Researchers building multi-weapon summer weeds arsenal

Key points

  • A range of options is needed for effective summer weed control
  • Researchers are working on control options for awnless barnyard grass, feathertop Rhodes grass, windmill grass, liverseed grass, sowthistle and fleabane
  • All these summer weeds have populations that are either resistant to glyphosate or naturally tolerant of glyphosate (such as feathertop Rhodes grass)

A leading northern region weeds researcher explains the multi-pronged approach that needs to be developed to control summer weeds

Photo of Feathertop Rhodes grass

Feathertop Rhodes grass is pictured as a flush of green. It has emerged from seed already distributed prior to burning. In this situation burning may be useful as a salvage operation.

Resistance to the all-round herbicide glyphosate has profoundly changed and complicated weed management in the northern region, especially during the summer fallow when weeds thrive with no competition from crops.

With no single answer to weed control, the situation poses a sizeable challenge to weeds researchers such as Dr Michael Widderick from the Queensland Department of Agriculture, Fisheries and Forestry (DAFF).

Dr Widderick and colleagues are working on a range of control tactics, including residual herbicides, double-knock approaches, strategic tillage and burning, underpinned by an improved understanding of weed ecology.

The weeds in question – awnless barnyard grass, feathertop Rhodes grass (FTR), windmill grass, liverseed grass, sowthistle and the notorious fleabane – are all a problem in summer fallows: “And all have populations either resistant to glyphosate or naturally tolerant to glyphosate (such as FTR). Plus we want to preserve the chemicals we use now, or may use.”

Photo of burning grass patches at research trials

Feathertop Rhodes grass patches being burnt in research trials.

The GRDC-supported research is multifaceted and at different stages but is already showing that no one tactic is going to provide 100 per cent control 100 per cent of the time.

The research – in the glasshouse and in the field – is also showing that control strategies vary in effectiveness between weeds and approaches.

For example, research into the effect of double knocks on barnyard grass, FTR and windmill grass found optimum intervals between knocks differed among the three species, particularly for the glyphosate-based double knocks.

The research trialled two double-knock approaches, one with glyphosate and then paraquat and a second with a Group A herbicide (PER13460, PER12941, PER11163) followed by paraquat.

For the glyphosate-based double knock, the best intervals were varied; a seven-day interval was found best for controlling FTR, a seven to 10-day interval best for windmill grass and a 14-day interval best for barnyard grass.

This information helps growers to better time their double-knock applications, Dr Widderick says. “The delay between knocks needs to maximise the efficacy of both the first and second knock. If the delay is too short, the first knock may not have had full effect and if the delay is too long, any survivors of the first knock may become too big, therefore putting too much pressure on the second herbicide application.”

For the Group A-based double knock, intervals of one, two and four days between applying the Group A and then paraquat were optimum across species and experiments.

“But we need to be wary about using Group A herbicides in fallow, as they are designed for in-crop use, and herbicide resistance to this group develops very quickly, often within six to eight seasons.”

To broaden the herbicide ‘toolbox’, Queensland DAFF’s research is also considering the efficacy of residual herbicides, work that began in August and will have results early next year.

Although residual herbicide use does restrict rotational choices, Dr Widderick says that once tested and registered, these herbicides may offer growers an extra tool, but he warns that any complacency towards their use could lead to herbicide resistance.

As ‘last resorts’, two practices are also being investigated – tillage and burning – with tillage showing mixed results, mostly due to seasonal conditions.

Photo of single-pass tillage

Researchers use single-pass tillage with finger harrows to influence weed emergence.

For example, a wet season in the first year’s trial saw high levels of weed emergence for all six weeds (awnless barnyard grass, FTR, windmill grass, liverseed grass, sow thistle and fleabane) in zero-tilled plots compared to tilled plots.

“We thought, ‘Wow tillage could work’. However, these results changed as the seasons changed over the course of the next two years,” Dr Widderick says.

In the third year, it was hot and dry and zero-tilled plots generally had the least amount of emerged weeds. Dr Widderick thinks this is because the hot weather decreased the viability of surface-lying weed seeds.

“There is more to answer in regards to tillage and weed emergence and weed-seed persistence. Tillage is likely to be more of a salvage option because you don’t know its potential effect until the season is upon you.”

For the moment, he concludes that soil disturbance does have an impact – with one-way disc treatment seemingly superior to harrow use – but results vary across seasons and weeds.

Field-trial data found that seed burial depth had a big impact on weed emergence. In general, emergence decreased as the depth of burial increased for all weeds apart from liverseed grass, which had the highest emergence from seeds buried below two centimetres.

Burning was tried too, but is also seen as a salvage option. In 2013, Queensland DAFF researchers investigated the impact of burning FTR patches, finding that burning reduced the numbers of viable seeds on the soil surface by 93 per cent, but this still had little impact on reducing overall seed set.

With no competition against weeds in a fallow, another option for weed control could be cover cropping. “We are in the initial stages of this work and need to monitor how much stored soil moisture is used up.”

The cover crops being trialled in different combinations are French millet, lablab beans and cowpeas. “It’s a new area of research for us but we will hopefully find a fit for cover crops in our summer fallow phase,” Dr Widderick says.

He is hopeful that all measures can contribute to a suite of strategies: “The key is to reduce the seedbank.”

More information:

Dr Michael Widderick,
07 4639 8856,

A fact sheet on Group A in fallow is available at:

A fact sheet on Effective double-knock herbicide applications – northern region is available at:


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