Herbicide Resistant Weeds and Their Management Eric P. Prostko and A. Stanley Culpepper Extension Weed Specialists The University of Georgia Updated October 2005

Herbicide Resistance Definition  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.

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).

Why Are Plants Resistant to Herbicides?  altered site of action  enhanced metabolism  sequestration

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

Herbicide Resistance  How does it occur?

Herbicide Resistant Weeds Selection Pressure Source: J.L. Gunsolus. Herbicide Resistant Weeds North Central Region Extension Publication 468.

Herbicide Resistance Around the World  304 Resistant Biotypes  182 Species (109 dicots and 73 monocots)  over 270,000 fields Source: Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet. October 20, 2005.

The Beginning of Weed Resistance  1968 (Washington)  nursery crops  common groundsel  atrazine  simazine Photo: The Missouri Flora Web-Site

Weed Characteristics That Favor Resistance  reproductive capability  seed dispersal mechanisms

Source: Dr. Ian Heap ( Worldwide

Average Number of Seed Produced per Plant 400,000 40,000 9, , , , , , , , , ,000 Palmer Amaranth goosegrassannual bluegrass common cocklebur Top 4 Herbicide Resistant Weeds in US peanut - 175; cotton - 250; corn - 800

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!!

Herbicide Resistant Weeds By Mode of Action

Commonly Used ALS Herbicides  Accent  Ally  Beacon  Cadre  Classic  Envoke  Exceed  Express  Harmony Extra  Peak  Permit  Pursuit  Python  Scepter  Staple  Strongarm

Herbicide Resistance in Georgia

Other Counties in Georgia with ALS- Resistant Palmer Amaranth  Tested by BASF in 2005 –Colquitt –Cook –Mitchell  175 locations were also sampled by UGA weed scientists in 2005 * **

Glyphosate Resistance Around the World (8 species)  rigid ryegrass (1996)  goosegrass (1997)  horseweed (2000)  Italian ryegrass (2001)  hairy fleabane (2003)  buckhorn plantain (2003)  common ragweed (2004)  Palmer amaranth (2005)

Glyphosate Resistant Horseweed in US (confirmed in 12 states)  DE (2000)  TN (2001)  IN (2002)  MD (2002)  NJ (2002)  OH (2002)  AK (2003)  MS (2003)  NC (2003)  OH (2003)  PA (2003)  CA (2005) Horseweed pictures from SWSS Weed ID Guide

Other Weeds in the U.S. that have Developed Resistance to Glyphosate Palmer amaranth (GA-2005) Common ragweed (MO-2004) Rigid ryegrass (CA-1998) Italian ryegrass (OR-2004) SWSS L. Hall - UGA USDA

U.S. Acres Treated with Glyphosate Source: USDA - National Agricultural Statistics Service – Agricultural Chemical Usage Reports

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.

MSMA Resistant Cocklebur in NC (Dr. Alan York – NCSU))

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!!!

Herbicide Resistant Weeds Strategies 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

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% Source: Weed Technology (1997): 11:

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

More Information About Herbicide Resistance  UGA Weed Science Web-Site *  International Survey of Resistant Weeds *