Drilling a companion crop with oilseed rape can mean an extra £124/ha through cost savings and extra yield, according to one French expert. Louise Impey finds out more.

A companion planting technique developed in France for winter oilseed rape could help UK growers reduce fertiliser and agrochemical inputs, while improving soil structure, claims the company behind the innovation.

Seed and plant health specialist Jouffray-Drillaud is marketing a mixture of legumes that are sown with the oilseed rape crop in August to grow alongside it for a few months, before being killed by winter frosts and releasing their nitrogen back to the soil.

The legumes are specifically chosen for their susceptibility to frost, as well as their ability to release nitrogen at different rates over a period of time, explains international commercial manager Jerome Vasseur.

“They grow slowly at the beginning,” he says. “This lets the oilseed rape crop get away without interference, while also benefiting from the weed control effect provided by the legumes’ ground-covering growth habit.”

Companion planting can have numerous benefits, he continues. “When it’s done right with the optimum mix, it increases yields, helps with weed control, provides a good nitrogen supply at the end of the winter and improves soil structure.”

But further benefits have been recorded in France, he stresses, where 2,000ha of the technique were drilled last autumn.

“Growers have reported that they are using one less insecticide per crop,” reveals Mr Vasseur. “Having other plants in the field seems to have a dilution effect, so reducing the attraction of the oilseed rape plants.”

Further work is planned on this unexpected pest benefit, he adds. “Whether it’s because insect behaviour is disrupted through visual disturbance, a barrier effect or a repellent effect, we’re not sure yet. But flea beetle populations come down from 10 to 25 pests per plant to just five to 10 pests per plant when companion plants are present.”

The mix consists of one clover and two vetches, says his colleague, product manager Laurent Victor. “They are all summer vetches, which makes them more sensitive to frost. And all three species have a different C:N ratio, which affects the mineralisation process and ensures that nitrogen is released over a period of time, rather than in one hit.”

Species selection is critical, he stresses. “We have a specific variety of berseem clover, Tabor, which is adapted to this use and comprises 20% of the mix.”

The other two are common vetch, Nacre, and red vetch, Bingo, he continues. “Our trials have shown that these have the characteristics required and will grow well with oilseed rape.”

The same work also suggests that nitrogen applications can be reduced by 30kg/ha of nitrogen. “The advice is to stick to the same fertiliser strategy as before, but to reduce the amount going on in the second application.”

Herbicide treatments have to be altered, so that the legume mix survives. “We’ve tried different pre- and post-emergence herbicides and have come up with different programmes for high and low weed pressure situations.”

Should winter frosts fail to materialise and remove the legumes, they can be treated with 0.3-0.4 litres/ha of Dow Shield (clopyralid) after the middle of February, he advises.

The yield benefit from the technique has been in the region of 0.3t/ha, notes Mr Victor. “There’s no doubt that some situations are better for this technique than others. Our advice is to avoid land with very high weed populations and to use direct drilling.”

Drilling can take place simultaneously or by making two passes, with the companion plants drilled first. “Obviously growers prefer to complete the task in one operation. It’s best to drill in batches of 4-5ha, as this avoids any settling out of seed in the hopper.”

The seed mix costs €60 (£50) for 20kg, which is enough for 1ha, he says. “But the savings in crop inputs, plus the additional yield, come to €150. So for every €1 invested, the grower gets €2.50 back.”

Distributor view

Colin Button of Hutchinsons is keeping an open mind about the companion planting technique, but believes that it deserves further investigation.

“There could be something in it,” he says. “It’s an opportunity to make cost savings and bring environmental benefits. Reducing the crop’s nitrogen requirement by 30kg/ha is worth £30/ha alone.”

He would like to introduce the concept into the company’s trials programme. “There are unknowns, such as whether any of the companion plants harbour phoma, for example, which we would need to explore.”

Engineering solutions may also be required, he notes. “If you’re drilling 3kg/ha of rape and 20kg/ha of the companion plants mix, then you need the technology to be able to do that.”

There’s also a psychological step to overcome, he says. “UK growers will have to accept plants other than the crop growing in their fields.”