Planting Legumes as Cover Crops
Legumes are interesting species from an agronomic point of view due to their ability to fix atmospheric nitrogen through a symbiotic association with a soil bacteria of the genus Rhizobium. This process allows nitrogen to be supplied in agricultural systems naturally and at low cost.
1. Presentation
Characterization of the technique
Description of the technique:
The interest of establishing a legume in cover crops lies in returning nitrogen from the air to the following crop thanks to the decomposition of crop residues. Except, of course, in cases where the cover crop is harvested then removed from the system as a catch crop.
The most easily degradable residues, such as leaves and non-woody stems, will decompose and mineralize nitrogen quickly, providing available nitrogen within a few weeks. Woody parts, such as stems and roots, mineralize more slowly.
Trials have shown that establishing a cover of crimson clover at 15 kg/ha in August, destroyed in February, provides 60 to 80 kg of nitrogen per hectare to the following corn crop[1].
In cover crops, legumes can be established in mixture with other species (oat, mustard, phacelia, sunflower…) and sometimes in pure stands (pea, faba bean, clover, vetch…) in the case of catch crops. In the case of a nitrate catch crop (CIPAN), it is forbidden to grow legumes in pure stands, although exemptions exist for organic farming.
To combine the objectives of "nitrogen accumulation" and "trapping" of available nitrogen in the soil, crop associations are preferred ("mustard + faba bean", "oat + vetch", ...). They increase the chances of successful cover crop establishment (development) and also increase the biomass produced.
Details on the technique:
Due to the temperature accumulations required for their development, legumes should be established earlier (late July/early August) than conventional cover crops. Moreover, legumes can be more sensitive to drought than other cover crops.
Broadcast seeding is possible but must be followed by shallow tillage or rolling to slightly bury the seeds and firm the soil.
No-till direct seeding or row seeding techniques are preferable and allow better control of seed distribution uniformity.
The cover crop seeding can also be done during the previous crop in spring, to ensure a relay until the next crop and allow greater biomass production. The success of relay crop establishment is highly dependent on climatic conditions (water availability...) and is mainly practiced in organic farming (less competition from the commercial crop for light).
Given their relatively slow growth rate, legumes are preferred for long cover crops (before spring crops).
To maximize nitrogen accumulation and avoid any risk of nitrogen leaching after cover destruction, destruction in mid-January is necessary. For "early sowing" crops (e.g., early February for spring barley), destruction of the cover in early January is necessary; in the case of mid-April sowing (sunflower, corn), the cover can be maintained until early February.
In all cases, cover destruction should be done 1 to 1.5 months before the establishment of the next crop.
Mode of valorization of the technique
The destruction of the legume after the cover crop will return nitrogen to the soil for the following crop if it is not harvested.
Another possible valorization mode is to harvest the legume grown to constitute a forage resource (e.g., association of grasses and legume).
Implementation period During the cover crop
The technique applies to the cover crop, but practices in organic farming show that seeding can also be done under the previous crop cover.
Spatial scale of implementation Field
Application of the technique to...
All crops: Sometimes difficult to generalize
On early harvest crops and legumes.
In rotations including legumes as cash crops, host species of common pathogens (e.g., aphanomyces) should be avoided. It is also useless to establish legumes before a "protein crop" (pea, lupin, faba bean, soybean, ...).
All soil types: Easily generalizable
The technique is easily generalizable to many soils provided the choice of species is adapted to the soil type, particularly regarding ease of destruction (frost-sensitive species or not).
The development levels of cover crops (biomass produced per hectare) are strongly influenced by soil type, especially by soil water availability (useful reserve).
All climatic contexts: Sometimes difficult to generalize
This technique is generalizable provided the choice of species is adapted to the climate: the climatic context strongly influences emergence and development levels of cover crops (biomass produced per hectare). Climates that are rather "dry" in summer and autumn (e.g., Midi-Pyrénées, Poitou-Charentes) do not allow cover crops to develop even if they were established under very good conditions. In this case, the objective sought by the technique is not achieved.
Regulations
In vulnerable zone, the 5th action program of the nitrate directive (2014) requires soil cover for a minimum of two months for long cover crops. Legumes are allowed in mixtures up to 50%. Pure legume covers are prohibited as CIPAN but exemptions exist in organic farming notably.
2. Services provided by the technique
Nutrient supply
Legumes will return nitrogen to following crops thanks to their ability to fix atmospheric nitrogen (symbiosis with a soil bacteria of the genus Rhizobium). This process provides nitrogen naturally in agricultural systems by decomposition of crop residues that mineralize.
Effect level: MEDIUM if technique used alone, to be combined
Confidence index: HIGH
COMPLEMENTARY TECHNIQUE(S)
Introducing a cover crop in vegetable production
Relay sowing of a winter cereal with a legume established in spring
Growing diversified species in rotation
INCOMPATIBLE TECHNIQUE(S)
Physical stability and soil structuring
Establishing legumes in cover crops allows:
- To cover the soil during the cover crop period and thus limit crusting and erosion.
- To pump water from hydromorphic soils in winter. This facilitates sowing of the following spring crop.
Some legumes also have a positive effect on soil structure notably through their taproot systems (alfalfa, faba bean).
Effect level: MEDIUM if technique used alone, to be combined
Confidence index: MEDIUM
COMPLEMENTARY TECHNIQUE(S)
Growing diversified species in rotation
Establishing catch crops or double crops
Introducing a cover crop in vegetable production
Growing annual species associations
Practicing direct seeding under cover
Returning and leaving residues of the previous crop on the surface
INCOMPATIBLE TECHNIQUE(S)
Weed management
Covering the soil during the cover crop period can limit the development of weeds through competition (soil cover), allelopathy or biofumigation.
Effect level: MEDIUM if technique used alone, to be combined
Confidence index: MEDIUM
COMPLEMENTARY TECHNIQUE(S)
Growing diversified species in rotation
Establishing catch crops or double crops
Introducing a cover crop in vegetable production
Relay sowing of a winter cereal with a legume established in spring
Growing annual species associations
Practicing direct seeding under cover
Establishing allelopathic or biocidal intermediate crops - biofumigation
INCOMPATIBLE TECHNIQUE(S)
Management of beneficial insects enemies of pests
Legumes in cover crops can be used for their properties to attract beneficial insects, such as predatory bugs for example.
Effect level: MEDIUM if technique used alone, to be combined
Confidence index: MEDIUM
COMPLEMENTARY TECHNIQUE(S)
Growing diversified species in rotation
Practicing direct seeding under cover
Introducing a cover crop in vegetable production
Growing annual species associations
Establishing catch crops or double crops
Relay sowing of a winter cereal with a legume established in spring
Establishing intermediate crops attractive to beneficial insects
INCOMPATIBLE TECHNIQUE(S)
Management of pollinator beneficial insects
The cover can serve as a refuge for pollinator beneficial insects at a time of year when resources are limited.
Effect level: MEDIUM if technique used alone, to be combined
Confidence index: MEDIUM
COMPLEMENTARY TECHNIQUE(S)
Growing diversified species in rotation
Relay sowing of a winter cereal with a legume established in spring
Establishing intermediate crops attractive to beneficial insects
Practicing direct seeding under cover
Growing annual species associations
INCOMPATIBLE TECHNIQUE(S)
Forage autonomy
Legume cover crops can be used by mixed crop-livestock farmers as forage resources. An association of grasses and legumes, such as vetch and oat, for example, will constitute quality forage.
Effect level: MEDIUM if technique used alone, to be combined
Confidence index: MEDIUM
COMPLEMENTARY TECHNIQUE(S)
Growing annual species associations
Harvesting cereal-protein crop associations at an immature stage
Growing diversified species in rotation
Establishing catch crops or double crops
INCOMPATIBLE TECHNIQUE(S)
Protein autonomy
Due to their protein richness, legumes can help increase the protein autonomy of the farm when harvested for livestock feed (forage or grain).
Effect level: MEDIUM if technique used alone, to be combined
Confidence index: MEDIUM
COMPLEMENTARY TECHNIQUE(S)
Growing annual species associations
Harvesting cereal-protein crop associations at an immature stage
Growing diversified species in rotation
Establishing catch crops or double crops
INCOMPATIBLE TECHNIQUE(S)
3. Effects on the sustainability of the cropping system
Environmental criteria
Effect on air quality: No effect (neutral)
Effect on water quality: Increasing
The cover captures nitrogen, phosphorus, potassium, and other mineral elements from the soil, preventing their leaching. Moreover, there are effects (still little known) on the degradation kinetics of phytosanitary molecules in the soil and their transfer to water, linked to microbial activity around the root zone and increased soil organic matter (SOM) (SOM tends to favor the "storage" of pesticides in the soil and limit their transfer to water).
However, establishing frost-sensitive legumes in cover crops can be accompanied by increased use of herbicides for their destruction.
Effect on fossil resource consumption: Variable
This technique causes a slight increase in fuel consumption for seeding or cover destruction. However, nitrogen input by the legume reduces fertilization on the following crop, thus reducing energy consumption and CO2 emissions related to mineral fertilizer production (conventional agriculture).
GHG emissions: Variable
Fuel consumption for cover management (seeding/destruction) generates CO2 emissions. However, the cover produces biomass that stores carbon, and nitrogen input by the legume reduces fertilization on the following crop, thus reducing energy consumption and CO2 emissions related to mineral fertilizer production (conventional agriculture).
Agronomic criteria
Productivity: Increasing
Normally, the following crop benefits from the improvement of soil fertility (as do subsequent crops if the approach is repeated and sustained over time).
This improvement is only appreciable and significant after several years of implementing the technique.
Warning: in case of poor management of cover crops, particularly their destruction (poor conditions, partial destruction, ...), yield losses may be observed.
Production quality: No effect (neutral)
Soil fertility: Increasing
This technique allows the recycling of soil mineral elements (N, P, K, …) and activation of biological activity.
Water stress: Variable
The establishment of cover crops can cause water availability problems for the following crop if they are destroyed too late or if the autumn and winter climate of the year ("dry") does not allow the soil water reserve to be replenished. To avoid any penalizing effect, destruction before 02/15 (before sunflower, maize) is preferred.
In the long term, the systematic introduction of cover crops has positive effects by contributing to enriching the soil in organic matter; this practice favors an increase in the soil's capacity to store water.
Functional Biodiversity: Increasing
The cover will provide refuge and food for game, beneficial insects, and pollinators at a time of year when resources are limited. The number and diversity of cover species increase the impact on biodiversity.
Weed control: Variable
The cover, if well developed, will compete with weeds and prevent their development, thus reducing the seed bank of the plot.
However, care must be taken that the cover crop does not become a weed for the following crop.
Economic criteria
Operational costs: Variable
This technique incurs costs for seed purchase (40 to 100 €/ha, slightly higher for legumes than for other species), and cover destruction costs, but reduces fertilizer costs for the following crop and possibly herbicides (weed smothering).
Mechanization costs: Variable
This technique increases mechanization costs: sowing + cover destruction. Costs vary depending on the techniques used:
- sowing: from 5 €/ha (broadcast sowing + shallow tillage) to 40 €/ha (shallow tillage + combined rotary harrow seeder)
- destruction: 0 (frost) to 40 €/ha (mowing or glyphosate).
However, the "restructuring" effect of the cover limits soil work for establishing the following crop.
Regarding fuel consumption, it can increase or decrease.
It increases if:
- the cover develops little or not at all
- sowing/destruction specific to the cover are fuel-consuming (row sowing, plowing, ...).
It decreases if:
- the sowing/destruction techniques are simple (e.g., broadcast sowing/frost-killed cover)
- the cover limits shallow tillage and reduces nitrogen inputs on the following crop
Margin: Variable
This technique involves costs for seed purchase and cover destruction.
However, nitrogen is recycled, which implies a reduction in nitrogen fertilizer purchase.
Moreover, this technique allows soil structuring and enrichment in organic matter, which reduces mechanization costs for soil work.
Productivity gain varies according to the following crop and the system's intensification level: it can be significant for demanding crops and/or low-input systems (inputs) (e.g., maize, organic farming), but will be lower for less demanding crops (e.g., sunflower) and/or intensive systems.
Social criteria
Working time: Variable
Sowing cover crops is done at a time when labor availability may be low (August-September) or in competition with other tasks (10 to 35 minutes/ha for establishment and 0 to 35 minutes/ha for destruction).
Relay cropping (under cover of a commercial crop in spring) is less labor-intensive, but success is highly dependent on climatic conditions.
Peak period: No effect (neutral)
Effect on farmer health: No effect (neutral)
4. Favored or disadvantaged organisms
Favored bioagressors
| Organism | Technique impact | Type | Details |
|---|
Disadvantaged bioagressors
| Organism | Technique impact | Type | Details |
|---|---|---|---|
| weeds | MEDIUM | weeds | Covering the soil during the intercrop period can limit weed development through competition, allelopathy, or biofumigation. |
Favored beneficials
| Organism | Technique impact | Type | Details |
|---|---|---|---|
| Pollinators | MEDIUM | Pollinators | The cover can serve as refuge for pollinators |
| Predatory or granivorous bugs | MEDIUM | Natural enemies of bioagressors | Legumes in intercrop can attract predatory bugs |
Disadvantaged beneficials
| Organism | Technique impact | Type | Details |
|---|
Favored climatic and physiological accidents
| Organism | Technique impact | Details |
|---|
Disadvantaged climatic and physiological accidents
| Organism | Technique impact | Details |
|---|
5. Further reading
- Intermediate Crops Minette S. (CRA Poitou-Charentes) Intermediate Crops
- -Minette S. (CRA Poitou-Charentes)
Technical brochure, 2009
- Intermediate crops - Legumes to supply nitrogen to the following maize
- -Cohan J.P., Pauget J. (Arvalis)
Agricultural Perspectives no. 380, pp.44-48, Press article, 2011
- Cover Crops Dossier
- -Thomas F. (TCS)
TCS. no.33, Press article, 2005
- Establishing a legume in the intercrop: a double-effect nitrate trap
- -Le Souder C., Labreuche J. (Arvalis)
Agricultural Perspectives no. 333, pp.63-65, Press article, 2007
- Legumes: how to use them as intermediate crops?
- -Minette S. (CRA Poitou-Charentes)
Technical brochure, 2010
- Legumes to supply nitrogen in the rotation
- -Agro-Transfert
Technical brochure, 2017
- Insertion of legumes in cropping systems: opportunities, interests, results (parts 1 and 2)
- -Jeuffroy M.H. (INRAE)
2017
2018
- Lengthening rotations by introducing forage legumes
- -Delesalle M., (Agro-Transfert RT)
2016
- Reducing nitrate leakage through intermediate crops
- -Justes E., Beaudoin N., Bertuzzi P., Charles R., Constantin J., Dürr C., Hermon C., Joannon A., Le Bas C., Mary B., Mignolet C., Montfort F., Ruiz L., Sarthou J.P., Souchère V., Tournebize J., Savini I., Réchauchère O., (INRAE)
Professional report, 2012
6. Keywords
Bioagressor control method: Cultural control
Mode of action: Barrier
Pesticide use strategy type: Redesign
Appendices
Est complémentaire des leviers
- Crop association
- Relay sowing of a winter cereal with a legume established in spring
- Growing annual species associations
- Growing mycorrhizal species
- Growing diversified species in the rotation
- Growing legumes / fabaceae
- Establishing catch crops or double cropping
- Establishing intermediate crops with allelopathic or biocidal - biofumigation effect
- Establishing intermediate crops attractive to beneficials
- Introducing an intercrop cover crop in vegetable production
- Multiplying and inoculating indigenous mycorrhizal fungi
- Practicing sowing under cover crop
- Harvesting cereal-protein crop associations at immature stage
- Returning and leaving residues of the previous crop on the surface
Contribue à
- Forage autonomy
- Protein autonomy
- Nutrient supply
- Management of beneficial enemies of bioagressors
- Management of pollinator beneficials
- Weed regulation and management
- Physical stability and soil structuring
S'applique aux cultures suivantes
Favorise les auxiliaires
Défavorise les bioagresseurs suivants
- ↑ J-P. Cohan, Arvalis, Advantage to legumes - Agricultural perspectives, 2010. https://www.perspectives-agricoles.com/file/galleryelement/pj/9e/57/68/d4/363_1487753829275939358.pdf
