Scott BJ, Podmore CM, Burns HM, Bowden PI, McMaster CL (2013). Developments in stubble retention in cropping systems in southern Australia. Report to GRDC on Project DAN 00170. (Ed. C Nicholls and EC (Ted) Wolfe). Department of Primary Industries, Orange NSW pp 103.
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This review updates the 2010 Graham Centre Monograph, Stubble Retention in Cropping Systems in Southern Australia: Benefits and Challenges (Scott et al. 2010). It expands on research data and extension materials from the past few years for south-eastern Australia, including the results from GRDC funded grower groups. This update also includes some issues not referenced in the original monograph (Scott et al. 2010).
Prior to 2010, eight years of dry seasons made moisture conservation a priority for growers and during this period stubble retention was generally well adopted. During the past three years (2010-2012), with above average rainfall and heavier stubble loads, growers have been forced to burn stubble to address management issues including machinery trash flow, crop establishment, herbicide-resistant weeds, and pest and disease problems. Machinery development and stubble handling have become key priorities for growers.
Until the early 2000s the emphasis of conservation farmers had been natural resource management (NRM) and fuel and labour savings, with erosion mitigation and soil protection the main drivers for implementing stubble retention practices. While the NRM initiatives fostered commitment among conservation farming advocates, they have not provided the compelling economic evidence needed to persuade mainstream growers of the need to change from the traditional practice of burning stubbles to full stubble retention. Few research trials conducted in the last decade have conclusively demonstrated the economic benefits of full stubble retention.
Low frequency of stubble burning during the Millennium Drought seasons (2002-2009) indicated a large proportion of growers in the GRDC southern region will opportunistically
retain stubble. However, better seasons from 2010 onwards have seen a return to heavier stubble loads and extensive burning of stubbles. Regional surveys and consultation with growers indicate variable capacity and commitment of individuals to manage multiple components of complex stubble retention systems.
The benefits of conservation farming are assumed to accrue through improved soil health with subsequent benefits to crop yield and quality or reduced inputs to the farming enterprise. This may be true, if in a mixed farming system, the ratio of crop to pasture is stable. There is evidence this stability is not the case, and the farming system itself has changed to more cropping years and fewer pasture years. Any advantage of conservation farming may not necessarily be expressed as improved soil health, but as the potential for more cropping.
Heavy stubble loads are frequently encountered within the medium and high rainfall zones of the GRDC southern region. This frequency is the result of high grain yield, but also of high stubble loads, immediately post-harvest, at any given grain yield. For example, trials carried out across Australia show that a 2 t/ha grain crop in Queensland would appear to average about 1.8 t/ha of stubble immediately post-harvest. Reports from Western Australia indicate a stubble estimate of 3.6 t/ha, and at Wagga Wagga NSW 5.7 t/ha of stubble would be expected. The amount of stubble present at sowing is also influenced by the rate of breakdown of stubble between harvest and sowing of the subsequent crop.
Standing stubble effectively reduces wind speed at or near ground level. However, the effect of stubble on soil water storage during fallow is highly variable and depends on timing and amount of rainfall. This variability has recently been modelled and modelling offers the potential to better describe the effects of stubble retention on moisture storage across different seasons. The 'pulse paradigm' considers pulse size of soil water in relation to the frequency and duration of rainfall and soil moisture movement to depth. Rainfall variability is viewed as different possible sequences of rainfall events that create pulses of water into the soil, which are then lost to evaporation from the surface. For a single rainfall event retained stubble will only delay the evaporative loss of the infiltrated water. Cumulative evaporation from the system with residue will catch up to that without residue if it is not followed by a second 'pulse'. This means the benefits of retained stubble are determined by the soil water pulse size and duration, frequency and timing and the evaporative demand. If a second pulse occurs before full drying of the first pulse, water can move further down the soil profile in the delayed system (with residue). This becomes ‘stored’ soil water if it is pushed beyond the evaporation zone of the soil profile.
It is reasonable to assume that retained stubbles will eventually return the nutrients they contain back into the farming system. However, many claims of loss of nutrients from stubble burning are probably exaggerated. At Wagga Wagga, NSW after 21 years of a wheat-lupin rotation in a no-till system where stubble was either burnt or retained, the annual rate of change (loss) of total soil nitrogen was -13 kg/ha/year for retained stubble and -28 kg/ha/year for burnt stubble. The difference (15 kg/ha/year) is the result of retaining stubble compared with burning stubble. Any advantage of retained total nitrogen would not all be available to the plant, but can be seen as a long-term benefit in retained stubble systems in conserving rather than accumulating nitrogen.
Burning stubble produces smoke and contributes to particulates in the air that are a potential cause of health problems. Frequent high particulates in autumn at Wagga Wagga, NSW and to a lesser extent Albury, NSW were attributed to stubble burning. The high frequency of particulate exceeding standards at Wagga Wagga in 2002–2009 was probably due to dust resulting from dry conditions. The exceedances in autumn were probably a result of this being the time of the year with minimal groundcover. However, the contribution of stubble burning to particulate levels in the air remains undefined, but is recognised as probably less important than previously thought.
The ability of retained stubble to increase soil organic carbon (SOC) levels has generally been slow to negligible in Australian no-till cropping systems. As an example, at Wagga
Wagga SOC declined over 26 years by 52 kg/ha/year when stubble was retained, and was lost at 98 kg/ha/year where stubble was burnt. These loss rates were not statistically different from one another and not different from zero. Recent scientific reviews on the subject in Victoria conclude there is presently limited potential for carbon accumulation in soils either there, or more generally in Australian agricultural soils. Suggestions as to why SOC is not accumulating under no-till stubble retained systems include inadequate stubble loads (i.e. low carbon input) and/or that other nutrients essential to sequester carbon are limiting.
The adoption of no-till systems using disc seeders (zero-till) is currently increasing in continuous cropping systems as it enables minimal soil disturbance and retention of greater
amounts of stubble, particularly at narrow row spacings. Widening of row spacing is primarily a machinery modification to assist the passage of sowing equipment through heavy
stubble loads. At higher potential grain yields of wheat and wider row spacings it is clear the yield losses can be substantial - at 4 t/ha in 18 cm rows doubling row spacing to 36 cm reduces yield by 9.5%. Canola yields of 2.5 t/ha at 18 cm row spacing are reduced by 7% by doubling row spacing to 36 cm.
The adoption of inter-row sowing is another innovation aimed at handling heavy stubbles. Growers using inter-row sowing have commonly adopted row spacings of between 22.5 and 30 cm, with installation of 2 cm guidance systems enabling accuracy for row placement. Inter-row sowing enables machinery to avoid stubble for ease of sowing and herbicide application. It also has advantages in slowing crown rot infection of emerging seedlings in wheat-on-wheat sowings. Herbicide carry over effects in inter-row cropping can limit the choice of the second crop after the application of some herbicides to the first crop.
Grazing stubble has associated benefits including reducing high stubble biomass before sowing, utilisation of residual grain and consumption of green summer plants. But there are potential disadvantages of grazing stubble including hoof damage to soil structure and reduced water infiltration rates. However, there is relatively little (<10%) or no effect on subsequent crop growth and yield if excessive grazing is avoided. The risks can be minimised by avoiding overgrazing to maintain groundcover and avoiding excessive grazing in wet conditions. In southern NSW, soil nitrogen levels have been shown to be higher, and wheat yield and protein were the same or higher, following grazing by sheep. This finding was attributed to more rapid nitrogen cycling in grazed systems.
Stubble may carry over for more than one year, adding to the amount of stubble to be sown through. Carryover stubble also can be a source of disease, not only in the first crop sown into retained stubble, but also for a second crop.
Of the wheat diseases carried over on stubble, yellow leaf spot (YLS) was frequently reported as misdiagnosed in the NSW and Victoria. Symptoms were confused with nitrogen
deficiency, herbicide phytotoxicity, frosts and aluminium toxicity. These misdiagnoses can lead to unnecessary fungicide treatments.
Crown rot is an important disease of wheat in the GRDC northern region and appears to be of increasing importance in the GRDC southern region. A survey of 76 paddocks in southern NSW during 2012 revealed crown rot to be ubiquitous.
Complete control of rhizoctonia has been reported at Avon, South Australia, 5-10 years after adoption of full stubble retention, in systems with limited grazing and high nutrient inputs. Disease suppressive soil activity was considered a function of microbial populations, composition and activity. However, the SA experience must be kept in geographical context as there is no evidence for suppressive soil activity in other areas of the GRDC southern region.
The GRDC southern region provided abundant evidence of pest-related issues associated, at least in part, with stubble retention. Changes in farm management practices and varying climatic patterns are contributing to a shifting complex of invertebrate and vertebrate pests. Snails, slugs and false wireworms are long-standing issues, the severity and geographic distribution of which seem to be increasing. The associated increase in pesticide use has influenced pest complexes and accelerated selection pressure for resistance. Emerging invertebrate issues include weevils, bronzed field beetles, earwigs, millipedes and slaters.
Mouse plagues have increased in frequency as a result of stubble retention, a range of diverse crops, reduced cultivation, and reduced grazing pressure from livestock. This has led to both an increase in mouse numbers and greater damage for the same number of mice. Whereas during the past mouse plagues occurred every 6 to 7 years (before 1970), they are now likely to occur once every four years. Changes in cropping systems have not only improved feed supply and quality, they have provided a longer time period during which high quality food and shelter are available. Also mouse plagues are now reported in consecutive years, which rarely occurred historically.
Herbicide efficacy is a challenge in stubble retained systems where the stubble can interfere with the application of herbicides. Herbicide retained on stubble can volatilise, breakdown or wash onto the soil with subsequent rainfall. For pre-emergent soil applied herbicides, increased water volumes (100 -150 L/ha) are recommended in order to get more herbicide to the soils surface. Higher ground speeds at sowing can be accompanied by 'soil throw' from tined implements. Soil from the sowing row is thrown to the inter-row space reducing the effective rate of application of soil applied herbicide near the seed and increasing effective application rates in the inter-row space. The combination of wide row sowing and higher ground speeds at sowing have enabled higher application rates of pre-sowing soil-applied herbicides.
In addition, some useful herbicides are limited to a maximum of 50% groundcover of stubble, which is well below the stubble loads experienced in commercial cropping systems.
Furthermore, there are proposals that could lead to regulation of useful chemicals and could see their loss from agriculture.
Conservation farming systems rely heavily on herbicides for weed control, and this has led to the evolution of herbicide-resistant weeds in southern Australian cropping areas. Nonchemical weed control offers an alternative control as part of an integrated weed management (IWM) system. Non-chemical methods of weed control can include cultivation, burning stubble, in-crop competition with weeds and collection of weed seeds in the harvester and their subsequent destruction. Other options include forage conservation, or green and brown manuring of paddocks, often with legume crops. These crops are then either incorporated by cultivation or, in the case of brown manuring, sprayed with non-selective herbicides before herbicide-resistant weeds set seed.
Conservation farming practices that retain stubble on the soil surface do not involve tillage. However, the sustained use of no-till can create problems in some situations, including stratification of nutrients, inability to incorporate lime and herbicides, increased pest populations (for example, slugs), inability to manage herbicide-resistant weeds, increased disease incidence, consecutive high stubble loads and soil compaction by livestock. Strategic tillage could have a place in conservation farming systems to ameliorate the effects of no tillage.
But there is concern that some benefits of no-till are cumulative, and cultivation could eliminate or reduce these benefits.
This review of stubble retention in cropping systems in southern Australia has identified potential gaps and opportunities in extension of disease diagnosis and clarification of the
effects of no-till on SOC. Development and research gaps exist in (1) understanding the effects of stubble and stubble arrangement on storage of soil moisture and nitrogen cycling, (2) improving herbicide efficacy and (3) the evaluation and adaptation of the destruction of weed seed at harvest in the GRDC southern region. Further advances in inter-row sowing as a technique for areas of high and medium rainfall is a topic that provides opportunities which may enhance herbicide efficacy, improve moisture storage and increase grain yield.