Break crops in retained stubble systems in the Wimmera and Mallee

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  • Crop sequences are most profitable when break crops are included to solve agronomic problems (grass weeds, root disease, low soil N).
  • Trial work has demonstrated that crop sequences involving a broadleaf break crop can be more profitable than continuous wheat. This is despite break crops sometimes being less profitable than continuous wheat in an individual year.
  • Termination timing of legumes will affect biomass production, soil water and nitrogen fixation. Vetch biomass of 3-5 t/ha can fix a total of 200-300kg N/ha. An early termination of vetch (at 2 t/ha biomass) can be lower risk in the Mallee if the seasonal outlook is dry.
  • Standing stubble improves the micro-climate for lentils and increases yield, regardless of seasonal conditions.

A break crop is sown to provide diversity to the cropping rotation. In the Wimmera and Mallee, legume and oilseed crops and pastures are considered break crops in a traditionally cereal dominant cropping system.

Broadleaf break crops can be perceived to be higher risk than cereals in medium to low rainfall environments due to lower water use efficiency and poor profitability in drought years. However, in recent years, profitability has varied with high value break crops.

Break crops increase flexibility in the management of weeds, diseases and nutrition. Despite sometimes being less profitable than continuous wheat in the year in which break crops were grown, trial work has demonstrated that crop sequences involving a broadleaf break crop can be more profitable than continuous wheat.

BCG research

GRDC water use efficiency break crop trial (2009-2012)

An experiment was established in 2009 to identify low risk, profitable break crops and end-uses (grain, hay, brown manuring) and quantify their benefits to subsequent wheat crops. At a site near Hopetoun in the Mallee region of north west Victoria on two soil types (clay and sand), three different broad-leaf break crops, wheat and long-fallow were grown in 2009 or 2010 and followed by wheat in 2010 and 2011.

Wheat grown after juncea canola, peas and vetch (sand soil) and vetch and peas (clay soil) yielded more than continuous wheat.

At the clay site pea hay in 2009 followed by wheat in 2010 and 2011 had the highest mean gross margin (Figure 1). At the sand site, canola grain, pea hay and vetch hay followed by wheat were more profitable than continuous wheat or fallow followed by wheat (Figure 1).

bcg break crops graph

Figure 1: Mean three year gross margin ($/ha) for each crop type and end use grown in 2009 followed by Correll wheat in 2010 and 2011 at the clay and sand sites.

As reported by BCG in the 2012 Australian Agronomy Conference Paper: Break Crops Pay in the Victorian Mallee, crop sequences are most profitable when break crops are grown to solve agronomic problems (grass weeds, root disease, low soil N) in continuous cereal rotations. The risk of losing money on break crops can be greatly reduced if growers make their crop choice based on the amount of soil water, nitrogen and timing of the autumn break, and if inputs are kept to a minimum and applied in response to favourable conditions, such as in-season top-dressing and fungicides, and end-use selection based on seasonal conditions (eg. cut for hay in a poor season).

Soil-borne diseases

Grass-free canola, mustard, chickpeas, field peas, vetch medic pasture and fallow can result in significant reductions in the inoculum in a cropping sequence but the rotation benefits last for only one crop season.

Non-cereal crops can be infected by rhizoctonia however, most do not allow the build-up of inoculum. Grass free canola and medic pastures reduce rhizoctonia inoculum levels resulting in subsequent cereal yield increases. Other legumes such as field peas, chickpeas, and vetch also showed limited or no inoculum build-up. Importantly, the effect of rotations generally lasted for one crop season only. (ref: Management of soil-borne rhizoctonia disease risk in cropping systems and How crop rotation and summer rainfall influence rhizoctonia).

Brome grass

Results from trials undertaken at Chinkapook in the Victorian Mallee, when sub-soil water was present 2011-2014, indicated that growing high value canola and legume break crops was profitable in the Mallee.

Where brome grass populations were high, break crop sequences of longer than two years were necessary to decrease numbers and keep them down. (Source: BCG 2014 Season Research Results, Weed Management, Crop sequences and brome grass, page 119).

BCG crop sequences graph

Figure 2: Annual gross margins ($/ha), cumulative gross margin ($/ha) and 2014 Brome grass numbers in-crop (plants/m2). Stats: 2011 gross margin LSD 126 $/ha; 2012 gross margin LSD 179 $/ha; 2013 gross margin LSD 106$/ha; 2014 gross margin CV 22%; Cumulative gross margin LSD 303 $/ha; Brome grass in-crop LSD 0.5 plants/m2.

Lentils

Standing stubble improves the micro-climate for lentils and increases yield, regardless of seasonal conditions.

Research results showed a greater yield advantage could be achieved from standing stubble rather than slashed stubble, especially when lentils are sown later.

Sowing lentils into standing stubble may provide further benefits to harvestability eg. increased biomass, better plant and pod height and less lodging.

Further information: Pulse Australia update 2014

Legume termination timing

The timing of legume termination (chemically terminated, cut for hay, grazed, crop failure, or grain fill) will affect biomass grown and subsequent legume residue. Legume termination timing will also affect soil water use and nitrogen fixation, which have consequences for the following cereal crop. These consequences will depend on the season the cereal crop experiences.

Research at Birchip between 2012-14 evaluated the effects of five termination timings (June, July, August, September and November) of March sown vetch on soil water, nitrogen and the performance of two subsequent wheat crops. Unfortunately the trial experienced three seasons of decile 1-2. However, the trial found:

  • Early termination (June, July) conserved soil water for the following wheat crop and reduced the risk of that crop haying off due to high nitrogen compared with later termination (August, September).
  • Vetch biomass production of around 2t/ha (fixing up to 120kg N/ha) is unlikely to cause a subsequent wheat crop to hay off if dry seasonal conditions (decile 3 or less) are forecast.
  • Residual nitrogen from vetch brown manure can carry over for two years and influence cereal crop growth, but the conservation of soil water is more critical for following crops when seasons are decile 2 or lower.
  • Vetch biomass of 3-5 t/ha can fix a total of 200-300kg N/ha. An early termination of vetch can be lower risk in the Mallee, where soil water is often limited and there could be a dry spring for the following cereal crop.

A Grain & Graze 3 trial at Beulah in 2014-2015, evaluated two termination timings (June and September) on May sown peas, vetch, medic and fallow. Again, two decile 1 seasons were experienced so crop production was low. In 2015 wheat grain yield did not respond to termination timing, but did have a small grain yield benefit (90kg/ha) for wheat that followed field peas or vetch.

Despite crops having more biomass in September than July, crops weren’t able to fix more nitrogen between July and September (they actually utilised soil nitrogen), probably due to the dry conditions. This may have implications for termination timing in poor seasons, and warrants further investigation.

BCG Farming Systems site 1999-2013

The BCG farming systems site was established in 1999 in response to conjecture from Wimmera and Mallee farmers about which farming system was most profitable, productive and sustainable. This systems trial existed for 14 years (1999-2013) and examined the following farming systems:

  • A ‘Fuel Burner’ system which featured mainly cereals, the regular use of tilled fallow (commenced prior to harvest), low intensity livestock and full disturbance tillage at sowing.
  • A ‘Hungry Sheep’ system which featured intensive cropping (mainly cereals), intensive grazing, winter lambing, grazing over summer to take advantage of stubbles and to control weeds, early sown cereal/pasture forage for feed and generally full disturbance tillage at sowing.
  • A ‘No-Till’ system which featured minimum soil disturbance seeding (knife points, press wheels, 30.5cm row spacings) and no livestock. Initially this system included the high use of break crops but later cereals were predominant and some chemical fallow.
  • A ‘Reduced-Till’ system which took a flexible approach – tillage or full disturbance sowing, mainly chemical weed control, a mix of cereals, canola and other break crops and, in early years, some livestock over summer.

The analysis conducted on the farming system trial since 2000 demonstrated that no one system is more profitable than any other (2012 BCG Season Research Results pp. 176). The research highlighted that it doesn’t matter which system you employ, provided you manage that system well.

The severe subsoil limitations discovered within the highly alkaline calcarosol soils at the site drove home understandings about the inability of such soils to support pulse and canola crops, particularly in dry seasons. The hard lessons learned by the ‘No-Till’ system which suffered failed canola and pulse crops in the early years helped growers appreciate both the challenges of subsoil constraints and the value of a chemical fallow. Once the ‘No-Till’ rotation was altered to a continuous cereal with chemical fallow it’s performance results improved markedly, in 2005 growing the highest yielding wheat crop at the site (on a chemical fallow). Read more about the BCG Systems Site here or read the BCG News – May 2014 web edition.

Further research

Break crop residue over summer is generally less than that of cereal stubbles in the Wimmera and Mallee. Less groundcover therefore remains to protect the soil, particularly sandy soils from wind erosion. Less stubble from break crops, however, may enable greater herbicide efficacy for pre-emergent herbicides that are readily stubble bound. The challenge is to choose and manage the right break crop in the right sequence in the right conditions.

Continued research into these issues is being carried out through the stubble project will be extended by BCG and will hopefully address these challenges and opportunities.

By  De-Anne Ferrier and Alison Frischke, BCG
E: alison@bcg.org.au

This research iGRDCLogoStacked_TM_CMYKs 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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