Companion cropping – should we be considering it?

Take home messages

• Diversity in farming systems can be increased using multi-species mixtures with different strategies including cover crops, grazed mixtures, companion crops terminated in-season, or intercrops harvested for grain.
• Potential benefits sought usually relate to the increased shoot or root biomass potential of the mixture, or specific impacts of the components such as nitrogen (N) fixation, weed or pest suppression and soil structural improvement.
• Case studies in southern NSW reveal the practical adoption of companion cropping has been successful when sown in mixes for grazing, but can be more challenging in grain only scenarios. Companion cropping systems are more complex and need to be well planned in advance to help achieve objectives and minimise losses.

Background

The idea of growing more than one crop/species on the same land in a mixture is not new – it has been common in subsistence agriculture and in organic agriculture for generations and is thought to currently provide 15%-20% of global food production (Fletcher et al., 2016). However, the application in broadacre mechanised agriculture has been limited. The biological mechanisms that underpin the benefits of mixtures have a sound scientific basis, but in large scale mechanised agriculture where labour can be a scarce or expensive resource, monocultures are generally more efficient, as less complex management is required (e.g. sowing, harvest, weed management, physical handling). In general, rotated monocultures are the norm.  As precision machinery and new varietal options (e.g. herbicide tolerant) emerge, new opportunities may exist to more effectively capture the benefits of crop mixtures at a broader scale. Recently Fletcher et al. (2016) reviewed the current literature and considered not only the ’resource’ dimension benefits to crop mixtures, but the ’farming systems’ dimensions that are usually overlooked in scientific studies. A farming system benefit might derive from increased productivity of the mixture (related to better resource capture), but also reduced lodging, improved harvest-ability, reduced input needs, improved product quality, benefits to subsequent crops, nutrient recycling, and provision of cover. In addition, there may be a reduction in season-to-season yield variability, or a reduction in risk which may be as important to the farming system as an increase in mean yield. In mixed farming systems, grazed forages provide further opportunities to consider the potential benefits of mixtures. This paper briefly reviews the mechanisms underpinning the benefits of mixtures, outlines different strategies being used to manage mixtures, and then provides some case studies from southern New South Wales (NSW) where growers have been trialling some of those strategies, their rationale for doing so and some outcomes to date. Discussions of summer cover crops grown between two cash crops are not included.

Farming systems, which include companion cropping, are inherently more complex. To mitigate this, a small group of long term no-till growers in southern NSW have worked together to trial what works with their machinery, rotations and soil types. They have sourced information on companion crops from growers overseas and locally to develop systems that help improve soil biology and cycling of nutrients and reduce risk and costs. The experience has not been without challenges — from dealing with weeds and herbicide plant-backs to managing seasonality with frost or even sourcing seed. They remain committed to including companions in their farming systems and are making adjustments every year based on the trials and experiences from within the group.

Mechanisms underpinning benefits from multiple species

Biomass ’over-yielding’

Mixtures of species often are more productive per unit land area than sole crops as a result of the way in which the resources for growth (light, water and nutrients) are more effectively shared and used. In general, these effects can be complementary — species use resources separated in space or time (e.g. different rooting depth or pattern, or different canopy structures to intercept light); facilitative — one species facilitates better resource use by the other; better resource use — same total resource use, but improved efficiency due to impacts of one component on the other (e.g. reduced disease or weeds, changed soil microbial profiles).

Nitrogen fixation from legumes

Legume components as intercrops provide N-fixation capacity to potentially improve the N economy of the mixture and increase N availability to both the non-legume component (by not requiring N uptake), and to the subsequent crop through increased N input.

Weed, disease and pest suppression

Some species have suppressive capacity to weeds by means of vigorous growth and resource capture, while both weeds and diseases can be suppressed by the release of specific compounds or by other plant characteristics that can deter pests and diseases.

Soil structural improvement

The root systems of some plant speciescan improve soil structure by either improving aggregate stability through extensive fine root systems and root exudates, or by producing large continuous and stable bio-pores that provide areas for the rapid infiltration of water and air, and channels for subsequent roots to access deeper soil layers (primer crops).

A recent and relatively simple example of the benefits of mixtures can be seen in Table 1, which summarises work from Couedel et al. (2019) on the benefits of legume and brassica mixtures compared to sole crops. Legumes are known for their N-fixation and green manure value, while brassicas are known for their deep taproots, vigorous growth, efficient capture of N and sulphur (S) from the soil and their ability to suppress weeds, pest and disease (i.e. ’biofumigation’). The individual crops can provide the maximum benefit in specific areas, but the 50:50 mixture provided much more than half of each benefit, so that overall, the multiple benefits are maximised in the mixture.

Table 1. Summary of the services provided by brassica and legume mixture compared with the sole crops. Services provided by the best sole-crop are set to 100%. In each case, the mixture delivers more than the average of the sole crops (from Couedel et al. 2019).

In some cases, there may also be potential for negative effects of mixtures or ’disservices’. For example, greater water use by the mixture may leave less for the following crops, or components of the mixture may host pests or pathogens of subsequent crops. In considering mixtures, it is important to be clear about what benefit is being sought, what unintended problems may arise, and that the activity does not create unacceptable levels of management complexity.

Practical strategies to improve diversity with mixtures

Multiple species mixtures for grazing only

A range of different species are selected for the mixture specifically for increased forage production throughout the season which helps with improved animal intake and live-weight gain. Grazing mixes are usually terminated as a spring fallow to retain groundcover and soil protection over the summer and for ease of crop establishment the following season. The mixes are sown with a similar objective as brown manure pulse crops to improve soil N status, control resistant grass weeds and build soil water prior to high risk crops such as canola. In contrast to brown manure mixes, these crops are grazed and primer species, such as tillage radish and turnips, are included to penetrate dense clay subsoils and leave stable biopores.

Dual-purpose mixtures

A cash crop that is intended for grain harvest is included in the mixture (e.g. a grazing wheat or canola) and the remaining components of the mixture are terminated after grazing to allow grain recovery in the cash crop. Benefits sought are diverse feed sources for livestock to minimise metabolic issues, improved nutritive value and provision of additional dry matter through different growth habits or seasonality. Common examples include mixing vetch and tillage radish with cereals such as grazing wheat or oats. Winter canola for grazing is also mixed with vetch to improve feed diversity and soil N input and to also provide a source of arbuscular mycorrhizal fungi (AMF) with non-host species such as canola.

Companion cropping

In ungrazed systems, companion crops are sown with the main cash crop, but are terminated in-season. Benefits sought are weed suppression, N-fixation, soil structural benefits and AMF (mycorrhizae) hosting. Common mixes include wheat and tillage radish or wheat and pulses such as vetch, faba beans or field peas. Termination timing in southern NSW systems has been targeted during late winter-early spring prior to stem elongation of the cash crop.

A recent example of this used commercially is the frost sensitive faba bean (and some other species) that is sown as a companion with winter oilseed rape in France, where it is primarily aimed at repelling insect pests in young canola and reducing the need for early sprays. It also provides competition with weeds and some N-fixation, and the faba bean is killed naturally by frost in winter.

Intercropping (both crops harvested)

A mixture of two cash crops is sown with the intention of harvesting both crops. The benefits sought are the over-yielding arising from synergies and complementarities of the crop mixture which may relate to N dynamics (in legume mixtures), reduced disease epidemics, and improved nutrient uptake. Harvesting two crops at the same time can be difficult with header setup and separation. Cleaning grain can also be a challenge and adds further cost, but it is often worth the effort given lower costs of production compared to monoculture crops.

Field peas and canola have been recognised as suitable companion crops for many years and ’peaola’ has been used by growers with various levels of success (Fletcher et al. 2016). Non-host species such as canola are mixed with pulses to improve AMF levels. Locally, these mixes have been challenging to get right with seasonality of the different crops difficult to manage. For example, field peas sown as companions with canola have been affected by bacterial blight when sown earlier than their preferred window. In Canada, growers have found mixing species, such as chickpeas and linseed, has helped reduce the risk associated with growing high value crops such as chickpeas, primarily through a reduction in disease epidemics.

Research at the University of Western Australia (UWA) by Bai Li et al. (2016) suggested that when faba beans and corn were intercropped, root interactions between the species boosted faba bean biomass and grain by 35% and 61%, respectively. Plants can communicate through the release of root exudates, which act as underground highways, promoting N-fixation between neighbouring plants.

Case studies

Case study 1

Grower and farm description: Damien and Brian McKelvie, Marrar. Red loam soils. Mixed farming operation with wheat, barley, canola, vetch, lucerne pastures and prime lambs.

What mixture is being used and how? Grazing mixes with vetch, radish, turnip and wheat sown in early autumn into cereal stubbles. The mix is grazed through winter and spring then spray fallowed in late spring to control grass weeds and retain soil moisture.

What benefit is being sought from the mixture? Provides high quality feed for their composite sheep flock through winter and spring. When dual purpose canola or wheat crops are locked up for grain recovery in late winter, the sheep move onto the vetch mix allowing lucerne pastures to be spelled. The mix replaces the need to sow a grain only pulse to get a legume into the crop rotation.

What has been the result – good and bad? The grazing mix has been used for several years with success to finish lambs to target weights with minimal supplementary grain plus include a low risk pulse (vetch) in the rotation. Sourcing soft seeded vetch has been a challenge and stocking rates need to increase to prevent radish flowering during spring.

Case study 2

Grower and farm description: Hugh and Libby Cruikshank, Coolamon. Red loam and sodic grey clay soils. Mixed farming operation with wheat, barley, canola, vetch, lupins, lucerne pastures, merino ewes, wethers and prime lambs.

What mixture is being used and how? Vetch, radish, turnip, wheat mix for grazing, a pulse added into the rotation to help improve dense sodic subsoils in combination with stubble retention, no-till and gypsum. Grasses are selectively controlled in late winter then the remaining vetch mix is spray fallowed in late spring.

What benefit is being sought from the mixture? Grazing mix provides a low risk pulse alternative, feeds merino wethers during winter and the deep tap-rooted brassicas (radish and turnip) help penetrate dense sodic subsoils providing pathways for subsequent roots to follow.

What has been the result – good and bad? Dry starts have limited early dry matter production from the vetch mix with limited feed production during late breaks and getting the ratios of pulse to cereal has been difficult. Observations of soil structure the year following the vetch radish mix have shown improved infiltration and aggregate stability.

Case study 3

Grower and farm description: Brendan and John Pattison. Red loam soils. Continuous cropping canola, wheat, barley, faba beans, lupins, companion winter and opportunity summer crops.

What mixture is being used and how? Canola and faba beans growing together with volunteer faba beans left in a canola crop or both sown as a mixed companion with both crops harvested. Wheat and barley are sown with tillage radish and vetch at low densities. The radish and vetch are terminated in late winter, with a goal of adding diversity with rooting depth and improved biological activity.

What benefit is being sought from the mixture? Provide a source of AMF (mycorrhizae) for non-host species such as canola, improve soil biota, reduce costs and build resilience in their cropping systems by minimising inputs such as N fertiliser and fungicides.

What has been the result – good and bad? Faba bean and canola mixes are still in the development stages, but the mix has shown good potential to date, albeit with low yields from the faba beans. Radish and vetch mixes have been successful when terminated in late winter in wheat grain crops. Soil water use by the companion crop has not reduced grain yield of the wheat or barley being harvested.

Case study 4

Grower and farm description: Matt and Belinda McKinley. Red loam, granite and grey clay soils. Winter companion crops, wheat, barley, faba beans, linseed, canola, chickpeas, lupins and opportunity summer crops. Trade cattle on mixed species grazing crops.

What mixture is being used and how? Grazing mixes of cereal rye, millet, corn, oats, tillage radish, vetch, field peas. Sowing mixes of faba beans and canola, field peas and canola, chickpeas and linseed.

What benefit is being sought from the mixture? Aiming to improve soil health through carbon cycling plus develop a resilient, low cost farming system.

What has been the result – good and bad? Achieved success with grazing mixes sown for cattle in late summer or early autumn using a disc seeder into tall cereal stubbles. Faba bean and canola mixes sown into double cereal stubbles have worked well on elevated paddocks, but yields have been reduced by frost in lower blocks. Field peas and canola mixes sown into cereal stubble have also been subject to frost issues, particularly with bacterial blight infecting the field peas. The field pea and canola crop was cut for silage in 2018 and sold to local livestock producers as a high quality feed source. Chickpea and linseed companion crop established well, but with limited herbicide choices, the broadleaf weed density was problematic at harvest.

Conclusion

Companion cropping potentially offers a range of benefits for improving soils, reducing disease and insect pest pressure along with helping to minimise risk. Whilst there is limited research data in Australian cropping systems to quantify the practice, growers are looking to adopt companion cropping to help diversify their rotations whilst reducing the cost of production. Companion cropping is not without its challenges and growers or advisers are urged to plan ahead to reduce any problems with issues such as herbicide plant-backs or machinery configurations.

Useful resources

The potential for companion cropping and intercropping on UK arable farms. Andrew Howard, UK Nuffield Scholars Report 2015

Intercropping – the growing of two different crops simultaneously. Bryan Granshaw, Australian Nuffield Scholars Report 2016.

Intercropping expands the mix of crops in Saskatchewan.

References

Bai Li et al. (2016). Root exudates drive interspecific facilitation by enhancing nodulation and N2 fixation. Proceedings of the National Academy of Sciences.

Couedel, A., 2018. Provision of multiple ecosystem services by crucifer-legume cover crop mixtures. Univ. Toulouse PhD Thesis 188. DOI: 10.13140/RG.2.2.25222.01606.

Fletcher et al. (2016). Prospects to utilise intercrops and crop variety mixtures in mechanised, rain-fed, temperate cropping systems. Crop and Pasture Science.

Jahanansooz M.J (1999). Wheat-Chickpea yield performance, competition and resource use in intercropping, under rainfed conditions of South Australia. PhD Thesis.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the author would like to thank them for their continued support.  Thank you to those growers who have shared their experiences both good and bad with companion cropping over the past five years. Ben Beck, Matt McKinley, Brendan Pattison, Daniel and David Fox, Hugh Cruikshank, Trent Gordon, Keith Walton, Warwick and Di Holding, and Michael Molloy.

Contact details

John Kirkegaard
CSIRO Agriculture and Food
GPO Box 1700, Canberra ACT 2601
0458 354 630
john.kirkegaard@csiro.au
@AgroJAK

Greg Condon
Grassroots Agronomy
PO Box 73, Junee NSW 2663
0428 477 348
greg@grassrootsag.com.au
@grassrootsag