Herbicide resistance in wild radish populations randomly collected in Australian surveys

Abstract

The GRDC-funded national random surveys conducted at harvest in 2024 reveal that herbicide resistant weed populations are widespread across Australia. However, the incidence of resistance varies considerably between weed species, cropping regions and states. A total of 130 wild radish populations were collected from cropping paddocks in Western Australia, South Australia, Victoria, Tasmania, New South Wales, and Queensland. These populations were treated with a range of commonly used herbicides to assess resistance levels. High levels of resistance (greater than 20% survival) to several herbicides were detected. Resistance to the ALS-inhibiting herbicides was widespread, with 77% of populations classified as highly resistant to chlorsulfuron. Additionally, 38% of populations showed cross-resistance to the ALS herbicide mixture imazamox + imazapyr (Intervix). Resistance to the synthetic auxin 2,4-D was observed in 42% of populations, while around 30% were resistant to diflufenican. Developing resistance (1–19% plant survival) was also common for these herbicides. In contrast, resistance to atrazine and the mixture pyrasulfotole + bromoxynil was rare, detected in less than 5% of populations. No resistance to glyphosate was identified in this survey. Wild radish populations with some level of herbicide resistance were more common in Western Australia than in other states. Compared with the 2020 national survey, overall resistance levels increased slightly for most herbicides over the four-year period, although changes in resistance frequency varied between herbicides and states.