Date: 07/2021

New DSV trials are highlighting how companion cropping could develop to provide greater pest protection for oilseed rape crops in the future, says the company’s Sarah Hawthorne.

Whilst everybody is looking for a silver bullet to address the cabbage stem flea beetle (CSFB) problem now affecting many parts of the country, a single management action, new active or emerging genetic trait is highly unlikely to solve the problem anytime soon.

Few agrochemical interventions seem to make a big difference in the parts of the country where the CSFB numbers are at their highest.

The debate about early drilling versus later timings continues with strong advocates of both approaches but what seems right in one year is often contradicted in the next one.

One approach that is proving more reliable than others is the use of high vigour hybrid varieties that allow crops to establish quickly with continued rapid growth giving them the best chance of growing away from pest attack as they develop.

High yielding varieties like DSV Temptation, Darling and Dazzler also have a series of core characteristics such as TuYV resistance that help protect growing plants against abiotic stresses that can give weakened plants the best opportunity to recover after pest attack

It’s the same with resistance to light leaf spot and RLM7 + resistance to phoma stem canker. Such characteristics have a vital role to play in keeping plants healthy so they have the best chance of compensatory growth mitigating against the affects of early plant damage.

Such varieties are also characterised by deep taproots and well-branched rooting systems that act as a store of vital energy for early growth and then later as a strong defence against droughts.

But for all these attributes to play their role in protecting young plants and creating the right plant architecture for high yields subsequently, seedlings have to establish and avoid the worst ravages of CSFB attack. This where companion cropping brings some hope.

Companion planting in OSR has two main objectives. The primary one is to reduce nitrogen fertiliser inputs by using legumes to fix atmospheric nitrogen and the secondary one is repelling or distracting insect pests.

It can be argued that in the UK, the most important of these is better pest control. For pest control, non-leguminous companion plants have been shown to prevent insect damage in autumn with species which reliably die off in winter or can easily be controlled with herbicides proving to be the most suitable.

New companion crop trials 

In DSV trials carried out in the UK three clear characteristics of the best companion crops are becoming clear. 

Firstly, the companion crop has to grow at the same rate as the oilseed rape. If it lags behind it will make the oilseed rape more exposed to CSFB attack and if it grows too quickly it will affect main crop growth from being too competitive with it. 

Compared to the DSV Clearfield variety Phoenix CL, crimson clover, sweet clover, berseem clover all showed a similar growing pattern in the first 10 days but then grew away rapidly, apparently suffering less damage than the oilseed rape as they did so. 

Secondly, the crop has to be attractive to the CSFB over the oilseed rape. Out of 20 potential crops looked at in the trials, only five were eaten to any degree by the pests. These were oil radish, stubble turnip, abyssinian mustard and two types of white mustard. 

At 20 days after drilling some 80% of the leaf area of the abyssinian mustard had been consumed by CSFB followed by the oil radish at 50% and then the stubble turnip at around 40%.  

Of all the varieties tested, only the Abyssinian mustard and the stubble turnip acted as true sacrificial crop evidenced by emergence being badly affect by the CSFB attack.  

Thirdly, a sacrificial role is not the only way a companion crop can benefit oilseed rape crops. Other crops can act as protectants, physically shielding the emerging seedlings from attack.

 After 20 days, the crops showing the best protectant function were false flax, niger, berseem clover, buckwheat and small burnett. 

As expected, it’s unlikely one variety is the best companion crop option with the data suggesting the best way forward is through specific mixes of plant types with some providing a sacrificial action and others offering protectant cover to young plants. 

These could be augmented by specific pulse crops to provide vital Nitrogen fixing. Previous trials have shown the best of these can fix up to 30 kg N/ha before winter, so these too would have a valuable role to play.

The trials are continuing with the hope an optimum mix of varieties can be identified to provide the best multi-faceted action to the benefit of oilseed rape crops up and down the country.