibition of protoporphyrinogen oxidase 14 E
Inhibition of pigment synthesis (bleaching) / F
Inhibition of PDS / F1
Inhibition of 4-HPPD 27 F2
Unknown target 11, 13 F3
Inhibition of DOXP synthase / F4
Inhibition of EPSP synthase 9 G
Inhibition of glutamine synthetase 10 H
Inhibition of DHP 18 I
Inhibition of microtubule assembly 3 K1
Inhibition of microtubule organization 23 K2
Inhibition of cell division (VLCFA) 15 K3
Inhibition of cellulose synthesis 20, 21 L
Uncoupler of oxidative phosphorylation 24 M
Inhibition of lipid synthesis (not ACCase) 8, 26 N
Synthetic auxin 4 O
Auxin transport inhibition 19 P
Unknown mode of action 17, 25 Z

The herbicides with these mechanisms of action were concentrated in 1930-1990. The earlier ones were synthetic hormones in the 1940s, PS II inhibitors in the 50s-60s, PPO inhibitors in the 60s, glyphosate, and ammonium glyphosate in the 1970s, with a difference of about 5 years. ACCase was born in the 1970s, and ALS and HPPD inhibitors were born in the 1980s.


The importance of recognizing the herbicide class and mode of action

vegetation management

Herbicides are essential tools in modern agriculture, horticulture, and vegetation management. They help control weeds and unwanted vegetation, increase crop yields, enhance aesthetics, and ensure efficient use of resources.

Accurately mastering and understanding the types and modes of action of herbicides is of great guiding significance for the correct selection of resistant weeds.

When weeds are resistant to herbicides with a certain chemical structure or mode of action, herbicides with the same chemical structure or mode of action cannot be effectively controlled. The best solution is to choose compounds with new mechanisms of action. To learn more information about our herbicides, please feel free to contact us at info@hb-p.com.