What are the options to manage glyphosate- and paraquat-resistant ryegrass?
with Dr Roberto Busi, senior research fellow, AHRI
Fence lines are an ongoing challenge as they are frequently the source of herbicide-resistant weeds in cropping systems.
Dr Roberto Busi, senior research fellow at the Australian Herbicide Resistance Initiative (AHRI) says the rising number of annual ryegrass populations with high-level resistance to both glyphosate and paraquat demands a change in fence line weed management.
“Dual resistance in annual ryegrass to glyphosate and paraquat was first confirmed in broadacre cropping systems in 2022 in Western Australia, and since then we have identified six similar populations,” he says. “The management of fence line weeds like ryegrass that readily evolve resistance to herbicides requires careful consideration and high priority in a weed management program.”
Roberto and Farmanco agronomist, Brent Pritchard, collected seeds from these six populations in 2022 and 2023, then tested them against various herbicide double-knock and tank-mix treatments.
“We conducted trials using seed collected from these resistant populations and found that under optimal glasshouse conditions, it was possible to achieve up to 100 per cent control,” says Roberto. “However, field application is rarely ideal, and our trials demonstrated that control levels of around 40 to 50 per cent were more likely. This allows growers to suppress a blow-out but does not provide a long-term strategy.”
In addition to glyphosate and paraquat, these populations were also resistant to clethodim, diquat (e.g. Reglone) and paraquat+diquat (e.g. Spray.Seed). At two of the trial sites, the resistant ryegrass had already moved from the fence line to the adjacent cropping areas, posing considerable challenges to the grower.
The WeedSmart Big 6 strategy embraces herbicide and non-herbicide weed control tactics to keep weed numbers low in cropping systems.
Can the double-knock control glyphosate and paraquat-resistant ryegrass?
These populations were highly resistant to both herbicides used in a traditional double-knock treatment. Under field conditions, glyphosate applied on its own achieved 18 per cent control, while a stand-alone application of paraquat achieved 30 per cent control.
Control significantly improved when a double-knock treatment was applied. However, it was clear that most of the improved control was due to the paraquat component. Both the traditional glyphosate followed by paraquat and the ‘double paraquat’ double-knock strategies achieved around 45 per cent control.
What were the best mixtures to control these populations?
Herbicide mixtures that included either glyphosate or paraquat also improved the level of control to around 45 per cent under field conditions. In the glasshouse trial, the paraquat-based mixes were almost twice as effective (80 per cent) compared to the glyphosate-based mixes (42 per cent).
The recommendation is to consider using a tank mix of paraquat with other modes of action, including Groups 13 [Q], 14 [G], 15 [J/K] and 34 [Q], to suppress a ryegrass population with increasing resistance to glyphosate and paraquat.
Switching to paraquat mixed with pre-emergent products with residual activity on ryegrass, such as BoxerGold, Mateno Complete, Overwatch, Sakura + Terrad’or, propyzamide (e.g. Rustler) or Voraxor will help extend the efficacy of paraquat, the last reliable knockdown herbicide available in many farming systems.
What are the long-term options for fence line weeds?
The first step is to stop using glyphosate and paraquat as stand-alone weed control treatments on fence lines (or anywhere else). In the short term, implement a double-knock strategy when the weeds are small and actively growing. Where possible, add other modes of action as a tank mix with the paraquat application.
Resistance testing will provide useful information and highlight herbicide options to help stop the build-up of herbicide resistance genes in the weed seed bank.
Roberto has conducted preliminary glasshouse tests that have confirmed specific products for fence line weed control (e.g. Alion, Terrain, Uragan) are fully effective at their fence line registered rates against glyphosate- and paraquat-resistant ryegrass. He plans to conduct field validation of some of these promising options this year.
For longer-term management of resistant ryegrass on fence lines, it is necessary to look for alternative, non-herbicide strategies to stop seed set.
The optimal strategy is to create competition for resistant weeds by establishing perennial plant species that are easily controlled if the seed blows into the cropped area. If no herbicide is applied to the fence line area, then herbicide resistance will not be a problem. Other options include cultivating or planting the crop as close to the fence as possible to provide strong competition for weeds, then slashing, grazing or baling around the field’s perimeter.
In the future, non-chemical technologies like electric weeding could provide another option for managing fence line weeds.
GRDC project: UWA2007-002RTX
More resources:
- GRDC Update paper – How we best manage glyphosate and paraquat resistance in annual ryegrass
- AHRI Insight – Resistance testing is power
- Mix up your approach to fence line weeds
- Management of herbicide-resistant weeds on fence lines
