Multiple tactics needed for the north

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Fast facts

  • 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.

Photo of Feathertop Rhodes grass invading a sorghum crop in Queensland's central Darling Downs

Feathertop Rhodes grass invading a sorghum crop from a fenceline on Queensland's central Darling Downs.

PHOTO: Tony Cook

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.”

Engineering hopes

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.”

Fenceline treatments

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.

Mr Cook says more effective weed control could result in cleaner cropped paddocks and the elimination of the green bridge between crops, which has the potential to carry diseases and pests.

Attention turns to fenceline hygiene

Photo of fenceline

Poorly managed fencelines are a refuge for an array of weeds that are likely to afffect adjacent cropped paddocks. With glyphosate resistant weeds on the increase, it is time to look for other options for controlling weeds and fenceline in non-crop areas.

By Tony Cook, Bill Davidson and Rebecca Miller

Even with the best possible control of in-paddock weeds we are still prone to invasion from adjacent weed-infested, non-cropped areas such as fencelines, irrigation channels and farm tracks. The biggest threat from non-crop areas is the windblown weeds that can spread large distances.

With respect to control, a new five-year project in the northern region has confirmed that bromacil can be mixed with other herbicides to broaden the range of weeds controlled in non-crop areas.

Work in the southern region has already confirmed bromacil’s effectiveness at controlling ryegrass, but growers in the north are looking for options to control a much wider range of weed species.

Fleabane, common sow thistle, windmill grass and feathertop Rhodes grass are widely distributed and of particular concern in the northern region. Annual ryegrass is more of a problem in central and southern NSW.

In the past, growers have relied on glyphosate to control weeds in non-crop areas. But with glyphosate resistance now found in all of these weeds, it is clear that a major change of approach is required. Search for new treatments

This project aims to identify new treatment options with alternative modes of action.

Attitudes about fenceline hygiene vary between regions. Clean fencelines in southern and western regions were driven by the need to have a good firebreak, with most farms already managing these non-cropped areas. However, in the north, fenceline management is a mixed picture.

To improve this situation, it needs to be more widely understood that fenceline weed management can no longer be an ad hoc farm practice. It requires a dedicated application of treatments, either pre-emergence to weeds or at the early growth post-emergence stages.

Finding suitable treatments for irrigation channels is a more difficult task. The list of ‘aquatic approved’ products from which to select is much more restrictive, but potential options are being evaluated.

The key aim of this project is to collect data to support registration of new weed treatments with the Australian Pesticides and Veterinary Medicines Authority.

Once registered, these options will be promoted to enable growers to better protect their crops from windblown weeds.

GRDC Project Code US00084
More information:
Tony Cook, 02 6763 1250,

More information:

Richard Daniel,
0428 657 782,;

Dr Michael Walsh,
02 6799 2201,

Tony Cook,
02 6763 1250,


Decide who is calling the shots


Narrow rows aid the battle for living space

GRDC Project Code NGA00003, US00084

Region North