Field trials examined the persistence of weed seeds under a range of treatments, including different crop rotations.
Weeds that are glyphosate resistant or tolerant and difficult to control, are making the control of summer fallow weeds in the north more complex; adopting a diverse range of tactics can help keep weed seedbanks low.
The cropping region of northern Australia has a diverse range of cropping systems and weeds. A fallow phase is commonly required between crops to enable the accumulation of stored soil water in these farming systems dominated by reduced tillage.
During the fallow phase, weed control is important and is heavily reliant on herbicides, the most common of which has been glyphosate.
As a result of an over-reliance on glyphosate for fallow control of weeds, there are now seven weed species confirmed as being glyphosate resistant:
awnless barnyard grass
sweet summer grass.
In addition, a reduced-tillage system in combination with reliance on glyphosate has resulted in difficult-to-control, glyphosate-tolerant weeds such as feathertop Rhodes grass dominating summer fallows.
As a result, the control of summer fallow weeds is becoming more complex.
Dr Michael Widderick, principal research scientist with the Queensland Department of Agriculture and Fisheries (DAF), is leading a project focusing on awnless barnyard grass, flaxleaf fleabane, liverseed grass, windmill grass, common sowthistle and feathertop Rhodes grass.
Areas of research include weed ecology, chemical and non-chemical tactics, glyphosate resistance and resistance surveys.
Three field experiments on the seedbank dynamics of barnyard grass demonstrated the impact of crop rotations and agronomic practices on emergence and long-term persistence of seed in the soil.
A wheat/summer fallow rotation had up to 50 per cent less emergence of barnyard grass seedlings compared with other crop rotations, and regular tillage decreased barnyard grass emergence.
However, zero-tilled fallows with high levels of wheat stubble increased the window of barnyard grass emergence.
Two field trials have been conducted to quantify the persistence of seed of feathertop Rhodes grass and windmill grass planted at 0, 2 and 10 centimetres below the soil surface.
For both grasses, the persistence of seed decreased rapidly for the first six months and by the end of the experiment (18 months) no seed of either grass had persisted.
Seeds of feathertop Rhodes grass persisted for 10 to 12 months irrespective of the depth of burial, while less than one per cent of windmill grass seed persisted up to 12 months after burial at 2 and 10cm.
The results of both trials indicate that the prevention of seed set in both feathertop Rhodes grass and windmill grass will result in rapidly depleted seedbanks.
The impact of different forms of tillage on seed burial and subsequent emergence was investigated in three field and two pot experiments using seed of barnyard grass, feathertop Rhodes grass, windmill grass, liverseed grass, sowthistle and fleabane.
In all three field experiments harrows had the least seed burial (majority in 0 to 2cm) and one-way discs the most (majority buried below 5cm); however, different weed species responded differently to these treatments.
Emergence of fleabane was reduced by the greatest amount irrespective of tillage treatment compared with zero tillage, with the one-way disc treatment producing the greatest reduction in emergence.
Similarly, the emergence of feathertop Rhodes grass seedlings was reduced by all types of tillage, with the one-way disc treatment being the most effective.
One field experiment in southern Queensland was used to investigate the impact of burning feathertop Rhodes grass patches on surface soil seed viability.
Burning significantly reduced the number of viable feathertop Rhodes grass seeds on the soil surface by an average of 93 per cent. This may be a useful tactic to target small patches of the weed.
Dr Widderick says herbicide options have been identified in the field.
“Glyphosate alternatives have been recently identified for the summer grass species, including knockdown and residual treatments at Queensland DAF’s research farm where we have an established population of barnyard grass and feathertop Rhodes grass,” he says.
As a result of confirmation of glyphosate resistance in common sowthistle, work has commenced to identify and evaluate glyphosate alternatives for this weed, including identifying double-knock treatments and defining optimal time between knocks.
In addition, a coordinated and comprehensive survey of common sowthistle glyphosate resistance is taking place across the northern region.
Dr Michael Widderick
07 4639 8856
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