Weed management in stubble retained systems in the Wimmera and Mallee

Guidelines for growersBCG_portrait

  • Reducing rye grass concentrations in a cereal intensive cropping system requires effective strategies that reduce the soil seed bank, avoid the development of herbicide resistance and do not incur a yield penalty.
  • A combination of chemical, cultural and mechanical weed control options will extend the life of a herbicide by reducing resistance enabling a continuous intensive stubble retained cereal cropping system to continue.
  • The benefits of a stubble retained farming system, are acquired through improved soil health, improved water use efficiency and increased grain yield.

The overwhelming majority of farmers in the Wimmera Mallee region have adopted no-till farming practices and are retaining stubble. The retention of stubble has helped growers in these regions to improve soil structure, reduce wind and water erosion and recycle nutrients more effectively.

However, while the benefits of stubble retention are widely acknowledged and embraced, growers have been challenged by a number of weed management issues which have emerged as a result the change in farming practices.

BCG Research

Integrated weed management for ryegrass control (2009-2011)  

A three year research trial conducted by BCG from 2009-2011 determined the effectiveness of weed set control strategies by reducing the seedbank without reducing the yield in cereal-intensive cropping systems. The trial was established at Jil Jil  on a clay loam soil in the Mallee region of Victoria.

This trial compared a range of weed control strategies including crop topping, chaff carts, cutting for hay and narrow windrow burning to control ryegrass.

Figure 1: Windrow burning in action

Figure 1: Windrow burning in action

Figure 2: After burning the windrow for 20 seconds at 400 degrees all ryegrass seeds should be destroyed.

Figure 2: After burning the windrow for 20 seconds at 400 degrees all ryegrass seeds should be destroyed.

bailed hay

Figure 3: Cutting for hay.

To determine the effectiveness of each strategy soil cores from each plot was taken to measure ryegrass seed bank numbers. Rye grass, brome grass and wild oat head numbers, grain yield and quality were also measured.  Gross margins were calculated for each treatment.

The results showed that cutting for hay significantly reduced ryegrass heads and ryegrass seed emergence, however this treatment also produced the lowest gross margin ($/ha). While cutting for hay was the least profitable treatment it was by far the most effective at controlling the weed seed bank (Table 1.). The reduction in profitability needs to be balanced against the need to reduce the seedbank, so that future crops can be more profitable.

Table 1. Annual gross margins and cumulative gross margins 2009-2011; and rye grass panicle counts and seed emergence in 2011.

Gross margin ($/ha) Ryegrass heads in-crop (heads/m2) Ryegrass seed emergence (plants/m2)
Treatment 2009 2010* 2011 2009-2011 2011 2011
Chaff cart 156 683 498 1337 127 2511
Crop topping 132 667 482    1281 99 1169
Windrow burn 156 683 498 1337 76 1870
Hay 147 301 502 950 7 72
Control 156 527 498 1181 94 1703
Sig. Diff. P<0.001 P<0.001 P=0.019 P<0.001 P<0.001 P<0.001
LSD (P<0.05) 7 41 11 43 48 795
CV % 2.5 3.8 1.2 1.9 20.7% 54%

*2010 delivered higher than average rainfall and this is reflected in the gross margins.

From 2009 to 2011 ryegrass emergence increased for all treatments except cutting for hay which had significantly lower ryegrass emergence. By cutting for hay you are preventing ryegrass seed set therefore reducing the amount of ryegrass emergence the following year.

wordpress graph

Figure 1. Ryegrass population as influenced by harvest weed seed management.

Chaff cart

Using a chaff cart was the least effective at reducing ryegrass levels; ryegrass emergence more than doubled over the trial period. A late harvest allowed the ryegrass to lodge and shed seed and increase the seedbank, reducing the efficiency of the chaff cart. The use of the chaff cart had a cumulative gross margin which was significantly higher than that produced by the hay and crop topping treatments or the control, but was equivalent to windrow burning.

Chaff collected can be used as sheep feed with research suggesting only 6 per cent of weed seeds pass through the sheep undigested. By utilising chaff carts and retaining stubble cover, the positive effects of stored soil moisture, less wind and water erosion, fewer weeds and increased soil microbial activity are possible.

Crop topping

From 2009-2010 there was a reduction in ryegrass heads but a significant increase in seedbank emergence. This trial demonstrated that crop topping is highly dependent on weed growth stage. Crop topping can be used to pick up the left over weeds still present as a late post emergent. A potentially major risk of crop topping is increased herbicide resistance especially those that also play a role in other areas of the farm systems such as glyphosate.  It is therefore vital, wherever possible, to rotate herbicide choice, i.e. gramoxone or diquat, or combine it with other practices such as windrow burning to limit the number of escapes.

Hay cutting

After 2009 cutting for hay reduced ryegrass seeds in the seedbank by 78 per cent and also reduced ryegrass heads by 92 per cent in the same year. A herbicide application, in conjunction with cutting for hay, will reduce ryegrass heads that would have set seed. Initially there was a reduced gross margin, and growers should be aware the benefits of cutting for hay, that predominantly come from reducing the seedbank, may take a few years.

Windrow burning

Previous studies in windrow burning have shown that the success of this treatment is reliant on the temperature of the burn. In this trial ryegrass head counts were reduced but ryegrass emergence increased. Potential reasons why this treatment was not as effective as expected could be due to insufficient biomass not achieving the adequate temperature to kill the seeds and/or the weeds may have lodged before harvest and didn’t pass through the header and consequently was not raked into the windrow. To destroy a ryegrass seed the biomass must sustain a burn of 400°c for 20 seconds.

In summary cutting for hay is the most efficient for reducing the weedseed bank. Chaff carts were not seen as effective because during 2009 the crop and weeds lodged and consequently was lower than the cutter bar which prevented material from passing through the header. Windrow burning and crop topping are both highly dependent on the right timing and conditions.

BCG crop sequencing and brome grass trial 2012-2014

BCG crop sequencing trial work looked at how a range management tactics over three years with different rotations influenced brome grass populations.

It was concluded that when brome grass numbers are increasing the use of break crops for at least two years can significantly decrease the weed seed bank. Continued crop and fallow management is required to maintain low weed seed numbers.

BCG row direction and row spacing trial 2015

Crop sowing direction can have an impact on weed growth and seed production in cereal crops. (Brooke, G 2014).

The ability of row direction to influence weed suppression will be dependent on factors that affect light availability: latitude and time of year (sun angle), and crop type and the season (affecting canopy size and competition).

In 2015, a low rainfall year (Decile 1), a BCG trial found no effect of sowing direction or row spacing on grain yield. Weeds established faster where row spacing’s were wider (38cm), but by late tillering all treatments had similar weed numbers (Angel, K. 2015).

Row direction had no influence over weed biomass production or seed set. It is likely that the seasonal conditions were too dry to develop a sufficient canopy that could offer competition for light and nutrients, so no potential differences in orientation could be realised.

When conditions are right for row direction to be effective, it can be a cheap, non-chemical alternative to add your suite of weed management strategies.

Further research

  • What are the effects of livestock grazing stubble and the implications for sowing difficulties into trampled stubble?
  • Does the structure (architecture) and quantity of the stubble of various crops differ in maximizing the benefits in a stubble retained system?

For further information regarding the opportunities present for growers to expand their weeds approach view https://www.youtube.com/watch?v=mkds8En3Iu4

Further information

  • Developments in stubble retention in cropping systems in Southern Australia – GRDC December 2013
  • BCG crop and pasture manual production manual 2005-2006 – Managing Resistant Ryegrass

By  Jessica Lemon
BCG Research and Extension officer
E: jessica@bcg.org.au

GRDCLogoStacked_TM_CMYKThis research is being conducted by BCG 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 the Birchip Cropping Group (BCG) 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. BCG 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. BCG 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 for BCG 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.
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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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