Southern growers' expanding weeds toolkit

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Fast facts

  • Weed populations are changing and require many different approaches to control.
  • A diverse cropping sequence provides the biggest range of opportunities to manage weeds.
  • Some form of harvest weed seed control is required in continuous cropping systems.

Photo of delayed brome grass in barley crop

Brome grass emergence is delayed in this barley crop, demonstrating the higher level of seed dormancy in cropping paddocks compared to non-cropped areas.

PHOTO: Sam Kleeman

Growers are taking a diverse approach to reducing weed seed-set to overcome herbicide-resistant weeds and shifting dormancy patterns

Continuous cropping, no-till farming and a shifting climate are among the trends contributing to changes in farming systems that have shaped the weed population growers face today. As weeds adapt to our farming systems, an ever-evolving weed management plan must take advantage of multiple opportunities for control.

The removal of tillage and the pasture phase from our farming systems has placed increasing pressure on herbicides to control fallow and in-crop weeds. While resistance to both glyphosate knockdown and post-emergent Group A and B herbicides is common, we are now starting to see resistance developing to pre-emergent herbicides.

Dr Peter Boutsalis from the University of Adelaide says low levels of resistance to trifluralin and tri-allate were detected in south-west Victoria during a random survey in 2014. “The only way to determine which herbicides will still control a weed population is to conduct a resistance test,” he says.

“To get the most out of pre-emergent herbicides we need to maximise their efficacy by understanding some of their characteristics and how they will perform under different conditions, such as with retained stubble or heavy rainfall.”


A shift in seed dormancy may also have contributed to a change in the type of weeds we are finding in crops. Growers in southern Australia have reported increasing density of brome and barley grass in their cereal crops. The adoption of earlier sowing or dry sowing is not the only explanation for the emergence of these weeds.

“Research has clearly shown that barley grass and brome grass populations collected from in-crop have higher levels of seed dormancy than those collected from non-crop situations, including fencelines, roadsides and long-term pastures,” Dr Gurjeet Gill says. These results suggest weed populations from intensively cropped paddocks are likely to have greater seed dormancy than those from mixed pasture-cropping situations.

“Because individuals with greater seed dormancy will escape pre-sowing weed-control tactics such as tillage or knockdown herbicides, over time we have selected for increased seed dormancy in barley and brome grass. The biotypes with low dormancy have been removed over time.”

Likewise, a shift in rainfall patterns to more summer rain at the expense of growing season rain in combination with reduced tillage has given summer weeds a free kick. Weeds such as fleabane and common sow thistle are able to quickly flourish when there are good spring/summer rains and can be very tolerant to glyphosate when plants are water-stressed.

Diversity works

The best defence against weed threats is to aggressively push the weed seedbank down by using tactics such as hay, harvest weed-seed management and vigorous, competitive crops and then work to keep it there with effective weed management in grain crops.

“The more diverse the farming system the more opportunities there are to reduce weed seedbanks,” says Dr John Kirkegaard from CSIRO Agriculture and Food. “Where practical, a pasture phase provides an excellent opportunity to manage weeds.”

In trials at Eurongilly, near Wagga Wagga, NSW, break crops such as canola and grain legumes provided a profitable option to manage grass weeds including herbicide-resistant annual ryegrass. Using different crops and different end uses allows different chemical and non-chemical control measures such as crop-topping in legumes and narrow windrow burning.

“A two-year break with complete control of ryegrass was required to reduce large seedbanks to manageable levels,” he says. “Fallow or different end uses such as hay or brown manure provide the opportunity to drastically reduce seed-set using non-selective herbicides with different modes of action (the double-knock).”

Competitive crops can also be very effective at reducing weed seed production by reducing the amount of light and other resources available to weeds. This is particularly valuable when combined with sowing early into warm soils, giving crops an advantage over weeds that germinate as pre-emergent herbicides wear off.

“The need for vigorous and competitive crop canopies has seen a recent trend back to narrower rows on some continuous cropping farms, particularly those in higher-rainfall areas,” Dr Kirkegaard says.

Other methods to increase the competitive ability of crops include sowing east–west to shade the inter-row space and practising good agronomy.

Harvest weed seed control

Dr Kirkegaard believes that some form of harvest weed seed control (HWSC) is essential in continuous cropping systems, particularly those that do not include hay crops or a high frequency of crops that can be crop-topped in the crop sequence.

HWSC exploits the common weakness of many grass weeds, such as ryegrass, in not shedding their seed before harvest. Narrow windrow burning is most commonly used in lower-rainfall areas and has the advantage that fewer nutrients are lost when compared to burning the whole paddock. However, many growers are now moving to chaff tramlining in which chaff is concentrated on to the wheel tracks in controlled-traffic systems, where it decomposes as it is trafficked and is easier to control with targeted spraying. Chaff carts are more suited to growers with mixed farms where the chaff can be fed to livestock in containment areas.

Adapting these techniques to the different environments in southern cropping systems will take time and is now the subject of GRDC investigations.

More information:

Dr Gurjeet Gill,
08 8313 7744,;

Dr John Kirkegaard,
02 6246 5080,


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GRDC Project Code UA00134, UA00149, UA00156, CSP00146

Region North, South