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Herbicide Resistance
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Herbicide Resistance

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  1. Herbicide Resistance

  2. Discussion Topics What is resistance? What causes resistance?

  3. Terminology

  4. Mode of Action Codes • ACCase inhibition • ALS inhibition • Microtubule assembly inhibition • Synthetic auxin • PS II inhibition, not groups 6 or 7 • PS II inhibition, not groups 5 or 7 • PS II inhibition, not groups 5 or 6 • Lipid synthesis inhibition, not ACCase inhibition • EPSP synthase inhibition • Glutamine synthase inhibition • Carotenoid biosynthesis inhibition at PDS • Carotenoid biosynthesis inhibition, unknown target • PPO inhibition • Inhibition very long-chain fatty acids • Unknown MOA • Auxin transport inhibition • PS I electron diversion • Inhibition of HPPD

  5. Herbicide Resistance Defined (WSSA definition) Inherited ability of a biotype* of a weed to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type. *Biotype is a subset of the population having some genetically controlled characteristic not common to the population as a whole

  6. Herbicide Resistance Defined (WSSA definition) Inherited ability of a biotype of a weed to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type.

  7. Herbicide Resistance vs Tolerance Resistance: herbicide used to kill that weed; it no longer does; we have selected for resistance. Example: Hoelon and ryegrass Tolerance: the herbicide never did kill that species. Example: 2,4-D and ryegrass

  8. Cause of Herbicide Resistance • Herbicides do not create resistance. • Herbicides can select for resistant • individuals already in the population.

  9. Resistant individuals initially rare Initial frequency of resistance: 1 in 100 thousand 1 in 1 million 1 in 10 million 1 weed/4.3 square feet = 10,000 weeds/acre = 1,000,000 weeds/100 acres Frequency of 1 in 1 million equal to one resistant plant per 100 acres

  10. Example of Cross Resistance Italian ryegrass Hoelon-susceptible biotype 0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X Hoelon-resistant biotype, Union Co.; Treated with Hoelon Group 1, ACCase inhibitor 0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X Hoelon-resistant biotype, Union Co.; Treated with Axial Group 1, ACCase inhibitor 0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X

  11. Example of Multiple Resistance: Italian Ryegrass Hoelon-susceptible biotype 0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X Hoelon-resistant biotype Treated with Hoelon ACCase inhibitor, Group 1 0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X Hoelon-resistant biotype Treated with Osprey ALS inhibitor, Group 2 0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X 0X 1/4X 1/2X 1X 2X 4X 8X 16X 32X

  12. Quiz Multiple resistance is greater concern than cross resistance. Why?

  13. Discussion Topics What is resistance? What causes resistance? What is the extent of resistance? What species have resistant biotypes?

  14. Herbicide Resistance in US by Site of Action* Nitriles HPPD inhibitors Carotenoid biosyn. inhibitors Resistance reported with 14 SOAs and 76 species; 128 species by SOA combinations, due to multiple resistance Arsenicals PPO inhibitors Thiocarbamates Paraquat DNA’s Ureas/amides Auxins Glyphosate ACCase inhibitors PS II inhibitors (triazines) ALS inhibitors *Current as of Dec. 6, 2010 ACY 2008

  15. Glyphosate-resistant biotypes in U.S.

  16. Glyphosate-resistant biotypes in U.S.

  17. Glyphosate-Resistant Weeds in the US 7 Broadleaf weeds 5 Grasses Palmer amaranth Italian ryegrass Tall waterhemp Rigid ryegrass Common ragweed Johnsongrass Giant ragweed Goosegrass Hairy fleabane Annual bluegrass Horseweed Kochia

  18. Glyphosate- Resistant Weeds in NC

  19. Discussion Topics What is resistance? What causes resistance? What is the extent of resistance? What species have resistant biotypes? Impacts of herbicide resistance?

  20. Impacts of Herbicide Resistance Increased cost of weed management; reduced profitability Increased complexity of weed management Fewer herbicide options Fewer crop options Impacts of Herbicide Resistance Increased cost of weed management; reduced profitability Increased complexity of weed management Fewer herbicide options Fewer crop options Compromise conservation tillage

  21. Impacts of Herbicide Resistance Increased cost of weed management; reduced profitability Increased complexity of weed management Fewer herbicide options Fewer crop options Compromise conservation tillage Impacts of Herbicide Resistance Increased cost of weed management; reduced profitability Increased complexity of weed management Fewer herbicide options Fewer crop options Compromise conservation tillage

  22. Discussion Topics What is resistance? What causes resistance? What is the extent of resistance? What species have resistant biotypes? Impacts of herbicide resistance? Basics of resistance management

  23. How does herbicide resistance develop? • Prerequisites for Resistance • At least one resistant plant must be present • 2. Selection pressure must be put on that resistant plant

  24. Herbicide Resistance Management • The focus must be on • reducing selection pressure.

  25. Diversity in management options reduces selection pressure on herbicides.

  26. Weed Management in Cotton: Before RR • Conventional tillage • PPI and PRE herbicides • Early POST or POST-directed herbicides • Late POST-directed herbicides • Cultivation

  27. Weed Management in Cotton: with RR • No-till or strip-till • No cultivation • No PPI herbicide; little to no PRE • Multiple in-crop Roundup applications • Very little other chemistry

  28. Herbicide Resistance Management (Reducing selection pressure) • Minimize seed bank; avoid seed production

  29. Impact of Crop Rotation and Good Control in Rotational Crop (whole field in soybean in 2009; all treated alike in 2009) Carthage, NC 2009

  30. Impact of Crop Rotation and Good Control in Rotational Crop (whole field in soybean in 2009; all treated alike in 2009) Soybean 2008 Tobacco 2008 Carthage, NC 2009

  31. Herbicide Resistance Management (Reducing selection pressure) • Minimize seed bank; avoid seed production • Think whole-farm • Good control in all crops in rotation • Avoid seed production on ditchbanks, turnrows • Avoid seed movement

  32. Herbicide Resistance Management (Reducing selection pressure) Herbicide Resistance Management • Minimize seed bank • Do not depend on a single herbicide (or a single SOA) • Multiple SOA’s within a crop, starting with a PRE or Preplant • Crop rotation, multiple and different SOA’s

  33. Herbicide Resistance Management (Reducing selection pressure) Herbicide Resistance Management • Minimize seed bank • Do not depend on a single herbicide (or SOA) • Multiple SOA’s within a crop, starting with a PRE. • Crop rotation, with different SOA’s • Use full rates, including sequentials and tank mixes