AGR 3102 Principles of Weed Science Herbicide

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AGR 3102Principles of Weed ScienceHerbicide

• Muhammad Saiful Ahmad Hamdani

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Unit 6 Topics Covered

• Herbicides
• Classification cont
• Formulation

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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.

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• 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.

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• 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
• organisms
• - Widely used in 1900s for aquatic and woody
• terrestrial plants, many no longer applicable
  as
• herbicides nowadays

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• 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

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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

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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).

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• 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).

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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).

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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.

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• 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.

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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.

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• 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).

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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.

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• 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.

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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.

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• 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).

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8. Pigment Inhibitors (Bleaching Agents)

• Divided into 2 MOA Carotenoids synthesis
  inhibitors HPPD (4-hydroxyphenyl-pyruvate-dioxyg
  enase) 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).

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• 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.

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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.

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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.

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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.

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• 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).

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• 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.

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• 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
  (gt50) a.i, water soluble. Need initial agitation
  to dissolve.

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• 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.