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AGR 3102 Principles of Weed Science Herbicide. Muhammad Saiful Ahmad Hamdani. Unit 6 – Topics Covered. Herbicides: Classification cont… Formulation. Herbicides Classification cont…. F. Chemical Structure Common method of grouping herbicides: herbicide chemistry and MOA

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agr 3102 principles of weed science herbicide

AGR 3102Principles of Weed ScienceHerbicide

Muhammad Saiful Ahmad Hamdani

unit 6 topics covered
Unit 6 – Topics Covered


Classification cont…


herbicides classification cont
Herbicides Classification cont…

F. Chemical Structure

  • Common method of grouping herbicides: herbicide chemistry and MOA
  • Classification based on similarity/dissimilarity of the herbicides chemistry.
  • Herbicides with a common chemistry are grouped into “a family.”
Herbicides in different/same families have the same mode of action, grouped into “similar MOA.”
  • Based on the MOA: classification by a numerical system (WSSA) and alphabetical system (HRAC).
  • Importance: helpful in knowing what groups of weeds are killed, specifying application techniques, diagnosing herbicide injury problems, and minimising herbicide resistant weeds.
2 types of herbicides:
  • Inorganic compounds* - “old school”

- Copper sulphate, ammonium sulphate,

ammonium sulphamate, ammonium nitrate &

ammonium thiocyanate

- Act as desiccant/desiccating agent

- Highly persistent in soil & highly toxic to other


- Widely used in 1900s for aquatic and woody

terrestrial plants, many no longer applicable as

herbicides nowadays

Organic compounds
  • extensively synthesized following 2,4-D

& other PGR commercialization

  • widely used in agric and non-agric areas
  • differ from one to another by their chemical characteristic and MOA
  • In general, divided into several important groups based on MOA:*
1 plant growth regulators pgr synthetic auxins
1. Plant Growth Regulators (PGR) / Synthetic Auxins
  • First commercialized in the 1940s to primarily control dicot weeds in wheat, corn, rice, and many cereal crops.
  • MOA: response similar to those of natural, growth-regulating substances called auxins.
  • Selective systemic, post-emergence: effective on annual & perennial broadleaf plants and usually have no activity on grasses or sedges, except at high application rates.
  • Short soil residual; average half-life of 5 - 14 days.
  • Comprise of phenoxy-carboxylic acid, benzoic acid, pyridin carboxylic acid, quinoline carboxylic acid & phtalamates
2 amino acids biosynthesis inhibitors
2. Amino Acids Biosynthesis Inhibitors
  • Divided into 2 major MOAs: Acetolactate synthase (ALS) inhibitors and 5-Enolpyruvylshikimate 3-phosphate synthase (EPSPS) inhibitors. Post-emergence, systemic.
  • ALS inhibitors effective mostly on annual broadleaves (selective), while EPSPS inhibitors have activity on grasses, sedges, and/or perennial dicot plants (non-selective).
  • Consist of sulfonylureas, imidazolinones, triazolopyrimidines, pyrimidinyl thiobenzoate (ALS) & glycines (EPSPS).
These herbicides cause the shutdown of metabolic activity with eventual death of the plant.
  • Glyphosate – single EPSPS herbicide, for example, is a broad-spectrum herbicide and has activity on all types of plants.
  • ALS herbicides and glyphosate considered among the safest herbicide because ALS & shikimate-pathway exists only in photosynthetic plants and not in animals.
  • Soil residual: half-life varies among herbicides, generally about 2 months – 3 years (long persistence).
3 fatty acids accase biosynthesis inhibitors
3. Fatty Acids (ACCase) Biosynthesis Inhibitors
  • Selective-commonly referred to as graminicides (grass killers), but have no activity on other monocots (i.e. sedges). Also have no activity on dicots. Post-emergence, systemic.
  • They inhibit meristematic activity, stopping growth almost immediately, especially young grass.
  • Comprise of aryloxyphenoxypropionates (APPs), cyclohexanediones (CHDs), and phenylpyrazoline (PPZ).
  • FOPs are sold as esters of their acids. The esters are rapidly absorbed by plant foliage. Once inside the plant, the ester is converted to the acid by carboxyolesterase. The acid is considered to be the phytotoxic form of these herbicides.
  • Soil residual: half-life 3-5 days (DIMs), 17 days (PPZ), 9-60 days (FOPs).
4 seedling growth inhibitors root shoot
4. Seedling Growth Inhibitors (Root & Shoot)
  • 2 types of MOA: microtubule inhibitors and cell wall (cellulose) biosynthesis inhibitors.
  • They inhibit root and shoot development (depending upon where they are absorbed) by interfering with cell division in meristematic areas.
  • Effective only on germinating, small-seeded annual grasses and some broadleaves (seedling selective, systemic, pre-emergence).
  • Have little to no post-emergence activity.
Consist of dinitroanilines, nitriles, pyridines, benzamides, carbamates and benzoic acids (DCPA).
  • Depending upon the herbicide and crop, often incorporated into the soil (esp. dinitroanilines).
  • Have relatively long residual activity (half-life more than a year).
  • Water insoluble and are subject to photo-degradation.
5 seedling growth inhibitors shoot
5. Seedling Growth Inhibitors (Shoot)
  • Divided into 2 MOAs: lipid synthesis inhibitors (not ACCase) & cell division inhibitors (VLCFA synthesis). Applied as pre-emergence/planting.
  • Considered to be seedling shoot growth inhibitors. These herbicides cause abnormal cell development or prevent cell division in germinating seedlings.
  • Seedling-selective & systemic. Most effective on annual grasses and sedges, some can control small-seeded annual broadleaves.
Lipid synthesis inhibitors need soil incorporation, absorbed readily by emerging shoots.
  • Cell division inhibitors absorbed by emerging shoots and roots.
  • No control of established weeds.
  • Have a relatively short persistence in soil. Half-life 4-8 weeks for VLCFA inhibitors and 3-6 weeks for cell division inhibitors.
  • Consist of chloroacetamides, oxyacetamides, acetamides (VLCFA inhibitors) & thiocarbamates, phosphorodithioates, benzofurans (lipid synthesis inhibitors).
6 photosynthesis at ps ii inhibitors
6. Photosynthesis (at PS II) Inhibitors
  • Divided into 2 types: mobile/slow acting – soil applied herbicides (pre-& post-emergence); and non-mobile/rapid acting – foliar applied herbicides (post-emergence). Both systemic.
  • Triazines, triazinones, uracils, phenyl-carbamates, ureas, amides (soil applied) & nitriles, phenyl-pyridazines, benzothiadiazinones (foliar applied) are structurally diverse chemical groups but have similar MOA.
  • These herbicides are effective primarily on annual broadleaves (selective), however at certain rates, some provide control of grasses as well.
Foliar applied; absorbed by leaf, block the photosynthetic process so captured light cannot be used to produce sugars.
  • Soil-applied, these herbicides permit normal seed germination and seedling emergence, but when the seeds’ food supply gone, the seedlings die of starvation.
  • These herbicides are more effective on seedling weeds than on established perennial weeds.
  • Some herbicides under ureas family such as prometon and tebuthiuron are considered soil sterilants.
  • Relatively long persistence. Soil half-life: 60 – 120 days.
7 cell membrane disrupters
7. Cell Membrane Disrupters
  • Consist of two MOA: Protoporphyrinogen IX oxidase (PPO) inhibitors (selective) and PS I inhibitors (non-selective). Both post-emergence.
  • PPO inhibitors: diphenylethers, phenylpyrazoles, N-phenylphthalimides, thiadiazoles, oxadiazoles, pyrimidindiones & triazolinone; PS I inhibitor: bipyridiliums (paraquat & diquat).
  • PPO inhibitors control almost all broadleaves, although some have some activity on grasses.
  • PS I inhibitors provide broad-spectrum control of many different species.
Paraquat: extremely toxic, no antidote so far. 5-10 ml fatal to human.
  • Referred to as contact herbicides & kill weeds by destroying cell membranes.
  • They appear to burn plant tissues or desiccation within hours or days of application.
  • A very good coverage of the plant tissue and bright sunlight are necessary for maximum activity.
  • The activity of these herbicides is delayed in the absence of light.
  • PS I inhibitors are extremely tightly bound to soil colloids, especially clay (no root uptake). Because of binding to soil, bipyridyliums have no phytotoxicity in soil (no/very limited soil activity).
  • Soil half-life for PPO inhibitors: varies among chemical groups. Can be as short as 5 days or long (100-280 days).
8 pigment inhibitors bleaching agents
8. Pigment Inhibitors (Bleaching Agents)
  • Divided into 2 MOA: Carotenoids synthesis inhibitors & HPPD (4-hydroxyphenyl-pyruvate-dioxygenase) inhibitors.
  • These herbicides provide control of many annual broadleaves and some grasses (non-selective, systemic, some pre-, some post-emergence, some both).
  • Consist of pyridazinone, pyridinecarboxamide, triazole, isoxazolidinone, diphenylether (Carotenoids synthesis inhibitors) & triketone, isoxazole, pyrazole (HPPD inhibitors).
Referred to as “bleachers” since they inhibit carotenoids (chlorophyll protector) biosynthesis or the HPPD enzyme by interfering with normal chlorophyll formation.
  • Without carotenoid pigments, the sun damages chlorophyll pigments and the plant becomes “bleached” and dies.
  • Absorbed by roots and shoots (pre-) or leaves (post-emergence).
  • Soil half-life ranging from 25-145 days.
9 phosphorylated amino acid nitrogen metabolism disrupter
9. Phosphorylated Amino Acid (Nitrogen Metabolism) Disrupter
  • Phosphinic acids derived herbicide glufosinate-ammonium. Provides broad-spectrum control. Group H (HRAC) & 10 (WSSA).
  • It affects growth by disrupting nitrogen metabolism (glutamine synthesis), important for amino acids synthesis.
  • Generally considered a contact herbicide (thus, much more effective on annuals than perennial weeds) although has slight translocation (systemic) throughout the plant.
  • Good spray coverage and sunlight are important for maximum efficacy.
  • Little to no soil activity because the herbicide is very rapidly degraded by microorganisms.
10 unknown herbicides
10. Unknown Herbicides
  • Group of herbicide chemicals with unknown MOA, but likely differ from aforementioned herbicides. Basically contact but some have limited apoplastic translocation.
  • Contain arylaminopropionic acid, pyrazolium, organoarsenical & other unknown family.
  • Relatively non-selective, post-emergence, & many are foliar-applied products.
  • DSMA and MSMA have no soil activity and tightly bound to soil, others unknown.
  • Dazomet and metam are considered soil fumigants. These products are applied to the soil and covered with a gas-tight tarp. Kill everything in the soil.
herbicides formulation
Herbicides Formulation
  • The active ingredient (a.i.) =chemical that has phytotoxicity & controls the target weed.
  • Herbicide product rarely made up only of a.i.
  • Often diluted in water or a petroleum solvent, and other chemicals are added before sale.
  • Other chemicals (additives): adjuvants, extenders or diluents (no herbicidal activity) = inert ingredients.
Purpose: make the product easier to mix & apply, increase its efficacy, improve formulation stability & improve shelf-life.
  • Mixture of a.i. and inert ingredients = formulation.
  • Herbicides are sold in various formulations, depending upon:

(1) the solubility of the active ingredient in water

(2) the manner in which the product is applied (i.e.

dispersed in water or applied in the dry form).

Several available herbicide formulations and abbreviations*:
  • Emulsifiable concentrate (E or EC)

A concentrated liquid (a.i., petroleum solvents, emulsifier), oil and water soluble.

  • Water Soluble Concentrate/Solution (WSC/S)

A liquid formulation (a.i., water, sometimes a surfactant), water soluble.

  • Oil Soluble (OS)

A liquid formulation (a.i. dissolved in oil or some other organic solvent. Must be applied in an oil-based carrier such as diesel fuel or kerosene.

Liquid Flowable (F or LF)

A thick slurry-like liquid (a.i., water, and stabilizers), water soluble. Spray tank agitation is necessary to avoid settling.

  • Wettable Powder (W or WP)

A dry powder (a.i., a diluent, and surfactants), water soluble. Spray tank agitation is necessary to avoid settling.

  • Soluble Powder (SP)

A dry formulation that contains a high percent (>50%) a.i, water soluble. Need initial agitation to dissolve.

Water-Dispersible Granules (WDG) & Dry Flowable (DF)

A dry formulation, water soluble. Easier to handle and measure than wettable powders. Spray tank agitation is necessary to avoid settling.

  • Granules (G)

A dry formulation (a.i., coated or adhered to some type of inert granule). Applied just as they are purchased with no mixing. Special granular spreader is required.

  • Pellets (P)

A dry formulation - similar to granules only much larger. Pellets are applied directly to the target area by hand or with special spreaders.