Low rainfall grass weeds need whole-of-system strategy

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Photo of Rick Llewellyn presenting at a field day

Rick Llewellyn is part of a team evaluating options to control difficult grass weeds in the low-rainfall zone.

PHOTO: Courtesy of Rick Llewellyn

Fast fact

  • CSIRO and Mallee Sustainable Farming are offering workshops to growers and advisers who would like to evaluate the options for brome and barley grass over several years.

Tools to help growers and advisers evaluate a multi-pronged approach to weed control over several years have been developed and are being used to assess the most profitable strategies for difficult grass weeds

Brome grass and barley grass are prospering under no-till systems in the low-rainfall zone, but many of the weed control options that work well on other major weeds, such as annual ryegrass, are not as effective on these weeds. Herbicide options are limited and with these weeds being common on potentially erodible sandy soils it also limits the appeal of cultivation and burning as control methods. The need to consider whole-of-farming system management strategies has become clear.

Reducing seed-set

Seed retention of brome grass at cutting height of more than 15cm at 0, 14 and 28 days after barley crop maturity

Figure 1 Seed retention above harvest cutting height (>15cm) of brome grass at 0, 14 and 28 days after barley crop maturity. Bars represent SE values around the mean.

Work conducted by Mallee Sustainable Farming (MSF) and the Eyre Peninsula Agricultural Research Foundation (EPARF) has shown that harvest weed seed control will not be as effective on weeds that shed seeds early, such as brome grass and barley grass, as it has been for ryegrass, with more than 50 per cent of seed often already shed by harvest time (see Figure 1).

Clearfield® technology is still widely effective in many low-rainfall districts, but there is the risk of herbicide resistance and persistent residues limit the flexibility of future crop choices.

As post-emergence options are limited and cheap pre-emergence herbicides can be unreliable, the importance of crop competition to suppress weeds has become more critical in the management of seedbanks.

Recent work as part of the GRDC’s ‘Maintaining profitable farming systems with retained stubble’ initiative has shown that improving crop density can often reduce grass seed-set by more than one-third. In one example, sowing into the previous year’s crop furrow on non-wetting sandy soil at Karoonda, South Australia, in 2014 and 2015, provided better crop establishment leading to higher early crop growth and major brome seed suppression (72 per cent less seed-set) when compared to sowing into the inter-row space.

Dedicating whole crop or pasture years to preventing brome and barley grass seed-set is also an effective option that was demonstrated by MSF research on brome in crop rotations near Mildura (see ‘MSF Two-year breaks’).

Testing the options

With no silver bullet solutions, it is not easy for an adviser to say whether a practice that doesn’t lift yield in any one year and only reduces weed seed-set by less than 50 per cent is worth the investment. But it is a question many growers are asking and the answer can only be provided by considering the whole cropping system across multiple years.

Modelling tools have been developed to help growers and advisers weigh up the costs and benefits of particular weed control practices in the medium term. The Ryegrass Integrated Management tool has been adapted for brome grass by CSIRO and other tools such as the Land Use Sequence Optimiser can be used to evaluate the impact of grass management on different crop rotations.

Growers and advisers in the southern region who are interested in applying these tools for brome grass and barley grass control can contact Dr Llewellyn or Michael Moodie (at MSF) to learn about a series of workshops being offered as part of the GRDC project.

More information:

Dr Rick Llewellyn,
08 8303 8502,
rick.llewellyn@csiro.au;

Michael Moodie,
03 5024 5835,
michael.moodie@msfp.org.au

Useful resources:

Two-year breaks profitably reduce agronomic constraints in the Northern Victorian Mallee – Mallee Sustainable Farming

Ryegrass integrated management – Australian Herbicide Resistance Initiative

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New options for southern fenceline weed control

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Tackling the brome grass seedbank

GRDC Project Code CSP00186, MSF00003, EPF00001

Region North, South