1. Herbicide Resistance: Evolution, Prevention and Control
Jamshid Ashigh
Extension Weed Specialist/ Assistant Professor

2. Chemical weed control
Herbicides are the most efficient technology for large-scale weed control.
The introduction of new herbicides to replace those herbicides failing due to resistance is essential for weed management.

3. Herbicide resistance world wide

4. Herbicides development and use
The rate of introduction of new herbicides has slowed dramatically.
Due to the high cost
Ten multi-national corporations
In 2004 there were four remaining
There is a strong imperative to use the currently available herbicide resources in more sustainable ways.
Sustainable agriculture
IWM

5. Herbicide Resistance
Herbicide resistance is the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type (WSSA).
Similar to resistance to:
Insecticides
Fungicides
Antibiotics

6. Evolution of herbicide resistance
Weed populations are diverse
Most individuals are susceptible to herbicides
A few individuals are naturally resistant (1X10-8 to 1X10-10)
After the herbicide is applied they are the only ones to survive

7. Evolution of Herbicide resistance
Survivors put seeds in the seed bank
The following year
A few more survivors
Not noticeable
Several years (3-10 years) of selection before it becomes visible
30% of the total population

8.                                                         
                                     
Resistance selection
Source: J.L. Gunsolus. Herbicide Resistant Weeds North Central Region Extension Publication 468.

9. Major factors influencing the evolution of resistance
                                     
Major factors influencing the evolution of resistance
Selection pressure
Initial frequency of herbicide-resistant individuals
1X10-8 to 1X10-10
Gene flow
Pollen and seed movement
Resistance controlled by single gene (e.g., ALS-inhibitors resistance)

10. Selection pressure Efficiency of the herbicide Frequency of use
Sensitive weeds
Frequency of use
Applied alone
Duration of effect

11. Weed characteristics that influence the evolution of resistance
Annual life cycle
Produce large numbers of seed
Genetic variability
Outcross

12. What makes the weeds resistant?
Altered site of action
Change in target enzyme
Enhanced metabolism
Ability to degrade herbicide
Decreased absorption and translocation
Herbicide does not get to its site of action
Sequestration
Herbicide not available to the plant (stored in vacuoles)

13. Altered site of action
Alterations in the site of action that prevent the herbicide from binding are the most common mechanism of resistance
Caused by mutation(s) in the gene(s)
Change in the amino acids

14. Triazine mechanism of action
Thylacoid
membrane
Chloroplast
Source: Dr. J. Christopher Hall

15. Plastoquinone Binding
Source: Dr. J. Christopher Hall

16. Triazine Binding
Source: Dr. J. Christopher Hall

17. Triazine Resistance
Source: Dr. J. Christopher Hall

18. Confirmation of resistance in the field
Other causes of herbicide failure have been ruled out (e.g., plant size, time of application etc).
The same herbicide or herbicides from the same family have been used year after year.
One weed that is normally controlled is not controlled while other weeds are.
Single weed species in patches and they are spreading.
Mix of dead and healthy weeds from the same species are seen in the field.

19. Source: Dr. Earl Creech

20. Confirmation of resistance in the lab
Seedlings.
Pre emergence
Post emergence
DNA sequencing.
Absorption, translocation and metabolism S R R S
Pre- emergence
Post- emergence

21. Herbicide resistant weeds
Worldwide:
323 Resistant Biotypes, 187 Species (112 dicots and 75 monocots)
United States:
Resistant weeds have been reported from 46 states.
In New Mexico:
One confirmed case (Kochia)
Suspected glyphosate-resistant Palmer amaranth.
Source: Dr. I. Heap.

22. Glyphosate-resistance
Glyphosate-resistant (Roundup ready) crops are the most widely used transgenic crops.
98% of cotton in US.
Glyphosate is also used extensively in other crops.
Nine weed species including Palmer amaranth have developed resistance to Glyphosate.
Resistance is threatening the ongoing sustainability of Glyphosate.

23. Glyphosate-resistant palmer amaranth
Glyphosate-resistant Palmer amaranth in Georgia, Tennessee, North Carolina, South Carolina and Arkansas.
In late 2007, a Palmer amaranth population was reported to have survived Glyphosate applications in New Mexico.
Palmer amaranth is an out-crossing plant:
Spread faster than self-pollinated species.

24. Preliminary Results
Resistant
Susceptible 2X 1X

25. It is important to adopt proactive weed management strategies to prevent/control resistance.

26. Prevention and management
Prevent seed production introduction
Monitor the field
Follow label direction
Rotate herbicide
Rotate crop
Use less herbicide
Integrate other methods of weed control with herbicides
Use herbicide mixtures

27. Problem with rotating herbicides
Herbicide rotation has not precluded the development of some types of resistance (e.g., multiple- or cross-resistant wild oat in Canada).
Especially if the label rates are not followed
Due to stacking of resistance genes
Triazine resistance + ALS resistance

28. Criteria for good herbicide mixtures
Same residual (persistence)
Same efficacy (control the same spectrum of weeds)
Different mode of action

29. Example of a bad mixture
Herbicide
Mode of action
Residual
Pursuit
ALS
Long
Basagran
Photosystem II
None
Follow label direction for mixture options

30. Label Legally binding Contains critical info: How to mix & apply
Restrictions
Use rates
Spill info/contact
Soil activity
Species
Time of application
Mixture options