This year's extreme drought is once again complicating the establishment of plant cover in many regions, despite the fact that it represents a major agronomic lever for moving towards a system that is more robust in the face of climatic hazards. They are also one of the main factors in improving the carbon balance. Here are a few tips on how to maximise the success of your cover crops in these conditions.
As Matthieu Archambeaud (from Icosystème) mentioned in the first episode of the Radio Carbone podcast devoted to plant cover crops, sowing as early as possible (after the harvest) is generally a factor in success. If the right techniques are used, a cover crop sown early, even in dry conditions, generally already has roots that enable maximum use to be made of the first rains, and accumulates sums of temperature that are fundamental to its future development. If you wait for the first shower before sowing, you run the risk of making less use of the water in a surface horizon that will quickly dry out again.
In many situations, using a no-till drill with fine no-till tines is the technique that maximises the quality of planting (rapid intervention, conservation of residual moisture, etc.). In dry conditions, rolling the seed improves soil-seed contact and closes the row effectively. This "direct seeding" approach also requires very good distribution of straw and chaff, which can be supplemented by using a straw harrow or very shallow stubble ploughing (less than 5 cm).
A significant sowing depth of 4 to 5 cm is ideal for optimising emergence. At this depth, the seeds will not germinate until the profile has been sufficiently moistened, which then reduces the risk of the seedlings drying out. For direct seeding, it is once again preferable to use a tine seeder in dry conditions, as it penetrates the soil surface more easily than a disc seeder.
While very shallow stubble ploughing can help to keep the soil cool and limit evaporation by capillary rise (like hoeing), take care not to go below the seeding depth and ensure that the seeds are placed on the working surface. The most unfavourable scenario for summer sowing is a succession of deep stubble ploughings that dry out the soil, followed by shallow sowing in completely dry soil (e.g. rapeseed sowing in 2021).
If late sowing cannot be avoided (due to technical or logistical constraints, etc.), it is preferable to postpone the date of destruction until after winter, so that the cover crop has time to fulfil its function of protecting and structuring the soil. If the cover crop contains a high proportion of legumes, it can be destroyed easily and quickly, and will also be able to release fertility for the following crop. Species suitable for late sowing include oats, phacelia, faba bean, linseed, hairy vetch, mustard, radish, etc. On the other hand, you should avoid all "summer" species such as sunflower, sorghum, moha, clover, etc.
Diversifying the species sown is also a key factor in the success of cover crops. Four to five different species are often considered a minimum to ensure optimum development and coverage whatever the conditions of the year. To make up the mix, the general principle of association is to divide the pure sowing rate of each species in the mix by the number of species present in the mix, possibly adapting the densities according to the speed of growth (under-dosing crucifers or overdosing legumes).
Against a backdrop of rising fertiliser prices, let's not forget one of the major advantages of planting intercropping cover crops: the return of nutrients to the crops that follow. Let's take the example of a 'biomax' cover crop made up of radish (2 kg), phacelia (2 kg), peas (20 kg) and vetch (12 kg). It is well suited to planting from mid to late August for autumn and winter intercropping. If it produces 3 tonnes of dry matter, it provides on average (simulation using MERCI software) the equivalent of 110 kg of nitrogen, 15 kg of phosphorus and 115 kg of potassium to the following crops (including 41 kg of nitrogen available for the following crop), which represents more than €250 in 'fertiliser equivalents', including phosphorus and potassium.
The current dry conditions are having an impact on the sowing dates for plant cover crops this year, but they are still feasible. Some farmers have been able to sow directly after the harvest, but for those who have not yet been able to start, other scenarios are possible for planting cover crops.
We are confident that they can be planted in the coming weeks. In addition to their favourable impact on the carbon balance, the nutrients provided by cover crops make them, now more than ever, an essential ally that should be maintained in the current context of rising fertiliser prices.
Matthieu Delespesse, Gilles Duhaubois, Max Morelle & Nicolas Verschuere
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