A MAMMOTH herbicide resistance testing task has been completed by the Australian Herbicide Resistance Initiative (AHRI), with the work consisting of 579 ryegrass samples from 298 farms, across four Australian States.
A total of 21 herbicides were tested - 12 standalones and nine two-way mixtures - with more than 15,000 individual resistance tests performed.
All States have significant clethodim resistance, with Western Australia leading the pack, New South Wales had the highest levels of resistance to glyphosate, while South Australia and Victoria were epicentres of resistance to Sakura (pyroxasulfone), prosulfocarb and trifluralin.
The huge testing task was undertaken by AHRI research fellow Roberto Busi.
AHRI content writer Peter Newman said the most significant result of Dr Busi's fidnings was that herbicide mixtures rock.
"The resistance frequency to standalone pre-emergence herbicides ranged from 10 per cent to 34pc, yet resistance to herbicide mixtures ranged from 0pc to 6pc," Mr Newman said.
"Herbicide mixtures are exceeding our expectations and have a big part to play in managing resistant weeds, with Luximax and Triallate being the winner in the mixtures competition."
The testing program showed that a big problem was looming with sakura, trifluralin and prosulfocarb resistance, especially in NSW and South Australia.
While trifluralin resistance has been known for a long time, the levels of sakura and prosulfocarb resistance were concerning.
In other results, both Luximax and propyzamide did well.
"Luximax is new, but there is a risk with any new herbicide that there may be existing cross resistance from all of the herbicides we have used in the past," Mr Newman said.
"We had previously reported that Luximax gets its own box and this research is further confirmation.
"There is still very little or no resistance to propyzamide in Australia at this stage which is both pleasing and remarkable."
When it came to clethodim resistance, WA may have led the pack, but every State showed significant levels of resistance.
There were only 25 ryegrass populations tested from NSW in total, all from the low rainfall, western districts.
The agronomists taking those samples found that their area does have low levels of resistance to pre-emergent herbicides, but they weren't expecting the results to be quite as low as what they were.
"NSW is leading the country when it comes to glyphosate resistance," Mr Newman said.
"This is likely due to all 25 NSW samples coming from the low rainfall area where chemical fallow has been a significant part of the rotation.
"The agronomists who took those samples were alarmed, but not surprised."
In the good news column no paraquat resistance was discovered in this research, however it is known that paraquat resistant ryegrass exists and several populations were confirmed resistant in 2021.
"We also know that many growers have switched to double paraquat knockdowns (paraquat followed by paraquat) in NSW and Victoria with great success," Mr Newman said.
"We need to develop strategies to look after paraquat with this practice and a good start is to ensure that we mix a group 14 (G) herbicide plus a pre-emergent herbicide with the second paraquat."
As part of the analysis, 579 ryegrass populations were collected from 298 farms at the end of the 2019 growing season - 352 from WA, 25 from NSW, 98 from South Australia and 104 from Victoria.
Fields were chosen according to the grower or consultant's interests, so it was not a completely random survey, with seed samples screened between February and March last year as part of the summer-autumn season in a cooled glasshouse.
The herbicides Luximax, Sakura, prosulfocarb, trifluralin, glyphosate and paraquat were tested in winter with the rationale of assessing herbicide efficacy at a lower temperature.
