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Herbicide Resistance

  • Herbicide resistance is defined as ‘the evolved ability of a weed population to survive a maximum dose rate of herbicide that was previously known to be lethal’.
  • Resistant weed populations may by-pass the herbicide action by two mechanisms: target-site resistance (TSR), where simple mutations prevent the herbicide from binding effectively to its site of action and non-target site resistance (NTSR), where complex multigenic changes, allow the weed to metabolically detoxify or degrade the herbicides to an extent where they are ineffective.
  • Resistance is further exacerbated by the lack of alternative herbicide types, forcing growers to repeatedly use the same active ingredients.
  • Herbicide resistance is a global problem with 500 unique resistance cases being reported, globally until 2019.
  • Herbicide Groups A (ACCase) and B (ALS) pose a very high risk of resistance.
  • NTSR has been reported to be the common type of resistance to glyphosate, and also plays a key role in resistance to ACCase and ALS inhibitors. Although, NTSR is slower to develop, NTSR resistant weeds are widespread in major tillage crops. For example, chlorotoluron and pendimethalin actives can affect NTSR to some degree.
  • Herbicide resistance can also occur through spread of resistant genes, in contrary to independent-endowing mutations.
  • Sensitivity tests are valuable to provide direction to correct management strategies

Herbicide Resistance Testing

Approximately 25% of wild oats samples submitted to the ECT project in 2019 have confirmed herbicide resistance and preliminary tests on black-grass would suggest that there is a range of resistance to ACCase and ALS herbicides in the black-grass populations here in Ireland. It is very important where you suspect resistant that it is tested as quickly as possible in order to confirm if resistance is present and if so to what family of herbicides. This will allow a robust IPM strategy to be formulated based around the resistance profile of the grass weed population.

Anyone who suspects that they have a herbicide resistant grass weed on their farm can send samples into the Enable Conservation Tillage project in Oakpark. When submitting a sample for testing please fill in the information on the Grass Weed Seed Collection Form and follow the sampling protocol to ensure that the sample provided is suitable for testing.