- Stubble management requires an integrated management approach, often combining several strategies, that may vary between seasons.
- Stubble management during fallow is important for pre-sowing weed and pest management and stubble characteristics to maintain groundcover and enable crop sowing and establishment.
- Tools to handle stubble during fallow include mulching (slashing, chaining, rolling), incorporating (cultivating), grazing, or removal by baling or burning.
To successfully establish a crop into a stubble retained system requires an integrated approach incorporating stubble management at harvest, during fallow (post-harvest/pre-sowing) and at sowing time. This approach may also need to be flexible because there is no one perfect stubble management strategy – each year there will be different stubble loads, rotations, weeds, disease, pests, and potential sowing issues. Having a flexible approach to managing stubbles means crops can be harvested at different heights, and then the remaining stubble managed to the best economic advantage or paddock preparation outcome.
Pests, diseases and weeds during summer fallow
Crop stubbles provide a cool, moist habitat that favours pest survival. Pests such as slaters, millipedes earwigs and weevils have become increasingly common in broadacre crops over recent years with the use of minimum/no-till and stubble retention practices. These pests are found in many crops, but canola seedlings are often most susceptible to damage. Other pests include slugs and snails in higher rainfall years, and mice.
Stubble must be managed before sowing to control these pests (e.g. by grazing, cultivating or burning), as once the crop has germinated cultural and chemical control options are limited.
Soil and stubble borne diseases can be higher in minimum/no-till and stubble retained systems, as has been the case for root diseases such as cereal cyst nematode (CCN), rhizoctonia, root lesion nematode and stem nematode, and leaf diseases yellow leaf spot, crown rot, eyespot and take-all.
The level of these pathogens and nematodes can be tested during the fallow period, allowing time to make management decisions about resistant varieties, crop rotations, use of chemical or biological controls, or grazing, to hopefully avoid resorting to tillage or burning.
Growers also need an integrated approach to weed control as minimum/no-till and stubble retention practices have placed a greater reliance on chemicals to control weeds. At the same time, less tillage reduces pre-emergent herbicide incorporation and increased stubble hinders soil penetration or foliar contact of herbicides. Stubbles left high can also shade and slow crop establishment reducing competition with weeds.
Weed seed management can begin at harvest by cutting low and concentrating weed seeds into windrows for burning, or chaff carts heaps to be burnt or grazed later. 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. Summer weeds are best targeted with herbicide applications early to prevent them growing large enough to cause seeder blockages and hinder crop emergence.
Early weed control is also best in the case of early summer rain to give time for weeds to break down, and minimize soil moisture and nutrient loss. A four year BCG field trial examined the effects of summer weed management and 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).
Having a diverse cropping sequence/rotation provides benefits for stubble retention as each crop is sown into a less antagonistic stubble by reducing the pests, disease and weed constraints. Legumes also build soil nitrogen that is used to counteract nutrient tie up with the stubble breakdown process.
Stubble management strategies during fallow
Stubble can be managed several ways during the fallow period. The selection of the appropriate tool or technology, set up and operation of that method, will help achieve the desired levels of stubble components and characteristics for maintaining ground cover and for ease of sowing the next crop.
It’s recommended that a minimum of 50-70 percent ground cover (about 1-1.5 tonne dry matter/ha) remains on paddocks to prevent wind erosion (Figure 1). Wind erosion results in the removal of valuable topsoil that is regarded as rich in nutrients and organic matter. Most susceptible to erosion are lighter textured soils like sands and sandy loams, commonly found in the Mallee. Wind erosion will begin when the soil is dry and bare (cover is reduced below 50 per cent), the paddock is exposed to winds of 30 km/hour or more.
The way stubble is managed over the summer fallow period effects ground cover levels. Many farmers now elect to leave the stubble standing, while others manipulate stubble through mulching (chaining or rolling) or incorporating by cultivation.
To demonstrate the effect of stubble management on ground cover, Mallee Sustainable Farming (MSF) established demonstration sites over the 2012/2013 summer fallow period comparing standing stubble to cultivation, chaining and rolling. Treatments were applied in January 2013 and monitored until April 2013. Groundcover results and photos of treatments can be seen in Stubble Management: A guide for Mallee Farmers by MSF (pages 13-16). Rolling or chaining reduced ground cover levels by 5-40% relative to the standing stubble treatment, whereas incorporating stubble through cultivation dramatically reduced ground cover levels below the critical 50% ground cover level in April. The demonstration sites were established in a very dry summer with only 25-50mm January – April rainfall. You could expect greater stubble breakdown in wetter summers, especially where stubble has been brought into contact with the soil surface.
1. Stubble retention
Crops grown in low rainfall areas generally produce stubbles that can be managed by most sowing systems. However, crops grown in more favorable seasons, and in medium rainfall areas, can result in stubble loads that require management at harvest in a stubble retention system.
As outlined in guidelines Desired stubble characteristics for the WImmera and Mallee and Stubble management at harvest, decisions will be made at harvest that will determine the ideal harvest height and spread of chaff and straw across the width of the header cut in the paddock. These decisions will vary with the season (stubble bulk), harvest logistics (header and spreader setup) and the following paddock management (pest and weed management, sowing system and rotation).
Stubble height should be no more than 65% effective tyne vertical clearance (distance between ground surface and first major obstruction on the tyne shank or mounting head) (Figure 2.), and no more than 65%, preferably less than 50%, between tynes (Figure 3.).
Sowing into standing stubble will be easier if tramlines and inter-row sowing are used
2. Mulching and Incorporating
Mulching and incorporation require soil moisture, warm soil temperature, soil-stubble contact and nutrients for microbes to convert stubble into the humus fraction. Early mulching and incorporation allows time for the stubble to decompose and immobilise nitrogen (N) well before sowing, reducing the likelihood of reduced N availability.
Mulching breaks up and leaves stubble on the soil surface using techniques like slashing, prickle chains, rolling, disc chain and harrowing. Mulching creates a layer of stubble that reduces weed emergence and helps to conserve moisture. The mat of residue left in the paddock however, can cause issues at seeding, particularly with hair-pinning (stubble is bent rather than cut) in disc seeders. Mulching is most cost effective in hot, dry conditions, which is also when fire risk is the greatest. If a slasher is used, aim to slash at a height to match the capacity of tyne seeder (described above), using a forward speed of 6-8 km/hour. Slower travel speeds and lower blade settings and will result in a finer stubble cut.
Incorporation mixes stubble into cultivated soil (Figure 4.) using a disc plough, scarifier or disc implement such as Vaderstad Topdown, Grizzly, Speed tiller, Lemken Heliodor or Amazone Catros. Done early into moist soil, incorporation can increase stubble breakdown before sowing. However, it can leave stubble lying on the ground, often in high quantities, where it can cause seeder blockages, particularly in dry conditions between harvest and sowing when breakdown 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. On lighter sandier soils more prone to wind and water erosion, it is recommended to delay incorporation until 3-4 weeks before seeding.
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 residue7-10 days after spraying. This puts 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 chainson 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 barcut 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.
3. Grazing with livestock
Crop stubbles are often grazed by sheep and cattle, providing valuable feed for livestock in summer and autumn months in mixed enterprises. Livestock will increase the rate of stubble breakdown by trampling stubble onto the soil, and through digestion.
Grazing will lay stubble down which could lead to increased hair-pinning when sowing with disc seeders. If planning to graze stubbles, ensure that the harvest height of stubble is less than the sowing row width to avoid the trampled stubble causing seeding problems. While grazing, ensure some stubble remains standing as research indicates that standing, anchored stubble 10cm high is twice as effective at reducing wind erosion compared with loose flat stubble.
Stubble ‘bulk’ however, does not necessarily translate to animal performance. Stubble quality can vary between crops and seasons and can change quickly across a paddock, particularly once grazing begins, or after summer rain. The value of feed is determined by the amount of residual unharvested or spilt grain and green plant growth present, including summer weeds and shot grain.
Livestock will consume summer weeds and volunteer cereals, reducing the need for summer weed spraying and herbicide costs. Grazing of weeds can be uneven with current grazing systems however, so a paddock spray may be needed for full weed control to conserve stored soil moisture for sowing. Ensure Withholding periods for all products (Herbicides, Insecticides and Fungicides) are adhered to.
Grain and green-pick should be monitored carefully. The efficiency of modern harvesters and better weed control means there is now less grain and fewer weeds left in stubbles than 25 years ago. Livestock may begin to lose weight on a stubble paddock anywhere between 2 to 6 weeks of grazing, depending on type of stubble, season, paddock size and the stocking pressure (number and class of animal), so watch animal condition carefully and supplement to maintain production.
Monitor paddocks carefully to avoid overgrazing and exposing soil. To encourage more even grazing and reduce selective grazing, trampling and camping, graze with larger mobs for a shorter time (compared with small mobs for a long time). Alternatively, use temporary fencing to create smaller grazing areas and control stocking pressure, and exclude animals from erosion-prone areas. Place water points in central locations as best you can, and ensure water is clean and cool with a good flow rate so that mobs know they can come in and get a drink quickly, then move out again. Although there are varying points of view about grazing stubbles, numerous studies across Australia, including 3 years of livestock in no-till systems research by BCG with Grain & Graze in the Mallee and a long-term no-till controlled traffic grazing experiment by CSIRO at Temora, have shown that grazing stubbles will not compromise a cropping system – there are no negative impacts to soil health, moisture retention or subsequent crop yields, so long as sufficient stubble (70% groundcover, Figure 1 above) is retained to prevent wind erosion and maintain water infiltration. Any compaction by sheep is shallow and corrected by the next tillage pass.
Over seven years, the Temora experiment recorded a $44/ha increase in gross income where sheep were used to graze the stubbles and then retained (not burnt) compared to nil grazing if no grazing value was assumed. This increase was related to higher yields and grain quality in subsequent crops driven by greater N availability in the grazed stubble. There was a $159/ha increase if a grazing value for the stubble was assumed.
Another flow on effect of grazing is fewer mice as the livestock consume the grain. Grazing chaff heaps: To manage resistant weed seed banks at harvest, one strategy is to collect the chaff fraction from the header with a chaff cart, then dump in a heap to concentrate resistant weed seeds.
These heaps have been burnt in the past, but now some growers are grazing them first, and may not burn at all. There is mounting evidence that sheep grazing chaff heaps are performing better than those who are not, achieving better growth rates and lifting lambing percentages. Sheep help to knock down the heaps making the next sowing pass easier, and if burnt they burn faster which avoids having smouldering heaps for days. Barley chaff heaps may thatch and may need burning, preferably in winter to avoid the risk of a fire spreading.
Even though heaps are knocked down and spread out, WA research found grazing made no difference in ryegrass germination up to 8m from the edge of the heaps, indicating sheep do not spread ryegrass weed seeds. Research has also found sheep are better suited to grazing chaff heaps, showing that less than 3% of ryegrass seeds will survive the sheep rumen, whereas almost 30% will remain viable in cattle faeces.
To be most effective, harvest low to pick up weed seeds, and have the header set up with a high rotor speed to cope with more straw, and the back of the concave opened-up to efficiently separate seed and straw (weed seed ends up on the sieve, not out the rotor).
Stubble can be baled and removed as straw for stock feed. The grower can either:
- harvest high using stripper front if possible, then cut and bale
- harvest low and drop trash directly behind the header (no spreading) for baling
- harvest low, with trash fed directly into a trailed baler.
Removal by baling is not ideal as it removes nutrients tied up in the stubble from the paddock, and groundcover which is needed in dry conditions. However, baling may be necessary in high stubble situations as an alternative to burning stubble.
While best avoided, in strategic situations burning can be considered as an effective means of removing stubble, and reducing weed seeds and certain seedling pests. Burning stubble causes a loss of groundcover in the summer fallow, which can lead to moisture loss as well as a loss of nutrients. Approximately 4 kg N/ha is lost per tonne of stubble, or an average of 15-26 kg N/ha. Burning is best left until late, just before sowing, to minimise the time soil is exposed.
Field experiments indicate that a strategic burn will do little damage to grain yields. Burnt treatments in a long term trial at Harden NSW still producing some of the highest yields after 28 years of continuous cropping, while at Riverine Plains, Victorian HRZ and the MacKillop Farm Management group burnt treatments have usually had similar yields to stubble retained treatments. In seasons where crops grown on burnt compared with retained stubble have differed, burnt treatments had greater early vigour and subsequently suffered greater moisture stress when there was a hot, early finish.
If choosing a strategic burn, ensure you understand shire regulations about permits, requirements for the number of people and fire-fighting units, and when you can leave the paddock.
Narrow windrow burning
Forming narrow windrows at harvest can concentrate stubble, chaff and weed seeds. They’re created by removing straw spreaders or by using stubble chutes that place crop residues in a narrow windrow roughly 50cm wide, which can then be burnt. If used correctly, can have the benefit of removing stubble and weed seeds without burning the whole paddock.
To minimize the risk of the windrow burn spreading to adjacent stubble, sow and then harvest the crop low in the same up-and-back pattern, to allow the windrow to be positioned in-between two sowing rows, which will keep the windrow in place until it can be burnt. Conditions for narrow windrow burning need to be calmer and cooler than normal paddock burns that need heat and wind to carry the fire. One constraint with narrow windrow burning is if stubble loads are greater than 4t/ha, due to the risk of the fire spreading. Sometimes narrow windrow burning may be restricted to canola stubbles and other lower dry matter crops, suiting low to medium rainfall areas. A cool, wet autumn can severely reduce the efficiency of burns leading to weed strips in the paddock. You may need to wait two weeks for the chaff to dry out before attempting to burn.
The effectiveness of narrow windrow burning on weed types highlighted by Maurie Street, Grain Orana Alliance:
- Narrow windrow burning has shown to be very effective on weeds that retain their seed at harvest above 15cm, such as ryegrass and wild radish, making it easy to capture these weed seeds in the header front.
- Wild oats, brome or barley grass that shed or drop their seed may do so before the header has a chance to capture those seeds so these may not be reliable candidates for windrow burning.
- Short weeds, such as wireweed, that set seed lower than practical to capture in the header front are not ideal candidates.
- Weeds that are able to re-sprout or re-seed after harvesting, such as fleabane and sow thistle, are also not likely to be good targets for windrow burning.
To get effective control on weed seeds, you must have sufficient stubble load, residue concentration, and a slow, light wind (about 10km/hour or less) to achieve the required burn intensity. Dr Michael Walsh, from the Australian Herbicide Resistance Initiative (AHRI) says to guarantee the destruction of the weed seeds you need temperatures greater than 400 degrees for 10 seconds for ryegrass, and 500 degrees for 10 seconds for wild radish. Burning can be avoided by using a chaff cart, baler or Harrington Seed Destructor towed directly behind the harvester.
Development of an App for stubble management decisions
A Yeruga Crop Research project (GRDC Project YCR00003) is developing a computer/smart phone application (App) to assist growers and consultants to estimate the benefit or cost for different stubble management decisions. The App will estimate the effect on profit by comparing the cost of stubble treatment operation and value of nutrient loss, for practices such as burning, narrow windrow burning or baling a crop to reduce stubble.
When choosing the right stubble management tools for your paddock, do not compromise on managing weeds, pests and disease, nitrogen nutrition or being able to sow the following crop on time and with good establishment. There is no perfect stubble management strategy for every year, and it’s likely you’ll need a combination of harvest, fallow and sowing stubble management techniques. In the fallow phase stubble may be mulched, incorporated, grazed or baled without the need to burn which incurs the greatest loss of nutrients. However a one off, late strategic-burn can be used if stubble loads are too high to manage, or there are particular pest, disease or weed burdens, without comprising the system.
- 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
- GRDC Update paper, 2017. Maintaining profitable farming systems with retained stubble across various rainfall environments. Available at: https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2017/02/maintaining-profitable-farming-systems-with-retained-stubble-across-various-rainfall-environments
- 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. Weed management in retained stubble systems. Available at: https://thestubbleproject.wordpress.com/weed-management-in-retained-stubble-systems
- The stubble project: Victoria and Tasmania. 2016. Stubble management at harvest. Available at: https://thestubbleproject.wordpress.com/2016/06/01/stubble-management-at-harvest/
- Tools and technologies for managing stubble characteristics in the fallow period, SW Victoria HRZ. Available at: https://thestubbleproject.wordpress.com/2017/…..
- The stubble project: Victoria and Tasmania. 2016. Break crops in retained stubble systems in the Wimmera and Mallee. Available at: https://thestubbleproject.wordpress.com/2016/05/31/wimmera-and-mallee-breack-crops
- The stubble project: Victoria and Tasmania. 2016. Sowing into stubble: seeder set-up and selection. Available at: https://thestubbleproject.wordpress.com/2016/05/13/sowing-into-stubble-seeder-set-up-and-selection
- 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/
- Stubble Management: A Guide for Mallee Farmers (2013). Available at: http://www.msfp.org.au/publications/msf-stubble-guide
- BCG (2016). Keeping livestock productive on stubbles. Available at: https://www.bcg.org.au/keeping-livestock-productive-on-crop-stubbles/
- Hunt et al (2016). The effect of grazing and burning stubbles on grain yield and quality in no till and zero till controlled traffic farming systems in SNSW. Available at: https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2016/02/the-effect-of-grazing-and-burning-stubbles-on-grain-yield
By Alison Frischke Livestock and Farming Systems Manager (BCG) Alison@bcg.org.au
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).