Herbicide Resistance

From left to right: Plant 1 – Susceptible to flupropanate application, Plant 27 – Resistant to flupropanate application, Plant 25 – Unsprayed control. Source: DEPI Victoria

Herbicide resistance is the ability of a plant to survive and reproduce following exposure to a dose of herbicide that would usually be lethal. In a plant, resistance may usually occur through natural selection resulting from random and infrequent genetic mutations. It may also be induced through techniques such as genetic engineering. 

Through selection, susceptible plants are killed while herbicide resistant plants survive to reproduce. If herbicide treatment is repeated, resistant plants reduce and become dominant in the population. The appearance of herbicide resistance in a plant population is an example of rapid evolution and typically develops when a weed species has been exposed to 10-14 years of continual application from a particular herbicide group or type. Large weed populations increase the likelihood of developing resistance.

Serrated tussock resistance to flupropanate.

Flupropanate is a residual herbicide and four to five successive applications in one low-rainfall area over 10-14 years is equivalent to continual applications of a non-selective herbicide.

The serrated tussock seed bank may consist of millions of seeds, many of which will attempt to germinate each year. Like a lotto draw, eventually a serrated tussock seed will have a genetic code resist the effects of flupropanate and will survive to reproduce and spread the resistance.

The long term implications of serrated tussock herbicide resistance are its increased dominance as a weed, increased costs to land managers, more herbicide usage, and higher environmental pollution and damage. 

Ways to avoid herbicide resistance.

  • Do not rely solely on the use of chemicals for control. Integrate herbicide use with other control strategies.
  • Use immediate acting or short residual herbicides wherever possible. Use long-term residual herbicides (like flupropanate) wisely and not continuously on the same area.
  • Always apply herbicides according to label directions.
  • Regularly rotate herbicide types used with different modes of action. For example Group J (flupropanate) or Group M (glyphosate)
  • Always monitor and follow up control on any re-growing serrated tussock plants.
  • Check sprayed areas for resistant plants. If there are any plants that have not been affected by the herbicide application, always investigate the reason why it happened.
  • Practice good hygiene practices to reduce movement of weed seed.
  • Do not let any serrated tussock set seed.  

More detailed information on herbicide resistance can be found on pages 24-25 of the National Serrated Tussock Best Practice Manual or in the How to recognize, manage and prevent serrated tussock herbicide resistance brochure.