SA trial assesses different weed strategies

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Image of Dr Chris Preston.

Dr Chris Preston, from the University of Adelaide, told growers at the GRDC Crop Update at West Wyalong, New South Wales, that it was important to use long-term and integrated weed management to lower the weed seedbank.

PHOTOS: Nicole Baxter

Key points

To manage herbicide resistance:

  • keep weed numbers low
  • incorporate different herbicide modes of action and rotate
  • stop weed seeds from re-entering the seedbank
  • use non-herbicide weed management tactics 

Weeds, more than ever, can determine the size of a crop’s profit margin, which is why research into weeds management is seeking to extract every possible tactical edge in this ongoing agronomic and economic struggle

A long-term trial has clearly shown the gains that can be made by ensuring there is no let-up, even after apparent success, in suppressing the weed seedbank.

The trial, at Roseworthy in South Australia, sought to determine the impact of management strategies on the seedbank over four years, from 2009 to 2012.

Three different management strategies were tested (Table 1) to reduce a ryegrass population resistant to Group A and B herbicides.

After the first year of oaten hay, Dr Chris Preston, from the University of Adelaide, says the research team used low, medium and high levels of management and then measured the seedbank.

In the trial, the oaten hay crop reduced the high seedbank, of more than 4000 seeds per square metre, by more than 80 per cent (Figure 1).

However, the trial also showed that any failure to keep pressure on the population (management strategy 1) caused the ryegrass population to rebound immediately.

Two years of strong control (management strategy 3) reduced the seedbank by more than 90 per cent and allowed a further reduction with a pre-emergent herbicide in the wheat crop.

Dr Preston says the more intensive management in the field pea phase and the subsequent reduction in ryegrass seedbank resulted in increased yields and gross margins in the subsequent wheat and barley phases (Figure 2).

He says management strategy 3 resulted in an extra $541.50 per hectare over three years compared with the lowest intensity management strategy (1) and was considerably more profitable than management strategy 2.

Image of a line graph showing the effect of different long-term weed management strategies.

Figure 1 Effect of different long-term weed management strategies on pre-sowing (March) annual ryegrass (ARG seedbank at Roseworthy, SA, 2009–2012.

Image of a bar chart showing effect of long-term weed management strategies on total grain production.

Figure 2 Effect of long-term weed management strategies on total grain production at Roseworthy, SA, 2009–2012.

 SOURCE: University of Adelaide

 Management strategy
Table 1 Management strategies investigated for annual ryegrass control* at Roseworthy, SA, from 2009 to 2012.
 2009  2010  2011 2012
 1 Low
 Oaten hay
Kaspa field pea
Trifluralin incorporated by sowing
Axe wheat
Boxer Gold® incorporated by sowing
Scope barley
Trifluralin incorporated by sowing
 2 Medium
 Oaten hay
 Kaspa field pea
Trifluralin incorporated by sowing plus Select® post-emergence
Axe wheat
Sakura® incorporated
by sowing
Scope barley
Trifluralin incorporated by sowing
 3 High
Oaten hay
Kaspa field pea
Trifluralin incorporated by sowing plus Select® post-emergence plus Roundup PowerMax® crop-topped
Axe wheat
Boxer Gold® incorporated by sowing plus Roundup PowerMax® crop-topped
Scope barley
Trifluralin incorporated by sowing

* The annual ryegrass population was resistant to all of the Group A herbicides, including clethodim and all of
the Group B herbicides including intervix® and trifluralin.

Non-herbicide tactics

Consultant Greg Condon, from Grassroots Agronomy, says a long-term strategy for managing herbicide resistance using herbicide and non-herbicide tactics is important.

Some of his clients are using double breaks including crops of field peas, lupins or faba beans, either for harvest or brown manure, and then back to canola.

When growing field peas, he says growers can introduce diverse pre-emergent herbicides for grass control and sow later, which allows more knockdown opportunities before sowing.

Another option is long fallow using applications of glyphosate and paraquat through winter and spring, and then growing canola the following year to capture the benefits from stored water, carryover nitrogen and a double break on grass weeds.

Image of Greg Condon from Grassroots Agronomy.

Consultant Greg Condon, Grassroots Agronomy, says the
best crops in which to burn narrow windrows are lupins, canola and faba beans.

Mr Condon says crop-topping pulse crops and spray fallowing crops with heavy grass weed pressure are other alternatives.

He says two common non-herbicide options that are useful to drive down the weed seedbank are narrow windrow burning and making hay.

“You don’t have to make hay on every paddock every year. However, if you have a high-pressure paddock, making hay does drive down the weed seedbank,” he says. “But then it’s what you do after the hay that’s important.”

One suggestion is to follow the hay with canola or a double break crop.

Windrow burning

An alternative to hay making, Mr Condon says, is narrow windrow burning, where less than 10 per cent of the paddock is burnt. It involves a low, concentrated burn along the windrows.

Mr Condon says that over the years some of his clients have refined the chute used to concentrate the residue from the rear of the harvester.

“We were originally using chutes with 600 to 650-millimetre openings, which were too wide,” he says. “And just leaving the spinners off doesn’t work because you don’t achieve an intense fire.”

In Mr Condon’s experience, the best crops in which to burn narrow windrows are lupins, canola and faba beans.

In higher-yielding areas of southern New South Wales he says cereals can be “tricky” as whole paddocks tend to be burnt, which reduces the heat intensity in the windrow and therefore reduces weed-seed kill.

He says the new wheat variety LongReach Lancer has potential because it is a short variety, but other wheat varieties are more difficult. Barley is also difficult because whole paddocks tend to burn.

In Western Australia and SA he says most growers avoid grazing paddocks with narrow windrows; however, in NSW he has observed that Merinos tend to leave the windrows undisturbed. By contrast, cattle and crossbred ewes disturb windrows quite a bit.

In terms of the time to burn narrow windrows, he says mid-March is best, in light winds.

More information:

Dr Chris Preston,
08 8313 7109,;

Greg Condon,
0428 477 348,


Seed crusher eliminates need to burn


Team effort to solve the variability puzzle

GRDC Project Code UA00113, UA00121, UA00144

Region South, West, North