Scott BJ, Eberbach PL, Evans J and Wade LJ (2010)
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Late stubble burning just prior to sowing is commonly practised in the south-eastern cropping areas of Australia (central and southern New South Wales, Victoria and South Australia). However, stubble burning may be prohibited due to the perceived risks to public health from smoke. Regional information from southern Australia was reviewed in order to identify the basis for non adoption of stubble retention in southern and central New South Wales (NSW). We sought to highlight any gaps in knowledge of stubble retention practice, and impacts which may contribute to non adoption.
In the current monograph, "stubble retention" implies standing stubble or surface-applied stubble/mulch, and incorporated or buried stubble is rarely discussed. Similarly, stubble retention was separated from tillage practice, with direct drilling and/or no-tillage as the most relevant context for stubble retention.
In Western Australia (WA), the high level of adoption of stubble retention may have resulted from the risk of wind erosion on sandy soils, with the relatively low stubble loads not presenting a barrier to adoption. Similarly in Queensland and northern NSW, the intense summer rainfall and its associated soil erosion risk have contributed to the retention of crop stubble as the dominant practice. In contrast, erosion risks from water and wind have been lower in central and southern NSW and risk of erosive summer rainfall was very low in Victoria and South Australia (SA). Furthermore, the later burning of large stubble loads just before sowing practised in southern Australia, had some characteristics of stubble retention, by maintaining stubble over much of the summer/autumn period. The time of critical risk was after late stubble burning in March/April, when the soil surface was briefly exposed, and before the establishment of some ground protection, usually by early winter (June/July).
The presence of stubble can increase water infiltration and slow moisture losses through evaporation, increasing soil moisture storage at sowing. These effects appear to be of most value in Queensland and northern NSW, where production of winter cereal crops is highly reliant on stored soil moisture, but likely to be of less importance in the more southerly cropping areas where winter crops are more dependent on incident rainfall. In central and southern NSW there is a component of summer rainfall, which could be stored in the soil and be of benefit to a subsequent crop, particularly in lower rainfall years or environments. The effect of late stubble burning (compared with retention) on stored soil moisture is unclear.
In dryland crops, burning of stubble causes losses of approximately 4 kg nitrogen/t of wheaten stubble burnt; with average losses of 15-26 kg/ha of nitrogen (N) in high-yielding areas. These losses were less than suggested previously. Further, in stubble-retained systems, N may be immobilised. While immobilisation rates of 5-13 kg/ha of N with the decomposition of 1 t/ha of wheaten stubble can be derived from European research, the optimal rate of N fertiliser was only increased slightly by stubble incorporation in WA.
Soil organic carbon (SOC) was less in stubble-burnt or stubble-removed systems than in stubble-retained systems. However, there was no evidence of sequestering of C in stubble-retained systems; rather the amount of SOC in the soil declined at a slower rate with stubble retention compared with stubble burning in cropping systems. Where stubble retention was practised, SOC was greater in the shallow surface soil (0-5 cm) than when stubble was burnt. This increase in SOC may contribute to greater structural stability and water infiltration in the soil surface and greater earthworm populations.
Blockages of sowing implements by stubble were the primary reason for non adoption of stubble retention by farmers in southern and central NSW, where stubble loads were high. Traditional sowing machinery was limited to sowing through 2-3 t/ha of cereal stubble and modification of machinery combined with pre-treatment of stubble (slashing, harrowing) can enable sowing to be conducted through 4-5 t/ha stubble. Heavier stubble, typical of the eastern higher rainfall areas of central and southern NSW, would require the purchase of specialist machinery. Widening sowing rows in cereals, which reduces stubble blockage problems, is likely to reduce cereal yield.
Estimates from field reports indicate that 20-49% of the stubble quantity at harvest is decomposed and lost by the time of sowing in southern Australia, compared with 57-84% in Queensland where higher rainfall in summer would hasten decomposition of stubble.
Burning stubble, rather than its retention, reduced the carry over of diseases and pests to subsequent sensitive crops. The temperatures achieved in a stubble fire influenced the effectiveness of the fire in controlling some plant disease on the stubble and Australian field examples of the effectiveness of stubble burning in the control of crown rot, common root rot, eyespot and yellow spot are presented. Similarly, stubble retention increased the populations of some grasses in subsequent crops.
Conservation farming systems with stubble retention relied on herbicide use for weed control and this has led to a problem with herbicide resistant weeds, particularly annual ryegrass, wild oats and wild radish. The integrated management recommended for control of resistant weeds included a reversion to stubble burning and cultivation.
Acidification of the surface soil (0-10 cm) was greater with stubble retention than stubble burning, in both southern NSW and SA. The effect was confined to the shallow surface soil (0-5 cm). Some nutrients (P, Zn, Cu) accumulated in the soil surface under conservation tillage. Stubble retention may contribute to the stratification and increased fertiliser input or occasional cultivation were suggested as amendments. Similarly stratification of soil pH can be amended by the addition of lime and its incorporation through cultivation.
Stubble retention is claimed to increase cereal yield in some areas. The evidence reviewed in the current monograph, however, indicated that yield was not significantly improved with stubble retention compared with stubble burning or stubble removal and yield may frequently be lower with stubble retention. In most experiments, the small yield loss was not related to seasonal rainfall, but in a few experiments, the adverse effect on yield of stubble retention was greater in wetter seasons. This effect of growing season rainfall needs to be understood as yield reductions with stubble retention were high (up to about 1 t/ha of grain) in seasons of high potential yield.
The results of long-term experiments do not always predict the outcomes in commercial agriculture. Limitations were imposed on long-term experiments which were not important or not relevant to farming. Stubble retention may provide an earlier sowing opportunity than stubble burning due to preservation of soil moisture in the soil surface and this effect would not be accounted for in experiments sown on the same (or over a few) days. Also, in many areas of Australia, a pasture phase was typical of the farming system, but most experiments and/or treatments related to continuous cropping. Thus, the frequency of stubble burning in experiments was often far higher (every year), than would be observed in commercial agriculture. Any advantage or disadvantage of stubble burning may be over represented in the experimental results.
There appeared to be scope for breeding wheat better adapted to stubble retention systems. Sources of disease resistance had been identified and applied breeding is required. Longer coleoptile wheats were identified and they minimised the consequences of poor depth control at sowing, and assisted emergence through stubble. In the longer term, the competitive ability of wheat may be enhanced and specific allelopathic characteristics may be identified and incorporated into new cereal cultivars.
Research examining systems maintained in long-term conservation farming in which the system is "disturbed" by infrequent cultivation and/or stubble burning is required. These practices appeared necessary to control weeds, mix the surface soil to de-stratify nutrients and incorporate lime in acidifying soils. If the benefits of conservation tillage accumulate in the longer term then these disturbances may negate any benefits of conservation tillage.
Stubble retention has been widely recommended to farmers in southern Australia. However, the case for stubble retention in central and southern NSW has been more difficult to justify, as the key drivers of benefit and adoption of the practice are less clear. Erosion from intense summer rains, as in Queensland, is rarer and the soils are not as prone to wind erosion as some soils in WA. Further, as presented in the current monograph, the effects on grain yield of stubble retention are largely negative, using current technology. The farmer has additional costs, (machinery modification or purchase, and potentially increased field operations to roll/slash/harrow stubble, and potentially increased application of pesticides and nutrients) and may not be receiving an economic benefit. Other benefits of the change in stubble-retention practice are possibly accruing to the community, through reduced smoke pollution and reduced turbidity and nutrient concentrations in waterways.