Herbicide Resistant Weeds and Their Management in the Southeast

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Herbicide Resistant Weeds and Their Managementin
the Southeast

• Dr. Eric P. Prostko
• Extension Weed Specialist
• University of Georgia
• Tifton

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Herbicide ResistanceDefinition

• inherited ability of a weed or crop biotype to
• survive a herbicide application to which the
• original population was susceptible.

Biotype a group of plants within a species that
has biological traits that are not common to the
population as a whole.
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Herbicide Resistance

• cross resistance
• weed biotype that has gained resistance to
  more than 1 herbicide with the same mode of
  action. Same or different families.
• multiple resistance
• weed biotype that has developed tolerance to
  more than one herbicide brought about by
  different selection pressures (different modes of
  action).

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Why are plants resistant to herbicides?

• altered site of action
• enhanced metabolism
• sequestration

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Altered Site of Action
Source J.L. Gunsolus. Herbicide Resistant
Weeds. 1998. North Central Region Extension
Publication 468.
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Herbicide Resistance

• How does it occur?

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Herbicide Resistant WeedsSelection Pressure
Source J.L. Gunsolus. Herbicide Resistant
Weeds. 1998. North Central Region Extension
Publication 468.
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Herbicide ResistanceAround the World

• 284 Resistant Biotypes
• 171 Species (101 dicots and 69 monocots)
• over 270,000 fields

Source Heap, I. The International Survey of
Herbicide Resistant Weeds. Online. Internet.
December 1, 2003 .
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The beginning of weed resistance

• 1968 (Washington)
• nursery crops
• common groundsel
• atrazine
• simazine

Source The Missouri Flora Web-Site
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(No Transcript)
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Weed Characteristics That Favor Resistance

• reproductive capability
• seed dispersal mechanisms

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Source Dr. Ian Heap (www.weedscience.com)
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Average number of seed produced per plant
peanut - 175 cotton - 250 corn - 800
Top 4 Herbicide Resistant Weeds in US
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Herbicide Characteristics That Influence Weed
Resistance

• herbicides with a single site of action
• herbicides used multiple times during the
  growing season
• herbicides used for consecutive growing seasons
• herbicides used without other control strategies
• repeated use of a product for more than 2 years
  could develop a herbicide resistance problem!!

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Herbicide Resistant WeedsBy Mode of Action
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Commonly Used ALS Herbicides

• Accent
• Ally
• Beacon
• Cadre
• Classic
• Exceed
• Express
• Harmony Extra

• Peak
• Permit
• Pursuit
• Python
• Scepter
• Staple
• Strongarm

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Herbicide Resistance in Georgia
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Herbicide Resistance in Florida
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Herbicide Resistance in Alabama
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Herbicide Resistance in South Carolina
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Glyphosate Resistance Around the World (6 species)

• hairy fleabane (S. Africa)
• horseweed (USA)
• goosegrass (Malaysia)
• Italian ryegrass (Chile)
• rigid ryegrass (Australia, USA, S. Africa)
• buckhorn plantain (S. Africa)

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Glyphosate Resistant Horseweed in US (confirmed
in 9 states)

• DE (2000)
• TN (2001)
• IN (2002)
• MD (2002)
• NJ (2002)
• OH (2002)
• AK (2003)
• MS (2003)
• NC (2003)

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Glyphosate resistant horseweedin US - 2003
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U.S. Acres Treated with Glyphosate1996-2002
Source NASS
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Herbicide resistance should only be suspected
when ....

• other causes of herbicide failure have been
  ruled out.
• the same herbicide or herbicides with the same
  mode of action have been used year after year.
• one weed that is normally controlled is not
  controlled while other weeds are.
• healthy weeds are mixed with controlled weeds
  (same species)
• a patch of uncontrolled weed is spreading.

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MSMA Resistant Cocklebur in NC (Dr. Alan York)
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Causes of Herbicide Failures

• weed size
• moisture
• temperature
• humidity

• rate
• application method
• calibration
• others

All possible reasons for poor performance should
be investigated before considering the
possibility of resistance!!!
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Herbicide Resistant WeedsStrategies for
Control/Prevention

• proactive vs. reactive
• utilize other weed control tactics (cultivation,
  row patterns, etc.)
• rotate herbicides with different MOA
• rotate crops
• scout fields
• prevent seed production
• clean tillage and harvesting equipment

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How long does resistance last in absence of
further selection pressure?

• Andrews and Morrison (1997) - Canada
• DNA resistant green foxtail
• of resistant seeds after 7 years
• initial resistance was at least 90

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Does the use of reduced rates influence the rate
of herbicide resistance development ?

• some say yes, some say no
• single dominant gene or multi-genic?
• Is level of control is same?
• rate vs selection pressure
• infrequent use of the lowest rate that provides
  effective control

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More Information About Herbicide Resistance

• International Survey of Resistant Weeds
• http//www.weedscience.org/in.asp
• UGA Weed Science Web-Site
• http//gaweed.com