Update on companion plants

Article taken from the Agronomist & Arable Farmer

Agrovista is now in the third season of looking at growing companion plants among oilseed rape crops.  The concept which was born in France, primarily due to environmental restrictions is that the companion plants are sown alongside the oilseed rape crop in the autumn.

Growing alongside the oilseed rape they are essentially harvesting sunlight, which would otherwise be wasted, and converting it into organic matter.  Through this process, they mop up nutrients such as nitrogen and hold it in an organic form over winter, preventing it being lost due toleaching or de-nitrification.  The companion plant mix is designed to be frost-intolerant and dies over the winter, releasing captured nutrients back to the growing crop in the spring.

The trials to date have exceeded all expectations.  In addition to trapping up to 40 kg/ha of nitrogen over the winter and releasing it back to the oilseed rape crop at the start of spring growth, the companion plants have also demonstrated a number of other benefits.

The first interest finding was with establishment.  For two out of the three seasons at several of the trial sites throughout the UK, there has been very high slug pressure and it is evident that growing companion plants alongside the oilseed rape appears to reduce the slug activity on the oilseed rape.

While the reason for this is not clear, a number of theories have been raised, including simply that the increased biomass means the slug activity is diluted on the oilseed rape.

Secondly a theory raised from an organic organisation is that a slug satisfies its hunger quicker on a more varied diet hence eats less oilseed rape, and a third theory is that one or more components of the mix are repulsive to slugs.  Whatever the reason, it is very clear that companion plants reduce slug activity.

The other interesting pest area with companion plants is their effect on flea beetle.  While to date we have no evidence from the UK, French data would suggest up to 60% of farmers who have sown companion plants alongside their oilseed rape have been able to omit at least one flea beetle spray, which they believe is due to a confusion factor.

The species within the companion plant mix are also selected on their root exploration capabilities, and are able to break through areas of resistance in the soil which the oilseed rape root may not.

There appears then to a synergy, with oilseed rape roots appearing to ‘piggybank’ on the companion plant roots, and at all sites in each of the three seasons we have seen a consistently bigger root system from the oilseed rape grown alongside the companion plants.  This rooting benefit is also having a dramatic impact on soil structure by using natural ‘sun-powered roots’ to restructure soils.

Increasing organic matter in our soils is vital if we are going to increase yields, and companion plants and cover crops could be a very good way of achieving this and bringing some resilience back to our soils.  The following photos show the effects of companion plant roots on soil structure.  Yield results have also been very encouraging with over a tonne a hectare yield response to the inclusion of companion plants where slugs were particularly challenging.  The average yield response across all sites would, however, mirror the results from the Continent at around 0.4t/ha.

While throughout all the sites we have not yet seen a negative yield response, smaller yield responses appear to have followed poor establishment of the companion plants particularly in high trash conditions or where the establishment technique did not provide good seed-to-soil contact.  The yield benefit was also reduced where oilseed rape crop was too thick in the autumn, probably due to increased competition.

In addition to the standard vetch and berseem clover mix which Agrovista has been evaluating over the last three seasons, in looking to develop solutions for the future, Agrovista has tailored a new two-species companion plant mix based on a higher berseem clover content, specifically designed to improve rooting and soil structure, particularly in a banding scenario, and initial results are very encouraging.


18 March 2015

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