Herbicide Resistance: Evolution, Prevention and Control

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. 1998. North Central Region Extension Publication 468.

9. 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 • 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. Thylacoid Chloroplast membrane Triazine mechanism of action 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. www.weedscience.org

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 1X 1X 2X 2X Resistant Susceptible

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 • 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