A survey conducted in Western Australia’s Great Southern region has highlighted the value of checking weed seeds for herbicide resistance - a task best undertaken during harvest.
The localised 2015-16 annual ryegrass and wild radish resistance survey found more than half the growers surveyed had never tested weeds for herbicide resistance and 30 per cent incorrectly assumed which herbicides were working.
The survey was conducted by the Nyabing Farm Improvement Group (NFIG) through a Grains Research and Development Corporation (GRDC) Regional Cropping Solutions Networks (RCSN) Albany port zone project.
The project used weed seed samples from 44 paddocks on 32 properties located between Lake Grace and Jerramungup.
NFIG coordinator Fiona Hobley said knowledge about the herbicide resistance status of annual ryegrass and wild radish across the Great Southern was lagging compared with northern areas and could only be bolstered by testing.
“The survey was designed to involve the region’s growers in the testing process and highlight the value of checking weed seeds for herbicide resistance,” she said.
“Growers who see herbicide failure in a paddock have been assuming the presence of resistance but, potentially, testing shows the actives are working under ideal controlled conditions.
“Without testing for resistance levels, growers risk exhausting herbicide options and wasting money on ineffective herbicides.
“Testing may also help to rule out other factors that might be affecting poor herbicide performance such as application technique, weather conditions, soil moisture and compatibility.”
Under the project, Charles Sturt University assessed eight commonly-used herbicides on 37 annual ryegrass samples and five herbicides on 22 wild radish samples at half and full label application rates.
“The survey highlighted that annual ryegrass resistance, especially to clethodim, is a major challenge for many croppers in the Great Southern,” Ms Hobley said.
“In the annual ryegrass sample testing, 5 per cent of samples showed clethodim (Group A) resistance when the standard rate was used, while glyphosate (Group M) resistance was found to be at developing levels (5 per cent of samples) when standard rates of this herbicide were used.
When half rates of clethodim were used (250 mL/ha), 19 per cent of annual ryegrass samples were developing resistance or resistant.”
Ms Hobley said the incidence of wild radish resistance appeared to still be low, but was on the radar for growers as an emerging threat.
“The wild radish testing showed that 32 per cent of samples were resistant to the Group F herbicide diflufenican, but there was lower-than-expected resistance to Group B sulfonylurea active (41 per cent at full rate and 45 per cent at half rate),” she said.
Australian Herbicide Resistance Initiative communications leader Peter Newman said local growers were surprised that clethodim resistance levels were not higher in the annual ryegrass samples tested under the project.
“But the fact that the resistance testing occurred in summer – under ideal, warm conditions for clethodim – might explain this anomaly,” he said.
“Clethodim doesn’t perform well in cold weather, so when it is sprayed in winter and there is low level resistance in annual ryegrass, there will be many survivors - which is what growers are seeing in their paddocks.”
Mr Newman said a concerning outcome from the survey was that several of the annual ryegrass samples were found to have ‘developing’ levels of glyphosate resistance.
“However, an encouraging result from this project was that growers in the area still have some great options for wild radish control, and many growers were pleasantly surprised with the level of control available with Group B herbicides in many paddocks,” he said.
Information about herbicide resistance (or susceptibility) testing is available on the AHRI website, the WeedSmart website via and the GRDC Integrated Weed Management Hub.
Growers are advised to send samples to weed seed testing services before the end of January so that results are available before the beginning of the cropping season.
Fiona Hobley, Nyabing Farm Improvement Group
0448 880 585
Peter Newman, Planfarm/AHRI
08 9964 1170, 0427 984 010
Natalie Lee, Cox Inall Communications
0427 189 827