Sowing a cover crop

Climate resilience Soil cover Biodiversity Soil regeneration

Sowing an cover crop is a challenge. The goal is to achieve good emergence of the cover without overly encouraging the regrowth of cereals while keeping costs as low as possible. The establishment often takes place on a previous cereal straw crop without tillage.

The sowing date will depend on the chosen species

The sowing of cover crops spans a very long period: some farmers position it just after harvest, or even during, while others wait until the regulatory deadline in mid-September. Each strategy has its advantages, but from late August/early September, the choice of species becomes limited.

Very early sowings

They benefit from residual moisture at harvest and long days (high temperature sums) for growth. Very early sowing is therefore the strategy to aim for the highest biomass production.

However, some difficulties may arise:
- water stress during the summer period,
- lack of available nitrogen leading the cover to have a high C/N ratio (poor nitrogen return to the crop in case of non-legume),
- no possibility to destroy regrowth before sowing the cover, risk of seed setting of some short-cycle weeds.

Early sowings in July or early August suit most species except those with a rapid cycle that can flower quickly (e.g., white mustard).

Sowings a bit later, close to usual rapeseed dates

Facilitate field management.

They still allow reaching fairly high biomass provided adapted species are chosen and rapid emergence is obtained.

Legumes become "marginal" in this date window. It is necessary to choose the most vigorous species (vetches, lentil…).

For later sowings from early to mid-September

Temperature sums become truly limiting to produce biomass before winter. Less frost-sensitive species such as some grasses are among the most suitable.

The table below summarizes the adaptability of cover crop species to sowing dates.

The sowing technique must be adapted to the chosen species

Sowing a cover crop in the intercrop period implies dealing with the potential hindrance of straw and a high risk of drought (depending on the storms that may fall). When rain falls shortly after sowing, all establishment techniques are satisfactory. It is under limiting water conditions that the care given to sowing becomes crucial.

Several possible sowing techniques

Sowing under the cut: the seed is placed on the soil and covered with chopped straw. It requires early sowing. Results are rather good with small seeds (especially crucifers…) by utilizing the soil moisture remaining at harvest. This technique is very economical but requires some attention during harvest.

No-till sowing on stubble: it limits the stimulation of regrowth emergence, which is interesting for sowing very shortly after harvest. Emergence quality is variable in dry conditions (depending on the amount of straw the discs can clog in the furrow). Tine seeders adapted for no-till sowing provide better seed-soil contact than discs but stimulate regrowth emergence a bit more.

Broadcast sowing on a stubble cultivator just after harvest: sowing after stubble cultivation can give interesting emergence but using it directly on stubble will favor strong regrowth emergence, which can sometimes be problematic (smothering the sown cover, pest pressure…). Pneumatic seeders for small seeds can also be mounted on the stubble cultivator.

Sowing on cultivated soil: if rains or soil moisture allow regrowth emergence after harvest, these should be less abundant in the cover. Sowing a cover on cultivated soil facilitates the operation of some seeders, improves seed-soil contact but tends to favor soil water evaporation. Cover emergence is often more uniform on cultivated soil than in direct sowing on stubble. However, emergence is more dependent on rain because the cultivated layer is often dry. Cover sowing is also delayed by at least 10-15 days compared to sowing just after harvest, which can be problematic for early sowing of legumes north of the Loire.

Different sowing techniques usable on previously cultivated soil

Sowing with a traditional or no-till adapted seeder: ensures good burial of large seeds but is an expensive technique. The seeder's hopper allows good autonomy, even with large seeds.

Broadcast sowing on a cultivator: this can destroy the weeds emerged after the first cultivation. Depending on where the seeds fall (in front of soil-working elements like discs or near the roller), seeds will be more or less buried, which suits more or less depending on seed size. Broadcast sowing on a cultivator offers the best quality-price ratio for those wishing to sow a cover on cultivated soil.

Broadcast sowing (centrifugal or pneumatic), possibly followed by rolling: seeds remain on the surface or are barely buried. Emergence is entirely dependent on rain. Crucifers like mustard tolerate this sowing method quite well.

Performance of different sowing operations of cover crops after a previous returned straw crop:

Sources

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