Weed management in stubble retained systems in the south west

Guidelines for growersSFS_FINAL_Logo

  • Weed numbers are dynamic across a paddock. Any intervention to decrease weed seed set is not going to deplete the seed bank within a season.
  • Narrow windrow burning and harvest weed seed destructors are viable options when presented with the right seasonal conditions.
  • When controlling ryegrass in a canola crop using clethodim, ensure steps are taken to apply it correctly to improve efficacy and avoid developing resistance.
  • Weed management should be approached as a whole farm system and control should not be relied on by any single practice.

The burden of herbicide resistant weeds is an increasing problem for growers in the Victorian western district. For a region that is relatively new to the cropping game, the rise of herbicide resistance has been rapid in comparison to other cropping areas. The long growing season in the high rainfall zone (HRZ) generates an array of challenges, including a larger window for weed germination, sub-optimal spraying conditions and stubble loads that restrict the use of integrated weed management (IWM) techniques that have been effective in drier cropping zones.

Harvest weed seed capture

Common district practice embraces the integration of livestock on stubbles over summer, with the aims of reducing stubble load, grazing weeds and utilising lost grain from harvest. These attempts range in effectiveness, often resulting in a paddock being burnt in autumn because the producer is looking for greater ease of sowing and improved pre-emergent herbicide efficacy. There is also the broad consensus that harvest weed seed capture techniques do not work in the high rainfall zone due to the heavy stubble loads and a tendency for the grass weeds to lodge before harvest.

Southern Farming Systems (SFS) conducted a paddock scale trial in a wheat crop at Mininera, Victoria over the 2014 harvest to determine the effectiveness of harvest weed seed capture techniques in high yielding paddocks in the Victorian high rainfall zone.

The site has a mean annual rainfall of 526mm and would typically record cereal yields exceeding 6t/ha, however with the driest spring on record a 3t/ha grain yield was achieved, accompanied by a 5.4t/ha stubble load.

Three treatments: a conventional burn, windrow burn, and a harvester integrated weed seed mill, were aimed at control of annual ryegrass (Lolium rigidum) and great brome (Bromus diandrus).The paddock was harvested on 12 December 2014 with a Case IH Axial Flow 9120 at 15cm above the ground. Weed counts after sowing were completed on 16 June 2015.

There was an observable contrast in weed populations among treatments, but they were not significant. The windrow burn treatment appeared to have greater effect on weed reduction however the variability of the trial area produced a high coefficient of variance which rendered the data insignificant.

Table 1. Mean weed populations in 2015 after different harvest procedures in 2014.

Weed seed treatment

Annual ryegrass (plants/m2)

Great brome (plants/m2)

Conventional burn



Windrow burn



Weed seed mill



LSD (P=0.05)



CV (%)



The statistics in Table 1 do not convey the full story about getting weed seeds into the harvester. The effectiveness of any of the treatments is directly influenced by the capacity of the machinery to consume the reproductive body of the target weed. Pre-harvest cuts done across the trial revealed that as much as 35 per cent of annual ryegrass and 34 per cent of great brome seeds are located between 0-15cm above ground level. This begs the question why don’t we harvest lower? For a farmer in the high rainfall zone the proposition of harvesting lower then 15cm in a wheat crop would mean a substantial compromise in harvest efficiency.

Harvest height

Harvest weed seed trials in a 3.4t/ha Bolac wheat crop showed 65 per cent of weed seed were collected by the harvester in 2014 at Streatham. In a similar trial at Inverleigh in a short Hindmarsh barley crop 95 per cent of weeds seeds were collected when crops were harvested at 15cm high.

Table 4. Weed seed capture for harvest weed seed capture trials at Streatham and Inverleigh.



Streatham HWS

Inverleigh HWS
Comb height
>15cm (seeds/m2)

Comb height
>15cm (seeds/m2)




Giant Brome


Wild Oats


Comb height
<15cm (seeds/m2)

Comb height
<15cm (seeds/m2)




Giant Brome


Wild oats


Disc vs tyne sowing

In paired test plots comparing disc and tyne seeders at two sites there was on average 0.65 ARG/m² in the tyne sown areas compared to 0.19 ARG/m² in the disc areas (data not presented).

Use of clethodim to control annual ryegrass in canola

Break crops offer the opportunity to control grass weeds in a rotation. Group A resistant annual ryegrass (ARG) is an increasing issue in stubble retained and cultivated farming systems.  Clethodim (Select®) is popular choice of Group A herbicide to tackle ARG in canola crops and it is important that it be applied correctly to avoid resistance.

Improving the efficacy of spraying clethodim is key to increasing strike rate.

  • Poorly timed application of clethodim reduced the chemical’s efficacy. Weather conditions, machinery and water quality also play an important role, not just nozzle selection and rates.
  • Clethodim can be sprayed from two leaves to tillered growth stages but applying at early growth stages will enhance control of annual ryegrass (ARG) populations significantly. Controlling ryegrass early will also reduce seedbank and enhance crop yields.
  • When ARG was left to spray at late tillering, shading of weeds by crop and cooler weather reduced herbicide coverage.
  • At early growth stages of annual ryegrass, no significant difference was observed when using flat vs angled nozzles. Using angled nozzles provided greater control of ryegrass populations at later growth stages by increasing coverage, which shortened time taken to kill ryegrass.
  • Spraying clethodim 1-3 days before or after frost occurrence will reduce the product’s efficacy.

The benefits of capturing weed seeds at harvest has been established as a valid substitute to conventional practice in the western district. The drawbacks of this research is that it doesn’t define the full potential of these harvest weed seed capture. Unless is it paired with cumulative years of the treatment with a focus on weed reduction we cannot conclusively know the advantages of these techniques in high yielding crops. The fundamental advantage of harvest weed seed capture is increasing the ability to retain stubble in these systems. Any benefit to weed reduction is a bonus when we recognise the long term numbers game that it is.

One method alone is not going to be a silver bullet when it comes to the battle against weeds. The use of harvest weed seed capture when paired with other integrated weed management tactics such as ryegrass control in a breakcrop phase and sowing method, is profitable in the high rainfall zone.

By Aaron Vague
Research and extension officer
Southern Farming Systems
E: avague@sfs.org.au

GRDCLogoStacked_TM_CMYKThis research is being conducted by SFS as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 ‘Maintaining profitable farming systems with retained stubble in Victoria and Tasmania’).

Disclaimer: Any recommendations, suggestions or opinions contained in this publication do not necessarily represent the policy or views of Southern Farming Systems (SFS) or the Grains Research and Development Corporation (GRDC). No person should act on the basis of the contents of this publication without first obtaining specific, independent professional advice. SFS and GRDC and contributors to these guidelines may identify products by proprietary or trade names to help readers identify particular types of products. We do not endorse or recommend the products of any manufacturer referred to. Other products may perform as well as or better than those specifically referred to. SFS and GRDC will not be liable for any loss, damage, cost or expense incurred or arising by reason of any person using or relying on the information in this publication.
Stubble project overview: This guideline has been developed by SFS Farming Systems Group as part of the Maintaining Profitable Farming Systems with Retained Stubble initiative, funded by the Grains Research and Development Corporation (GRDC). The initiative involves farming systems groups in Victoria, South Australia, southern and central New South Wales and Tasmania collaborating with research organisation’s and agribusiness to explore and address issues for growers that impact the profitability of cropping systems with stubble, including pests, diseases, weeds, nutrition and the physical aspects of sowing and establishing crops in heavy residues.

About BCG

Birchip Cropping Group Inc. (BCG) is a not-for-profit agricultural research and extension organisation led by farmers in the Victorian Wimmera and Mallee.
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