Download Weed 3: Weed establishment

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
• 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:
1.
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terrestrial plants, many no longer applicable as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
2. 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
• 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
• 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 (nonselective).
• 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
• 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)
• 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)
• 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
• Divided into 2 types: mobile/slow acting – soil applied
herbicides (pre-& post-emergence); and nonmobile/rapid acting – foliar applied herbicides (postemergence). Both systemic.
• Triazines, triazinones, uracils, phenyl-carbamates,
ureas, amides (soil applied) & nitriles, phenylpyridazines, 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
• Consist of two MOA: Protoporphyrinogen IX oxidase
(PPO) inhibitors (selective) and PS I inhibitors (nonselective). Both post-emergence.
• PPO inhibitors: diphenylethers, phenylpyrazoles, Nphenylphthalimides, 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)
• 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 (postemergence).
• Soil half-life ranging from 25-145 days.
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
• 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
• 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*:
1. Emulsifiable concentrate (E or EC)
A concentrated liquid (a.i., petroleum solvents,
emulsifier), oil and water soluble.
2. Water Soluble Concentrate/Solution (WSC/S)
A liquid formulation (a.i., water, sometimes a
surfactant), water soluble.
3. Oil Soluble (OS)
A liquid formulation (a.i. dissolved in oil or some
other organic solvent. Must be applied in an oilbased carrier such as diesel fuel or kerosene.
4. 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.
5. Wettable Powder (W or WP)
A dry powder (a.i., a diluent, and surfactants),
water soluble. Spray tank agitation is necessary
to avoid settling.
6. Soluble Powder (SP)
A dry formulation that contains a high percent
(>50%) a.i, water soluble. Need initial agitation
to dissolve.
7. 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.
8. 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.
9. 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.