Multiple tactics needed for the north
GroundCover™ Supplement Issue: 128 May - June 2017 | Author: Liz Wells
- Growers are turning to double-knock and residual herbicides in response to glyphosate resistance.
- Strategic tillage can be a useful control for emerging weeds such as fleabane and feathertop Rhodes grass.
- Controlling weeds in non-crop areas such as fencelines will reduce the spread of weeds into paddocks.
- Non-herbicide treatments, including use of rotations, are the best way for growers in the northern region to control weeds.
The ability to grow summer and winter cereals, legumes and oilseeds gives some growers in the northern region the chance to make rotations their greatest ally in weed management, but the region also throws up circumstances that make the suppression of weeds far more complex than this.
Wildly variable rainfall, diverse soil types and an ever-expanding number of invasive species require northern growers to have a range of weed management options ready to use when conditions prevent rotations from working effectively.
Several of these options are being evaluated in GRDC-funded projects and their impact on farming systems is being taken into account as growers look for ways to minimise weed infestations without adding further pressure through herbicide resistance.
The projects are also looking into the phenology of weeds in the hope of finding periods of weakness in plants’ life cycles when they can be destroyed or prevented from germinating.
Residuals and strategic tillage
As growers are forced to move away from reliance on knockdown herbicides in no-till systems, the range of chemicals being used and the methods of applying them are widening.
“The main change we are seeing is a weed spectrum increasingly dominated by glyphosate-tolerant or glyphosate-resistant species and an increasing reliance on residual herbicides and double-knock,” Northern Grower Alliance (NGA) chief executive officer Richard Daniel says.
Mr Daniel says weed control is being researched by NGA through its Grower Solutions projects. The work has found that while double-knock treatments can be much more effective than single applications, they come at considerable cost and can still have variable levels of control.
“For instance, glyphosate mixed with 2,4-D followed by paraquat is still the most consistent option for fallow fleabane management, but will frequently only provide 70 to 80 per cent control,” Mr Daniel says.
“Double-knocks can be expensive in terms of cost of herbicide, labour and spray rig capacity, particularly when the level of weed control is still incomplete. So there are situations where growers should consider alternative strategies including tactical use of tillage or residual herbicides.”
Mr Daniel says strategic tillage can be a useful tool, particularly for surface-germinating weeds such as feathertop Rhodes grass. Its most likely fit in northern farming systems comes after low-stubble crops such as chickpeas.
“Feathertop Rhodes grass emergence, for example, can be reduced by up to 95 per cent if seeds are buried by cultivation to five centimetres. In addition, the seed appears short-lived, with burial for seven to 12 months reducing viability by greater than 90 per cent.”
As concerns about herbicide resistance rise, the use of residual herbicides is increasing, but their efficacy can be limited when application is not followed by regular rain events. Performance can also vary by soil type and seedbank distribution.
Mr Daniel also points out that residual herbicides will invariably lead to re-cropping restrictions. For this reason, the frontline weapon is still rotations.
“Sorghum is considered the blowout crop for feathertop Rhodes grass, so if you’ve got a paddock with a real problem, put mungbeans in that paddock and sorghum into the others. It’s about tweaking your system rather than changing everything.
“The key message is the need to put as much focus as we can on non-herbicide treatments.”
Narrabri-based University of Sydney researcher Dr Michael Walsh is running a project to evaluate engineering solutions to weed control.
Under assessment are electrocution, heating, lasering and microwaving, and commercially available units such as harvest weed seed control systems.
As research points more and more to site-specific weed control being the best option for growers, Dr Walsh says the feasibility of weed mapping is also being examined.
Dr Walsh believes that more targeted weed-control technologies such as these could reduce the cost of weed control by as much as 80 per cent.
“From an engineering perspective, it’s easy. The hard part is making the technology practical for on-farm use.”
NSW Department of Primary Industries technical specialist (weeds) Tony Cook is working on a project to develop new treatments for key weed species that often populate areas adjacent to crops such as fencelines and farm tracks (see breakout box).
He says fenceline weed management has become crucial to the overall suppression of weed populations, but now needs a more comprehensive approach than simply relying on glyphosate.
The project’s first experiments, on the Darling Downs and in northern and central NSW, suggest that bromacil can be mixed with other herbicides to broaden the spectrum of weed control to provide long-term control of broadleaf weeds and annual ryegrass.
GRDC Project Code NGA00003, US00084
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