Rotation & Variety choice

1. Introduction

Soybean is not only the most important grain legume crop grown in the world, because of its dual purpose oil and protein production, it also has important properties that allow a flexible and beneficial role in many farming systems. Specific crop management decisions such as nutrition, management of weeds, diseases and insects etc. are covered in subsequent modules in this manual. In this module we consider farming system related topics that influence the success of a soybean rotation crop including: 

• benefits of soybean as a rotation crop

• land management

• row spacing

• criteria for variety selection

• varieties adapted to different regions in Australia.

2. Soybean and its use in farming systems

Crop rotation plays an important role in spreading risks associated with seasons and markets. As our knowledge of the agronomic value of crop rotations increases – e.g. reducing disease severity, lowering the risks of herbicide resistance, controlling hard to kill weeds, reducing insect pressure, positively affecting soil physical properties and maintaining arbuscular mycorrhizae levels – we more appreciate the importance of crop rotation in sustainable farming systems. Further, if the rotation crop is a legume, the provision of symbiotically fixed nitrogen is an added bonus. Thus the ideal system uses broadleaf phases (legumes, canola, sunflower, cotton) in rotation with grass phases (maize, wheat, barley, triticale, oat, sugarcane, rice, pasture). Being a broadleaf crop and legume, soybean is ideally suited for rotation with grass crops.

It is also important to consider both summer and winter crops when looking for rotation species. With major winter crops being the various cereals, particularly wheat and barley, and soybean being a summer crop, it ideally suits as a rotation crop in many cropping areas in eastern Australia. Further, a ‘double-crop’ rotation of soybean and wheat, for example, on an annual basis will improve cash flow and reduce overhead costs. Even if it is not possible to grow both crops on the same land in any one year the growing of a summer and winter crop will spread the work load and seasonal risks to income and better utilise machinery and labour resources.

Soybean offers a wide diversity of uses. In addition to grain production for a range of markets, it can also be used for forage, hay or silage, or incorporated as a green manure crop. 

2.1 Grain soybean

The vast majority of soybean crops grown in Australia are harvested for grain. Grain soybean crops will deliver all the agronomic benefits of a legume in the crop rotation sequence. However, an important additional benefit is the cash income that can be generated by taking the crop through to grain harvest.

There are three main markets for grain soybean in Australia. The backbone of the industry is now the edible grain sector. Soybean grain can be processed into vegetable oils for further processing into table oils and margarine spreads and also for animal, poultry and fish feed, however, crushing markets for soybean grain have reduced in Australia since 2015. The crushing market in Australia is heavily influenced by the import parity price of soybean meal from North and South America. Soybean varieties with a dark coloured hilum can only be sold for oil and crushing as hilum colour is not critical for this market.

The oil from grain soybean can also be used for industrial applications such as biodiesel, plastic, wax, ink and the production of graphene membranes used in medical and industrial processes, including water filtration.

Edible grades of soybean grain can be sold into the high-end culinary markets to be processed into a variety of foodstuffs such as milk, tofu and baking additives such as kibble and flour. This market prefers grain with a high protein concentration, large size and only clear or pale coloured hilum varieties are accepted. Soybean grain that does not satisfy the superior quality required for milk and tofu but still meets all the other quality specifications for colour, protein and size can be sold into the edible flour or kibble markets.

2.2 Green manure crops

Soybean has gained popularity as a green manure crop option for the months between sugarcane crops on the east coast of Queensland and New South Wales (NSW). In the tropical regions of north Queensland managing the soybean crop for grain can be challenging due to insect and disease pressure and the frequency of rainfall can cause harvesting difficulties. However, as soybean grows very well under high rainfall conditions it is considered highly desirable as a break or ‘fallow’ crop in the sugarcane system where it has replaced more traditional green manure crops such as cowpea and lablab. This is partly because soybean produces more dry matter (Table 1) with a higher nitrogen concentration (3.0–3.5 % N compared with 2.0–2.5% for cowpea) and is more tolerant to waterlogging than cowpea and lablab.

As a green manure crop, soybean can be sown anytime from late October to December or earlier provided the risk of frost has passed and soil temperatures have reached at least 13°C for seven days and are rising. This usually fits well with the sugarcane cropping system between when the last sugarcane ratoon is cut and ploughed out and before the next sugarcane crop is planted, i.e. the sugar cane ‘fallow’ period.   

The inclusion of soybean in the sugarcane systems of eastern Australia provides a range of benefits, in addition to income from the grain if harvested, including:

• capacity to produce more dry matter and fix more nitrogen per hectare than traditional green manure legumes

• better tolerance of waterlogging than traditional green manure legumes such as lablab and cowpea

• potential soil health benefits by reducing soil-borne diseases and lesion nematode numbers in the sugarcane system

• broadleaf rotation crops provide an opportunity to control grasses and other weeds, which benefits the following plant cane crop. The sulfonyl urea tolerant soybean varieties Kuranda HB1* and Mossman HB1* released in 2022, remove the plant back period for certain halosulfuron methyl herbicides (e.g. Sempre®) used to control nutrass  

• good groundcover for erosion control between sugarcane ploughout and replanting.

The varieties Kuranda HB1*, Mossman*, Hayman*,  Leichhardt and Stuart are well adapted to tropical Queensland and suitable for use as a green manure crop. In sugarcane production areas of southern Queensland and northern NSW suitable varieties for green manure include Hayman*, Richmond*, Soya 791 and Asgrow A6785. The use of green manure crops in irrigated soybean cropping regions of southern NSW is not common, however, varieties such as Burrinjuck* would be suitable.

Traditionally green manure legumes in the sugar industry were broadcast and disced into the soil (i.e. not sown using a planter). Research and experience has shown that like most grain crops, the growth and benefits from the soybean crop can be greatly enhanced by establishing it with a planter to achieve an even plant stand, using raised beds or ridges to improve drainage, applying fertiliser and a pre-emergent herbicide. Good weed control in the legume phase will carry through to the next cane crop.

2.3 Forage soybean

Although not widely used for this purpose in Australia, soybean crops are suitable for both forage, hay and silage production, particularly in the sheep and dairy production systems of northern NSW. The best dry matter yields can be expected from early-planted, long-season varieties, cut around early to mid-podfill stage. 

Silage yields (at around 35% dry matter, cut at the milky-dough/pod fill stage) can be expected to be around 25 t/ha.  This equates to around 8 t/ha dry matter yield. 

NSW DPI and Local Land Services investigated the use of variety Hayman* for grazing by cattle and grain recovery. The results are described in this Factsheet, including feed value analysis.

3. Land management 

Good and even crop establishment is critical for success in all plant crops. Soybean seed is sensitive to soil moisture and planting depth during the establishment stage. If the soil is too wet the seed may rot and if too dry it may fail to germinate and/or die before emergence. Emergence may also fail if the seed is planted too deep (>5 cm) or into a hard setting soil. In hard setting soils, soybean may emerge more successfully when planted in hills or well-prepared beds, rather than on the flat. Controlling traffic to reduce compaction of the crop’s root zone is known to benefit crop establishment and later phases of the crop, particularly in high rainfall areas. Thus, land preparation is a very important consideration when planting soybean.

3.1 Conventional tillage

With conventional tillage the whole paddock is prepared prior to planting. Initial cultivations are usually done with discs, chisels or heavy scarifiers. The aim is to have 20 to 25 cm of loose soil, allowing the formation of raised hills or beds. However, land preparation will vary according to the particular soil type. Where soybean is grown in rotation with sugarcane, the cane can be sprayed out prior to cultivation or disced out during the land preparation. On light sandy soils, cultivation should not be excessive. When dry enough, heavier soils may require more working to obtain a satisfactory seedbed – care must be taken when working heavy clay soils to avoid causing further compaction and damage to soil structure.

3.2 Controlled traffic and permanent beds

Controlled traffic farming (CTF) has become popular for many cropping enterprises as increased knowledge is gained of its benefits and machinery with satellite GPS guidance and autosteer systems are more widely available. Soybean is no exception. 

CTF involves separating the traffic (wheel) zones from the crop root zone to avoid the adverse effects on crop growth of soil compaction caused by heavy machinery, especially when the soil moisture level is high. Soil compaction has long-term adverse effects on crop establishment and yield through limiting root penetration, water and nutrient supply to the crop and aeration of the soil.

Where in conventional farming the whole soil surface area of the field is worked to make it suitable for cropping, with controlled traffic only the planted area is worked and not the wheel tracks. The wheel tracks remain permanently located across crop cycles, allowing the compaction caused by the passage of heavy machinery to be minimised or completely isolated from the cropping zone.  

Controlled traffic requires that equipment be modified so that all wheel widths match, allowing tyres to run on the same permanent wheel tracks with every operation. This can be done by using guide arms to mark the rows, however, natural driver error limits the success of guide arms. The most effective way is to use satellite GPS guidance systems that minimise driver error.

The planted crop areas are commonly referred to as ‘beds’ and may be either raised to improve drainage in wet conditions or flat (drier areas or soil types not suited to raised beds).

Over time, as the permanent traffic lanes become more consolidated, it becomes easier to drive the machinery down the compacted wheel tracks, using less fuel and allowing earlier field access after rain. In the planting zones, the soil profile develops an improved structure and better moisture holding capacity. The establishment of permanent traffic lanes facilitates a movement towards, initially, zonal tillage (tilling only the crop area) and eventually, in some situations, use of minimal/strip or zero tillage can occur.

In the wet tropics, high rainfall or irrigated production areas, it is advisable to plant soybean on raised beds or hills to allow for better drainage during the wet summer months or irrigations. Ideally, hill or bed formation should be achieved with enough time for the soil to settle prior to planting. Depending on soil type and seasonal conditions, additional forming operations may be required to achieve the desired bed top and shoulder shapes. In areas with lower rainfall, soybean can be planted without forming beds.

3.3 Zonal tillage

As the name implies, tillage is restricted to zones. This concept is based on the premise that cultivation should only be carried out when absolutely necessary and that crop-growth zones and traffic zones should be separated on a permanent basis. Thus, zonal tillage is an integral part of a controlled traffic system and is important in maintaining the compacted traffic lanes and non-compacted cropping lanes. Other advantages include: being able to use smaller tractors, savings in time and fuel, non- compaction of the cropping zone benefiting root growth and drainage, compaction of traffic zone allowing quicker access onto the paddock following rainfall, and provision of a friable seedbed to facilitate crop establishment. Soybean can be planted in the cultivated zones (beds) with row crop planters. 

For success, aim to match wheel spacings of planters, harvesters, tractors and spray rigs to preserve the structural integrity of the beds and retain controlled traffic zones.

A system that has gained popularity in coastal farming systems, especially the sugarcane industry, is raised beds with furrows at 1.8 m spacing and a plantable bed top of around 1.2 m wide onto which two, three or four rows of soybean are planted. At the end of the soybean crop, two rows of sugarcane are directly planted into the beds through the soybean residue.

Cane farms using this system involving soybean rotations have reported 20% yield increase in the following sugarcane crop compared with a bare fallow between cane crops.

When sowing soybean into sugarcane or other crop trash, several issues must be considered. Soybean does not grow well in compacted soil. Use of a ripper tine along the cane stools will loosen the area where soybean is planted without disturbing the trash on the surface.

Zonal tillage can also be used between the rows of soybean stubble once the crop has been harvested to create a planting zone for the following crop, for example, sugar cane as illustrated in Figures 1 and 2 below.

3.4 Zero tillage

Zero tillage is an ultimate goal in developing a sustainable farming system. It is the next step from zonal tillage. After the permanent cropping and traffic lanes are firmly established the cropping zones or beds should remain friable enough to be able to be directly seeded into the undisturbed soil, however, friability may vary with soil type. Trash or stubble management will be an issue, especially with high biomass crops.

There are a number of advantages to this system in addition to those captured with zonal tillage:

• more time and fuel savings

• less capital invested in cultivation machinery

• can be done on slopes not suitable for conventional tillage due to erosion hazards

• improves soil structure long term

• planting moisture is not lost through tillage.

In coastal farming systems, zero-till systems have been developed in rotation with sugarcane where soybean is sown on either side of the cane row and cane is then planted into the soybean residue.  Volunteer cane can be controlled in the soybean phase with grass selective herbicides while the soybean can be controlled with a broadleaf selective herbicide. However, in most cases the soybean will be allowed to grow as cane will easily outcompete soybean.

4. Trash management

4.1 Managing grass (sugar cane, wheat) residue to facilitate soybean planting

Both grass and legume trash require careful management if the most is to be gained from a zonal/zero tillage system.

Fresh sugarcane trash in a green cane trash blanket system contains organic acids and other chemicals that can adversely affect soybean germination. To offset against poor germination, and thus poor soybean establishment, rake away the fresh cane trash from the line where the soybean is to be planted or delay planting the soybean crop until 4–6 weeks after cane harvest.

Another consideration when planting soybean into heavy trash situations can be nitrogen deficiency early in the life of the soybean crop (i.e. before the nodules are formed and start to fix N from the air). This is caused by the large amount of high carbon trash (e.g. up to 7–10 t/ha dry matter after sugarcane harvest) immobilising or ‘tying up’ available soil nitrogen as it begins to break down. The addition of some starter fertiliser with a low N level (20–30 kg/ha) at soybean planting will address this issue during the time prior to nitrogen fixation commencing.

4.2 Managing soybean residue to facilitate grass crop (cane, wheat, pasture) planting and to maximise the benefits of soybean

The amount of nitrogen contributed to the system by a soybean crop will depend on how the crop is managed – green manure or harvested for grain. With the latter, around 66% of the nitrogen in the crop will be removed with the grain.  

For example, an average soybean crop producing 8 t/ha total shoot (above ground) dry matter at 3.5 % nitrogen concentration will contain 280 kg N/ha. If the crop is harvested for grain, approximately 185 kg N/ha will be removed with the high protein grain, leaving around 95 kg N/ha of residual N contributed to the soil from the soybean crop residues. In addition, there will be a certain amount of nitrogen associated with the root system and rhizosphere. General estimates suggest that the root system and rhizosphere of the soybean crop contains approximately 30% of the N contained in the above ground biomass. Thus, if the shoot biomass contains 280 kg N/ha the amount in the rhizosphere and roots will be around 80 kg N/ha making a total estimate of residual N from a harvested soybean crop of 180 kg N/ha. These figures of course will vary with the situation and the vigour of the soybean crop, so they should be treated as a guide.

How the soybean residue is managed will also have an important impact on nitrogen availability to the following crop. If the residue is incorporated the nitrogen is mineralised rapidly. Although this is generally accepted as a positive outcome for the following crop or pasture, it can have negative effects with long-term crops such as sugarcane in high rainfall areas where leaching can rapidly move nitrogen out of the root zone on permeable soils and de-nitrification can become a major source of N loss on heavier clay soils.

Surface managing the legume residue can slow the mineralisation process and conserve the release of the legume nitrogen until later in the growing period of the following crop. Such a strategy fits very well into a zero tillage system. It  can also have positive effects on conserving soil moisture for the establishment of the following crop.

In mixed farming enterprises in Australia, soybean is used for pasture improvement with the pasture seed spread by aircraft or drone as the soybean crop is approaching harvest or just after harvest. The germinating pasture species make use of the high nitrogen soybean crop residues. A program known as ‘Beef and Beans’ was developed in the 1990s and is still used by graziers in NSW to improve grazing pastures, particularly on marginal soils/slopes and in systems where a restart is required, for example, following spray out of the pasture for weed management. The nitrogen from the soybean crop restarts the system.

5. Row spacing

Establishing the soybean crop in rows has economic and crop management advantages to older style ‘combine’ sowing methods. For example, row cropping allows precision placement of seed and fertiliser at sowing, the use of banded sprays, shielded sprays and inter-row cultivation for weed control. Rows are easier to access during the crop cycle to check for insects and diseases and facilitate the use of penetrating directed sprays for insect or disease control. Row cropping is also more amenable to controlled traffic and raised bed systems. 

Row spacing can be manipulated to manage some soybean diseases, for example, wider spaced rows allow more aeration and separation of plants in the crop, which is a management strategy to reduce white mould (Sclerotium sclerotiorum), particularly in high rainfall and humid growing conditions.

It is important to account for row spacing when calculating seeding rates to achieve recommended plant populations. Formulae to calculate target plant populations are covered in subsequent sections of this manual (Module 5 Agronomy). 

Soybean is adaptable to many row spacings, underpinning its usefulness as a legume rotation crop for many farming systems in Australia. Row spacing is generally determined by the dominant crop in the farming system (e.g. cotton, sugarcane, winter cereal, rice, maize or pasture), and the available soil moisture. Some particular regional comments regarding row spacing are as follows.

5.1 NSW rainfed

For dryland crops on the slopes and plains of northern NSW, a wide row spacing of 75–100 cm is suggested in order to help conserve soil moisture for pod-fill. In higher yield potential situations row spacing can be reduced from 75 cm.

In the higher rainfall areas of the Northern Tablelands and North Coast of NSW, soybean crops are usually planted at 30–50 cm row spacings. Wider rows are acceptable provided complete canopy closure is achieved by early–mid flowering and weeds are managed. Narrow rows are preferred when planting late or where weed management is likely to be a problem as the crop canopy closes more quickly.

In cultivated seedbeds, using conventional seed drills the row spacings will generally be between 18–36 cm. With minimum or no-till systems, specialised direct drills (preferably disc openers) or precision planters are necessary to ensure uniform seed placement and clearance of pasture residue or crop stubble.

5.2 NSW irrigated

In irrigated cotton farming systems in northern NSW and southern Queensland, soybean is traditionally planted on the same row spacing as cotton (e.g. single row hills at 100 cm spacing). However, grower experience suggests that soybean yield can be up to 10–20% higher using a narrower row spacing of 50 cm provided soil moisture is not limited.

In southern NSW irrigation areas, the industry standard configuration is raised beds with furrows at 1.8 m and two rows planted on each bed (90 cm row spacing). However, row spacing can range from 30–90 cm depending on the farming system, stubble loads, time of sowing and inter-row sowing capability. Planting soybean in the middle of a raised bed is only suitable on soils with very effective lateral water movement. Sowing more than two rows per bed may require more water to fill to the middle of the bed to field capacity where the centre row will be located.

Irrigated soybean crops grown in rows on raised beds produce higher and more consistent yields than soybean planted on a traditional ‘border check’ irrigation layouts in southern NSW/northern Victoria. Raised beds facilitate the sowing of soybean into a moist seedbed for successful plant establishment. Comparatively, border check layouts often result in soybean establishment problems, due to difficulties sowing into a moist soil that is not too wet to drive on. Often the soil surface dries out too quickly before, during and after sowing, resulting in uneven and low plant population. 

5.3 Queensland tropical

In North Queensland there is some flexibility in row spacing. Row spacings ranging from 45–75 cm are common and similar yields have been achieved when planted within this range. Row spacing decisions will normally be dictated by bed sizes and previous crop row configuration.

The ideal row spacing in the coastal tropics of Queensland is 50–75 cm. A 75 cm row spacing fits well with the Queensland sugarcane system, which uses single rows at 1.5–1.6 m spacing. With GPS guidance now widely adopted, planting sugarcane is easily achieved between the two rows of soybean with minimal requirements to cultivate.

6. Photoperiod

Soybean plants commence flowering and mature in response to increasing length of darkness (i.e.  shorter number of daylight hours). Traditionally, soybean varieties are highly photosensitive and in general the later they are sown in their recommended planting window the fewer days until flowering commences. This results in the plants being too small for adequate grain yield. Conversely, sowing a variety much earlier than its recommended window (or at latitudes much further south than its zone of adaptation) can result in the crop spending too long in the vegetative phase and growing too tall and bulky before flowering and maturity commences. This can then lead to lodging and the development of fungal diseases (e.g. white mould caused by Sclerotium sclerotiorum, or higher risk of Anthracnose in the wet tropics), difficulty in harvesting the crop, and reduced grain quality. 

Since 2012, the Australian Soybean Breeding Program has released varieties that are less sensitive to photoperiod (e.g. Hayman*) and less affected in terms of time from planting to flowering and maturity and, therefore, have a wider geographic range of adaptation.  

In an effort to expand soybean production, NSW DPI has been exploring the opportunity to plant soybean in spring (i.e. September or October) in warmer coastal production areas. The recently released variety Gwydir* has low daylength sensitivity and has demonstrated adaptation to very early (i.e. spring) planting in coastal NSW. Results of the NSW DPI on-farm experiments in 2021-22 with full gross margin analysis are available below:

Varieties that are planted later than their recommended time (or at latitudes north of their zone of adaptation) will not spend very long in the vegetative phase before they commence flowering. This can lead to shorter plants with pods set closer to the ground and less than optimal yield.

Planting in the early part of the recommended window is preferred where early growth is likely to be slower, such as where soil fertility is low, or where crops are direct drilled.

6.1 Regional adaptation

Planting a variety that is recommended for your region assists the crop to achieve maximum dry matter pre and post flowering, setting the crop up for maximum yield and assisting it to be fully mature at the correct time for harvest.

Varieties are also recommended for a region based on their tolerance to the particular diseases or environmental stresses of that region (eg. acidic soil, weathering tolerance, lodging resistance, Phytophthora etc.).  Many years of regional evaluation underpins the variety recommendations for Queensland, NSW and Victoria. Lists of the currently recommended varieties for the major soybean production regions are available through the agriculture departments in each state and are shown in Table 2 below.

When varieties are sown outside their range of adaptation they are not likely to perform to their best or achieve optimum yield. Imported soybean varieties are unlikely to perform as they do in their country of origin due to differences in diseases, soil types, climatic conditions and farming systems in Australia.

7. Variety selection

The soybean production regions of Australia cover a wide range of north to south latitudes and climates.  Thus, a range of varieties has been bred and selected to suit these varied environments. To give your crop the best chance of achieving optimal yield select a variety that is recommended for your region and plant it in the recommended planting window. Conversely, if you plant a variety that is not recommended for your region and/or outside the recommended window, expect less than optimal results. 

In addition to regional adaptation, processors in the human consumption sector of the market (e.g. soy milk, tofu, flour) may be specific about the varieties they wish to use. The rapid expansion of this sector of the soybean market in Australia and for export in the past decades drove the change in soybean breeding toward clear (colourless) hilum varieties with larger seed size and higher protein to allow growers to supply human consumption markets in addition to the traditional crushing markets for oil and animal feed, which will accept varieties of any hilum colour.

In general terms, clear hilum varieties can supply all of the market sectors provided that grain quality specifications are achieved by the grower. Dark hilum varieties are restricted to the crushing or stock feed sectors only, irrespective of grain quality.

If a large area of soybean crop is planned, consider selecting two of the varieties recommended for your region so as to spread the planting windows, seasonal risks, crop management and harvesting operations. If you are changing to a new variety for the first time, plant a portion of the area to variety that you are familiar with. This will assist in separating seasonal differences from varietal differences and allow you to assess the potential of the new variety in your situation.

Table 2 summarises the current recommended soybean varieties for the major soybean production areas in Australia ranging from far north Queensland to southern NSW and northern Victoria. Information sheets for the current soybean varieties are also available below.

8. Further reading and references

• Garside, A.L. and Bell, M.J. (2007).  The value of legume breaks to the sugarcane cropping system – cumulative yields for the next cycle, potential cash returns from the legume and duration of the break effect.  Proc. Aust. Soc. Sugarcane Technologists 29: 299 – 308.

• Garside, A.L., Berthelsen, J.E., Robotham, B.G., and Bell, M.J. (2006).  Managing the interface between sugarcane cycles in a permanent bed, controlled traffic farming system.  Proc. Aust. Soc. Sugarcane Technologists 28: 118 – 128.

• Garside, A.L. and Berthelsen, J.E. (2004).  Management of legume biomass to maximize the benefits to the following sugarcane crop in the wet tropics.  Proc. Aust. Soc. Sugarcane Technologists 25: (CD –ROM).

• Herridge, D.F. (2013).  Managing legume and fertiliser N for northern grains cropping. Grains Research and Development Corporation. 87 pp.