Guideline for growers
- Maintain a minimum of 5t/ha of stubble to maximise benefits of moisture retention.
- Monitor stubble load and if modifying stubble, do it early to maximise breakdown before seeding.
- Take machinery set up into account when modifying stubble.
Managing and monitoring stubble during the fallow period
- Start stubble management at harvest
Aim to cut and spread stubble in a way that suits your seeding equipment. There are many choppers and spreaders available to improve management of harvest residues (see Guideline 9: Sowing into stubble ). The JD premium package spreader chopper system, Redikop, DJ, Kirkby and MAV chopper have all been recommended for harvest residue management by farmers in the Mallee and Wimmera. This is discussed in detail in stubble guideline 4: Stubble management at harvest.
- Estimate stubble load
Stubble load (t/ha) = 1.5 x grain yield t/ha (works best for grain yields of 0.5-4t/ha). More estimates can be made where the harvest index is known with the stubble load (t/ha) = 1/HI.
- Estimate stubble cover
Managing stubble to minimise stubble cover requires working to a minimum surface cover threshold. There are good reference photos available for estimating cover in the Stubble Management Guide produced by Mallee Sustainable Farming (pp. 17-25).
- Know your seeder and how much stubble it can handle
Residue management is a primary concern of farmers when managing stubble load and harvest is the ideal time to target this by distributing residues evenly across the header. Uneven stubble can lead to poor weed control from pre-emergent herbicides, N tie up, poor trash flow under seeders and reduced seeding establishment. Although large amounts of on-ground residue is often more of an obstruction than the height of standing stubble, a general rule for avoidance of blockages for tyne seeders is that stubble height should be no more than half the length between tyne assemblies or any horizontal obstruction (eg. wheels), or 65 per cent of the distance from the ground to the lowest shank obstruction (Figure 1).
Disc seeders can handle taller standing stubble and it is best to minimise the amount of residue sitting on the soil surface by maximising standing stubble height. This reduces the risk of hair-pinning of stubble in the seed row, which limits the closing ability of the furrow at seeding and seedling emergence.
The stubble height specifications for seeders mentioned above are for a controlled traffic system. If a paddock is undergoing heavy traffic then stubble may need to be cut to shorter lengths as residue on the ground is more likely to cause seeder blockages. Inter-row sowing improves stubble handling by both disc and tyne seeders. The GRDC fact sheet ‘Strategies to manage winter crop stubble without reaching for the matches’ gives a benefit analysis of stubble related management options for both tyne and disc seeders .
Critical thresholds for stubble management
The stubble cover available in a paddock determines how susceptible it is to soil damage through erosion. A study of southern cropping areas in NSW produced estimates of the minimum ground cover required to reduce excessive run off and erosion and sustain productivity for a range of slope gradients and soil erodibility classes. In a Mallee environment 70 per cent stubble cover has been identified as the minimum level needed to avoid top soil erosion.
Table 1: Minimum ground cover (%) estimates to reduce erosion and run-off (Lang and McDonald, 2005).
Research conducted by BCG in partnership with Mallee Catchment Management Authority, as part of the ‘Protecting Mallee soils over summer’ project supported by the Australian Government, showed that soil water was not significantly influenced by stubble load, however stubble cover was essential for minimising soil erosion.
The handling abilities of the seeder will determine the maximum stubble load. Stubble loads should be small enough to enable easy sowing and large enough to reduce erosion and preserve soil moisture.
About 5-7t/ha of stubble is ideal for maximising fallow moisture storage and within the capability of most correctly set-up seeders to operate without major blockages. Wimmera farmers quoted 7-10t/ha as being thresholds where alternative stubble management practices were necessary to reduce the load.
At loads of less than 7t/ha most stubble can be managed using choppers and spreaders and didn’t present any major issues at harvest for appropriately set up seeders.
Some growers have expressed concern that a large amount of standing stubble could decrease yield by limiting light to plants. However, BCG research which involved seeding into a stubble load of up to 4.4t/ha, found little effect on yield. In fact, in this experiment the retained stubble treatments produced yields that were slightly higher than when stubble had been slashed and removed (Browne C. et al. 2012).
Some breakdown of crop residue will occur between harvest and sowing but in drier areas this is minimal. Approximately 70 per cent of stubble present at harvest remains at sowing. Canola and pulse stubbles break down more easily than wheat and barley stubble. Standing stubble acts as a trellis for lentils, improving growth, harvestability and yield, regardless of seasonal conditions.
If stubble cover and height is low enough that is unlikely to cause any hindrance to sowing machinery, then best practice is to leave it standing. Practices such as inter-row sowing can increase the amount of stubble handled by a seeder.
If stubble is at a threshold where management is necessary, there are a number of options available to make the residues more manageable or to reduce the overall amount of residue. When considering a stubble management option, farmers should take their seeder set-up into account to decide on the best practice for their machinery.
Stubble management strategies
This involves any practice where stubble is broken up and left on the soil surface including slashing, prickle chains, disc chain and harrowing. Mulching creates a layer of stubble that reduces weed emergence and helps to conserve moisture, however the mat of residue left in the paddock can cause issues at seeding, particularly with hair-pinning in disc seeders.
This is where stubble is mixed into the cultivated soil using a disc plough, scarifier or disc harrow. If done early into moist soil, mulching can increase stubble breakdown before sowing, however it leaves stubble lying on the ground, often in high quantities, where it can cause seeder blockages, particularly in dry conditions between harvest and sowing when decomposition is minimal. Cultivation can also be effective at controlling some weeds, however, if possible, it should be avoided as it damages the soil structure and increases the risk of erosion.
A good way to get value out of crop stubbles and reduce the stubble load is to use it as a feed source for sheep or cattle. This strategy is common in mixed farming enterprises. Flow on effects of grazing are fewer mice and less crop volunteers due to reduced stubble. Grazing physically removes stubble and increases its breakdown, however, it does remove valuable nutrients (and a carbon source for nutrient turnover) and causes the stubble to lie down which could lead to increased hair-pinning in disc seeders. Grazing has been found to have no effect on moisture retention or grain yield as long as the stubble load is maintained at approximately 2t/ha or 70 per cent ground cover, so regular monitoring is vital to prevent overgrazing. If grazing stubble then harvest may need to target shorter stubble lengths as long stubble which is subsequently pushed over by stock (or wheels) can cause additional seeding problems.
While best avoided, burning is an effective means of residue and weed seed reduction. Burning stubble causes a loss of cover in the summer fallow which can lead to moisture loss as well as a loss of nutrients. Approximately 4kg/ha of nitrogen is lost per tonne of stubble in dryland crops, or an average of 15-26kg/ha in higher yielding areas. If used correctly, windrow burning has the benefit of removing stubble and certain weed seeds while minimising the loss of biomass cover, which is important for reducing erosion and conserving soil moisture. If burning stubble is your only option, then try to do it just before sowing to minimise the period of time the soil is left bare.
Stubble can be baled and removed as straw for stock feed. Removal is not ideal as it removes nutrients tied up in the stubble from the paddock, however it may be necessary in high stubble situations. If stubble load is too heavy for seeding equipment to sow through effectively it can drag, creating areas of low establishment.
Weeds, pests, diseases and nutrient cycling
Pest, weed and disease management is an important consideration when managing fallow paddocks. Understanding the potential impact of stubble on pests, weeds and diseases, and in turn, the range of strategies that can be employed to manage them, will increase opportunities for growers to retain stubble.
If left to grow unchecked, summer weeds such as hogweed/wireweed, melons, marshmallow and bindii can cause sowing blockages by wrapping around tynes and discs. Such weeds are best targeted with herbicide applications early to prevent them getting large enough to cause seeder blockages and decreased emergence.
Early weed control is best in the case of early summer rain so there is capacity for weeds to break down and soil moisture and nutrient loss is minimised. A four year BCG field trial which examined the effects of summer weed management found that complete weed control was the most effective way of increasing soil moisture and had the potential to increase yield by up to 2t/ha (BCG 2011 p. 24). Methods for managing weeds in stubble retained systems are discussed in Stubble Management Guideline No. 2: Weed management.
Incidence of soil and stubble borne diseases can be higher in stubble retained systems which has been the case for yellow leaf spot, crown rot, eyespot and take-all. Diseases like yellow leaf spot can use stubble as a host and remain infective for several years, resulting in problems for subsequent crops. If disease is present it should be treated through a combination of tolerant varieties or crops and chemical treatments depending upon the mechanisms available. These are normally sufficient to manage the disease without resorting to burning or other stubble management techniques. Disease management in Victorian and Tasmanian stubble retained systems is being investigated as part of the stubble initiative.
The fallow period is the ideal time to monitor soil nematodes to aid in crop planning for the season. Pests such as cereal cyst nematode (CCN), root lesion nematode and stem nematode can cause issues in crop and effect yield. The level of these nematodes can be tested during the fallow period, allowing time for management decisions to be made such as rotations to lower the incidence of pests. Pest management is another area being examined in Victoria and Tasmania as part of the GRDC stubble initiative.
During the decomposition of cereal and canola stubbles, nitrogen is tied up, or immobilised, reducing its availability to plants in the following growing season. Immobilisation occurs when the micro-organisms decomposing the stubble acquire the nitrogen from the stubble and soil. This is particularly an issue in areas with low seasonal rainfall, where nitrogen tie up can be as much as 5-13kg/ha for every 1t/ha of stubble. If nitrogen levels are low the addition of nitrogen fertiliser may be necessary to meet crop requirements. Legume stubbles are high in nitrogen and immobilisation does not occur. Hence, soil nitrogen levels increases during decomposition of legume residues.
Managing brown manure crop stubble
Brown manure crops such as field peas and vetch can present their own management challenges over summer. If a brown manure crop has reasonable biomass and is left until seeding with no management except for summer weed control, then high stubble loads are likely to cause issues at seeding time, particularly if rainfall is too low for sufficient stubble breakdown. Some potential strategies for management include:
- Rolling residue 7-10 days after spraying. This gets the plant material on the ground to increase residue breakdown. This can be effective but relies on reasonable rainfall to increase breakdown. Coulters can assist with sowing into the remaining residue.
- Prickle chains on a hot day increases the break-up of stubble but can give mixed results as it often loosens residues, making it prone to blowing away. Sowing can also be difficult as the stubble is no longer anchored.
- Coulters on the seeder bar cut through residues making sowing easier. However, this works best when soils are dry and not wet and sticky. Coulters combined with rolling residue work well.
Disc seeders are generally designed to handle residue better than tyne seeders and can be an effective way of establishing crops into field pea and vetch stubble.
Browne, C., and Hunt, J., 2012. Water Use Efficiency (WUE): Yielding benefits through partnerships. http://www.bcg.org.au/cb_pages/water_use_efficiency_wue.php.
Summer fallow weed management a reference manual for grain growers and advisers in the southern and western grains regions of Australia, 2014. Available at: https://www.grdc.com.au/Resources/Publications/2014/05/Summer-fallow-weed-management
Mallee CMA, 2014. Protecting Mallee soils over summer [Video]. Published on May 13, 2014. Available at: https://www.youtube.com/watch?v=ZduGWZruIpI
Prime facts, 2009. Managing ground cover in the cropping zone of southern NSW. http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0005/302918/Managing-ground-cover-in-the-cropping-zone-of-southern-NSW.pdf
GRDC Stubble Management Fact sheet, 2011. Strategies to manage winter crop stubbles without reaching for the matches . Available at: http://www.grdc.com.au/uploads/documents/GRDC_FS_StubbleManagement.pdf
The stubble project: Victoria and Tasmania. 2015. Herbicide application in retained stubble systems. Available at: https://thestubbleproject.wordpress.com/herbicide-application-in-retained-stubble-systems/
The stubble project: Victoria and Tasmania . 2015. Inter-row sowing and row spacing. Available at: https://thestubbleproject.wordpress.com/inter-row-sowing/wimmera-mallee-inter-row/
NSW Department of Primary Industries. 2005. Maintaining groundcover to reduce erosion and sustain production. Available at: http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0018/162306/groundcover-for-pastures.pdf
BCG Birchip . (2015). Herbicide application in a retained stubble cropping system. [Video]. 16 March. Available from: https://www.youtube.com/watch?v=BbC-yGf0ziI
Charles Sturt University. 2010. Stubble Retention in Cropping Systems in Southern Australia: Benefits and Challenges. Available at: https://www.csu.edu.au/research/grahamcentre/publications/monograph/stubble-retention-in-cropping-systems-in-southern-australia-benefits-and-challenges
The stubble project: Victoria and Tasmania. 2015. Weed management in retained stubble systems. Available at: https://thestubbleproject.wordpress.com/weed-management-in-retained-stubble-systems
Stubble Retention in Cropping Systems in Southern Australia: Benefits and Challenges. 2010. EH Graham Centre Monograph No. 1. Available at: https://www.csu.edu.au/__data/assets/pdf_file/0007/922723/stubble-retention.pdf
By Amy Smith
BCG research and extension officer
This 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’).