Final Exam Flashcards
Triazine Injury on Soybeans
Group 14 PPO Inhibitors
Post Emergence Herbicides
Aryltriazinone
carfentazone → Aim
flumiclorac → Resource
fluthiacet → Cadet
Diphenylether
Acifluorfen → Blazer
Fomesafen → Flexstar, Reflex
Sharpen (Group 14)
Used for broadleaf weeds (pigweed, waterhemp, wild buckwheat)
Needs to have 0.5” water to activate
Tank-mixed with many herbicides
Triazines
Applied either as a soil (preemergence) or to the plant (post emergence)
If applied post then surfactants are normally added
Atrazine Herbicide Activity
Soil Applied
Herbicide translocated in the apoplast (xylem) and moves with transpiring water to the oldest leaves
Leaf Applied
Herbicide moves with water to the leaf margins
- must have good coverage to get good control
- Does not move to the youngest tissue
Site of Action for ALS Herbicides
Binds to and inhibit the enzyme acetolactate synthase (ALS enzyme)
Mode of action for ALS Herbicides
Blocks the formation of branched chain amino acids
- Leucine
- Isoleucine
- Valine
This leads to stoppage of protein synthesis
ALS Symptoms
Purpling of soybean leaf veins
Symptoms of ALS Herbicides
Slow to develop
May see reddish - purple shoots
(this may be due to variety or nutrient deficiency)
Stunting 10 days or more after application
Pinched corn ears if applied after labeled growth stage
Chlorosis, Necrosis
Bottle brush roots
ALS Inhibitor Sympton
Bottle Brush Roots on Corn
Prerequisities for Effective Herbicide Use
Must come in Contact with the Plant
- Has to Remain on the Surface of Seed or Leaf Long Enough to Penetrate
- Surfactants to Hold Chemical on the Leaf
- Rainfastness or Rain-free Period (30 minutes to 4 hours)
Must Reach the Site where it Disrupts the Vital Process or Structure
- A photosynthetic Inhibitor does no good in the Root
High Enough Concentration or Long Enough to Injure the Plant
- Reduced Rates may not have enough on Plant
- Rainfastness
- Soil Chemicals Leached to Levels below Optimum Rate
Methods of Classification
Time and Method of Application
Soil Persistence
Formulation
Herbicidal Activity
Herbicide Chemistry
Time of Application
Preplant (pp)
- Treatment before the crop is planted
- Need water to get herbicide into seed zone
Preplant Incorporated (ppi)
- Applied propr to Planting and Worked into the Soil
- Normally done because the Herbicide is Photodecomposed or Volatile
PP and PPI do not Interfere with Planting
PPI may not fit in a Minimum or No-till Operation
Time of Application
Preemergernce
Treatment made before Emergence of the Crop or Weed
Usually done at planting
Eliminates Early Cultivation
Treatment done without seeing the problem
- Unless it’s a burndown
Subject to Environmental Conditions, Soil Influences
Post-Emergence Application
Made after the emergence of crop or weed
If after weed emergence but before crop emergence (burndown)
- Can see the Problems
- If herbicides have ONLY foliar activity and no soil residual crop does not need to be tolerant to the herbicide
- Could use nonselective herbicides (e.g. Roundup, Liberty, paraquat)
- Apply when weeds are small (less herbicide needed)
- Done before critical weed free period (yield loss stopped)
If AFTER Crop Emergence
- Crop must be tolerant and at the correct stage of growth
Time of Application
Layby
Treatment made with or just after last culitivation
May have passes critical weed-free period
Cleans the field for harvest
- Rely on crop competition to keep any other weeds in check
Time of Application
Post Harvest
Done in Late Fall after Light Frost but before Killing Freeze
Controls Winter Annuals
Used to Give Better Controls for Perennials
- Carbohydrates move into roots and herbicide transported there also
Coverage of Target Area
Broadcast
- Applied to Entire Field
Band
- Applied only over crop row (more of a precision ag part)
Spot
- Localized application to scattered plants
Directed Sprays
- Treatment to weed but miss crop
(using drop nozzles that goes between the plants)
Foliar
Soil
- Surface/Incorporated or Knife
Type of Formulations
Water Soluble
Oil Soluble
Emulsifiable Concentrate
Ultra-Low Volume Concentrate (ULV)
Liquid Flowable or Slurry Type
Dry Flowable
Water Dispersible Granule (WDG)
Granules (G)
Soil Persistence
Persistent
When Applied at Recommended Rate will Harm Susceptible Crops in Normal Rotation (atrazine, picloarm)
Long Residual
Herbicide used as Soil Sterilants to Control for > 1 season
(use it around oil tankers/train tracks/pipeline)
Problems of Persistence Soil Herbicides
Crop Failure Neccesitates Replanting
Susceptible Crop Follow Short Season Crop within the Same Growing Season (sunflower,winter wheat)
Suscpetible Crop Follows year after Persistent Herbicide Applied (Trifluralin on Spring Wheat)
Decomposition is slow due to weather or soil conditions
Herbicide Formulation
Herbicidal prepartion designed for pratical use by the manufacturer for the grower
Herbicide Formulation
Made Up Of
Active Ingredients (AI) the part that is phytotoxic
Carrier - Serves to dilute the active ingredient (water,oil,clay)
Surfactants - to spread the herbicide on the folidge
Stickers
Other inert ingredients
Type of Formulations
Water Soluble (S or WS)
Active Ingredient totally water soluble (salts of acids like glyphosate, 2,4-D)
Easy to mix
DO NOT PENETRATE into plant weel
Must add a stickers to help get the herbicide into the plant
Type of Formulation
Emulsifiable Concentrate (E or EC)
Active Ingredient not water soluble, soluble in nonpolar solvents
Can penetrate the cuticle of the plant easier than water soluble
Problems with drift at high (> 80F) temperatures
Plant wax is nonpolar making the herbicide easy to get into the plant
Types of Adjuvants
Spray Modifiers (Type 1)
Dispersing agents
(Enhances the dispersal of a power in a solid-liquid suspension)
Thickener
(used to reduce the number of fine droplets from a nozzle)
Type of Adjuvants
Utility Modifiers (Type 2)
Compatibility Agents
- Used to keep different types of formulations in suspension
Antifoam Agents
- To reduce foaming in the tank
Emulsifiers
- Material that aids in the suspension of fine drops of one liquid in another, like oil in water
Degree of Response
Selective Herbicide
Only some plants of a mixed population are injured
Degree of Response
Nonselective Herbicide
All plants of a mixed population are killed (paraquat and Roundup)
Herbicide Movement in the Plant
Contact
No movement in plant
Kills only the tissue that is treated
Complete coverage of the plant is needed to get good control (best applied when plant is small)
Examples
- aciflorfen (Blazer) (soybean)
- bentazon (Basagran) (soybean & corn)
- paraquat (Paraquat) (orchards & other places)
Herbicide Movement in Plants
Translocated Herbicides
Moves in the plant from the site of application to other plant parts
Two Types of Movement
- Symplastic (phloem)
Moves with the sugars/typically moves to the newest areas of the plant
- Apoplastic (xylem)
Water moves in the xylem/going to the oldest tissue in the plant/applying to leaf it moves to the outer edges of the leaf
Symplastic Movement
Herbicide moves in the phloem
Moved with sugars
Moved to the YOUNGEST tissue were demand for energy greatest
Injury seen in youngest leaves and in the youngest roots
Good for control of perennials
Apoplastic Movement
Herbicides that move in the apoplast move in the xylem
Move with Water
- Move to the oldest, most actively transpiring tissue
If taken up by roots move to the oldest leaves
If taken up by leaves stays in the treated leaf
Apoplastically Translocated Herbicides Examples
Trizines (atrazine, metribuzin)
Uracils (diuron, monuron)
Herbicide Mixtures
Often done to control many types of weeds (mix broadleaf and grass herbicides)
To have the operation be ‘one pass’ (to lessen application costs)
To get synergistic responses (better kill)
To lessen the costs of the ‘expensive’ herbicide by adding a cheaper tank mix partner
Type of Mixture
Premix
Sold by the Manufacturer
- These already have what the manufacturer thinks is the best combination of ingredients
- A premix may have similar active ingredients at different concentrations and be marketed under different names
Examples
Storm (29% bentazon + 18% acifluorfen) vs Galaxy (33% bentazon + 7% acifluorfen)
Type of Mixture
Tank-Mix
Mixed in the field not as the formulation
- Caution must be taken
- Incompatible Formulations
The solution turns into sludge
Having AI precipitate out of solution
Mixing chemicals bound to clay with chemicals that are adsorbed by clay
(i.e. glyphosate and DF formulations)
Combining antagonistic chemicals (get no or little plant response)
Getting too high a response (Burn crop or residual lasts too long)
Not understanding all the ingredients already in a formulation
When mixed there can be 4 types of responses that occur
Responses to Herbicide Mixtures
Additive Response
When the response of a plant to each herbicide alone or in the mix is the same
Responses to Herbicide Mixtures
Syngeristic Response
When mixing 2 chemicals get a greater response than either chemical alone
- This can be good if get better weed control i.e. mix a some 2,4-D (cheap) with Tordon (expensive) and get a more phytotoxicity than either alone
- Bad if get more crop injury
Responses to Herbicide Mixtures
Antagonistic Response
When the mixture of 2 or more herbicides results in less than expected response (i.e. less weed control) than when any of them are used separately
- If you mix 2,4-D with some wild oat herbicides get no wild oat control
Graph Info
To get the same response as if the two chemicals were used alone, 1.2 to 1.5 times as much of either herbicide needes to be applied in the mix
Responses to Herbicide Mixtures
Enhancement
Mix a nonphytotoxic adjuvant (i.e. crop oil or other) with a herbicide and get a response that is greater than the herbicide is used alone
- Phytotoxicity increased usually results from increased leaf absorption
- May reduce crop selectivity (may get more injury such as burned leaves or stunted plants)
Mechanism of Action
Precise biochemical reaction that creates the herbicide’s ultimate effect
Herbicide Mode of Action
Sequence of events that occur from the herbicides contact with the plant until its final effect is expressed
Site of Action
Precise enzyme or target area affected by the herbicide
Mechanisms and Mode of Action
Auxin-type Growth Regulators
2,4-D
Dicamba
Picloram
Mechanisms and Modes of Action
Photosynthesis Inhibitors
Triazines
Mechanisms and Modes of Action
Disruption of Cell Permeability
Paraquat
Mechanisms and Modes of Action
Disruption of Cell Division (Mitosis)
Trifluralin
Mechanism and Mode of Action
Root and Shoot Inhibitors
Alachlor
Acetochlor
Mechansims and Mode of Action
Pigment Inhibitors
Clomozone
Balance
Mechanism and Mode of Action
Block Amino Acid Formation
Glyphosate (ESPS)
Immidiazilinone and Sulfonyl Ureas (ALS)
Mechanism and Mode of Action
Other
Aryloxy phenoxy
(stops long chain fatty acid syn)
Why Soil Applied
Herbicides not absorbed by foliage
- only enters plants through roots
May be too volatile
May be photodecomposed and lost if not soil applied
- Incorporation is needed
May have low water solubility
May only be root active and not translocated
Advantages of Soil Applied
Control weeds at very small growth stage
Not as much herbicide needed to kill weeds
Dont get much crop competition
Disadvantages of Soil Applied
Dont know which weeds will be the problem
Dont know where problem areas are
Some herbicides become environmental problems
Application Type
Soil Pre
Herbicides Applied to Soil (PRE Applications)
- Meant to have residual effect (if not burndown)
- Control germinating seedlings (if seedlings have emerged, some have no activity)
- More environmental problems with soil applied herbicides
- Some metabolites of the chemical remain toxic
Atrazine to deethylatrazine
Balance (isoxaflutole) (1/2 life 4 days) to DKN metabolite (highly active) (1/2 life 3 weeks plus) low sorption of each
Preemergence Herbicide/Soil/Plant Interaction
Must come in contact with developing plant
- Depth of application must be correct or need to leach to the zone needed
- Dormant structures are not affected
Herbicide must be there in high enough concentration
- Reasons why the concentration may be low even if applied at the correct amount
Degradation (microbial or chemical)
Binding to soil (sorption decreases amount available to plant)
Herbicide Uptake from Soil
Soil Water Must be Present
- Needed for seed germination, needed for herbicide uptake
Too Dry
- Photodecomposition
- Volatilization
- No Seed Germination
Too Wet
- Leach from Seed Zone
- Runoff
- Microbial Degradation
Chemical Fate in the Environment
Plant Uptake
1-10% of Applied Herbicide
Taken up either as
Toxic chemical or Nontoxic metabolite
Chemical Fate in the Environment
Fate in the Air
0 to 30% of Applied
Sprayed On - no movement (ideal and usually not the case)
Drift as spray droplets to nontarget areas (0 to 30% of applied)
Volatilization (vapor drift) with movement in the air deposition with rain and snow
Photodecomposition by UV light
Chemical Fate in the Environment
Fate in Soil
50-100% of applied
Sprayed On - lasts long enough to kill the target pest - no movement (ideal)
Remains where it’s at is nontoxic due to rapid breakdown (e.g. 2,4-D) or very high sorption (e.g. Roundup)
Remains where it’s at and continues to be toxic
Remains where it’s at degrades to another toxic product
Movement to offsite areas as parent or metabolite (can be toxic or nontoxic)
Mechanisms for movement (< 1% of applied but still detectable)
- Runoff
- Leaching
- Erosion
Herbicide Uptake
Herbicide moves in soil to the point of contact
- with water (mass flow)
- as a gas (diffusion)
Herbicide DOES NOT move
- Roots or shoots grow into and through the treated area (interception)
Note: Some herbicides cannot be taken by roots, some cannot be taken up by shoots
- Can use these properties to design where the chemical is placed to control the weeds of interest and not injure the crop
Routes of Herbicide Entry for Root Uptake
3 Major Routes
All Passive (through nonliving tissue)
All Active (through cells and plasmodesmata)
Mixture of Passive and Active
Routes of Herbicide Entry for Root Uptake
All Passive
(Through nonliving tissue) (e.g. cell walls)
Entry with water and movement in nonliving tissue (cell walls) and xylem
Routes of Herbicide Entry for Root Uptake
All Active
Through Cells and Plasmodesmata
- Requires energy, moves in cell and symplast
- If phloem only - the herbicide moves to the root actively growing cells, usually the root tip
Casparian Strip
Major barrier to herbicide movement
- reason for why some herbicides are not taken up
Herbicide Uptake
Underground Shoot Adsorption (Thiocarbamates)
- Coleoptile node of grass seedlings is the entry point
- This is part of the leaf so there is no Casparian Strip
- Hypocotyl and shoot of broadleaves are other entry points for herbicide
Soil Applied Herbicides
Shoot Inhibitors
Group 8
- Thiocarbamates
- Lipid Synthesis, not ACCase
Gropu 15
- Acid Amides also called acetamides
- Inhibits very long fatty acid synthesis
Soil Applied Herbicides
Root Inhibitors by Microtubule Disruption
Group 3
- Dinitroanilines
Soil Applied Herbicides
Bleaching Herbicides
Group 27
- HPPD Herbicides
- 3 Families
- Inhibits plastiquinone biosynthesis
Group 13
- DOXP Herbicides
- Inhibits isoprenoid synthesis
Shoot Inhitors WSSA Group 8
Thiocarbamates
EPTC (Eptam) & butylate
- Does not rely on post-application rainfall (but have to be incorporated due to high volatility)
- Less loss from dry soil than from moist soil
- Underground shoot absorption
- Entry Point - coleoptile node of grass seedlings and hypocotyl hook of broadleaf
- No problems with crop rotation restrictions
- Only translocated in the xylem
Shoot Inhibitors WSSA Group 8
Inhibit Lipid Biosynthesis
Grasses may not emerge
Reduces cuticular wax formation
Leaves tightly rolled and do not unroll properly
Broadleaf plants may have small leaves or ‘bud seal’
Roots may be brittle, short and thick
WSSA Group 15
Acid Amides (acetanilides) and Isoxazolinone
Shoot Inhibitors
- Stop very long chain fatty acid formation (VLCFA herbicides)
Drawstring (heart shaped) leaves
Acid amide injury to soybean
Symptoms of EPTC, butylate (Group 8) and acide (Group 15) Damage to Corn
Injury
Improper leaf unfurling
Twisted whorls
Buggy whipping
Conditions
Misapplication
Heavy rains soon after herbicide application that may leach away the herbicide safener
Stressed conditions (cool, wet soils)
Certain hybrids may be sensitive
Site and Mode of Action (Group 15)
Site of Action: Unknown
Mode of Action: Inhibits synthesis of very-long-chain fatty acids in the growing shoots
- membrane disruption
- Stops growth
- Elongase inhibition and inhibition of geranylgeranyl pyrophosphate cyclisation enzymes, part of the gibberellin pathway
Acid amide (Group 15) Symptoms
No emergence of susceptible weeds (mostly grasses)
Crops may be injured (cold, wet conditions; high application rate; improper placement)
Sorghum is sensitive to injury
Symptoms Include
Drawstring effect on broadleaf crop leaves
- Also known as heart-shaped leaves
May have buggy whipping on corn
Acid amide damage to Soybean
Acetamide Damage to Corn
Flufenacet (Define, Axiom)
Applied Preplant, PPI, Pre
Used in corn, soybean, wheat, sunflower
In corn and soybean mixed with metribuzin anad sold as AXIOM
Maybe applied up to 14 days before planting, 10 gpa carrier minimum
Used for grass and small seeded broadleaf control
Mode of Action is similar to acetochlor and metolachlor
- Stops very long chain fatty acid synthesis, so membranes are disrupted
Outlook - dimethamid
Used as preemergence or early postemergence (to the crop) but preemergences to weeds
Can be impregnated on dry fertilizer
Grass control and also some broadleaf control
- amaranth
- purslane
- ragweed
Pyroxasulfone (Zidua)
Used in corn and soybean
- 1-3.5 oz /ac depending on soil type
- Needs 1/2 inch of rain to active herbicide
- Minimum carrier volume is 5 gpa
Can be fall applied, early preplant, ppi, pre or early post
Grasses and Sedges
Dinitroanilines (WSSA Group 3)
Root Inhibitors
Trifluralin
Pendimethalin
Dinitroaniline Herbicides (WSSA Group 3)
Root Inhibitors
Used a dye intermediates
All highly colored (orange, yellow)
Some have fungicidal activity
Taken up by germinating seedlings
Dinitroaniline Family (WSSA Group 3)
Soil Applied Herbicide
Pendimethalin
- Applied pre to corn → DO NOT incorporate
Treflan
- If applied to corn, apply after corn is 8” tall and incorporate
- DO NOT apply preplant or preemergence as injury can be severe
Site of Action of Dinitroaniline Herbicides
binds to a-tubulin
- Mutations at certain points in this protein do not allow for binding and result in a resistant plant
Mode of Action Dinitroaniline Herbicides
Stops microtubules from assembling
Dinitroaniline Herbicide Symptoms
Stunted Plants
Roots Short and Thick
Adsorption and Translocation
Adsorbed by roots and shoots
Vapors can be adsorbed and kill plant
Concentrates in
- Areas high in lipid and proteins
- If very hihg lipid content, binds herbicde and no injury
Effects on Growth and Development
Roots
- Inhibits lateral and secondary roots
- Swelling of root tips and cell enlarge
Shoots → Dicots
- Decreased elongation, stunting
- Leathery appearance of cotyledons
- May be very dark green due to increasd chlorophyll
Bleaching Herbicides
Inhibit 4 HPPD (3 Families) (WSSA Group 27)
Isoxazole → isoxaflutole (Balance); pyrasulfotole (Huskie)
Triketone → mesotrione (Callisto)
Pyrazolone → topramezone (Impact)
Inhibit DOXP Synthase (WSSA Group 13)
Isoxazolidinone Family → Clomazone (Command)
Isoxaflutole (Balance Flexx)
Used pre or early preplant
Corn, Sugar Cane
Use for broadleaf weeds - excellent control of velvetleaf, control of ragweed, pigweed and some grasses (foxtails, barnyard grass)
HPPD Inhibitors (Group 27)
Site of Action
Inhibits p-hydroxyphenyl pryuvate dioxygenase (HPPD)
HPPD Inhibitors (Group 27)
Mode of Action
Stops plastoquinone biosynthesis which leads to inhibition of cartenoid synthesis
What are Carotenoids
Constituents (tetraterpenoid orangic molecules) of the plants that add pigment (color) to the leaf
What do Carotenoids do
Harvest Energy (i.e. Chlorophyll-photosynthesis)
- Taking in the light energy & splitting water (ATP)
Protect the leaf from free radicles and oxidative stress
(xanthophylls and carotenes)
Balance Flexx (Group 27)
Plants are bleached white
Tolerance of plants is due to rapid metabolism of the herbicide
(herbicide is broken into smaller molecules)
Degraded in soil by chemical and microbial means
Some problems with carryover seen in the cool Midwestern soils and leaching
Balance Flexx Injury Symptoms
White Tissue (happens first)
Poor Emergence
Stunted Plants
Growing point dies
Mesotrione (Callisto)
Used pre or post in corn
Used for control of Broadleaf weeds, cocklebur, velvetleaf, lambsquarters and some grasses
½ Life in Soil → 5 to 15 days
Same site and mode of action as Balance Flexx
- Inhibits HPPD, carotenoid biosynthesis, plants are bleached
Soil Applied Herbicides - Bleaching Type (Group 13)
Site of Action
DOXP Synthase (1-deoxy-D-xylulose-5-phosphate synthatase) that is a key component of isoprenoid synthesis
- Different site of action than Balance Flexx adn Callisto
Soil Applied Herbicides - Bleaching Type (Group 13)
Mode of Action
Similar to HPPD herbicides; stops carotenoid synthesis (pigment inhibitor)
- Same symptoms as HPPD herbicides
- Decreased carotenoid synthesis and bleached plants
Clomazone (Command) Group 13
Used in soybean and processing pumpkings (ornamental pumpkings are sensitive)
Soybean not injured the plant metabolizes the herbicide
Controls annual grasses and some broadleaf weeds such as velvetleaf
Problems with drift due to volatization problems from wet or moist soils
Post Emergent Applications
More Problems with Drift and Aerial Transport
Wind Speed
- Higher speed, more drift
Inversion Conditions
- Inversion more movement close to ground
Distance to the nontarget area
- Further the distance to offset areas (large buffer zone) less risk of contamination
Droplet size
Smaller droplets - further drift
Herbicide Uptake in the Foliage Depends on
Spray Timing (plant size/cover)
Formulation and Additives
Application Equipment (droplet size)
Plant properties (hairs, smooth, waxy)
Time to rain (rainfastness)
Foliar Uptake of Herbicides
Uptake more difficult than through root
Factors affection foliar sorption
Retention
- Dependent on carrier
Water “falls” off leaf - lack of wetting and spreading
Granules - Bounce off
To Help Retian Herbicide on the Leaf
Use Oils or Surfactants
Help water spread and adhere to leaf
Reduce surface water tension
Allow closer contact of the herbicide with the leaf surface
Help minimize runoff (help water stick to the leaf)
Foliar Retention
Dependent on Drying Time
- If dries too quickly get residues than will not enter plant
- If the herbicide does not dry quickly enough may get washed of by rainfall
Barriers to Herbicide Absorption by the Leaf
Cuticle
Cell Wall
Plasmalemma
Herbicide Movement through the Cuticle
By diffusion (passive process from high to low concentration)
Moves through
- Intermolecular Spaces
- Pectin Spaces (if water soluble)
- Embedded Waxes (if nonpolar)
Depends on the Chemistry of the Herbicide
Nonpolar Molecules (like EC formulations)
Can easily move through wax but dont move through water well
Polar Molecules (like amine formulations)
Cannot move through wax but once in the leaf can move in polar (pectin or water filled spaces
Bariers to Herbicide Absorption by the Leaf
Cell Wall
Made up of Cellulose Strands
Interspaces fill with water
Water soluble herbicides penetrate the cell water easier than EC type herbicides
Movement in the cell wall is by diffusion
Barriers to Herbicide Absorption by the Leaf
Plasmalemma
Outer membrane of the living cell that encloses cytoplasm
For most herbicides penetration into the cell is by active transport cell has to expend energy
If the herbicide does not get through the plasmalemma and into the cell - usually is ineffective
Soil and Foliar Applied Herbicides
PPO Inhibitors (Group 14)
Photosynthetic Inhibitors → Triazines (Group 5)
Meristem Inhibitors (ALS Enzyme Inhibitors) → IMI and SU (Group 2)
Inhibition of protoporphyrinogen oxidase herbicides
(PPO or Protox Inhibitors) Group 14
Mode of Action
Cell Membrane Disruption
Inhibition of protoporphyrinogen oxidase herbicides
(PPO or Protox Inhibitors) Group 14
Site of Action
Inhibiton of protoporphyrinogen oxidase (Protox or PPO Inhibitors)
Saflufenacil (Kixor) - Mechanism of Action 14
Sharpen (other applied post)
PPO is an enzyme of chlorophyll and heme biosynthesis
Inhibition of PPO leads to accumulation of a chlorophyll precursor (PPIX)
PPIX then absorbs light and causes single oxgen to form
Leads to membrane breakdown via lipid peroxidation
Saflufenacil (Kixor)
Used for broadleaf weeds (pigweed, waterhemp, wild buckwheat)
Applied at 2 oz/ac in coarse (sandy) soils up to 3 oz/ac in fine (clay) soils
Tank-mixed with many herbicides
Sulfonylurea Herbicides
Active at low rates
- 1/3 to ½ oz per acre (do not overapply)
Specific Herbicide Dictates
- Crop (corn, soybean, wheat, sugarbeet)
- Weed Spectrum (some best on broadleaves, some control grasses and broadleaf weeds)
Resistant Weed Problems
Resistant weed populations built up in as little as 2 cycles of use
Some weeds including kochia and prickly lettuce which were very well controlled after one application showed resistance after 2 or 3 applications
Use some of ALS type herbicide once every 48 months
Carryover Problems
Carryover (even if applied at ½ oz rate) to sensitive crops several years after application
Longer carryover if soil pH is 8 or greater
Example
Red River Valley North Dakota
Clorsulfuron Uses
Used for broadleaf weed control in wheat, barlet
Applied pre or post up to 2”
Problems with carryover, resistant weeds
Active at low rates
Clorsulfuron Soil Activity
½ life 4 to 6 weed if pH < 7
½ life up to 3 years > 7
- In high pH soils, problems with rotational crops such as sugar beets, sunflowers, lentils, chlorsulfuron at <1 ppb can cause problems
Breakdown is through hydrolysis at urea group and occurs much faster in low pH (7 or lower) soils
Not bound to soil - leaching and carryover problems
Uses of Classic
Annual broadleaf control in soybean
Used pre or pst
Used at higher rate than Glean (.5 to .75 oz/ac)
Antagonism if applied with post grass herbicide
Imazaquin (Scepter)
Used pre and post for broadleaf control in soybean
Use rate about 2 oz a.e./ac
½ Life
- Long persistence in some environments
- 1 year restriction on corn following application
Imazethapyr - Pursuit
Used in legume crops for broadleaf and grass control at 4 to 6 oz rate
- PPI
- Pre
- Post
Clearfield corn
Selected for resistance (nonGMO) and allows for Pursuit/Lightning to be used for control
Clearfield Rice/Canola/Sunflowers
Imazethapyr - Pursuit
Used for common cocklebur, wild buckwheat, NonALS kochia, Lambsquarters, barnyardgrass, foxtails
½ life 4 months but can be longer
IMI resistant corn (Clearfield or Lightning) developed for that reason
Problems with resistant weeds and cross resistnace to SU herbicides
Cloransulam (FirstRate) Uses
Used in soybean
Pre or Early Post
Used for broadleaf weed control - ragweed (common and giant), marestail, velvetleaf
Can be mixed with Roundup in RR soybean and other broadleaf and grass herbicides
Use rates .3 oz product/ac - post and .6-.75 oz product/ac pre
Flumetsulan (Python)
Broadleaf control in corn and soybean
Only applied preemergence (preplant, ppi or pre)
Restrictions on some rotational crops
- Sweet corn and sunflower - 18 months
- 26 months cannola, sugar beets
Foliar Applied Herbicides - Act like the Growth Regulator AUXIN
Group 4
Phenoxy (Aromatic carboxylic) acids (2,4-D; MCPA; 2,4-DB)
Benzoic Acids (Banvel, Clarity)
Picolinic Acids or Pyidines (Tordon, Milestone, Quelex halauxifen for wheat)
What is Auxin
Naturally occuring plant growth hormone (in form of IAA)
- Move from site of production to site of aciton
Effects on Plant Mechanims
Increased respiration due to uncoupling of oxidative phosphorylation
(ATP is no longer produced from ADP in the mitochondria)
Decreased photosynthesis
In plant roots inhibits
- Elongation of cells (plants have short stubby roots)
- Decreased water uptake by roots
- Leads to decreased transpiration (leaves get hot)
- Stomatal closure (no carbon dioxide enters)
- Even lower photosynthesis
Translocation of All Auxin Herbicides
Translocated (moved in the plant) in the symplast (living tissue of the plant; moved in the phloem)
Moves to actively growing shoot and root tips
- Shoots elongate, get epinasty (twisting and bending of stems due to ethylene production)
- Vascular tissue destroyed due to unchecked cell division
- Roots stop growing, get stubby roots
- If applied when flower initials are forming will cause infertility problems or malformed flowers
Selectivity of Auxin Herbicides
Selective Herbicides (does not control all weed types)
Control broadleaves weeds the best
- Although poor control of weeds in Polygonum (smartweed) family
Grasses
- May be killed if applied at seedling stage
- Injury occurs if applied at a susceptible stage (tillering and boot of cereals)
2,4-D was the first high potency herbicide (use rate 1 to 4 lb/ac)
Uses of 2,4-D
Used to control
Broadleaf Weeds in
- Corn
- Wheat
- Pastures
- Noncropland
Will kill or injure grasses if appliked at critical times
Tank-mix partner with many herbicides
Visual Effects of 2,4-D
Epinasty
- Bending of stems due to differentinal growth rates of elongating regions
Thickening of stems and leaves
Cupping and Twisting of stem
Brace root proliferation (corn plants)
Timing is key on grass plants to avoid injury (malformed roots, sterile flowers)
Dicamba Uses
Broadleaf control in grass (grain, corn)
- does not control mustard
Pre emergence applications can control some grasses
Drift and Volatility Problems
- Soybean very sensitive to dicamba
- Injury can occur at rates 50 x lower than 2,4-D
If applied too late to corn, cracking and brittle stems
Dicamba Herbicidal Activity
Absorbed by Foliage (some root uptake)
Translocated in phloems (some in xylem)
Accumulates in meristems
Can be exuded into soil and taken up by other plants
Acts like 2,4-D (alters nucleic acid synthesis, etc.)
Tolerant plants metabolize dicamba rapidly
Dicamba Soil Activity
Not adsorbed by soil
Can leach readily into and through root zone
Can be a problem if need to replant less tolerant crop
Picolinic Acids
Act like Auxin
Highly toxic - 100 times more active than 2,4-D
Used for broadleaf weeds in grass, pasture, range, forest
Adsorbed through roots and foliage
Damage looks like 2,4-D
Picolinic Formulation
Acid not water soluble so derivatized to:
- Na Salt
- Isopropyl amine salt
Premixed only with
- 2,4-D
Picolinic Uses
Broadleaf weed control in grainland, fallow, noncrop, forest, range, pasture
- Does not control mustards
- Do not rotate to broadleaf crops
Translocated mostly symplastically
Plant growth reponse similar to 2,4-D
Not metabolized in plants
Clopyralid (Stinger)
Known for Canada thistle control
Also used for other broadleaf weed control in
- Sugar beets
- Corn
- Wheat, Barley, Oats
- Rangeland, Pasture, Noncropland, CRP
Readily translocated in the phloem
Auxin Inhibitor - Distinct
Mode of Action
Inhibits polar transport of IAA
Auxin Inhibitor - Distinct
Site of Action
Abnormal accumulation of IAA which causes imbalances in shoots and roots and inhibits growth
Auxin Inhibitors - Distinct
Symptoms
Sensitive broadleaf weeds exhibit rapid and severe plant hormonal effects (e.g. epinasty) after application of Distinct
Symptoms are visible within hours
Plant death usually occurs within a few days
Corn tolerant becasue of rapid metabolism
Symplastically Translocated
Herbicides that look like Amino Acids
Glyphosate (Roundup)
Glufosinate (Liberty)
Glyphosate Site of Action
Enol pyruvyl shikimic acid phosphatase (EPSP) enzyme
Glyphosate Mode of Action
Inhibits aromatic amino acid synthesis
Amino acids not formed
- tyrosine
- phenylalnine
- tryptophane
This leads to problems with protein synthesis
Glyphosate Soil Activity
Very strong adsorptoin to soil
- Binds with Fe, Ca, Al
- Compounds not taken up unless in solution
Very little leaching
No carryover
Microbial degradation is major means of degradation in soil
Glyphosate Herbicide Activity
Absorbed by foliage
Translocated to meristems and root buds in the phloem
Visual Effects
- Sometimes deep purpling (wild buckwheat, corn)
~ Stunting
~ Chlorosis (yellowing of tissue)
~ Wilting
~ Tissue Necrosis (dead tissue)
- Seen after
~ 5 to 15 days depending on annual/perennial
Problems with Glyphosate Application
Do not combine with
- Wettable powders or dry flowable (binds irreversibly to the clay particles)
- Hard or dirty water (binds with iron, calcium and soil)
Maybe washed off by rain and have no activity
- Some formulations are better sorbed and have more rainfastness than others
- Use with surfactants and at times fertilizers to increase uptake (Use AMS not UAN)
Glufosinate
Amino acid Herbicide
Not water soluble - formulated as an ammonium salt (described as a.e.)
Nonselective Herbicide
- Kills grasses and broadleaf weeds
- Used post emergence only
- No soil activity
- Works best when the growing point of corn is emerged from soil (30” corn)
Liberty and Ignite are not easily taken up
Glufosinate Site of Action
Glutamine synthase an enzyme that converts glutamate + ammonia to glutamine
Glufosinate Mode of Action
When glutamine synthase is stopped toxic accumulations of ammonia are found in plant tissue
Glufosinate Symptoms
Chlorsis (yellowing of tissue) and Wilting
3 to 5 days after application
Necrosis (dead tissue)
1 to 2 weeks
Lipid Synthesis Inhibitors (ACCase Enzyme Inhibitors)
Aryloxyphenoxies (‘fops’, ‘props’ and ‘dops’)
Cyclohexanediones (‘dims’)
Pinoxaden (‘dens’)
Aryloxyphenoxy Site of Action
Blocks the ACCase (acetyl CoA carboxylase) enzyme
Aryloxyphenoxy Mode of Action
Stops acetyle CoA from going to malonyl CoA, the first step of fatty acid synthesis; inhibition of long chain fatty acid synthesis
Long Chain Fatty Acids needed for cell membranes
- No membranes, no cell integrity
- No new growth
- Plant death
Aryloxyphenoxy Symptoms
Grass Herbicides
Symptoms
- Root and shoot growth slow soon after application (seen as stunting)
- Purpling of Steam
- ‘Mushy’ growing point
Diclofop
Used for post-emergence grass control (wild oat, foxtail, crabgrass) in barley and wheat
Applied when susceptible grass is in the 1 to 4 leaf stage
Controls downy brome if applied ppi
Need Good Coverage
Inhibits Shoot Growth
Diclofop Symptoms
Slow to Develop
Chlorosis and Stunting of Plant
Degrades quickly
½ life → 10 to 30 days
Fluazifop
Post Emergence Grass Control
- Volunteer Corn
- Quackgrass
Absorbed by foliage and translocated
Inhibits ACCase activity
Fluazifop Symptoms
Symptoms
- Stunting
- Chlorosis
- Necrosis
- Decayed growing point
Symptoms slow to develop
Low soil mobility
½ Life → less than 1 week
Sethoxydim (Poast)
Used for post emergence annual grass control in alfalfa, soybean and sunflower
Increased uptake when mixed with crop oil
ACCase Inhibitor
½ Life → 2 to 13 days
Foliar Applied Contact Herbicides
Selective (only kill broadleaf) Photosynthetic Inhibitor
- Bentazon (Basagran) → Broadleaf (Group 6)
- Bromoxynil (Buctril) → Broafleaf (Group 5)
Selective (kill grass only)
Difenzoquat (Group 26, site unknown)
Selective (kills broadleaf and grasses, not crops)
PPO Inhibitors → Group 14
Nonselective (kills all plants)
Paraquat → Group 22
Bentazon Site of Action
Photosystem II in chlorophyll of plant - site B
Bentazon Mode of Action
Inhibits electron transport in Photosytem II
- Binds to an amino acid in the protein bound plastiquinone enzyme and does not allow electron transport to continue (stopping photosynthesis)
Bentazon Symptoms
Chlorosis
- If not good coverage, yellow spotting
Necrosis
Wilting
Stunting
Tolerant crops may show these symptoms form slight to moderate (depending on variety)
Bentazon Soil Activity
Very water soluble but not leached due to binding with soil
Soil persistence 3 to 5 months but not active cause
- Tightly bound
- Does not translocate well to the leaves of the plants (site of action)
Bromoxynil Site of Action
Photosystem II (same as bentazon)
Bromoxynil Uses
Appliked Post to:
- Wheat
- Barley
- Oats
For seedling broadleaf control
Best when weeds are small, < 4 leaf stage
Controls
- Wild buckwheat
- Annual smartweed
These are weeds that 2,4-D does not control well
Bromoxynil Mode of Action
Inhibits photosynthesis by stopping electron flow in PSII so no reducing power is generated in photosynthesis
- Secondary effect is lipid peroxidation which causes holes in membranes, loss of cell integrity follwed by loss of cell function
Bromoxynil Symptoms
Leaf Chlorosis (due to lipid peroxidation)
Leaf spotting if poor coverage
Necrosis
Diphenyl ethers Site of Action
Inhibits carotene biosynthesis (protoporyhyringen)
(Protox or PPO)
Diphenyl ethers Mode of Action
Oxidation of membranes and cell constituents (breaks down membranes) because singlet Oxygen builds up
- carotene buffers against singlet O formation
Diphenyl ethers - Symptoms
Water Soaking
Wilting
Foliage burn
Aciflurofen Uses
Early postemergence broadleaf and small control in soybean
Excellent control of
- Pigweed
- Mustard
- Black Nightshade
Poor control of large seeded weeds (sunflower)
Contact Herbicide - need excellent coverage
Paraquat (Group 22)
Nonselective Contact Herbicide
Used in lots of crops to dry down (i.e potatoes)
Used to dessicate weeds at the end of the season prior to harvest
Used as burndown in spring
Does not translocate - poor control of perennials
Paraquat Symptoms
Plant dessication
Rapid wilting due to disruption of membranes and water loss of cells
Cocklebur (mature and drought stressed) are tolerant to paraquat
Paraquat Mode of Action
A free radical is formed using paraquat and keeps reoxidizing to paraquat
- reoxidation results in Hydrogen peroxide (H2O2) production which destroys cell membrane
Paraquat Site of Action
Photosystem I in chlorophyll the paraquat molecule acts as a free radical and scavenges electrons from photosynthesis and also respiration
What is Herbicide Resistance
The inherited ability of a plant to survive and reprouce following exposure to a dose of herbicide that is lethal to the wild plant type
Tolerance
Ability of a plant to survive and reproduce after herbicide treatment
Cross Resistance
Plants that are resistant to several herbicides in a similar chemical group with a similar mode of action
Multipe Resistance
Plant is resistant to herbicides that are chemically unrelated and have different mode of actions
Factors that Influence Resistance Development
Over reliance/exclusive reliance on a single herbicide (or herbicide mode of action) for weed control
- Single site of action increases problems
(ALS, ACCase, glyphosate, PPO, PSII, all ahve single site of action)
An increase in the application frequency of a particular herbicide
Management to Prevent Resistant Weeds
Use integrated weed managment techniques
Prevention is the best method
Diverse program is needed
Tank mix at least 2 chemicals with different sites of action
Then:
- Rotate Culivation Techniques
Rotate
Use herbicides with short-term soil activity
Use optimum doses to prevent weed escape
Use cultural managment to prevent weed seed spread
Tank mix different sites of action
It is recommended not to use herbicides with the same site of action more than once every 3 years
Ask at Least 6 W’s
Which weeds
What method of control
- May not alwasy choose an herbicide
Where present
What mode of action or combination
- Whats been used in the past
What stage of plant growth (when to apply)
- Choose the right timing
What rate
