- Residual herbicides help to reduce the fleabane burden
- Double knocks are expensive and best used on a major weed flush
- When other options are exhausted a strategic till can be useful
With ‘single knocks’, ‘double knocks’ and residual herbicides, growers in the northern region are successfully fighting back against fleabane
Five years ago managing fleabane was a cause of considerable stress for many northern growers. Glyphosate was not effective and no residual herbicides were specifically registered for use against this prolific seeder.
Today it is a different story with many growers now using new, integrated weed management strategies and recently registered residual herbicides in the fight against fleabane.
It is a battle that the Northern Growers Alliance (NGA), with agriculture departments in Queensland and New South Wales, has been working hard to win.
However, as NGA chief executive Richard Daniel says, there has been no ‘silver bullet’, rather the development of a range of control strategies, including the use of residual herbicides, ‘double-knock’ approaches, earlier intervention and strategic tillage.
Fleabane is a wind-borne, surface-germinating weed that thrives in areas of low competition and which proliferated in the northern region as growers moved towards reduced and no-till farming.
Fleabane in an area without competition – a firebreak around a wheat crop.
PHOTO: Anthony Mitchell, NGA
Mr Daniel recalls it becoming a major problem in the early 2000s. “Although fleabane has always been around, changes in farming systems favoured it. It’s not an easy one to control with glyphosate and it thrives where there is no competition.”
By 2006, when the NGA began work to control the weed, fleabane management was the region’s top grower priority.
“Growers were finding that glyphosate was not effective. They were throwing everything at it, at a high cost, but were not getting acceptable levels of control. It had become a massive economic issue,” he says.
New ways of approaching fleabane were needed and the NGA began to investigate residual herbicides.
Unlike knockdown herbicides such as glyphosate, residuals remain active in the soil for weeks, months or years.
“But they are a double-edged sword,” Mr Daniels says – advantageous because they ensure long-term weed control, but limiting because rotations may need to be changed to avoid damaging a subsequent crop.
It is this residual activity that often led growers to favour glyphosate. However, with its effectiveness now so variable, residual herbicides are increasingly being used.
Mr Daniel suggests residual herbicides should be applied before the largest expected fleabane emergence flushes. For example,
in autumn/winter during the winter fallow;
within the winter cropping program; and/or
- within the summer cropping program.
For those concerned about residuals limiting rotation choices, he suggests starting with the worst paddock and working from there. “That way, the whole farm’s rotation does not need to be changed.”
However, residuals do not achieve 100 per cent control, so knockdown herbicides are still needed. Mr Daniel sees weed-seeker technology fitting well with this approach. “Using a weed seeker to spray fleabane after a residual herbicide reduces the weed burden dramatically, but also lowers herbicide costs.”
As well as the residual herbicide approach, the NGA has found a ‘double knock’ to be successful. The most consistent and widely adopted approach is a mix of glyphosate and 2,4-D as the first application, or ‘knock’, followed by a paraquat-based herbicide as the second knock.
“Glyphosate alone followed by paraquat will result in high levels of leaf desiccation but the weed will nearly always recover. That is why the 2,4-D is included.”
Richard Daniel: new approaches paying off.
Mr Daniel says labour and herbicide costs make double knocks expensive, so they are best used in specific paddocks during a serious weed flush.
“Double knocks are most likely used for summer fallow ‘blow outs’, whereas a satisfactory fleabane knockdown with just glyphosate and 2,4-D may be achieved when small (rosette-stage) weeds are targeted in autumn and winter.”
With both approaches Mr Daniel says to apply herbicides earlier: “One key lesson learnt has been to move the main battle from late spring and summer, when weeds are ‘hardened off’ and difficult to completely control, to earlier in the season,” he says.
“This way we can use either double knocks on smaller weed stages or incorporate residual chemistry into our fallow, or in-crop management on a paddock-by-paddock basis.”
Research has also investigated strategic tillage, finding that a light cultivation can take fleabane seeds down to a depth of two to three centimetres, creating poor emergence levels.
“But if you cultivate again, it will bring it to the surface, negating all the work,” Mr Daniel says.
“So when we talk about ‘strategic tillage’ what we are trying to say is this – cultivation will not be the primary tool, but use it when other options have been exhausted and when it will have the least impact on the paddock, perhaps after a chickpea phase when there is not much stubble.”
The strategic, integrated approach to fleabane management has been a great success for northern growers, and while the weed still persists, Mr Daniel says everyone is getting better at managing it.
This is what growers often encounter after the winter wheat crop is harvested – an understorey of fleabane.
PHOTO: Anthony Mitchell, NGA
What is fleabane?
Fleabane is a wind-borne, surface-germinating weed that thrives in situations of low competition.
When does it emerge?
Germination flushes usually occur in autumn and spring when surface soil moisture levels stay high for a few days, although emergence can occur at any time. Fleabane germinates on the soil surface where there are gaps, tending to proliferate when paddocks are in fallow or between crop rows.
Is it glyphosate-resistant?
Resistance has been confirmed, although there is variability in fleabane’s response to glyphosate. Many samples from non-cropping areas are still well controlled by glyphosate, while samples from reduced-till cropping areas have shown increased levels of resistance. Either way, knock-down control of large plants in the summer fallow is variable and very expensive.
What does fleabane do?
Where fleabane draws on moisture levels, crops have lowered levels of productivity.
Where is fleabane?
Fleabane proliferated in the northern region but has spread south and west as more growers adopt no-till and reduced-till farming.
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