Herbicide resistance this spring

Herbicide resistance was first reported in Australia in 1982, when annual ryegrass was first found to be resistant to glyphosate. This phenomenon has spread across Australia, with now almost 350 documented glyphosate-resistant populations of annual ryegrass.

Other species have also developed resistance, with confirmed cases in 37 grass and broadleaf species. Glyphosateb is no longer the only chemical involved; there is now resistance to 11 different herbicide chemical groups. Cases of multiple mode of action resistance are also more common.

Moving into spring and summer, agronomists and growers should be aware that the following grass species have developed resistance (affected mode of action groups in brackets):

  • awnless barnyard grass ( Group M)
  • barnyard grass (C)
  • barley grass (A, B, L)
  • crabgrass (A, B)
  • crowsfoot grass (A)
  • giant Parramatta grass (J)
  • lesser canary grass (A, B)
  • liverseed grass (C, M)
  • paradoxa grass (A, B)
  • serrated tussock (J)
  • windmill grass (M).

The following broadleaf species have also developed resistance:

  • arrowhead (Group B), in rice
  • black bindweed/climbing buckwheat (B), in cereals and fallow
  • Calomba daisy (B), in crops, pastures and roadsides
  • dirty Dora (B), in rice
  • flax-leaf fleabane (M), in crops, pastures and roadsides
  • small square weed/white eye (L), in crops and roadsides in northern Australia
  • starfruit (B), in rice
  • Stinging nettle/dwarf nettle (C), affects pastures and roadsides.

“Where one of these weeds occurs in a paddock, choose a moderate-risk mode of action herbicide and use it at full label rate,” Dow AgroSciences research leader southern Australia Greg Wells advised.

“Ensure control before seed set or collect seed at maturity to prevent it spreading. Consider crop topping if you’re growing a pulse or legume crop."

“Consider cutting the crop for hay or after harvest consider burning the stubble. Do not introduce or spread weeds by contaminated seed, grain or hay. Rotate crops and/or varieties where alternative mode of action herbicides can be used"

“Focus on crop management to grow the most dense and competitive crop possible, to out-compete these weeds. Accurate records of herbicide applications on a paddock basis will help identify any issues and will assist in developing a plan of management should resistance arise.”

Mr Wells advised farmers to attend training courses and field days on herbicide management and herbicide resistance to ensure they were informed of local developments. He reminded reseller agronomists, district agronomists and farm consultants that growers would be looking to them for up-to-date information.

CropLife Australia provides detailed herbicide resistance management strategies for each herbicide mode of action group. GRDC and WeedSmart are other excellent sources of information on weed management.

WeedSmart, sponsored by Dow AgroSciences, provides regular webinars focused on weed management. These webinars give access to industry leaders and the latest research. Prior recordings can also be accessed from the WeedSmart website.

Refer to the WeedSmart ’10 Point Plan’ for a detailed approach to managing herbicide resistance. If an issue hasn’t already been covered on the WeedSmart website, you can always ‘Ask an Expert’ via the online blog.

Mr Wells said herbicide resistance severely limited options for farmers.

“Using every possible strategy available (herbicidal and non-herbicidal) whilst staying informed will allow you to adjust tactics in response to new developments and help you to remain on top of this problem,” he said.