Herbicide Mode of Action

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Herbicide Mode of Action

• Cecil Tharp, MSU Pesticide Education Specialist

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Do you know what mode of action means?

1. Yes
2. No
3. Maybe

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Do you know the mode of action of all pesticides
you use?

1. Yes
2. No

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What is the mode of action?
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Mode of Action

• Sequence of events from absorption into the plant
  until the plant dies
• Systemic herbicides translocated from site of
  entry to area in plant where they are active.
  Glyphosate
• Contact herbicides - kill only the area where
  they are applied. Paraquat

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Further Break Down into Groups

• Weed Science Society of America has classified
  all modes of action into groups.
• These groups are usually on the pesticide product
  label. This is for Milestone.

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What does a group 4 mode of action mean?

• Use the handouts available.
• This would be a growth regulator herbicide or a
  synthetic auxin herbicide.
• Causes abnormal growth, extended leaf terminals,
  curled leaves and eventual death
• These target broadleaf weeds
• Many products including 2,4-D, dicamba, picloram,
  clopyralid, aminopyralid, aminocyclopyraclor,
  milestone etc.

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Common Leaf Symptoms from PGR Exposure
normal
cupped and blistered from PGR exposure
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We have many modes of action

• ACCase inhibitors (Group 1)
• Inhibit ACCase enzyme in grasses. Stunting,
  browning of grasses,
• Used primarily in broadleaf crops
• Some may be used to selectively control grasses
  in grass crops.
• Poast, Puma, Select
• Sethoxydim, clethodim, fenoxaprop

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Aromatic Amino Acid Synthesis Inhibitors

• Group 9.
• Glyphosate
• Inhibits amino acid synthesis.
• Slow process that may take weeks.
• Broadspectrum, yellowing of plants, stunting

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Plateau or Escort Injury Symptoms
Stunting
Chlorosis of youngest tissue
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ALS Inhibitors

• Inhibit amino acid synthesis
• Sulfonylurea herbicides
• Vulnerable to low pH conditions
• Also imazapic, imazamox, imazethapyr
• Resistance and cross-resistance is common.
• Targets a wide variety of grasses and broadleafs
  while protecting crops (selective read label)

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Do you mix your modes of action?

1. Consistently
2. Occasionally
3. Never

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Who Cares?

• Why do we need to group our modes of action?

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Plants, Diseases and Insects often develop
resistance to modes of action.
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History of Weed Resistance

• The first reported case of herbicide resistance
  in the United States was reported in the 1950s.
  Field bindweed resistant to 2,4-D was reported in
  Kansas in 1964.
• The greatest number has been observed with the
  ALS inhibitors (imidazolinones,
  pyrimidinylthiobenzoates, sulfonylureas,
  triazolopyrimidines).
• 25 herbicide families have confirmed resistance
  to various weeds.

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Glyphosate No Longer Effective in Managing Kochia

• From 1984 2013
• Resistant to
• Photo-System II 5 (atrazine)
• ALS Inhibitors 2 (chlorsulfuron, etc.)
• Sythentic Auxin 1 (dicamba, fluroxypyr)
• EPSP Synthase Inhibitor 9 (glyphosate)

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Wild Oat Resistance

• Cell elongation inhibitors (8)
• Lipid inibitors (8)
• Thiocarbanates
• Difenzoquat
• ALS Inhibitors (2)
• Imazamethabenz methyl
• ACCASE inhibitors (1)
• Clodinafop
• Diclopop-methyl
• Tralkoxydim

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Resistance to Persian Darnel, Green Foxtail and

• Persian Darnel
• ACCASE inhibitors (1)
• Diclofop-methyl
• Green Foxtail
• ACCase inhibitors (1)
• Diclofop-methyl
• Sethoxydim
• Horseweed
• EPSP synthase inhibitors (9)
• Glyphosate

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How does the plant develop resistance?
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How can you combat resistance?

• Mix modes of action from application to
  application
• Dont stay with only one mode of action
• Or use multiple modes of action in a tank mix
• Some pesticide products already deliver multiple
  modes of action
• Use highest rates
• Use multiple control tactics or integrated pest
  management.

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Are there other reasons to combine modes of
action?
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Tank Mixing Multiple Products

• Can cause unexpected outcomes
• Enhancement
• Increases efficacy by adding adjuvants
• Synergism
• By combining chemicals increased efficacy of
  either product is achieved
• Antagonism
• Efficacy is lowered by combining the chemicals
• Incompatabilities
• 2,4-D and glyphosate in hard water situations

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Common Mixing Issues

• WPs and ECs they can form a putty or paste with
  an oily layer that floats on the top of the tank.
  
• Many physical incompatibility problems
• Combinations of pesticides (ECs, for
• example) are mixed with liquid fertilizers.
• A few pesticides are available in special
  fertilizer-grade formulations that reduce
  incompatibility problems.
• Some poor mixtures result in lower efficacy from
  water issues.

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Alkaline hydrolysis High pH Water

• Dissociation is the breaking down of a complex
  molecule into simpler units.
• Efficacy often goes down when molecules are
  dissociated. Plants often wont absorb the
  chemical as well. At times this may totally
  inactivate the pesticide.
• pH 6 7 is ideal for most pesticides but it can
  vary.
• Weak Acid Herbicides are the most susceptible to
  alkaline (high ph) dissociation
• Roundup (glyphosate)
• Pursuit (ammonium salt of imazethapyr)
• Liberty (glufosinate ammonium).
• 2-4D salt

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Hardness of Solution Can Cause Problems in Tank
Mixes

• Hardness is the concentration of multi-valent
  cations (positively charged ions from minerals)
• Cations bind with negatively charged pesticide
  molecules
• pH and hardness work together to reduce efficacy
• Forms insoluble salts

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When is it a problem?

• If the sum of the concentration (ppm) for the
  cations exceeds 150 ppm action should be
  considered
• 2,4-D amine (gt 150 ppm)
• totally deactivated at 500 ppm)
• Dicamba (gt 150 ppm)
• Glyphosate (gt 150 ppm)
• Clopyralid (gt150 ppm)
• sethoxydim Poast (gt150 ppm)
• Imazethapyr Pursuit (gt150 ppm)
• Reduces efficacy of many
• surfactants
• Scale may plug sprayer

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If you have a problem

• Add an adjuvant containing sulfate or organic
  acids as they bind with hard minerals
• non-ionic surfactants increase efficacy
• Add ammonium sulfate (8.5 17 lb / 100 gallons)

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Prior to tank mixing

• Read the pesticide product label on compatibility
  issues or recommended tank mixes.
• Call your product manufacturer for more
  information prior to mixing.
• Last Resort Can perform a jar test
• Check for heat, gumming, foam and scum

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Order of Mixing

• Always consult the label of each product for
  product-specific mixing instructions
• 1. Fill the tank ¼ to ½ with carrier and
  begin agitation
• 2. Add compatability or defoaming agents if
  needed
• 3. Pesticides, in the order of WP, DF, WDG,
  F, FL, ME, EC, S, SP
• 4. Crop oils or surfactants

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Do you know what mode of action means?

1. Yes
2. No
3. Maybe

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Do you know the mode of action of pesticides you
use?

1. Yes
2. No

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Do you mix your modes of action?

1. Consistently
2. Occasionally
3. Never

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Thanks

• Cecil Tharp
• Montana State University
• Bozeman, Montana
• [phone]
• ctharp_at_montana.edu