Herbicide Tolerance

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Herbicide Tolerance Resistancein common
roadsideplants
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Herbicide Tolerance

• Def.

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Herbicide Resistance

• When plants are no longer controlled by a
  previously effective herbicide due to biochemical
  changes in resistant biotypes
• There are more than 230 resistant species
  documented world wide

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There are weeds resistant to these herbicide
families

• Aryloxyphenoxypropionates (Poast)
• Cyclohexanediones (Envoy)
• Dinitroanilines (Surflan)
• Imidazolinones (Arsenal)
• Sulfonylureas (Oust)
• Triazines (atrazine)

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Weed Characteristics Favoring Resistance
Development

• Prolific seed producer
• High germination rate
• Extended germination period
• Highly susceptible to the herbicide

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Herbicide Characteristics Favoring Resistance
Development

• Single site of action
• Very effective
• Long soil residual
• High use rate
• High use frequency

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Cultural Practices Favoring Resistance Development

• Growing or managing the same crop year after
  year
• Seed production from escapes
• Using herbicides with the same mode of action

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Managing Resistance

• Change management program (rotate crops)
• Rotate herbicide families
• Use herbicides with a different
  mode-of-action
• Use mixtures with different
  modes-of-action

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Managing Resistance

• Control escaped weeds in clear zones
• Prevent the spread of resistant weeds from clear
  zone areas to transition areas (backslopes, wide
  ROWs, interchanges)
• Use weed management methods other than herbicides
  (mowing, weedeating, etc.)

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Resistant Weeds

• Weeds that used to be controlled by a
  herbicide but are now tolerant
• Typically require a dose several times that
  normally needed for control
• May not be controlled at any dose level

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Resistant Weeds in Oklahoma

• Triazines - smooth pigweed
• DNAs - palmer pigweed, goosegrass
• Grass herbicides -
• johnsongrass (Fusilade)
• ryegrass (Illoxan)
• Arsenicals - cocklebur
• Amides - barnyardgrass
• ALS inhibitors - cocklebur, palmer pigweed

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Carefully diagnosis resistance

• Herbicide failure due to
• environmental conditions
• weeds that are too large
• poor application technique, rate, timing,
  coverage, etc.
• may be blamed on herbicide resistance

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Herbicide Resistance

• The first herbicide resistant weeds in the United
  States were discovered in the 1960s in a pine
  nursery in which triazine herbicides were used
  repeatedly

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Conventional Wisdom

• Weeds do not mutate or change to become
  resistant, but populations change
• The resistant weeds have always been present at
  low populations
• As the normal types are removed the population of
  resistant weeds increases

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Development of a Resistant Population
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Year 1
Year 2
Year ?
X susceptible
O resistant
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Things to Look For

• One weed, which normally should be controlled, is
  not controlled although other weeds are
  controlled
• A patch of uncontrolled weeds is spreading
• Healthy weeds are mixed with controlled weeds of
  the same species

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Resistance Mechanisms

• Target site changes
• Enhanced metabolism of the herbicide
• Changes in uptake and translocation

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Mode-of-Action ALS Inhibitors(acetolactate
synthase enzyme)

• Enzyme found only in plants
• Resistance to these herbicdes appeared within 5
  years of their introduction into the market place.

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ALS Roadside Herbicides

• Oust (sulfometuron)
• Escort (metsulfuron)
• Telar (chlorsulfuron)
• Outrider (sulfosulfuron)
• Arsenal (imazapyr)
• Plateau (imazapic)

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Oust-Resistant Ryegrass

• Oust resistant ryegrass found along Mississippi
  highways
• Caused by use of Oust for winter annual weed
  control and summer johnsongrass control

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Oust-Resistant Ryegrass (Mississippi)
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Cross Resistance

• ALS resistant plants are resistant to other ALS
  herbicides but not all
• Example Mississippi Scepter resistant cocklebur
  were susceptible to Classic (another ALS
  inhibitor)
• Missouri - Scepter resistant cocklebur were
  resistant to Classic and Broadstrike (before it
  hit the market)

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Mode-of-Action ACCase inhibitors(acetyl
coenzyme A carboxylase)

• Resistance with 5 years of 1st appl.
• Johnsongrass resistance in Arkansas
• Crabgrass resistance in northern states

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ACCase Agronomic Herbicides

• FOPs
• Fusilade
• Assure
• Acclaim
• Illoxan

• DIMs
• Envoy
• Vantage

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ACCase Turfgrass Herbicides

• Illoxan (diclofop)
• Acclaim Extra (fenoxaprop)
• Vantage (sethoxydim)
• Fusilade (fluazifop-methyl)
• Envoy (clethodim)

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Triazine Resistance
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Triazine Resistance

• Atrazine
• Princep
• Karmex

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Roundup Resistance

• Roundup resistant ryegrass
• found in Australia. Survives up to 5 times the
  normal rate.
• Dont forget Roundup resistant genes they are
  putting into crops these days (cotton, soybeans,
  corn)