Herbicide Resistance in Common Roadside Plants

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Herbicide Resistancein Common RoadsidePlants
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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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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

• Use herbicides with a different
  mode-of-action
• Use mixtures with different modes-of-action
• Control escaped weeds in clear zones

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Managing Resistance

• Prevent the spread of resistant weeds
• 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 inSouth Carolina

• DNAs Palmer amaranth, goosegrass
• ACCase Italian ryegrass
• Arsenicals - cocklebur
• ALS inhibitors Palmer amaranth

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Two Theories

• 1) Weeds mutate and become resistant
• 2) 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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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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ACCase Turfgrass Herbicides

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

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Roundup Resistance

• Roundup-resistant ryegrass
• found in Australia.
• Roundup-resistant horseweed found in Tennessee,
  Delaware, New Jersey.