Are cover crops a good ally for sunflowers?

Planting cover crops before growing sunflowers can increase yields, as trials at the French agri­cul­tural research centre – the Terres Inovia Insti­tute – have revealed.

Cover crops have been shown to have consid­er­able bene­fits – but how do they work in a rota­tion with sunflowers? “We have observed a number of bene­fits from cover crops,” says Matthieu Abella, sunflower expert at the Terres Inovia Insti­tute. “These include reduced nitrogen loss in winter, reduced risk of erosion, improved nitrogen miner­al­i­sa­tion, carbon storage, and better soil struc­ture.” Plant cover also makes it possible to meet regu­la­tory constraints imposed in vulner­able areas. And within a cereal rota­tion, they have another inter­esting effect: They reduce the grass weed burden, partic­u­larly Timothy and ryegrass.

Good mixtures

Aside from the agro­nomic bene­fits, what impact do cover crops have on yields?  “Mixes with legu­mi­nous and non-legu­mi­nous species maximise the bene­fits of cover crops, espe­cially in soils with low nitrogen supplies,” says Matthieu. For example, a phacelia / field bean mix is partic­u­larly well-suited to use before sunflowers.

“Cover crops should include species that estab­lish quickly and have a high nitrogen uptake capacity at the end of August / early September – like white mustard or phacelia.” However, growers should avoid niger, flax and buck­wheat, which increase the risk of mildew, verti­cil­lium wilt, and regrowth, respec­tively. 

Anti-erosion

Long-term effects also need to be taken into account, including the reduced risk of erosion. The clay-lime­stone slopes of the Laura­gais region, south of Toulouse, are highly suscep­tible to run-off. Cover crops tested in this area were planted at the end of the summer, before sunflowers were sown the following spring.

A legu­mi­nous cover crop sown with sunflowers may reduce height, but flower head size is main­tained.

After a storm seen in May, the soil was held in place where cover crops were used. “Not many soil deposits were observed at the bottom of the slope,” says Matthieu Loos, a Terres Inovia engi­neer. “On the other hand, at the lower end of the field where sunflowers were sown alone, erosion and soil deposits were observed. However, if the cover crops are sown during the period between crops, it’s impor­tant not to compro­mise sunflower estab­lish­ment as it will impact yield.”

Plant cover is no substi­tute for tillage where it is neces­sary for successful crop and root estab­lish­ment. In southern areas, the choice of species is similar to that made in the central-western area: Legume-based plant cover to manage the period between crops. But here, the impor­tant thing is timing their termi­na­tion correctly. “You need to inter­vene about two months before sowing the sunflowers to avoid compro­mising the crop,” says Matthieu Loos.

Intra-crop cover

Michael Geloen, an engi­neer at Terres Inovia, explains: “We tested three legu­mi­nous species sown with the sunflowers: White clover, lucerne and trefoil.” On average, combining these species with sunflowers reduced crop height by 10-20cm, while flower head diam­e­ters remained the same, at 17-19cm. However, the sunflower yield was 25-37% lower.

We have observed a number of bene­fits from cover crops.

Matthieu Abella

In prac­tice, late sowing of legu­mi­nous plants can avoid strong compe­ti­tion with sunflower estab­lish­ment. However, this tech­nique remains risky as legu­mi­nous plants do not always succeed in emerging, as three years of trials have shown. Tests are contin­uing to eval­uate this tech­nique and its bene­fits for soil struc­ture and nutri­tion.