- Inter-row sowing is critical to the success of establishing canola in retained stubble in the HRZ
- Leaving stubble upright increases success of canola establishment in retained stubble
- Slugs and insect pests can increase to levels that seriously damage canola seedlings in retained stubble
- Retaining stubble requires a carefully planned systems approach that considers every aspect of the system
Canola is an important crop in the Victorian and Tasmanian high rainfall zones (HRZ). It provides a profitable rotation that breaks cereal disease cycles and provides a wider range of grass weed control options for integrated weed management.
The 2012 GRDC Farm Practices survey reports that 22 per cent of crop areas planted in the Victorian HRZ is winter oilseeds. Much of this would be canola following a cereal crop.
Establishing canola in retained cereal stubble in the HRZ presents significant challenges for growers resulting from stubble loads as high as 10t/ha (Table 1).
In 2012, 42 per cent of all crops planted in the Victorian HRZ had previous crop residues burnt within one month of planting. This was largely driven by growers not having machinery suited to establishing crops successfully in retained stubble.
Burning is perceived as the most inexpensive and simplest solution to detrimentally reduce high stubble loads, however, loss of nutrition from stubble burning should be included to reveal the true cost. Regulations on stubble burning are being increased and this is reducing the attractiveness of burning. Developing a systems based approach to growing canola in retained stubble which maintains or increases profitability would alleviate concerns that many growers currently have.
Benefits of retaining stubble in the HRZ
The benefits of retaining stubble are well known. Examples include erosion reduction, increased rainfall infiltration, recycling of nutrients, decreased fire risk, time savings from eliminating burning/baling/mulching and increased harvest efficiency. In 2015 CSIRO researcher James Hunt (Hunt J R et al) demonstrated that retained stubble gave significant increases in rainfall infiltration and consequently soil profile moisture in comparison to grazing crops in winter and stubbles in summer.
Southern Farming Systems (SFS) conducted a disc versus tyne trial in 2015 where stubble retained areas, out yielded stubble burnt areas by 1t/ha. Dry seasonal conditions favoured the wheat crop sown into 30cm high wheat stubble which developed at a slower rate. This slower development reduced evapotranspiration rates and provided valuable moisture during critical grain fill periods.
Challenges of retaining stubble in the HRZ
There are also many challenges that come with establishing canola in retained stubble (Table 1). Challenges specific to the Victorian HRZ have been addressed in previous SFS stubble project work and will continue to be a focus in the next two years.
Overcoming challenges requires a careful systems approach that considers every aspect of the system and the impact on the system of each individual processes.
The main challenges and practices to increase the success of establishing canola in stubble include:
- Increasing soil to seed contact in the seed furrow. Poor soil to seed contact reduces germination and early vigour which in certain seasons has a detrimental impact on yield. Harvesting short, spreading chopped straw and trash evenly across the header swathe, keeping stubble standing erect by excluding livestock, auto steer inter-row sowing, adopting suitable row spaces, selecting crop rotations better suited to establishment in retained stubble, hybrid varieties with high percentage of large seeds and angling sowing direction by a minimum of 15 degrees across old rows can all increase success. In excessively high stubble loads (>6t/ha) removing all or part of the stubble as late as possible will provide most of the benefits of stubble retention and eliminate many of the problems that lead to costly failures.
- Reducing seedling damage from pests. For many growers this is a major concern as failure can be catastrophic. Some Victorian HRZ growers have removed canola from their rotations to eliminate this risk. Decomposing stubble provides an excellent environment for many canola pest species to survive and multiply. Insects such as European earwigs, Portuguese millipedes, slaters and false wire worm are detritivores that feed on decomposing organic matter. All have been suspected of causing serious damage to canola seedlings although there is no conclusive research on which to base management decisions. Conducting conclusive research that provides reliable information on threshold levels, life cycles, control measures and IPM practices has proven very difficult given the sporadic nature of pest infestations and under developed monitoring methodologies. SFS will focus strongly on pest issues for the remaining two years of the stubble project.
Growers and advisors lean towards adopting a safety first approach to insect pests when establishing canola in retained stubble. Insecticides on seed such as Cosmos® provide good early protection. Blanket applications of insecticides prior to or just after sowing are widely used. Some adopt a wait and see attitude and rely on careful monitoring of crops to alert them of insect presence and implement control measures if required. Many of these insect pests are nocturnal and control will be maximised by spraying insecticides at night. Baiting with Molluscicide is thought to have some benefits to insect control but more research is required to confirm the reliability of this practice.
Slugs, and to a lesser extent snails, are also major pests of canola seedlings in the Victorian HRZ. There are two major slug species that attack canola seedlings:
- Grey field slug. The grey field slug is particularly suited to stubble as it provides a refuge during hotter months. Moist conditions in the previous spring and summer are considered key indicators of potential problems the next sowing season. Moist conditions early in autumn can allow grey field slugs to multiply quickly and severely damage canola seedlings. Retaining stubble eliminates cultural practices such as burning and cultivation as slug control measures. Most growers apply slug bait immediately after sowing canola as insurance against damage to seedlings. Crops are then monitored carefully and additional baits are applied if required. As many as three to four bait applications can be required in a bad year. Other control practices used include rolling post sowing to squash grey field slugs and clods that provide refuge, sowing early before slug numbers are high and sowing high vigour seed to promote quick development of plants that outgrow slug feeding.
- Black keeled slug. The black keeled slug can burrow 20cm below ground and hibernate during summer to escape the heat. It is commonly thought (no supporting data) that a rainfall event of 30mm is required to allow them to emerge. These conditions are regularly encountered in the Victorian HRZ at sowing. Previous season climate is also considered to have a similar impact on black keeled slug numbers as detailed for the grey field slug. Control measures used are currently similar to that for the grey field slug.
- Minimising effects of cereal stubble on canola development. The physical presence of stubble can cause seeder blockages at sowing. The higher the stubble load the more likely it is to cause blockages. There is a plethora of variations in seeder design that will increase or reduce a seeders capability to handle high stubble loads. Generally disc seeders will handle higher stubble loads than tyned machines. The adoption of residue managers such as arricks wheels are critical to reducing the amount of straw that is ‘hair-pinned’ into the crop row and therefore reducing crop establishment with disc seeding machines. This has been essential in improving crop emergence in high stubble load retention systems. Wider row spaces will decrease the chances of blockages as will a longer distance between sowing units from front to back. Inter-row sowing using 2.5cm guidance greatly increases accuracy and reduces the potential for blockages. Lower harvest height and even spreading of straw and chaff across the header swathe will also minimise blockages. Any activities such as grazing or machinery traffic that reduce the amount of retained stubble standing erect in the row can decrease canola establishment.
Contact with cereal stubble can reduce canola’s early vigour. This can be a result of poor soil to seed contact or, allelopathic effects of cereal stubble on canola seedlings (Bruce S.E. et al), increased shading of plants and reductions in early N availability. Grassroots agronomy tested dry matter and grain yield variations for canola inter-row sown into 15cm, 26cm and 50cm high wheat stubble at Cootamundra in southern New South Wales. Early dry matter was as much as 50 per cent lower in the 50cm stubble compared to the 15cm in the early stages of growth. However at maturity the canola in the 50cm stubble had grown more dry matter. There were no differences in grain yield at harvest. Retained cereal stubble poses a considerable threat to the success of establishing canola. These threats can be successfully managed by the techniques outlined previously. Careful planning and experience with a range of situations over years has seen some growers adopt stubble retention and maintain profitability.
In 2016 SFS has continued the disc versus tyne site at Streatham. Canola was sown dry with a JD 1890 single disc opener, 25cm spaces, and an Equaliser tyne, 30cm spaces. The existing stubble was on 30cm row spaces and unfortunately we could not source a disc seeder with 30cm row spaces. This has compromised establishment for the disc plus stubble treatments in 2016. In 2015 the stubble was harvested at 15cm high for the tyne plots and 30cm high for the disc plots. Each plot is 24m wide in a fully randomised single block design. Each plot was split in half and 60kg/ha Urea was applied at sowing to randomised 12m strips. The Eastern end of the trial was burnt to give a direct comparison between burnt and retained stubble.
A bait mix of 2.5kg/ha of Mesurol® and 8kg/ha of Metarex® was applied in 4 x 10m wide strips at 90 degrees to the plots prior to emergence. Pest tiles were placed in each 24m plot where bait was applied and correspondingly where no bait was applied. This was done in burnt and stubble retained areas. (See trial plan Figure 1.) The aim of the on farm trial was to investigate how different seeders perform in different stubble architecture, the impact of urea applied at sowing and any variations in pest density and seedling damage. Temperature was also monitored over summer and in crop during winter.
The disc versus tyne trial saw noticeable differences in means of establishment between seeders in both retained and burnt stubble areas. The retained stubble areas were considerably lower (averaged 8-22 plants/m2) in establishment in comparison to the burned for both seeders (Figure 2).
The tyne plots were higher for establishment (3-17 plants/m2) in comparison to the disc in both burnt and retained stubble. This was thought to be a consequence of different row spaces for the disc, 25cm and the existing stubble, 30cm. However the differences were similar in the burnt area as well.
The tyne emerged faster than the disc in both areas. This was thought to be a consequence of moist soil being raised from depth by the knife point tyne that allowed the canola seed to germinate. Moisture content in each seed furrow was sampled and no difference was identified. The higher establishment in the burn area is reflected in higher NDVI for the two readings done thus far.
There was a noticeable reduction in percentage of plants damaged by insect pests where baits were applied in both the retained and burnt areas. The seedling damage was so serious in stubble retained areas where no bait was applied that the co-operator considered it necessary to re-sow.
The percentage of disc sown plants damaged by insects was much less than for the tyne plots. This may have been a result of the delayed germination in the disc plots but we are uncertain of why this happened.
The addition of urea at sowing caused a reduction in emergence for both seeders. This was a surprise for the tyne (15-20%) as the urea was placed underneath the seed. Fertiliser toxicity can reduce canola emergence in dry sowing conditions and this has been clearly demonstrated. The disc plots were less affected than the tyne for fertiliser toxicity which again was considered unusual.
Another demonstration site has been established near Cressy where stubble was harvested at 15cm high, 30cm high and 60cm high. The 60cm height was harvested using a stripper front. Establishment plus and minus inter-row urea, pests plus or minus baits, NDVI, weeds and yield are being monitored.
The problems that can occur regularly on farm when establishing canola into retained cereal stubbles have been well demonstrated in the SFS 2016 trials. Some grow highly vigorous crops such as faba beans or lupins following cereals and then plant canola in the following year. This may well be the most successful systems approach as it provides a range of other benefits. Legumes fix N for the high N use canola phase, provide other options for grass weed control that will allow two consecutive years of minimal grass weed seed set and ensure low disease carry over from cereal to cereal. There is concern that slug numbers will increase in the bean phase of this rotation.
More work needs to be conducted for a reliable canola establishment system for the high rainfall zone in Vicotria and Tasmania.
Hunt, J.R. et.al., Sheep grazing on crop residues do not reduce crop yields in no-till, controlled traffic farming systems in an equi-seasonal rainfall environment. Field Crop Research (2016)
Bruce, S. E., Suppression of canola through wheat stubble 1. Separating physical & biochemical causes in the field. Plant & Soil (2016) 281: 203-218
Grassroots Agronomy – Disc seeders experience a few years down the track. GRDC technical sheet.
Acknowledgements: GRDC, Myles Read, Scott Blurton, Charles Geddes, SFS Streatham branch committee, Tim Pilkington, Troy & Grant Keating, Michael Nash, Aaron Vague, Jim Caldwell
This 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’).