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Herbicide-resistant grass weeds: problems and the way forward

Herbicide-resistant grass weeds: problems and the way forward

Vijaya Bhaskar A.V., Teagasc, CELUP, Oak Park

The loss of herbicide classes coupled with the repeated use of herbicides with the same mode of action leads to reduced efficacy and resistance development. To date, 34 field populations of wild oats have been identified with herbicide (ACCase) resistance, since the first case was verified in 2016. To investigate the linkages between management actions, weed pressure and the incidence of resistance further, a nationwide survey of 145 farms was conducted from June to September 2020 as part of the on-going ECT project. The survey included a questionnaire, grass weed assessments, and the collection of seed samples for herbicide resistance testing. Glasshouse studies on 12 populations of sterile brome, identified a larger variation in susceptibility within and between populations to reduced rates of ALS-meso+iodosulfuron (Pacifica® Plus®) or ACCase-cycloxydim (Stratos Ultra®). This may indicate resistance evolution, which will likely accelerate with the repeated use of rates lower than the recommended label rates. Studies with 14 populations of black-grass indicate the presence of resistance to both ACCase and ALS herbicides. To limit the spread of resistant grass weeds, an integrated management approach with judicious herbicide use (e.g. using label rates) must be deployed to minimise seed return and ensure effective control with a limited herbicide armoury remains possible in the future.