Successful Cover Cropping

Technical articles - Centre National d'Agroécologie

Organic matter autonomy in vegetable farming Climate resilience Soil cover Biodiversity Soil regeneration CoAgroEco Weed control by cultural methods

A cover crop refers to a group of plants covering the soil either permanently or temporarily.

It has many benefits:

Captures and returns mineral elements
Maintains biodiversity
Improves soil structure
Activates soil biological activity
Protects against erosion

Not all these effects need to be sought simultaneously; it is important to identify the most important for your case and orient your choices accordingly. Whatever the objective, effectiveness depends on the amount of biomass produced.

Role

Nutrient recycling to crops

Cover crops are recycling units that prevent nutrient loss and make them available for the next crop:

The actual amounts of nutrients returned to crops depend on various factors: pedoclimatic conditions, farming practices, method of destruction, and vegetative stage of the cover at destruction time.
It can be considered that 30 to 40% of the nitrogen acquired by the cover will be made available for the following crop within 6 months after its destruction. The rest will be incorporated into the soil for its functioning, including some for humification.
The gradual release of organic nitrogen following cover destruction helps to moderate nitrogen availability in the soil. This acts as a buffer for the crop nitrogen nutrition index (ratio between nitrogen content in the aerial parts of the plant and the critical nitrogen content). This aspect is particularly economically interesting as it implies potential fertilizer savings.

The MERCI software (Method for estimating elements returned by intermediate crops), freely available, allows estimating the quantities of elements present in the cover based on biomass measurement of each species.

Feeding the soil

Cover crops activate the nutrients cycle by releasing root exudates.
Living roots serve as support for mycorrhizae which make nutrients soluble and available to plants in exchange for carbon chains (energy).
Through root exudates, the plant selects the microbial flora living in the rhizosphere.

A plant that exudes complex sugars can maintain proper nutrition and promotes a soil where pathogenic organisms have little available space (ecological niches are occupied by many different organisms).

Soil structure and available water reserve gain

Improvement of soil structural stability thanks to multiplication of soil microorganisms producing glomalin (“soil glue”) and stable humus.
Improved infiltration and water retention capacity in the soil, especially when combined with conservation agriculture practices.
Order of magnitude to keep in mind: 1 point increase in organic matter allows increasing the available water reserve by about 80 mm.

Protection against erosion

20% of French agricultural land is heavily affected by water erosion.

Soil vulnerability to erosion depends on its texture, structure, organic matter content, and permeability. Leaving crop residues on the surface and establishing cover crops are the most effective ways to combat erosion. The foliage reduces the impact of raindrops (“splash effect”), runoff, and wind. Roots create a network inside the soil that holds the soil in place.

Suppressive effect on weeds

The covering power of cover crops competes with weeds for light:

Covers must be well established and generate biomass quickly (like mustards or phacelia) to cover the soil: the effect appears from 80% coverage, especially when there is at least 2-3 tons of biomass/ha. Rapid coverage, such as with mustards or phacelia, is needed to establish effective competition.
Species complementarity in mixed covers increases this capacity.
If soil is covered at a given time of year, the dormancy-breaking conditions of some weeds are not met, preventing germination of corresponding weeds. For example, having a covered soil in September inhibits dormancy break of brome, a grass weed very common in cereal rotations.

Since the anti-weed effect works by competition for resources (water, minerals, light), the herbicidal effect will cease upon destruction of the cover crop.

Successful establishment and development of the cover

Sufficient seed quantity, sown under good conditions

To succeed in sowing:

Good seeding density: For example, for a winter cover between 2 maize, based on faba bean, at least 120 kg/ha is needed (e.g., 80 to 100 kg faba bean, 15 to 20 kg forage rye, and 2 kg radish and/or phacelia). Broadcast sowing requires higher density. It is also observed that the denser the sowing, the more productive the biomass. Target is 300 seeds per m².

Choose the sowing date well. The sowing date must be adapted to species. This choice is crucial for cover success. Depending on climatic conditions, species more or less tolerant to heat and drought will be chosen, as shown in this table:

Rapid establishment: To ensure good emergence, especially for small seeds, take advantage of residual moisture from the previous crop to establish the cover. Also, July-August days have much higher cumulative temperatures and sunshine duration than September. Species sown during this period will develop faster. Good soil firming also promotes rapid and uniform emergence. Sowing must be done carefully.

Good sowing conditions:
- For winter covers, sow as early as possible (ideally before October 15). Sow within 4 days after harvest to benefit from residual moisture or ensure watering within 15 days after sowing.
- Sowing depth should be set to the smallest depth suitable for the smallest seed (usually between 1.5 cm and 2.5 cm).
- Broadcast sowing is possible if soil structure is good (sufficiently crumbly to ensure seed-soil contact), no risk of phytosanitary product residues (sulfonylureas), and enough moisture.
- Direct seeding (seeders like Semeato or Great Plains) shows the least emergence loss.

Species adapted to the soil

Species must be chosen according to soil organic matter level. In soils with high levels and high C/N ratio (carbon/nitrogen), it is better to choose covers with low to medium C/N. These provide labile humus, more easily used by the following crop. Conversely, in soils with low C/N, the goal is to enrich organic matter with covers having high C/N (richer in fibers). Their direct return will thus be slower.

Source : Arvalis, ITB, Terres inovia, UNILET.

Species adapted to the following crop

Species choice depends on the upcoming crop and rotation. A cover is a service crop, so avoid introducing plants that could cause health issues in the next crop.

Manage your cover like a crop

Choose high-quality seeds (check recommended germination rate)
Irrigate when possible; 10 mm can ensure good establishment and development
Fertilize the cover to increase biomass production and give a head start to the next crop. (Regulations allow 50 kg effective nitrogen in organic form).

Active ingredient residues

Be very careful about active ingredient residues. For example, a sulfonylurea applied on a cereal before a cover crop, after early April, has a high risk of impacting cover biomass (or upcoming oilseed rape). So if you do a direct seeding costing €50/ha and put €40 seed, and the cover fails due to herbicide, the loss is €90/ha. Below are two illustrations to give an idea of residues.

Destroy at the right time and easily

Not too early to benefit from the agronomic role of the cover which will produce more biomass in early spring (especially legumes).

Nor too late to:

Avoid water competition on soils with low available water reserve (RU).
Allow soil warming before crop establishment.
Limit nitrogen hunger (in covers with grasses or crucifers): usually 3 or 4 weeks before sowing recommended. For mechanical destruction, timing must also consider cover species development stages: at flowering or heading for grasses, plants are more sensitive to destruction.

Mechanical destruction preferred: "Pre-digestion" of the cover on the surface: roller-crushers, shredders, rolofaca.

Beware of labour: avoid burying green matter whose fermentation would harm the next crop. Risk of regrowth with grasses: better to lay them down than cut.

According to crop type

Spring crops

Spring crops like sunflower, maize, sorghum, millet require a certain soil temperature after winter for good emergence.

Avoid too much straw mulch to favor good soil warming (especially in Central/Northern France).
Too much straw on soil can cause temperature differences of 2 to 6°C and affect germination and emergence in spring.
A cover composed of legumes in higher proportion plus cereals and/or crucifers. Cereals and crucifers fill gaps left by legumes during emergence. The cover will return carbon and nitrogen to the soil.

Example 1: between soybean and maize

Objective: optimize soil fertility.
Option: faba bean (120 kg/ha) with pure oat or oat/triticale mix (10-20 kg/ha).

The legume returns organic nitrogen and the grasses develop mycorrhizae in the soil. Early cover destruction (one month before maize sowing) preserves soil moisture and avoids potential nitrogen hunger. Having optimal available water reserve is essential for good start of spring crops.

Example 2: between two maize

Establishment: late October.
Option: faba bean base (100 kg/ha) - phacelia (5 kg/ha). It is possible to add a cereal in the mix (faba bean 80 kg/ha - phacelia 3 kg/ha - triticale 30 kg/ha for example).

Example 3: before soybean

Prefer to reduce legume dose: mono-species cover or cereal mix with low legume proportion to avoid increasing soil infectious potential.
Legumes take up soil nitrogen early in the cycle before fixing atmospheric nitrogen via symbiosis with rhizobium. Thus, having assimilable nitrogen in soil can help start.
Between two soybeans in rotation, a cover dominated by grasses (60-70%) like oat, rye, or triticale, 15-20% legumes, and the rest crucifers or others depending on seed availability can be considered.

Short intercrop covers

Short or summer covers are placed between two winter cereals.

Sow as soon as possible after winter cereal harvest to benefit from residual moisture

A simple mix of 3 species including one legume can be recommended. In drier areas, the key is to choose species more tolerant to drought.
However, always keep good farming sense: if climatic conditions are poor and soil is dry at depth, chances of successful emergence are compromised.
A poorly established cover is a cover without interest. In such cases, better to wait for better conditions and adapt strategy accordingly. For example, anticipating a biomax cover (>3 species) between two cereals after harvest, but if conditions come late (limit 1.5 months before cereal sowing), a simple buckwheat cover will be more effective for a short period.

Broadcast sowing

Another technique is to broadcast the cover mix one month before harvest. The idea is to compensate for difficult conditions, reduce costs, and save time, even in some cases achieve a double harvest.

Wait until first leaves start to yellow and let light reach soil (1 to 1.5 months before harvest).
Success depends on pedoclimatic conditions (low wind during sowing, regular moisture after sowing) and seed types (if TSW is below 4, broadcast sowing is impossible).
The first criterion is having a soil capable of supporting this practice: a living soil with good structure.

Between a winter cereal and oilseed rape, establishing an intercrop cover is not advisable

Better to opt for oilseed rape associated with companion plants.
The idea is to support oilseed rape during early vegetative growth to benefit from natural ecosystem services: nitrogen supply, pest control by insect disruption and auxiliary hosting, weed management, soil erosion protection, and improvement of soil structure and fertility.

Permanent covers

A permanent cover, or perennial cover, remains on the plot for at least 2 crop cycles: generally between 18 and 36 months.

With increasingly dry climatic conditions in some areas, semi-permanent covers are an interesting alternative to ensure soil cover when short covers produce little biomass.
Living cover can also temporarily mask the “monoculture effect”. It can limit yield declines related to monoculture, which is not always the case with intercrop covers.

The four main species for permanent cover are alfalfa, bird's-foot trefoil, sainfoin, and dwarf white clover.

Alfalfa thrives in calcareous soils, alkaline pH (pH>6.5), and non-hydromorphic soils with recommended seeding density of 8 to 10 kg/ha.
In slightly more acidic soils (5.5 to 6.5) and hydromorphic, bird's-foot trefoil and sainfoin offer good alternatives with seeding densities of 8-10 kg/ha and 40-50 kg/ha (hulled) or 160 kg/ha with hulls.
It is possible to mix several legumes, in which case doses are divided.

This technique requires good cover management to avoid competition with the crop for light, water, and nutrients.

Species mixtures

At least 4 species and 3 different families. This type of mixture allows:

Ensuring good coverage (possible compensation if one species develops poorly).
Optimizing space occupation with plants complementary at root and aerial levels. For the visible part, prefer species producing lots of biomass with erect habit. Underground, the goal is to work different horizons. Only mixtures achieve this, using different root systems (taproot and fibrous).
Combine different benefits (structuring effect of grasses, “water flushing” of crucifers, nitrogen supply by legumes, allelopathic action of some species like oat, rye, buckwheat, or crucifers…).
Break cycles of weeds and diseases with species little or not grown in the rotation.

It has been observed that beyond 5 species, biomass production no longer increases. The more species, the lower the seeding rate per species. Thus, with 10 species, emergence is ensured but species will be too weakly sown.

MERCI Measurement Network

To conclude, with cover crops, we know how much we have invested, but we still measure too little how much we have recovered. A good way to do this is to cut the cover crop close to the ground several times over half a square meter, weigh the harvested plants species by species, and enter the result into the (free) software MERCI, which will calculate the mobilized mineral elements as well as the humified carbon.

By quantifying the potential returns in N, P, K, the MERCI measurement allows reducing fertilizer inputs accordingly and thus achieving savings. Since 2017, the Chamber of Agriculture of Landes has been conducting these measurements with interested farmers, which provides local references for different types of cover crops, in various pedoclimatic contexts and with different types of sowing.

Consult on a map the cover crops showing the best results from 2017 to 2020 in the department of Landes. Find these results by clicking here. Click on a point on this map and the characteristics and results of the cover crops will be displayed. Zoom in if needed. Note that it is reasonable to take a safety margin on the restitution figures (reduce by 30%), especially if the cover crop is not buried.

This action "MERCI measurements" is conducted with the financial support of the Departmental Council and the Water Agency.

Warning: the management of cover crops in terms of species choice, duration of presence, and destruction methods may be regulated in different frameworks: zone vulnérable, CAP (EFA, OCACIA). To know the details of these regulations and to complete the technical information, consult the Chamber of Agriculture website (cover crop guide / destruction guide).

To go further

Appendices

See the following crops:

Sources