Break crops in stubble retained systems under irrigation

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  • Faba beans have relatively large seed reserves and are able to establish in cereal stubbles.
  • There is evidence that cereal stubbles also act as a deterrent to aphids that have the capacity to infect emerging faba crops with viruses such as bean leaf roll virus.

As defined by the GRDC, “a break crop is any crop sown to provide diversity to help reduce disease, weed and pest levels in a paddock” (Break crop benefits fact sheet, March 2011). In Victorian cropping regions a break crop it is generally accepted as a non-cereal crop.

While break crops grown in dryland systems tend to be regarded as riskier than cereals (from a yield and subsequent profit aspect), irrigation reduces the risk of growing these crops to the point where they can be more profitable than cereals.

In irrigated cropping systems in the Murray Mallee region of Victoria, the two major break crops grown are canola and faba beans. A typical rotation is wheat, barley, faba beans and canola. This rotation provides many agronomic advantages:

  1. Faba beans can contribute significant amounts of nitrogen (N) into the cropping system prior to the high N requirements of subsequent canola and wheat crops.
  2. Two years of non-cereals offers the opportunity to control grasses with a range of herbicide groups.
  3. The rotational sequence produces stubble loads that can be handled relatively easily by the break crops.

However, for Murray Mallee growers wishing to retain their stubble, the inclusion of faba bean and canola crop can present challenges in terms of cereal stubble management.

An obvious difference between canola and faba bean seed is the size. This seed size is indicative of the energy reserves that the seeds have to establish a plant. Therefore canola has relatively small reserves to draw upon during germination.

As irrigated cereal stubbles may be in the region of 8 t/ha and crops are often at narrower row spacing’s than on dryland farms, without some form of stubble management, this presents an inhospitable environment for a germinating canola seed. This is because the stubble intercepts the light and tends to result in ‘leggy’ seedlings that use more energy (seed reserves) to establish a plant which can weaken the crop.

Conversely, a faba bean crop produces less stubble and also breaks down more quickly than a cereal stubble, reducing the negative effect on canola establishment.

As part of the GRDC-funded stubble project (BWD00024), ICC has undertaken research examining how stubble management influenced the establishment of canola and faba bean crops.

ICC research

From field research trials and farmer experience, ICC is working towards identifying how irrigated farmers can retain stubble without compromising plant establishment.

Establishment of irrigated canola
The best management practice target sowing date is from mid-April to Anzac day. In most seasons, this will require irrigation, either pre-irrigation and sow into moisture or dry sowing and watering up. Having adequate moisture at sowing means that starter fertiliser (DAP or MAP) can be safely put ‘down the tube’ at rates up to 125 kg/ha with little detrimental effect.

If watering up, crusting can prevent establishment. The use of soil ameliorants such as gypsum can be beneficial as well as increasing organic carbon levels and reducing cultivation to improve and preserve soil structure.

If canola is to be established into cereal stubbles, the stubble must be managed to reduce the potential negative effects. Mulching, incorporation and burning all have their place.

Incorporation can be successful, particularly if it is combined with added fertiliser and irrigation a month or so before sowing, but nutrient tie-up can still be an issue.

Mulching can also be a useful management tool as long as the stubble load is not too great as to affect the ability of the sowing implement to accurately place the seed at a shallow sowing depth.

Establishing canola on cereal stubbles

To improve soil carbon levels, stubbles have to be incorporated and rapidly broken down by soil microbes rather than slowly decomposing on the soil surface. However if incorporated stubble does not break down before sowing, there can be an issue with trash flow and/or nitrogen (N) tie-up due the microbes trying to break down the stubble as the crop establishes.

Fortunately we have irrigation to provide the necessary moisture for the microbes to breakdown the stubbles, but stubble breakdown also requires enough nutrients for the microbes, in particular N, P and S. A rule of thumb derived by Clive Kirkby, CSIRO, is for every tonne of stubble, there needs to be 5 kg N and 3 kg P.

Demonstrations were conducted in 2014 and 2015 to look at the effect of stubble incorporation and the timing of fertiliser application on the speed of stubble breakdown. They found that incorporated stubble provided no issue with trash flow at sowing, and establishment was similar across all treatments. If plots were N deficient, plant size decreased and had lower vigour. These plots also were less competitive with ryegrass. Plots that had adequate N grew and yielded more.

Establishment of canola using soil ameliorants

The soil on the trial block is prone to slaking if overworked, which can become an issue when trying to establish canola, particularly when watering up.

Calcium, either as gypsum or lime, can assist in improving soil structure and hence reduce soil crusting and slaking. Gypsum is widely used as it is available locally, saving transport costs. It does not affect soil pH and is relatively soluble compared with lime.

In 2014, rates of gypsum, lime, gypsum/lime and PAM (polyacrylamide, which acts in a similar manner to calcium in that it helps aggregate the soil particles) were applied to the soil surface (which was crusting after pre-irrigation) and incorporated by sowing.

The 2014 results showed an improvement in establishment where gypsum was used at 3t/ha over all other treatments. All other treatments except for the PAM were an improvement over the control (no ameliorant).

The trial was oversown in 2015 following irrigation. This time, no treatment had any effect on establishment. In order for the soil to slake post-sowing, it has to be wet enough to breakdown the soil structure. If it doesn’t get particularly wet, then the soil can resist slaking and the seedlings can emerge unhindered. Results were therefore not surprising given the small rainfall events in May.

Canola plant growth (PGR) regulators

After no results in 2014  a canola PGR trial was repeated in 2015 looking at alternative timings for the application of the PGRs and two products that were not tested in 2014.

Note: There are currently no PGRs registered for use on canola in Australia, although there are products available here intended for alternative purposes that are registered for use as a PGR overseas. Therefore this information is only presented as experimentation and not for broadacre use.

The PGRs applied had no effect on either total dry matter produced, plant height, depth of podding or grain yield. The only treatment that may be worth pursuing is the chlormequat/tebuconazole mix at an earlier application (budding).

Establishment of irrigated faba beans

Fabas have the ability to handle cereal stubbles reasonably well. The target sowing date is late April to early May and seed should be treated with insecticide to reduce the chance of aphid-borne virus infections.

Fabas can be sown into pre-irrigated or watered up soil, with pre-irrigation giving the advantage of being able to apply a knockdown herbicide prior to sowing allowing faba beans to be sown a little deeper to chase moisture if necessary.

Molybdenum, row spacing, plant population and PGRs in faba beans

In 2015, several trials were conducted at Kerang to evaluate different agronomy practices on faba bean production. They investigated whether adequate molybdenum was present for nitrogen fixation, and after lodging issues in 2014 – the effect of row spacing, plant population and use of plant growth regulators (PGR) on lodging and grain yield.

In summary trials found:

  • Applying molybdenum to faba beans did not effect nodulation, plant molybdenum levels or grain yield.
  • Faba beans sown at 25 plants/m² on 7½ inch row spacings produced the highest yields.
  • PGRs applied on 14 August before flowering had no effect on the yield, height or lodging of faba beans. Timing of application needs to be investigated as the relatively late applications do not seem to be having an effect, but applying too early may reduce the height of the first pod and increase harvest losses.
  • Faba beans sown with a precision planter may benefit crop performance.

More details can be found here.

By Damian Jones
ICC trials manager

This research is bGRDCLogoStacked_TM_CMYKeing conducted by ICC 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 Irrigated Cropping Council (ICC) 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. ICC 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. ICC 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 ICC 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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1 Response to Break crops in stubble retained systems under irrigation

  1. Pingback: Break crops in retained stubble systems | The stubble project: Victoria and Tasmania

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