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Stubble Retention

A.P. Doyle,

Special Research Agronomist, NSW Department of Agriculture,

Agricultural Research Centre, Tamworth. NSW 2340

In discussing stubble retention, we are really considering alternate management techniques for our soils between subsequent crops. In so doing we must come to terms with the question raised by the Soil Conservation Service in their excellent 1978 journal - should we consider stubble as Trash or as Treasure? i.e. is stubble something we burn or else graze and trample to get rid of it as soon as possible, or is it a valuable farm management aid?

There is no doubt that the easiest way to deal with stubble is to put a match to it. Weeds are then easily controlled by repeated cultivation. Put this easy system extracts a heavy toll on the soil.

A more recent approach involves reduced cultivation, substituting herbicide application for weed control, or else direct drilling into the uncultivated field after spraying for weed control. In most systems used in southern Australia, grazing is an integral part of fallow management. In contrast in most of north eastern Australia grazing is not included in stubble management systems.

An alternate approach is to retain the stubble, by incorporation, by surface mulching or by seeding into undisturbed stubble residue.

The stubble management system most suited to an area will he largely controlled by the environment, and in particular, the rainfall pattern.

It is necessary to differentiate clearly between the Direct Drill system as practised in Southern Australia and the No-till system being developed in northern New South Wales and southern Queensland. The Direct Drill system involves no cultivation prior to sowing directly into undisturbed soil, with stubble residues normally removed by grazing or by burning. No till is practised where rainfall must be conserved in the soil during the fallow period. Cultivation during the fallow for weed control is replaced by herbicide application, and crops are sown through the stubble residue into undisturbed soil.

Effects Of Stubble Retention On The Soil


Soil erosion is arguably the greatest problem Facing Australian agriculture. Stubble management systems are a major tool in the reduction of soil erosion, whether by wind or by water. This is vividly demonstrated in results reported from southern Queensland (Table 1).

Table 1. Soil loss from tillage treatments at Creenmount, Southern Queensland l978/79 - 1980/81 (Freebairn and Wockner 1982).

Stubble Treatment

Soil Erosion Loss t/ha





Surface mulch


No till


Water erosion losses in southern Australia are normally not as dramatic as those caused by heavy summer storms in north eastern Australia - but they are a major concern, particularly as cropping is increased relative to pasture leys, leaving the soil surface increasingly unprotected. Dr. Pratley has quoted examples in his paper from work at Wagga and Canberra.

Retained stubble reduces the effect of rainfall impact on breaking down the surface structure, and it slows down the rate at which water runs off the surface. The reduced run off velocity, and hence reduced energy in the run off water decreases its capacity to carry soil with it.

Similarly, standing stubble reduces the wind velocity near the soil surface, and greatly reduces wind erosi on. In fact, stubble retention systems in mechanised farming were developed in response to the devastation of wind erosion in the dust bowl area of the U.S.A.

The effects of stubble on water erosion are complimentary to those of structural soil control works such as contour banks (Table 2). Stubble retention methods reduce the extent of structural works required, or alternatively in association with correct structural works, enable cropping of areas of higher erosion potential than is currently advisable with bare fallow methods - such as steeper slopes and more highly erodible soils.

Table 2. Percent soil erosion loss compared to continuous bare fallow. (Based on universal soil loss equation -Marston, P. and Donaldson, 52.

Stubble Treatment

Soil Loss %


No Structural Works

With Structural Works







Surface mulched



No till



There is little Australian information on the levels of stubble required to reduce erosion to an acceptable level. As an indication Table 3 shows levels of stubble calculated in the U.S.A. to reduce soil loss to two tonnes per hectare per year on an 8 per cent slope for various soil types. Also indicated is a crop yield to leave this amount of straw, at 150 kg wheat straw per 100 kg grain yield.

Table 3. Stubble amounts required to reduce soil erosion loss to 2 t/ha/year on land with an 8% slope (Woodruff et al 1q66).

Erosion type

Soil type

Flattened wheat stubble required

Equivalent crop grain yield


Loamy sand




















Loamy sand



The effects of grazing and of individual cultivation operations on the reduction in stubble cover and consequent loss in erosion protection are discussed in an article by Marston (1978).

The devastation of erosion is increased by the fact that the portion of soil removed by both wind and water erosion is much higher in nutrients than that left behind. For example Marsh has reported yield losses in Western Australia of 8 to 20% in the following crop as a result of nutrients lost in wind erosion.

In some instances where the soil surface sets very hard, runoff may be reduced where the soil surface has been roughened by limited cultivation compared to complete absence of cultivation, though stubble retention is still an advantage.

Soil Structure

As outlined by Dr. Osborne, soil structure refers to the way that the small soil particles are held together In larger units or aggregates. Each cultivation breaks some of these naturally formed aggregates apart. Organic matter is the source of the glues that hold these particles together, hence continued cultivation and stubble burning lead to a long term break down in soil structure. The problem is exacerbated by the fact that Australian soils are naturally low in organic matter compared to most overseas cropping soils.

A further effect of cultivation is to break up and fill large holes and cracks in the soil left by rotting roots and soil fauna such as earthworms, thus reducing the rate at which water can percolate into the soil.

Experiments at a number of centres throughout the Australian wheat belt have shown that retention of stubble in association with greatly reduced cultivation, or preferably elimination of cultivation, will halt the structural breakdown, and can begin the road to recovery. However, the soil structure improvement is slow under Australian conditions so new practices must be employed in the long term - two years of pasture will not rectify the problems caused by years of cultivation.


Retention of stubble generally improves the trafficability of soils in wet conditions, and this may allow greater timeliness of seeding. However, in Southern Australia greater difficulty may be experienced in sowing uncultivated soils in dry conditions where direct drilling is practiced. In contrast, no-tillage systems in the north normally retain a moist surface longer than cultivated soils, hence improving the chance of optimum time of seeding in dry conditions.

Moisture Retention

Surface retention of stubble by mechanical means generally does not improve total soil moisture retention over the season unless very high rates of stubble are present and/or frequent rainfall occurs. In contrast, where successful chemical weed control is substituted for cultivation, increases in soil retained moisture are normal. This has important farming implications for those areas where moisture conserved during the Fallow is critical for successful cropping.

In the more reliable and higher winter rainfall areas of Southern Australia, there is generally no advantage in attempting to store summer rainfall. However, workers such as French and Fettell have shown advantages of fallowing from the previous spring or of conserving summer rain in parts of southern Australia and of central western New South Wales, depending on soil type and rainfall pattern. The lower and less reliable the winter rainfall, the greater is the probability of benefiting from fallow water storage. Under these conditions a no till programme is very likely to improve fallow moisture storage.

Stubble Grazing.

This aspect has been largely covered by Dr. Pratley. Relevant here is the fact that, whilst grazing and trampling by stock will reduce the difficulty of seeding through stubble residues, any reduction in stubble cover reduces the erosion protection afforded by that stubble. This is particularly important in wind erosion liable areas, where protection is greatly enhanced when stubble is standing and anchored.

Effects Of Stubble Retention On The Crop

Reduced Plant Vigour.

The early growth of direct drilled wheat in southern Australia can be reduced by up to 30 per cent compared to crops sown under conventional cultivation. There is conjecture as to whether this is caused by reduced root penetration in the denser soils, by unavailability of nutrients (particularly nitrogen and phosphorus), by cooler soil temperatures, by plant toxins, or by a combination of these. In any case, the direct drilled plants normally catch up in growth by the time of harvest. In fact, in drier seasons or environments the reduced growth can be an advantage by reducing the early use of limited soil moisture reserves, and thus reducing water stress during the critical grain setting and grain filling periods.

A major problem associated with the reduced early plant vigour is greater susceptibility to weed problems because of reduced competitiveness of the crop.

Nutritional Effects.

Burning of straw results in loss to the atmosphere of nitrogen, sulphur and some phosphorus.

Nitrogen. Incorporation of straw leads to an initial tie up of nitrogen by the microorganisms that decompose the straw in the soil until the stubble is broken down. In addition, cultivation leads to the breakdown of organic matter with consequent release of nutrients, particularly nitrogen, which then become available for subsequent plant use. It was therefore anticipated that crops grown under stubble retention systems would require additional fertilizer nitrogen.

Although some experiments have indicated a greater response to nitrogen tinder stubble mulch, No till or Direct drill conditions, most experiments have been inconclusive, or indicate no increase in nitrogen requirement. It may well be that differential effects between management systems are small under our conditions, and that such differences really only show out under conditions of marginal deficiency. In any case, it is widely acknowledged that much more research is needed on this aspect of stubble management systems.

Phosphorus. Phosphorus uptake is reduced in direct drilled wheat, apparently due to the accumulation of phosphorus on the soil surface. There maybe also some limitation to phosphorus uptake due to restricted seedling root growth in direct drilled seed beds. The problem of surface accumulation of phosphorus will be accentuated where pastures are topdressed in rotation with direct drilled crops.

In areas where sulphur responses occur on pastures, the possibility of sulphur deficiency in direct drilled cereal crops should be considered.


Some of the compounds returned to the soil in stubble are toxic, or are converted in the soil to substances toxic to subsequent crops, resulting in reduced or distorted growth, or in some cases, death of seedlings. Although this phytotoxicity has been commonly reported to be associated with stubble retention in the field overseas (e.g. U.S.A. and Britain), and in pot experiments in Australia where fresh straw is used, phytotoxic residue effects have not been commonly reported in the field in Australia. This is probably due to the fact that residues are well weathered by the time of sowing the next crop, and that residue levels in any case are generally lower than in situations where injury is reported elsewhere.


Yellow leaf spot (Pyrenophora tritici-repentis) has been reported to be associated with stubble retention throughout New South Wales. The disease survives between crops on undestroyed stubble, and Rees (1981) has indicated that yield losses of 10 to 15 per cent are common. In the medium to long term, genetic resistance should be possible in wheat. In the short term, greater attention will need to be paid to rotation where the disease is present on the wheat stubble. Barley is not affected, and so is a suitable rotation crop. Control can be considered on a paddock basis because yellow leaf spot spreads only slowly from its original source of infection. In severe carry over situations where wheat must he sown, it may he advisable to burn the stubble just prior to planting.

Take all (Gaeumannomyces graminis var tritici) survives in infected roots and crowns, and is favoured by cool moist conditions. Some, hut not all, Australian reports indicate a build up in take all with direct drilling. The situation is complicated by the fact that stiibhle retention conditions favour the build up of organisms antagonistic to take-all.

Rhizoctonia bare patch (Rhizoctonia solani) has been reported to increase in severity with direct drilling.

Two separate south Australian reports indicate cereal cyst nematode to be decreased by direct drilling.

Weed Control

The elimination of cultivation prior to seeding precludes the use of pre-emergent incorporated herbicides. Large quantities of surface stubble can also restrict the effectiveness of surface applied pre and post emergent herbicides, leading to the necessity to use the more expensive in-crop herbicides.

Most failures with direct drilling are due to lack of adequate weed control. Success of direct drilling is more likely if the programme is begun in a situation where weeds are not out of hand.

Control of weeds will require an integrated management system, including crop rotation, grazing management and herbicides and may require strategic cultivation.

Stubble Management

Having suggested that stubble is available resource, rather than a nuisance to be disposed of at the first opportunity, we must now consider how to best utilize the stubble. The management system adopted will depend on the problems being faced, and may require a compromise between competing requirements.

Where wind erosion is a problem or where water erosion is likely during the fallow period, the aim must he to keen stubble standing and anchored. The greater the grazing of the stubble, the less will be the effectiveness of erosion control.

Stubble Mulching

In situations where moisture needs to be conserved during the fallow, weeds must be controlled either mechanically or by herbicides. With mechanical control, maximum surface residues are maintained with a blade or sweep plough - which will bury only about 10-15 per cent of the stubble at each pass. These are at times used in combination with a rod weeder, which buries only about 5 to 10 per cent of the stubble. Weed control will be inadequate with a blade plough if conditions are too wet.

A chisel plough followed by trash workers will bury about 20 per cent of the stubble at each pass. Again, because soil disturbance is less than for conventional scarifiers, weed control is dependent on optimal soil moisture conditions.

The major feature of these stubble handling implements is greater clearance between tynes (hence the use of sweeps and duck-foot shares) and greater clearance between the frame and the soil.

Retention of stubble in this way requires a new approach to seeding, because of the large amount of retained trash. The usual approach is to use an air seeder fitted to a tyned implement capable of passing through the trash.

Where fallow erosion is a problem, it is better to retain the stubble and to harrow burn just before seeding if no suitable seeder is available, rather than to clean cultivate. Burning is, however, a second best, and the aim must be to get maximum benefit from the stubble.

No till

As every cultivation breaks down soil structure, the aim has been to eliminate cultivation. The complete No-till system, using herbicides in place of cultivation can give winter cereal yields equal to (or in drier years greater than) conventional fallows, provided weed control is adequate. Unfortunately, at this stage, chemical weed control costs are usually higher than that of conventional systems, limiting its widespread adoption. Stubble retention systems, however, are rapidly gaining popularity.

In contrast, higher grain yields are almost always obtained for summer crops tinder a no-till system. yield increases of about 500 kg per hectare are usual in no-till sorghum (Holland, pers. comm.). We can expect to see rapid expansion of no till summer copping in the north of the state.

Stubble Incorporation

Offset or tandem discs do an excellent lob of weed control, and they bury about 90 per cent of the stubble at each pass. Stubble is better buried than burnt where cultivation is to take place, but the massive soil disturbance of the disc and consequent destruction of soil structure eliminates most of the advantage of incorporation of the stubble. Also, of course, as the stubble is buried, It loses most of its value for erosion control. Again - our aim should be towards minimum soil disturbance.

Direct Drilling

In terms of the direct drill system, I have both the advantages and disadvantages of being an outsider looking in.

There are many who are more qualified than I to outline the methods and to discuss the finer details of direct drill farming. However, some comments are warranted.

There are long term problems In any system which includes burning of stubble, though in the short term stubble burning may be desirable on occasion for disease control, or may be necessary to enable seeding through straw with available machinery. Any cultivation has adverse effects on soil structure (though on some hard setting soils surface roughness may be an advantage). On the other hand, substitution of herbicides for weed control must not lead to residue problems.

Reduced cultivation, with substitution of herbicide application for some cultivations, is a step towards better soil management. The ultimate for this region appears to he direct drilling into substantial stubble residues. Whilst direct drilling has been done with little or no modification to existing combines, other than using rigid or strengthened sowing tynes, sowing into heavy stubble will require modified machinery, with coulters, spaced seeding tynes and press wheels. Triple discs have generally compounded soil smearing effects, and have not found wide acceptance.

Many reports on direct drilling indicate an initial period of one or two years when direct drilled crops have lower yields than conventionally sown crops, after which the yields are at least comparable if not better. I suspect that much of the early lowered yields are a result of Inexperience whilst learning how to handle the system. There is no doubt that a direct drill system requires greater management skills than does conventional farming.

The crux of the direct drill system appears to be weed control, with many workers emphasising the need to begin direct drilling where there is not a weed problem, and to keep control of the weeds. In fact, the success of direct drilling was really only assured after the release of in crop selective weed control chemicals such as Hoegrass (B).

The question of advantages of stubble in addition to direct drilling is under investigation in the Wagga region in a programme which is also studying the place of crop rotations. In this programme, reported by Taylor and Lill (1982) grain yield of direct drilled lupins after the stubble was mulched was 50% higher than for those where stubble was burnt and lupins were sown after three workings. Yields of both wheat and lupins were increased where stubble was retained rather than burnt (Table 4).

Table 4. Grain yield of wheat and lupins under varying stubble management systems (Taylor Pers. comm.).


Grain Yield t/ha


Stubble Treatment

Direct Drilling

Reduced Cultivation

Conventional Cultivation







Not Burnt










Not Burnt




This particular piece of local research seems a suitable point for conclusion - in their report Taylor and Lill (1982) emphasise weed control. They are pointing to rotations and they are looking to a future where stubble is used as a valuable management aid - not lust trash to be trampled and burnt at the first opportunity.


Comments in this paper are based on the written and verbal reports of many workers. I have only included in the references work where I have quoted specific results.


1. Freebairn, D. M., and Wockner, C. H. - 1982. Conf. on Agric. Eng. Armidale, N.S.W.

2. Marsh, B. d’B. - 1981. Proc. National Workshop on Tillage Systems for Crop Production. Roseworthy S.A. p.B-22.

3. Marston, D. - 1978. .Jnl. Soil Con. Serv. N.S.W. p.228.

4. Rees, R. C. - 1981. Proc. National Workshop on Tillage Systems for Crop Production. Roseworthy S.A. p.E36.

5. Taylor, A. C. and Till, W. 3. - 1082. Proc. 2nd Aust. Agron. Conf. Wagga Wagga. p.196.

6. Woodruff, N. P. et al. - 1Q66. Trans.A.S.A.E. 9 p.849.

Further Reading

1. Proceedings of a national workshop on tillage systems for crop production. Roseworthy S. Aust. Aug.-Sept. 1081.

2. No-tillage crop production in northern New South Wales Project Team Meeting. Tamworth. April 1982.

3. Proceedings of the second Austra1ian Agronomy Conference. Wagga Wagga. July 1%2.

4. Journal of the Soil Conservation Service of N.S.W. Vol. 34. No. 4. Oct. lQ7g.

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