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What is the best cropping sequence for the farm?

If a farm has been in continuous wheat for many seasons and yields are poor or declining, the introduction of alternative species (break crops) into the cropping sequence might boost yield considerably. Interactions between the wheat and break crops are complex because there are variable roles of nitrogen, water, disease and weather. This chapter examines these complexities.

It explores how and why they might influence the performance of wheat in different sequences and how they might impact on the economics of the farming system. It then outlines trials to test these interactions. These three-to four-year trials when complete should provide the basis for crop sequence management packages for the local region.

To complete the story you will need to refer to the chapters Constraints to cereal-based rainfed cropping in Mediterranean environments and methods to measure and minimize their effects, Why do any of your research on-farm?, Optimizing variety x sowing date for the farm, and Optimizing nitrogen use on the farm.

Which farms could benefit from these trials?

Terminology: sequences versus rotations

This chapter deals with crop sequences not rotations. In rotations different crops are grown in a fixed order year after year but in sequences the order is not necessarily repeated. The term ‘sequence’ captures the flexibility that is a feature of aware farm management, continuously responding to changing market demands and changing on-farm constraints.

Here the focus is on short- to medium-term crop sequence trials rather than the fully phased long-term rotations traditionally conducted on experimental stations.

Background to cropping sequencing

Generally, the yields of crops grown in continuous monoculture decline as a result of a build-up of soil or stubble-borne disease that is specific to that crop species.

This problem can be overcome in some cases by the use of varieties that are resistant to the disease, but for some soil-borne diseases there is no current genetic resistance.

Chemical control is usually not economical so that the only control strategy for these diseases is to grow a non-host or break crop that is not infected by the disease, so that the levels of disease in the soil are reduced prior to the next wheat crop. Crop rotation of this sort has been practised for thousands of years.

As well as reducing the levels of cereal disease, legume break crops, such as peas, lupins and lentils, also fix nitrogen from the air. This provides a beneficial N contribution to following cereals that may persist for longer than one year.

Finding an appropriate sequence crop

The challenge is to find break crops that farmers are willing to include in their crop sequence. Farmer adoption will usually depend on the following:

Break crops must be adapted to the soils and climate of the area and be accompanied by a good agronomic management package if they are to be adopted successfully by farmers.

What influences a crop’s response to preceding break crops?

Disease control

The most significant benefit of break crops to following cereals is the reduction in soil and stubble-borne disease. In Australia for example, the yield of wheat after canola or linseed crops has averaged 20 percent higher than for wheat, after wheat largely because of disease reduction.

The dominant pathogens within a crop sequence differ between environments, soils and seasons. You must identify the pathogens in your system and understand what influences their activity if you want to realistically assess the chance of success of a crop sequence.

Impact of a break crop on the incidence of ‘whiteheads’ in a following wheat crop


Nitrogen benefits

In addition to providing a disease break, grain legume break crops such as peas, lupins, lentils and chickpeas may provide residual N to following wheat crops.

In some regions these benefits have boosted yields of following wheat crops by up to 50 percent, similar to the effects of applying adequate N fertilizer out of a bag. However, the magnitude of the N benefit depends on effective N fixation by the legume, the amount of biomass produced and how much N is removed in legume seeds or stubble.

Some non-legume break crops, such as linseed, may also leave any deep residual soil N untapped because they have a shallow root system and use soil N inefficiently. Since residual N influences both yield and protein content of following cereals, it is important to consider the N balance of the whole system when attempting to understand crop sequence effects (see chapter on Optimizing nitrogen use on the farm).

Benefits in N nutrition may also arise from non-legume break crops simply because they effect a healthier wheat root system, enabling the wheat crop to use soil N and applied N fertilizer more efficiently and thereby yield better.

Water use by preceding crops

In drier environments or in dry seasons the amount of water removed from soils by preceding crops can have a major effect on the growth of following crops. In annual cropping cycles the inclusion of deep-rooted rotation crops that remove significantly more water than wheat may cause yield loss in some seasons.

Impacts on nutrients other than N

Be aware of the nutrient requirements and likely removal by prospective break crops. Brassica crops such as canola, remove large amounts of elements from the soil, particularly S and Zn. These crops may induce deficiency in following crops if adequate amounts are not supplied.

Arbuscular mycorrhizal fungi (AMF) colonize the roots of most crop plants and symbiotically help acquire immobile elements such as Pand Zn. In general, tap-rooted crop species are dependent on AMF for Pand Zn nutrition in low Psoils while fibrous rooted cereals are less dependent.

Canola and some lupins are among the few species that are not hosts of AMF so that AMF inoculum levels in soil are usually low following these crops. If following crops in the sequence are AMF dependent or if your soils are low in P or Zn, you may need to apply the required nutrients.

Stubble allelopathy and herbicide history

Other interactions between crops in the sequence can arise from chemical interference caused by leached compounds from retained residues (allelopathy) or from residues of herbicides applied to the previous crop.

Seasonal interactions

All the above-mentioned factors influenced by crop sequence also interact strongly with seasonal conditions. The complexity of the many possible interactions can lead to outcomes that may be difficult to understand or explain.

For example, dry periods between seasons increase carry over of soil-borne diseases (dry conditions maintain inoculum levels). Root infection is increased by wet, cool conditions during early growth while dry conditions during grain fill, allow expression of the diseases as “whiteheads”. These heads have small pinched grains or no grains.

Alternatively, wet periods prior to sowing may negate any differences in disease inoculum. This may be by reducing inoculum directly. However, it may also be via the indirect route of improving the following crop by refilling the profile with water and mineralizing large amounts of soil N, so making that crop less susceptible to disease.

Such seasonal ‘weather’ interactions are largely responsible for variations in response to preceding crops. So although published studies have shown an average response of wheat yield to non-legume break crops of a 20percent increase, this spans a 104 percent increase to a 12 percent decrease.

As there are so many complex variations and interactions, the only way to get a reliable picture across your region is to actually conduct crop sequence trials at several sites over several seasons.

The human resource needed will clearly be considerable, ideally being interested, collaborating farmers working on their farms.

Each farmer will benefit by defining together with you the appropriate sequence for their location while you, as coordinator, will learn the driving forces for the region.

The overview information should give you confidence to design sequence packages for any site within the region that will have a high chance of working.

Some subtreatments in one replicate of year 1 in the case study

Quantifying the advantages of sequences using a gross margins analysis

A good place to start assessing a proposed crop sequence is to do a rough cost and benefits analysis (gross margins).

Decide on the crops you might use in the sequence then sum the likely income from each crop in the several-year sequence. Next, subtract the variable costs from that sum (seed, fertilizer, labour, fuel and machinery wear).

Compare the result with a sum calculated for continuous wheat crops. How much would the yield of the wheat crop in the sequence have to improve to make an overall improved profit compared with continuous wheat?

In some sequences, reduced direct income from the break crop might be offset by an increased income from a higher yielding wheat crop and/or by a reduced requirement for N fertilizer in the following year.

Other costs and benefits, which may be more difficult to quantify, are those associated with risk.

On the negative side, the risk of crop failure may be increased if the break crops are not well adapted to the region. On the positive side, having product diversification in years of cereal failure or low cereal prices may lessen risk. The costs and benefits of spreading sowing and harvest workloads of the different crops and on longer-term improvements in soil fertility are also difficult to assess.

How break crops alter wheat yield - an example case study

The case study is from two trials (I and II) investigating how break crops, in an area normally in continuous wheat, affected yield of the following wheat crop. It also details the resultant gross margins for the sequences.

The key difference between the studies was that in trial I the post break crop wheat was grown in a dry season (180 mm) while in trial II the season was wet (300 mm).

Field cropping history and methods

Wheat was grown on all plots in year 1. In year 2, field pea (a legume), canola (a Brassica) or wheat were grown in specific plots. Finally, in year 3, wheat was again grown in all plots. Yields of wheat crops from the final year are shown in the figure.

The effective N contribution of the legume to the following wheat crop was examined in a subtrial by comparing its effects on the wheat yield with the effects on yield of three fertilizer N treatments, 0, 50 and 100 kg N ha-1.

Effects on crop performance

The wheat crops grown in the dry year (trial I) had a limited yield potential (note that all yields in the left hand graph of the figure fell between 2 and 4 t ha-1). Despite this, they still had a sizable response to fertilizer N (compare the black points at 0 and 50 kg ha-1).

In this dry year the wheat crop following canola performed little better than that following wheat (compare the yellow and black lines). This is because in a dry year, disease barely develops, so the impact of a disease-cleansing crop such as canola cannot be demonstrated.

A positive impact of the preceding legume break crop on wheat yield of 1.5 t ha-1 can be seen clearly when no N was supplied to the wheat. This equated with fertilizer applied at 50 kg N ha-1.

The preceding pea crop had a grossly negative impact on wheat yield when the large amount of 100 kg N ha-1 was applied.

The residual N together with the applied N would have amounted to 150 kg N ha-1 equivalents being available to the wheat. This caused excessive vegetative growth, rapid use of available stored water and finally severe “haying-off” (see chapter on Optimizing nitrogen use on the farm) when the crop ran out of water in the dry spring of that year.

For the wheat grown in the wetter year (the right hand graph) the level of soil disease was very high and as a result, wheat grown after wheat had low yield, and when N fertilizer was applied, there was an even lower yield than in the dry year.

The positive impacts of the preceding break crops in this season of high disease were large, amounting to the more than doubling of wheat yield when fertilizer was applied.

Yield of wheat grown in three crop sequences, after wheat, canola and a legume, in two contrasting seasons, dry (180 mm rain) and wet (300 mm) with impact of applying N to the wheat crop shown

What about gross margins?

The gross margins for crops in the break crop year of a cropping sequence will depend upon yield of crops and their relative prices. Prices may be similar for wheat and canola but possibly lower for peas, which are sometimes used for stockfeed. So a sequence with peas would commonly be least profitable.

By contrast, in the wheat year of the sequence following the break, the gross margin for wheat in dry years may be highest after peas due to a saving in costs of applied N. In wetter seasons, the gross margin may be higher for the canola-wheat sequence due to the higher gross margin for canola in the first year and its excellent break crop benefits, and a large response to applied N for the following wheat crop.

The choices are clearly broad. There is no hard and fast rule for all areas. In general, however, in dry environments break crops may be a poor option for farmers because of lower yields and risk of crop failure of the break crops themselves, combined with a small positive response to the break of the following wheat crop. In wetter years and areas the break crop and N benefits can be very positive throughout the sequence and may persist into later wheat crops.

In dry years break crops may be a poor economic option for farmers but in wetter years and wetter areas benefits of break crops on reducing disease and supplying N to following wheat crops can be very positive.

Your on-farm trial

You will need to discuss with collaborating farmers which alternative (break) crops to wheat are best to include in the trial.

The control crop sequence for the trial will be wheat, wheat and wheat over three seasons. The alternative sequences will be wheat, break crop, wheat.

The break crops must be crops that can be handled with current equipment; crops that will produce a positive effect on following crops; crops that interest the farmer; and preferably crops that can be sold in the region without large costs for cartage.

From the outset, make sure that collaborating farmers really understand the potential economic benefits of using the break crops and when and why the benefits might occur as well as the risks involved.

Propose an alternative cereal such as oats or triticale, an oilseed like linseed or canola, and a grain legume (pea, lentil, chickpea) for inclusion. These will provide the most information on mechanisms of crop response that can be explored with the farmers as a joint learning exercise.

For farmers who have never grown break crops, or who are half-hearted about the trial, it may be better to concentrate on one promising break crop that is easy to grow. Do not be unduly adventurous with the farmers’ time and resources.

What you need before starting

Selecting a suitable area of land for a crop sequence trial is critical. Ask the collaborating farmers about the cropping and disease history of the land. Discuss the likelihood as well of future possible expansion of subtreatments as new questions arise. Before you both finally decide on the trial site, talk through the following requirements:

(1) Crop sequence experiments run for several years on the same area of ground so it is vital to commit that chosen land for two to four years. The farmer may not be prepared to ‘lose’ that land to trials unless there is an agreement that the research organization will cover any risks of crop failure at an agreed price.

(2) The trial will probably require a large area as it is usually necessary to:

(3) Field history and the current state of the field will play a major role in influencing the impact of a break crop. For example, a field that has grown wheat continuously for many years will respond differently to one that has grown wheat then an alternative crop prior to the trial.

The field chosen should have a cropping history common in the area and represent farmers’ practice. Preferably choose a site to exaggerate treatment effects (with high levels of disease or N deficiency).

Avoid fields with patchy areas of grass weeds that host cereal diseases because the patches will mask treatment effects. Also, avoid locating treatments under trees or by old fence lines and gates. Aim to identify suitable sites in the year prior to the intended trial.

Choosing species/varieties for the trial

An agronomic management package for successful growth of the break crops must be agreed upon prior to the trial. You may first need to consult your colleagues with expertise in the crops for detailed management requirements as well as the farmer to ascertain available resources.

Choose break crops that can be handled with the farmer’s present machinery and with a market for the produce. Explain if necessary, that the break crops must grow well to generate income in year 1 and maximize benefits to following crops. They are not a simple fallow replacement. Legume crops must be inoculated with the correct Rhizobium strains to optimize N fixation and seeders must be used that handle the large legume seeds carefully during sowing. A poor start will lead to a poor crop.

Layout of a crop sequence trial over two seasons, following a farmer’s wheat crop. Alegume (peas) and a Brassica (canola) are used as break crops. This trial can be duplicated on an adjacent site, but started one year later, to compare the impact of different weather patterns on the sequence

Oilseed crops such as canola may require insecticide sprays to control establishment pests and may require windrowing.

For the break crop year of the sequence choose varieties of the break crops that are reputed to be well-adapted to environments like at the site.

For the following wheat year of the sequence select a commonly grown wheat variety so that farmers can gauge the results against personal experience. If feasible, include a second wheat variety that may be more tolerant to one or more of the diseases thought to be present. This will check the role of the interaction between diseases and break crop in any yield response.

Afield layout and general methods to use

The diagram below shows one design for a crop sequence trial. It is based on that in the case study.

The trial investigates the impact of three different crop species on the growth of a following wheat crop.

Two nitrogen treatments (-N and +N in the diagram, which could be 0 and 50 kg N ha-1) are applied to the following wheat crop.

The trial can be duplicated on an adjacent site, but started one year later, to compare the impact of different weather patterns on the sequence.

Plot sizes

The design is called a ‘split-plot’ because the main treatments (previous crops) are not fully randomized with the subtreatment (N rates) but are kept together in ‘main plots’. This is done because it is difficult to physically manage small areas of different crops.

Generous buffer plots (shown in yellow on the plot layout) have been included at the junctions with previous crop treatments to allow herbicides to be applied without damage to neighbouring experimental plots of a different species.

The size of these buffer areas depends on the farm’s ability to spray herbicides without significant spray drift.

If the farm rarely uses sprays, suggest that accuracy can be achieved with covered boom sprays and by avoiding spraying when it is windy.

If herbicides will not be used these areas can be small or eliminated though they are also useful to reduce movement of surface stubble (and diseases) between plots during the off-season.

Individual plots should be 2 m wide at the absolute minimum to ensure that enough bordered rows are available for harvest samples, and to allow for spatial impacts of different diseases and lateral root exploration and normal water-use patterns of the crops.

The plots need to be long enough to accommodate any destructive sampling required during the season as well as a yield sample at least 2 m2. Usually plots should be at least 10 m long.

Part of a crop sequence study. Note long plots to allow for insertion of subtreatments in later years

Since disease incidence is one of the measured effects of the study and is notoriously patchy, either longer plots or more than three replicates (blocks) are recommended.

Marking out

Prior to the break crop season it is important to install permanent pegs near the trial plots that can be used to precisely relocate the plots from year to year. These can be placed on nearby fence-lines or buried in the soil at the edges of the plots below cultivation depth.

Plots can be located square from a fence line using a tape measure for distance. Plot pegs can then be re-located from the permanent pegs on the fence without the need to leave them in the field in the off-season.

Marking permits any crop residues that have been blown off plots or moved by animals over time to be relocated back onto plots before sowing the following crops in the sequence.

It is also critical to control weeds during the off season as weeds growing in the plots can host diseases and alter the water and N profiles which were left by the previous crops.

Other subplots

In the example provided, the subplots in the second year are N treatments. Other possible treatments for the second year might include wheat varieties with different tolerance to important diseases, different sowing dates which influences disease carry-over, or stubble retention versus stubble removal.

The treatments selected will depend on the nature of the major diseases present (soil or stubble-borne) and how they interact with treatments, and on the normal management strategies of the farmers.

It should be considered from the outset that it might be necessary to change the management of the following wheat crop in a sequence to maximize the benefits from break crops (e.g. more N required, earlier sowing).

Observations to make and data to collect in the year prior to the trial

After the wheat crop preceding the trial has been harvested and you and collaborators have some time, peg out the trial area in the field.

During the pre-sowing fallow ensure that crop residues are evenly spread or removed and that weeds are controlled. Baseline soil samples can be taken for key soil properties such as pH (acidity) and EC (electrical conductivity) to ensure the range is acceptable for the intended crops (e.g. canola is sensitive to acid soils, low S and waterlogging). Pre-sowing soil samples can also be used to assess disease inoculum using glasshouse bioassays, and to determine soil water and N profiles.


Rainfall data will be required throughout to interpret plant responses and max/min temperatures are very useful to gauge the speed of the season and for linking crop performance to extreme temperature events. If such data are not available within 1 km of the site, a rain gauge at least should be put in the trial area. Check in the chapter on variety x sowing date for guidelines on preparation of tables for recording rain and other data.

While discussing design of tables with collaborators, construct a table that will be the basis for the economic analysis of the crop sequences. Discuss how to assess and record the costs of all inputs and activities used to grow the crops. This table of details should enable you to make a summary table (shown later).

Data from first year plots and crops

From the first year you will need data on final biomass, yield and nutrient removal by the different crops and residual levels in the soil of disease inoculum, water and nutrients. The soil measurements can be taken following the harvest of the crops (post-harvest data) and immediately prior to sowing the following wheat crops (pre-sowing data).

If only one measurement is possible the pre-sowing one is preferred as this describes the conditions for the forthcoming wheat crop.

Water content and soil N can be measured gravimetrically from soil cores taken from the plots (see the chapter on nitrogen use for details of methods and calculations). Within a 2 m x 10 m plot at least two cores are required to the previous crop’s estimated final root depth.

To allow for the high variability of soil N in the surface layer, collect up to ten subsamples at random from the surface 0-10 cm layer and bulk them.

Appropriate amounts and depths of soil needed for disease bioassays depend on the disease. Ask a pathologist for advice and if possible involve him/her actively in this and other aspects of the study, as characterizing and quantifying disease is a vital component of the work.

Involve collaborating farmers in any on-farm assessments as it is essential that they should recognize symptoms, not only for the trial, but also for the future.

During the second year wheat crop

The pre-sowing measurements of soil water, N and disease you will already have taken will be essential for interpreting crop response during the season.

Recommended wheat crop measurements during the season are plant density at the two to three leaf stage and final biomass, yield and yield components at maturity (methods for collecting these data are detailed in the chapter on Constraints to cereal-based rainfed cropping in Mediterranean environments and methods to measure and minimize their effects). Observations of disease development are also required at these times.

Each root disease (e.g. Rhizoctonia, nematodes, Fusarium, Eyespot) has different symptoms and different systems for scoring severity. Check the literature and consult your local pathologist for methods best suited to your requirements. Be aware that some symptoms of root disease can be confused with those of drought or frost.

Disease development can be assessed using visual ratings of symptoms or by more quantitative assessments. For example, severity of take-all infection can be assessed either by scoring seminal root black lesions at the four leaf stage as in the accompanying picture, or by the proportion of ‘whiteheads’ to total spikes in the crop during the grain filling stage, or by the proportion of culms at maturity showing characteristic blackening of the lower stem base.

An example of scoring disease. As infection increases from slight (score 1) to severe (score 3) fewer healthy roots (yellow) are produced and at the same time the proportion of black infected roots increases

How to interpret the data

It is useful to start by making an estimate of yield potential at your site using the rainfall data you have collected. Use the following rough guide.

For each mm of growing season rainfall above 100 mm expect 15 (to 20) kg grain ha-1.

Thus for 300 mm growing season rainfall, yield potential is (300-100) x 15 kg ha-1 = 3 (to 4) t ha-1.

This relationship errs on the conservative side and varies somewhat between regions, but try it anyway to indicate roughly how well your treatments are performing. You will notice that it considerably underestimates production in the earlier case study where management was optimized.

If treatments are yielding well below the calculation, it is likely that there is an underlying cause of yield limitation that may not be related to your treatments such as saline subsoils or soil compaction.

Now have a session with collaborating farmers to decide exactly what the data say. You will want to have this assessment session after each season.

Many of the things that should be discussed will be obvious, like how did the break crops fare in their own right; which was the best break crop; were there any real problems associated with the break crops that could be overcome in any repeat study?

Before getting into debating and explaining in detail why particular things happened, a good starting point will be discussion of the cost benefit analysis or gross margins analysis. In a nutshell, this will show whether the sequences were worthwhile and whether further optimization of the sequences is required. An example follows.

Working out gross margins

The calculations in the following table use actual relative prices (with a nominal currency) and actual data collected from a study similar to that proposed as your on-farm experiment. It considers three sequences, wheat-wheat, canola-wheat and peas-wheat, i.e. wheat in the second year of all sequences.

Add your own data to a similar table but use local currency and adjust costs and values of crops to reflect local markets. You might decide to rework the example with local prices.

The example table shows the second year either as a dry season or a wet season. Only the zero N treatment is presented for the dry season as it is often impractical to add N under dry conditions. For the wet season, a 50 kg N ha-1 treatment is shown after wheat and after canola, as a top dressing would be appropriate in such a season. The added N cost is approximately US$50. It is unnecessary to add N after peas.

Check the following conclusions using the data from the table and look for parallels and differences in your own data.

Gross margins calculations

For break crops in year 1, for the wheat crops of the following year (year 2) and for the combined crop sequences, wheat-wheat, canola-wheat and pea-wheat (last column) Year 2 examples are for a dry year, when no N was applied to the wheat crop, and for a wet year. In the wet year 50 kg Na ha-1 was applied after wheat and canola, but not after peas. Price for wheat varies with protein content




4 (2x3)

5 (4-1)







$ yr 1

$ yrs 1 +2

Year 1



















Year 2: eg of a dry year after year 1 (0kgN/ha)






















Year 2: eg of a wet year after year 1 (50kgN/ha)






















Taken on that year alone, canola seemed to be a suitable economic alternative for wheat.

Wheat after peas at zero N does very well in economic terms (US$900 versus US$240 for wheat after wheat). Due to carry over of N from the pea crop it yields well (4 t/ha) and has a high value protein content (US$250 versus US$220 per tonne grain). Despite a poor economic performance in year 1, the two-year gross margin for the pea-wheat sequence in column 6 is good (US$1150 versus US$920 for wheat-wheat) or 25 percent better than wheat: wheat.

In your case, this benefit would be reduced if you receive no price advantage for higher protein content grain (12 percent better than wheat: wheat.)

Considering gross margins for the two-year sequence, using break crops virtually doubled farm income.

Most outcomes from break crop trials can be explained in terms of levels of disease, nitrogen or water.

How do you explain your results?

There are many possible outcomes to your trials because of the potential interactions between break crops, diseases, soil nutrition and weather. You and the farmer could have difficulty explaining the results.

To help you, several scenarios follow. Work through them with collaborating farmers and see what applies to your situation. They might explain why there were benefits from the sequences or why they failed. They will also help determine what further treatments or subtreatments might be required to optimize a crop sequence for the farm.

If break crops improve yield by reducing disease you would expect:

If there are high levels of disease even after break crops, it may be because:

If there is no disease in the wheat-wheat crop sequence it may be because:

If soil N analyses showed N benefits following the break crop you would expect:

If there are no obvious benefits of N to growth such as no increase in tillering or biomass it may be because:

If there are N benefits to growth but not to yield it may be because:

If the degree to which crops conserved soil water was important you would expect:

Results you cannot explain

Do not be surprised if there are inexplicable results. There are many aspects of plant response to crop sequence that have not been resolved. However, the majority of crop responses can be explained in terms of disease, water and nitrogen. You should also consider impacts of other nutrients in the sequence (P, K and S), impacts of residues of different crops on plant growth, or herbicide carry over. These effects are usually expressed during seedling emergence and early growth.

Further reading

Clarkson, J.D.S. & Polley, R.W. 1981. Diagnosis, assessment, crop-loss appraisal and forecasting. In M.J.C. Asher & P.J. Shipton, eds. Biology and Control of Take-all. Academic Press, London, UK.

Kirkegaard, J.A., Gardner, P.A., Angus, J.F. & Koetz, E. 1994. Effect of Brassica break crops on the growth and yield of wheat. Australian Journal of Agricultural Research 45, 529-45.

Kirkegaard, J.A., Hocking, P.J., Angus, J.F., Howe, G.N. & Gardner, P.A. 1997. Comparison of canola, Indian mustard and linola in two contrasting environments II. Break crop and nitrogen effects on subsequent wheat yields. Field Crops Research 52, 179-96.

Heenan, D.P. 1995. Effects of broad-leaf crops and their sowing time on subsequent wheat production. Field Crops Research 43, 19-29.

Hocking, P.J., Kirkegaard, J.A., Angus, J.F., Gibson, A.H. & Koetz, E.A. 1997. Comparison of canola, Indian mustard and linola in two contrasting environments I. Dry matter production, seed yield and quality. Field Crops Research 52, 162-78.

Peoples, M.B., Ladha, J.K. & Herridge, D.F. 1995. Enhancing legume N fixation through plant and soil management. Plant and Soil 174, 83-101.

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