Cost-effective weed control in the Mallee

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Photo of Cameron Taylor

Birchip Cropping Group’s Cameron Taylor.

PHOTO: Rebecca Barr, AgCommunicators

Better use of rotations and cultural methods, rather than reducing chemical rates, is likely to be more cost effective in controlling weeds in the Mallee, according to a Birchip Cropping Group (BCG) and University of Adelaide trial at Pira in Victoria.

BCG commercial services manager Cameron Taylor says although the trial is still in progress, they have already identified that when both wild radish and brome grass are problematic, chemical control alone is unlikely to be a cost-effective solution.

“It is important in the Mallee to minimise costs, as grain yields are often not high enough to justify high input costs. However we’ve shown that it is not a good idea to try and reduce costs by reducing application rates,” Mr Taylor says.

The trials are measuring the weed seedbank over three years, across eight chemical and rotation treatments, with a focus on brome grass and wild radish. The site has confirmed Group B resistance in wild radish. A summary of key results from the first year is shown in Table 1.

“We’ve seen in the first two years that the district practice of a relatively late, low-rate Tigrex spray is ineffective at reducing the weed seedbank and potentially leads to resistant plants,” Mr Taylor says.

“Best-practice chemical control, including multiple herbicide chemistry groups, has been effective at controlling the weeds but is too expensive to be practical.”

Mr Taylor recommends growers use crop rotation and cultural control such as narrow windrow burning rather than chemical control alone to keep on top of weeds. Break crops allow the use of different herbicides.

“While Clearfield varieties can be very effective at controlling brome grass, there is a big risk of IMI (imidazolinone) resistance, so break crops including legumes can be a safer option,” he says.

The GRDC-funded crop-sequencing project, which ran from 2009 to 2013, found in trials at Chinkapook and Mildura that incorporating a two-year break into a rotation was more profitable over four years than continuous wheat, as long as at least one of the breaks made a profit.

A two-year break was found to reduce brome grass populations from 450 plants per square metre in continuous wheat to 6 to 12 plants/m2 after a two-year break including legumes.

Treatment Early application (GS13)
Standard application (mid-late tillering)
Total season herbicide cost* ($/ha)
Estimated wild radish seed set (sdds/m2)
 Final wild radish count (plants/m2)
Final brome grass count (plants/m2)
Table 1 BCG Pira trial results – 2014.
District practice
Nil Tigrex® 750ml/ha
67.3 885 7.4 2.9
ClearfieldTM best practice
MidasTM 900ml/ha then JaguarTM 500ml/ha
PreceptTM 1L/ha and LexoneTM 100ml/ha
114.8 0 0
0.4
Break crop: peas (brown manure)
Nil
Nil 55.6
1 0.3
0

*This cost includes knockdown, pre-emergent, early, standard and crop-topping costs. For full results contact Cameron Taylor (details listed below).

IMI Resistance in brome grass in the Mallee

While imidazolinones (IMIs) are one of the most useful herbicides for controlling brome grass in the Mallee, growers should understand the risk of resistance, with GRDC-funded surveys showing Group B resistance is on the rise.

University of Adelaide weeds researcher Dr Peter Boutsalis says that between 2010 and 2015 IMI resistance in ryegrass in western Victoria increased from 18 per cent to 31 per cent.

In the 2015 survey, 15 per cent of brome grass samples in western Victoria were resistant to sulfonylurea (SU) herbicides.

No brome grass samples were resistant to the IMI herbicides in any region in South Australia or Victoria, however increased reliance on this herbicide could change this situation.

“Brome grass resistance to Group B SU herbicides is increasing quickly,” Dr Boutsalis says.

“For example, SU resistance has gone from zero to 50 per cent in the SA Mallee since 2010.”

Dr Boutsalis says that IMI resistance can form even when the chemistry has never been used on a weed population, due to cross-resistance with SUs.

“Unlike some other resistances, Group B resistance will not go away over time if the chemistry isn’t used for a few years,” he says.

“The mutation that causes Group B resistances doesn’t disadvantage the plant so it will just remain in the population. This means simply taking a break from SUs and IMIs isn’t enough to reduce resistance.”

Dr Boutsalis’s tips for controlling IMI resistance

  • Reduce reliance on chemical control by using rotations and harvest weed-seed collection for weed control.
  • Healthier crops will compete better against weeds by reducing seed-set.
  • Pre-emergent herbicides that are cost effective for the Mallee are unlikely to give full control, but even 50 to 60 per cent control will leave fewer weeds to manage later in the season.
  • Remember that resistance starts when the plant sets seed, not after a spray. If weeds are not dying after an IMI application, go back and do a double-knock, crop-top, or collect the weed seeds at harvest to stop the resistant plants setting seed. It might be necessary to patch out small regions of a paddock.
  • Perform resistance testing. There are a number of different types of resistance and sometimes there might be cheaper chemical options still available. Testing can be performed at any stage of plant growth.

More information:

Cameron Taylor, BCG commercial services manager,
03 5492 2787,
cameron@bcg.org.au;

Dr Peter Boutsalis, University of Adelaide and Plant Science Consulting,
0400 66 44 60,
info@plantscienceconsulting.com.au;

Crop Sequencing Project;

Long-term strategy for brome grass control

Risk of addiction to IMIs’ – GRDC Update paper

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Summer weed control needs to set up next season

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GRDC Project Code UA00149, UA00144, DAS00119

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