Herbicide Resistant Weeds and Their Management in the Southeast Dr. Eric P. Prostko Extension Weed Specialist University of Georgia Tifton

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. 1998.

North Central Region Extension Publication 468.

Herbicide Resistance  How does it occur?

Herbicide Resistant Weeds

Selection Pressure Source: J.L. Gunsolus. Herbicide Resistant Weeds. 1998.

North Central Region Extension Publication 468.

Herbicide Resistance

Around 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 .

The beginning of weed resistance      1968 (Washington) nursery crops common groundsel atrazine simazine Source: The Missouri Flora Web-Site

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

Source: Dr. Ian Heap (www.weedscience.com)

Average number of seed produced per plant 450,000 400,000 350,000 300,000 250,000 200,000 150,000 100,000 50,000 0 400,000 Palmer Amaranth 40,000 goosegrass

Top 4 Herbicide Resistant Weeds in US

40,000 annual bluegrass 9,000 common cocklebur 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 Exceed Express Harmony Extra        Peak Permit Pursuit Python Scepter Staple Strongarm

Herbicide Resistance in Georgia Weed Year Herbicide goosegrass 1992 DNA Italian ryegrass 1995 Prickly sida 1993 Hoelon IMI Palmer amaranth 2000 SU/IMI

Herbicide Resistance in Florida Weed American Black nightshade goosegrass Year 1985 1996 Herbicide paraquat paraquat

Herbicide Resistance in Alabama Weed Annual bluegrass goosegrass cocklebur Year 1980 Herbicide simazine 1987 1988 DNA MSMA/DSMA

Herbicide Resistance in South Carolina Weed Year Herbicide goosegrass 1974 DNA cocklebur Palmer amaranth Italian ryegrass 1990 Palmer amaranth 1985 MSMA/DSMA 1989 1997 DNA Hoelon IMI

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)

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)

Glyphosate resistant horseweed in US - 2003

U.S. Acres Treated with Glyphosate

1996-2002

1996 1997 1998 1999 2000 2001 2002 100 80 60 40 20 0 4 4 5 9 9 13 9 Corn 13 14 30 36 56 57 58 Cotton Crop 25 28 46 62 62 73 78 Soybean

Source: NASS

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)

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?

  DNA resistant green foxtail  % of resistant seeds after 7 years  Andrews and Morrison 100 (1997) - Canada initial resistance was at least 90% 75 50 25 0 87 81 98 0 DNA 1 DNA

DNA'a Applied in 7 years

2+ DNA

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

  single dominant gene or multi-genic?

 some say yes, some say no 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  International Survey of Resistant Weeds * http://www.weedscience.org/in.asp

 UGA Weed Science Web-Site * http://gaweed.com