Herbicide resistance levels across Australia in wild oats and sowthistle - 2020 national survey results

Abstract

Up to date knowledge of the extent of herbicide resistance in cropping weeds is of vital importance to Australian growers and advisors. For this reason, a national random survey was undertaken in 2020 collecting numerous weed species from both winter and summer crops. From 2861 paddocks visited (2395 winter and 466 summer) 707 wild oats and 591 sowthistle samples were collected for resistance screening.
The wild oats were screened to clodinafop, clethodim and pinoxaden (Group 1 herbicides), mesosulfuron (Group 2), flamprop-methyl (Group 0), glyphosate (Group 9) and the pre-emergent herbicide triallate (Group 15). Resistance was highest to clodinafop with 16% of samples resistant followed by flamprop-methyl (7%) and pinoxaden (5%). Three samples were resistant to mesosulfuron and two to clethodim while none were resistant to triallate. Resistance levels varied between states, for example 25% of samples from NSW were resistant to clodinafop, compared with 21% from Queensland, 12% from Victoria, 8% from South Australia, 4% from Western Australia and none from Tasmania.
Sowthistle were screened to chlorsulfuron (Group 2), 2,4-D amine (Group 4) and glyphosate. The majority of samples (73%) were resistant to chlorsulfuron with resistance to 2,4-D amine significantly lower at 3% of samples while no samples were resistant to glyphosate. For all states except Western Australia and Tasmania more than 66% of samples were resistant to chlorsulfuron while resistance to 2,4-D amine was only recorded in samples from Queensland (8%) and New South Wales (3%).
Coupled with the significant levels of resistance in both species a large number of samples had surviving plants but at less than the requisite 20% survival required for classification as resistant.
These findings confirm herbicide resistance is becoming common in these species and the need for on-going surveys to obtain data to allow the grain industry to make informed weed management decisions.