Guidelines for growers
- Stubble management starts at harvest: management decisions about choppers, spreaders, cutting height and your row spacing will affect harvest efficiency, weed control (herbicide options) and the subsequent crop.
- Stubble height should be determined according to weather conditions, risk to standing crop, how quickly harvest needs to progress and row spacing. Then consider crop rotation, stubble load, the paddock’s weed burden and management strategy and the capabilities of sowing machinery.
- Crop residues will create fewer issues at sowing if they are finely chopped and spread evenly across the paddock.
The characteristics of the stubble produced by the harvest process can create both benefits and challenges for the farming system. While the type and quantity of stubble and residue that remains in the paddock after harvest is predominantly determined by crop type and yield, actions taken at, or sometimes after, harvest can influence the height and amount of standing stubble and the composition and distribution of crop residue. This, in turn, will have an effect on spray efficacy, sowing and, ultimately, the performance of the next crop.
Desirable stubble characteristics include an even height, uniform distribution of straw and chaff, and upright standing stubble for inter-row sowing and the control of summer wiry weeds which will exacerbate seeder blockages.
Stubble height and evenness across the paddock can affect seeding efficiency, crop establishment, spray efficacy and nutrient availability to the subsequent crop. The degree to which the straw is slashed can either aid or inhibit microbial decomposition with 5cm of straw estimated to take approximately 50 days to decompose (rainfall permitting).
The length of stubble will also greatly influence the ease of inter-row sowing. Straw lengths of 20cm or less can allow tyne machines to work effectively with stubble loads of up to 5t/ha of stubble dry matter (GRDC Stubble Management Fact Sheet, March 2011). Disc seeders can sow into stubble that is much taller.
Work has been carried out at the Agricultural Machinery Research and Design Centre, University of South Australia looking specifically at the relationship between stubble height, machinery design and potential modifications to improve stubble flow. Their research shows that stubble height should be kept to 60-65 per cent of the effective vertical clearance the tyne (the distance between the ground and the first major obstruction on the tyne shank or mounting head). Stubble length should also remain less than half the lowest value of the inter-tyne spacing. That is, the narrowest clearance between components of any two tynes, or between tyne and wheel, in any direction. (2007, Desbiolles).
Current common practice across the Wimmera and Mallee is to harvest at a height that is no greater than the width of the row space. This is to avoid seeder blockages at sowing (particularly if operating a tyne system) and to achieve adequate spray penetration, while maintaining adequate groundcover to avoid erosion.
There are incidences, however, where a grower may choose to cut stubble shorter or taller.
For harvest weed seed capture, narrow windrow burning or if hay is planned for the paddock the following season, shorter stubble may be preferable.
Alternatively, some pulses such as lentils and field peas will benefit from inter-row sowing between taller standing stubble which will act as a trellis, improving harvestability and yield.
Importantly, the height the cutter bar is set on the header will also effect harvest efficiency and a grower’s ability to harvest their crop within the appropriate harvest window.
A field trial comparing three different harvest heights undertaken by Southern Farming Systems in 2014 showed that harvesting low (15cm) to the ground was considerably less efficient (Table 1) and more expensive (Table 2) than harvesting higher (30 or 50cm).
Table 1: Variation in harvest speed due to harvest height at 90 per cent engine load for a Case IH 9120. Streatham, Bolac wheat 2014.
|Engine load %||Speed km/hr||Av. km/hr||Per cent
difference in km/hr
|Time 100ha @ 7.5ha/hr|
|1||15||90||4.3||4.35||15cm v 30 cm -20.18%||15cm
|3||30||90||5.1||5.45||30cm v 50cm -22.70%||30cm 10.66hrs|
|5||50||90||6.9||7.05||15cm v 50cm -38.30%||50cm 8.26hrs|
Table 3: Cost analysis 15cm v 30cm harvest heights.
|Stubble height||Time/100ha||Harvest cost $||Fuel $||Cost $/100ha||Cost $/ha||Saving $/ha|
NB. Preliminary data for 9120 Case IH header from Table 1 suggests that there is a $10/ha cost to harvesting at 15cm compared to 30cm. For a 100ha cereal crop it would take 2.67 additional hours to harvest a crop at 15cm compared to 30cm. Costed at contract harvesting rates of $400/hr this equates to $10.68/ha. The 15cm height uses an additional 155l of fuel for every 100ha. At $1.50/l this equals $2.32/ha. Higher harvest heights are expected to provide additional savings of approximately $7.50/10cm of extra height. *Estimated not recorded
Straw and chaff
When it comes to dealing with crop residues, growers are advised to “chop it, distribute it, and do it evenly”, except if they are planning to bale straw or windrow burn. The chopper should chop the straw evenly and spread straw and chaff uniformly across the header width. This will ensure sowing efficiency is maintained and emergence is less effected by areas in the paddock where there might be heavy or thick crop residues.
Stubble breakdown is largely reliant upon soil moisture after harvest. Many farmers will choose to mulch (straw that is broken down into smaller pieces) their stubble if they have high stubble loads. Mulched stubble will speed up the process of microbial breakdown. It will also improve trash flow at sowing.
Although leaving stubbles higher at harvest can increase harvest efficiencies, the inability to use some seeding equipment in these tall and high stubble loads may demand the need for an additional process such as mulching which needs to be factored into the overall efficiency of operations. Mulching is discussed further in Guideline No. 8 – monitoring stubbles during the fallow period.
Yield penalties from retained stubble can be caused by nutritional tie-up, that is, nutrients are not released from the stubble as it breaks down.
Different crop stubbles will have different thickness, quality, handling ability, nutrient distribution and ability to assist in moisture conservation. The various crop types will also determine how easily the stubble is broken down post-harvest and the amounts of nutrients released. For example, a wheat stubble contains the same amount of carbon as canola stubble (400kg per one tonne of stubble), but the canola stubble will contain almost three times as much potassium as wheat (2010, Vadakattu).
At Horsham in the Wimmera, BCG is hosting a trial in collaboration with CSIRO that is measuring the flux (mineralisation and immobilisation) of nitrogen (N) under different stubble management methods (incorporated, burnt, removed, and standing stubble). Results so far have shown that only 31 per cent of the applied urea nitrogen (N) was taken up by the wheat crop in 2014, but unused N in the soil can benefit the following crop. In a decile 1 season, different stubble retention practices had no effect on microbial biomass and N supply potential (avg. 58kg N/ha).
Table 3. Crop performance, N uptake and N fertiliser use efficiency from the 2014 wheat crop.
HI – Harvest Index. *Estimate of fertiliser 15N uptake which doesn’t include N in roots.
Higher levels of stubble cover equates to greater amount of water infiltration and reduced water runoff. A study conducted by CSIRO found that 30 per cent stubble cover is the minimum level required to reduce soil water runoff. Stubble coverage of 50 per cent or greater will be of further benefit for soil moisture storage (Thomas,et al., 2007).
Retaining stubble will reduce the impact of raindrops and decrease potential water and wind erosion. This is achieved by 30-40 per cent stubble cover (Figure 1). Note that the amount of stubble cover will vary for different crops (soilquality.org.au).
Weed control with windrow burning
If windrow burning is being used as a weed management strategy, then stubble is best left short (beer can height) with crop residues in narrow windrows. Australian Herbicide Resistance Initiative consultant Peter Newman (Weedsmart), says as a general rule, aim for 20t/ha minimum of biomass in the windrow, otherwise you risk not getting an effective burn. Also be cautious of heavy stubble loads because the whole paddock may burn.
The following calculation can be used to workout windrow biomass.
Hypotheticial example: a 1t/ha yielding cereal crop, will have 1.5t/ha of residue (40% harvest index). Windrowing will put half the residue (0.75t/ha) in the windrow. If a header front is 10m, and the windrow chute is 0.5m, there is a multiplication factor of 20. So 20 x 0.75 = 15t/ha in the windrow. This could get to 20t/ha with a narrower windrow chute (0.375m).
The desired biomass range is normally achieved by a 2t/ha crop with a windrow chute that has an opening 500mm wide (www.bcg.org.au/cb_pages/news/windrowburning.php).
Midwood J. & Birbeck P., 2012, Managing Stubble, Accessed at: http://www.grdc.com.au/Resources/Publications/2012/05/Managing-Stubble
Soilquality.org.au, 2015. ‘Benefits of retaining stubble factsheet’. Accessed at: http://soilquality.org.au/factsheets/benefits-of-retaining-stubble-in-qld
Swella, G. B. Flower K. Ward P.R., Siddique K.H.M., Effect of stubble height and architecture on soil water capture, The University of Western Australia. Accessed at: http://aciar.gov.au/files/node/14068/effect_of_stubble_height_and_architecture_on_soil__49509.pdf
Thomas GA, Orange DN, King AJ., 2008, ‘Effects of crop and pasture rotations and surface cover on rainfall infiltration on a Kandosol in south-west Queensland’, Soil Research pp. 46, 203–209. Accessed at: http://www.publish.csiro.au/paper/SR07162.htm
Vadakattu, G., 2010. ‘The biological and nutritional value of stubble retention’, Mallee Sustainable Farming 2010 Results Compendium, pp. 117-120. Accessed at: http://msfp.org.au/wp-content/uploads/2013/10/Biological-and-nutritional-value-of-stubble-retention.pdf
By Jessica Lemon
Research and Extension Officer
Birchip Cropping Group
This guideline was produced 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’).