Choosing a cover crop

Technical articles

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

Cover crop (field bean)

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

Choice of cover

Definition and prioritization of its purpose

It is important to prioritize the objectives of the cover crop:

• generate biomass
• restructure the soil
• provide nitrogen
• reduce fertilizer inputs
• attract beneficial insects
• produce forage
• create biodiversity
• combat weeds
• control pests

Taking constraints into account

To make the right choice of a cover crop, certain criteria must be considered :

• the rotation of crops on the plot
• soil type
• crop type
• duration of the intercrop period
• the sowing date (early or late): depending on climatic conditions, species must be chosen accordingly
• whether or not it will be used as forage
• method of destruction
• cost
• available equipment: single hopper cereal drill (prefer seeds of uniform size to avoid sedimentation in the drill), precision drill, double hopper drill...
• nitrogen supply
• positive impact on pollinators
• regulations
• the CAP

Avoiding increased disease risk

Generally, it is advisable to avoid choosing a cover crop from the same family as the following crop to prevent increased risks of diseases or pests.

• For example, in a short intercrop, it is not recommended to sow oats before wheat or, in a long intercrop, nyjer before sunflower.
• The crops present in the rotation must also be considered to avoid increasing medium-term risk : before corn, in a rotation including pea, it is better not to choose a cover sensitive to Aphanomyces (pea cover, lentil, alfalfa and vetch for example). Before corn in a rotation with frequent oilseed rape return, crucifers (mustard, radish covers…) should be avoided as they may increase clubroot risk on the upcoming oilseed rape.
• If legumes are present in the rotation. Some legumes can increase fungal Aphanomyces risk (lentils, vetch, forage pea), others less so (field beans, fenugreek, Alexandrian clover).

Avoiding increased pest pressure

Given the frequency of dwarf yellowing of barley in some regions, limit the proportion of grasses in cover crops and/or destroy the cover in winter, during a period unfavorable to aphid flight. Similarly, take-all is becoming more frequent. This is one reason to favor sowing a cover crop rather than leaving regrowth of cereal straw that maintains the fungus.

• 
  Elements to consider when choosing a cover crop (ARVALIS, Terres Inovia, UNILET, ITB)
• 
  Legend

Choice of sowing characteristics

Sowing date

Depending on climatic conditions, species more or less tolerant to heat and drought will be chosen.

Germination conditions of cover crops

Sowing density

Densities for each species are calculated based on pure dose percentages. The principle is to divide the sowing rate of each species in the mixture by the number of species present. However, vigorous species at establishment such as crucifers sometimes need to be under-dosed to avoid suppressing others. Conversely, it may be wise to increase the density of slower species (field bean, crimson clover). In agriculture, the economic aspect must always be kept in mind. Locally present species or seeds produced on site should be favored to optimize costs.

Recommended sowing densities for different mixture types (source: Arvalis-infos.fr)

Sowing method

Characteristics of different sowing techniques

Techniques	Advantages	Disadvantages	Tools	Cost €/ha
Broadcast sowing	Low establishment cost Fast Avoids all constraints related to plant debris (if previous soil work was well done)	Equipment calibration issues (centrifugal) and sometimes uneven cover Not suitable for all species (early sowing)	Centrifugal spreader, DP12, sowing under cutting bar	15 to 35
Conventional row drill	Ensures optimal sowing depth, uniform distribution and rapid emergence Compatible with all species No investment	Expensive and slow technique Wear of agricultural equipment Residues from previous crop Limited working width	Mechanical and pneumatic drill	30 to 65
No-till sowing	Ensures good seed placement Single pass sowing Seed-soil contact without excessive soil disturbance	Risk of clogging due to residues Wear of agricultural equipment Limited working width Abandonment of stubble cultivation	Specific drill required (Unidrill, Semeato, Gaspardo Directa)	25 to 60

Emergence quality depending on sowing type, Arvalis, 2011

Impact of sowing density on biomass produced

The denser a cover crop is sown, the more productive it will be in terms of biomass (optimum: 300 seeds per m²).

How many species in a cover?

Above 5 species, the amount of biomass produced no longer increases; the more species, the lower the sowing rate per species. Thus, with 10 species, emergence is ensured, but the species that emerge will be too weakly sown.

Making your choice with DSS

DSS (decision support tools) can help make the right choice.

• Cover crop choice offered by Arvalis.

• ACACIA (Aid for Choice and Adaptation of Intercrop Cover Crops in Crop Rotations) offered by GIEE MAGELLAN.

For a summer cover

Short intercrop covers, for example between 2 wheat crops, play a key role in improving the overall "fertility" of your plot. Ideally, the cover should be composed of a mixture of several species to enhance biomass production and maximize benefits.

The summer cover is to be established before harvest or quickly after harvest depending on equipment and constraints (shared equipment…). It can also be complementary, to follow with a relay cover (long intercrop cover, winter cover).

Yield increase

Trials conducted by Arvalis have shown that, regardless of species in the cover, the average yield of the 2nd wheat is improved compared to the control without cover.

• Grasses in cover bring on average +1% yield;
• Crucifers +3%;
• Legumes and cover mixtures with legumes +6%;

For wheat yielding 7 t/ha, a 3% yield increase represents 210 kg/ha more. At €200/t, the gain is €42/ha (the cost of the cover and seeding pass are partly reimbursed just by this yield gain, not counting other agroecosystem benefits).

To achieve more than 3%, species choice is important. It is recommended to choose species that develop quickly, are less sensitive to heat, and have different root systems, as well as legumes to fix nitrogen.

Choosing small seeds increases the chances of success (they have lower water needs for germination). Rates should be adapted according to local conditions and possibilities, to be discussed with the local agronomist:

• Phacelia
• Radish/rape
• Nyjer
• Alexandrian or squarrosum clover
• Flax
• Sorghum
• Hairy vetch
• Mustard (variety "Verte", later maturing)

Nitrogen capture

The summer cover is also a very good trap for residual nitrogen and a good way to capture nitrogen from the air:

• A legume-only cover: up to 200 U/ha stored and 4-5 t DM/ha;
• A multi-species cover with legumes: up to 250 U/ha and 8-9 t DM/ha;
• A multi-species cover without legumes: up to 100 U/ha and 8 t DM/ha.

Adding legumes in the cover also maintains a medium/low C/N ratio ranging from 10 to 20, which allows rapid release (2-3 months) for most organic elements during destruction stages (start of first flowers), without causing nitrogen hunger.

Advantages and disadvantages of different species

Table from the article "Which mixtures for cover crops?" (AgroLeague)

	Grasses (Poaceae)	Crucifers (Brassicaceae)	Legumes (Fabaceae)	Sunflower (Asteraceae)	Nyjer (Asteraceae)	Phacelia (Hydrophyllaceae)	Flax (Linaceae)	Buckwheat (Polygonaceae)
Advantages	The fibrous root system of grass cover crops provides a surface soil structuring effect for the following crop. Good capacity to produce mycorrhizae, important to boost biological activity in the soil. Frost tolerant, they are easy to chemically destroy. High Carbon/Nitrogen ratio (straw has a C/N ratio of about 100, compared to legumes around 20). This is important for adding carbon to soils with low organic matter content.	The taproot system, complementary to grasses in a species mixture, provides deep soil structuring. Used as cover crops, this family is generally aggressive and fast-growing. In presence of nitrogen, crucifers have a strong potential for recycling mineral elements and releasing them to the following crop (especially sulfur, calcium, and potassium). Crucifer establishment brings an allelopathic effect: crucifers contain glucosinolates, substances that can impact development of some pests like nematodes and some pathogenic fungi (biofumigation effect when crucifers are incorporated into the soil).	Legumes can fix atmospheric nitrogen through symbiotic relationships with Rhizobium bacteria present in root nodules. Living roots provide carbon to mycorrhizae, which in turn supply assimilable nitrogen compounds. They thus contain high nitrogen content (2 to 3 times more than grasses). Cover crops mainly composed of legumes have a lower Carbon/Nitrogen ratio and mineralize faster for the following crop. Residue degradation is faster. This family includes a wide diversity of species with varying cycle lengths, allowing selection of species suited to local pedoclimatic conditions.	Adapted to hot and dry conditions. Its taproot and fibrous root system provides good soil structuring at surface and depth. Non-photoperiodic, allowing late summer sowing. Good support plant for legumes in mixtures and frost sensitive at winter onset.	Related to sunflower, also adapted to hot and dry conditions. Its small seeds allow higher density and a sowing rate three times lower than sunflower. Can be sown pure before a cereal.	Phacelia is one of the most used cover crop plants. Its root system (many superficial micro-roots) provides a good seedbed for the following crop. Seed shape allows good spatial distribution and ability to "fill gaps" in mixtures. This family is not found in crops, which can help break cycles. It attracts pollinators. Often used in organic farming.	Flax is a good precursor for cereals. It has good capacity to capture mineral elements and release them to the following crop. Easy to establish under limiting conditions.	Sometimes used as a catch crop to supplement margin per hectare. Adapts well to acidic and poor soils. Drought resistant and melliferous. Has allelopathic effect against other plants during growth (no risk for following crop).
Disadvantages	Grasses are generally very common in rotations, so care must be taken that the cover does not become a weed for the following crop. Grass covers in intercrop can be host plants for aphids and promote virus transmission of yellowing to the following crop. High C/N ratio of grasses can lead to nitrogen and water consumption during straw degradation.	Unlike grasses, crucifers have low mycorrhizal capacity. Establishing an intercrop cover composed only of crucifers does not benefit from this natural symbiotic capacity with soil microbial activity. Crucifers have higher nitrogen needs than grasses (e.g., to produce 100 kg seed, oilseed rape needs on average 7 units of nitrogen, compared to about 3 units for milling wheat and 2.2 units for forage barley).	The sine qua non condition for mycorrhization is good gas exchange between root and soil. In compacted or hydromorphic soils, poor air circulation can impair nodule development.	Host plant for Sclerotinia, which can be problematic if oilseed rape is in the rotation.	Very sensitive to low temperatures. Unlike sunflower, it must be sown early. More expensive seeds than sunflower.	High cost. Demanding in sowing quality. Sensitive to heat (temperatures > 25°C affect emergence).	Difficult to mechanically destroy.	Very sensitive to cold conditions: flowering stops at 0°C. Better to avoid sowing a legume after buckwheat (potential weed control issues).
Examples	Winter grasses: Black oat Brazilian oat Forest rye, winter rye Triticale Barley Italian ryegrass Summer grasses: Sorghums Mohas Millets	White mustard Abyssinian mustard Brown mustard Forage radish Chinese radish Forage rape Camelina	Annual legumes: Field bean Forage pea Common vetch Purple vetch Hairy vetch Lentil Fenugreek Perennial legumes: Alfalfa Bird's-foot trefoil White clover Alexandrian clover

Mono-species covers have economic, logistical, and technical advantages. A pure cover can address a specific issue on a plot. For example, mustard can be interesting to combat nematodes on an infested plot.

Species mixtures

Cover crops can be sown as mono-species, simple mixtures (2-3 species), or complex mixtures (>3 species, called "biomax").

Species mixtures introduce a form of homogeneity in agroecosystems:

• In unfavorable pedoclimatic conditions, a mono-species cover will perform less well than a mixture.
• A mixture of different species adapts better to heterogeneous conditions within a plot and avoids "putting all eggs in one basket".
• If nitrogen supply in the soil (residual and post-harvest mineralization) is sufficient, crucifers and grasses tend to express better and produce more biomass than legumes.
• If nitrogen supply is limiting, legumes will dominate thanks to their ability to fix atmospheric nitrogen.

Species mixtures introduce a form of resilience in agroecosystems:

• Each family and species has a different root and aerial system: Mixing plants increases light capture and root exploration capacity to remobilize mineral elements for the following crop.
• Diversity of root exudates and complementarity of mycorrhizal communities make mixtures better than pure species.
• Mixtures allow progressive release of nutrients to ensure balanced nutrition for crops. Once the cover is destroyed, nutrient release rates differ by species according to their characteristics. Field bean with its upright habit has less soil contact (40% of its biomass will mineralize quickly, stems remain in place). Whereas vetch or fenugreek with their sprawling habit have 100% of their aerial biomass in contact with soil, thus releasing nitrogen faster.

Non-exhaustive list of main species usable in cover cropping

Main species usable in cover cropping (non-exhaustive list)

Families	Species	Destruction by frost	Root system
Legumes	Fenugreek	-7 °C	mixed
	Spring Faba bean	-5 °C	taproot
	Winter Faba bean	-10 °C	taproot
	Vetch	-10 °C	fibrous
	Forage Lentil	-7 °C	mixed
	Bird's-foot trefoil	-10 °C	mixed
	Lupin	DND	mixed
	Alfalfa	DND	taproot
	Sweet clover	DND	fibrous
	Minette	DND	fibrous
	Spring Pea (protein pea)	DND	taproot
	Winter Pea (protein pea)	DND	taproot
	Spring forage Pea	-2 °C	taproot
	Winter forage Pea	-10 °C	taproot
	Sainfoin	-10 °C	taproot
	Soybean	DND	mixed
	White clover	DND	mixed
	Alexandrian clover	-5 °C	mixed
	Crimson clover	-10 °C	taproot
	Red clover	DND	fibrous
	Persian clover	-10 °C	mixed
	Spring Vetch	0 °C	fibrous
	Winter Vetch	-7 °C	fibrous
	Legume regrowths	DND	mixed
Grasses	Spring Oat	0 °C	fibrous
	Winter Oat	-13 °C	fibrous
	Brazilian diploid Oat	-4 °C	fibrous
	Foxtail millet	-1 °C	fibrous
	Millet	DND	fibrous
	Italian ryegrass	DND	fibrous
	Rye	ND	fibrous
	Sorghum	-1 °C	mixed
	cereal regrowths	DND	mixed
Crucifers	Camelina	-10 °C	taproot
	Winter forage Rapeseed	DND	taproot
	Winter Rapeseed	DND	mixed
	Rapeseed regrowths	DND	taproot
	Brown mustard	DND	taproot
	Classic white/yellow Mustard	DND	mixed
	White/yellow Mustard "nematodes"	DND	mixed
	Winter forage Turnip	-13 °C	taproot
	Forage Radish	-13 °C	taproot
Composites	Nyjer	-1 °C	taproot
	Sunflower	-3 °C	taproot
Hydrophyllaceae	Phacelia	-6 °C	mixed
Polygonaceae	Buckwheat	-2 °C	mixed
Linaceae	Spring Flax	-10 °C	mixed
	Winter Flax	-13 °C	mixed

Regional reference values for DM, N, P and K contents of different cover crop species

Family	Species	% of DM depending on cover age			% nitrogen in DM depending on biomass				Root correction coefficient for nitrogen	% P in DM	% K in DM
		< 60 days	< 90 days	> 90 days	< 1 t	< 2 t	< 3 t	> 3 t
Composites	Nyjer	13	17	21	2.5	2.5	1.6	1.2	1.1	0.4	3.0
	Sunflower	12	13	16	1.9	1.9	1.9	1.6	1.1	0.4	3.0
Crucifers	Crucifers (average)	16	17	18	3.1	2.7	2.6	2.3	1.2	0.4	3.4
	Forage cabbages	22	22	22	4.2	2.7	2.6	2.3	1.2	0.6	3.5
	Winter rapeseed	14	15	18	3.4	2.5	2.0	2.0	1.2	0.6	3.5
	White mustard	13	16	18	3.6	2.8	2.5	2.3	1.1	0.5	3.0
	Turnip rape	10	13	21	2.9	2.6	2.4	2.1	1.2	0.6	4.0
	Radish	11	13	15	3.3	3.0	2.7	2.4	1.2	0.6	4.0
	Chinese radish	12	12	12	2.4	2.4	2.3	2.2	1.3	0.6	4.0
	Winter rapeseed regrowths	20	23	25	3.2	2.5	2.5	2.5	1.2	0.6	3.5
Grasses	Grasses (average)	18	20	21	3.1	2.6	2.2	2.1	1.2	0.3	2.2
	Spring oat	16	19	21	3.0	2.7	2.4	2.3	1.2	0.3	2.5
	Winter oat	16	19	21	3.7	2.8	2.6	2.5	1.2	0.3	2.5
	Strigose oat	18	18	20	2.3	2.3	1.5	1.5	1.2	0.4	2.5
	Winter soft wheat	23	23	23	3.0	2.1	2.1	2.1	1.2	0.4	2.5
	Spring barley	12	14	17	3.1	2.6	2.2	1.6	1.2	0.4	2.5
	Winter barley	17	19	24	3.1	2.6	2.2	1.6	1.2	0.4	2.5
	Italian ryegrass	16	16	19	2.8	2.5	2.0	1.7	1.2	0.5	0.3
	Hybrid ryegrass	16	16	19	2.2	2.0	1.8	1.7	1.2	0.5	0.3
	Winter soft wheat regrowths	18	18	23	2.7	2.4	2.1	1.7	1.2	0.4	2.5
	Classic rye	16	16	17	3.7	2.9	2.3	2.3	1.2	0.5	2.5
	Hybrid rye (J.D.)	23	23	24	3.3	2.7	2.3	2.1	1.2	0.5	3.3
	Forage sorghum	14	14	14	3.0	3.0	2.4	1.9	1.2	0.4	2.5
Hydrophyllaceae	Phacelia	8	9	11	3.1	2.7	2.5	2.4	1.1	0.6	5.0
Legumes	Legumes (average)	16	16	14	3.6	3.5	3.3	3.2	1.3	0.4	3.2
	Fenugreek	16	16	16	3.2	3.2	3.2	3.2	1.3	0.6	3.5
	Faba bean (winter & spring)	14	14	14	4.0	4.0	4.0	3.2	1.3	0.6	3.5
	Vetch	16	16	14	4.4	4.1	3.9	3.4	1.3	0.6	3.5
	Lentil	18	18	15	4.3	3.5	3.5	3.2	1.3	0.6	3.5
	Alfalfa	32	32	32	2.3	2.3	2.3	2.3	1.3	0.6	3.5
	Sweet clover	30	30	30	2.0	2.0	2.0	2.0	1.5	0.6	3.5
	Minette	19	19	19	3.0	3.0	3.0	2.4	1.3	0.6	2.5
	Forage pea	12	12	12	3.2	3.2	3.2	3.2	1.3
	Protein pea	19	19	19	3.2	3.2	3.2	3.2	1.3	0.4	1.9
	White clover	24	24	24	2.2	2.2	2.2	1.8	1.3	0.4	3.3
	Alexandrian clover	10	11	13	3.7	3.2	3.0	2.5	1.3	0.4	3.3
	Crimson clover (and hybrid clover)	21	21	22	3.3	3.3	2.7	2.2	1.3	0.4	3.3
	Subterranean clover	24	24	24	1.8	1.8	1.8	1.8	1.3
	Red clover (and Persian clover)	17	17	17	3.3	3.3	2.7	2.2	1.3	0.4	3.3
	Vetch (winter & spring)	21	21	20	4.1	3.9	3.9	3.7	1.1	0.5	3.5
Linaceae	Flax	22	22	22	2.5	2.2	2.0	2.0	1.1	0.3	2.6
Polygonaceae	Buckwheat	22	23	25	2.6	2.2	2.0	1.8	1.1	0.6	2.3
Others	Mercurialis	15	15	15	3.5				1.2

To delve a little deeper into the subject, you can consult :

• Arvalis cover crop datasheets

To go further

• Cover crops
• Sowing a cover crop
• Succeeding with a cover crop
• Destroying a cover crop
• Rolling cover crops during the intercrop period
• Permanent cover crops

Other interesting documents

• Cover crops during intercrop - Chamber of Agriculture Ile-de-France region
• Intermediate crops in long intercrop - Chambers of Agriculture Pays de la Loire
• Cover crops - GRAB HN
• Cover crops, superficial techniques, Organic Farming and Agroforestry - GABB 32
• Choosing and succeeding with your cover crop during the intercrop period in organic farming - ITAB
• Innovative cover crops - Sem-Partners
• Intercrop cover crops - TCS Magazine
• Establishing cover crops Keys to success - Simplified Cultural Techniques
• Cover crops to improve soil fertility - GRAB
• Cover crops : A forage opportunity? Livestock Institute
• Intermediate crops - IBIS
• Impacts of inserting intercrop cover crops in cropping systems - Chambers of Agriculture Languedoc-Roussillon Midi-Pyrénées
• Cover crops. Which species for which sowing periods? - Green Science City
• The intercrop cover crop guide - Chamber of Agriculture Isère
• New cover crops - Sem-Partners
• Cover me!! - Agrivulg
• Guide on mechanical destruction of cover crops - CUMA 640
• Forage crops, cover crops - UNEAL
• Intercrop in Alsace. How to VALUE it? Chamber of Agriculture Alsace
• “Living Soil” cropping systems : What concrete solutions? - Agro d'oc
• Conservation agriculture. Cover crops - EUSKAL HERRIKO LABORANTZA GANBARA
• Cover crops Keys to success! - Chamber of Agriculture Meurthe-et-Moselle
• Cover crop, a crop in its own right - Chamber of Agriculture Brittany
• Cover crop trials after potato 2018-2019 - Bretagne Plants – Syndicat de l’Elorn et de l’Horn – Triskalia/capinov
• Better managing the intercrop for agronomic and environmental benefits - Chamber of Agriculture Poitou Charentes
• Legumes, how to use them as intermediate crops? - Chamber of Agriculture Poitou Charentes
• Multi-annual technical summaries - Green Science City
• Cover crops - Simplified Cultural Techniques
• The cover crop essential for soil fertility - Terra
• Species to sow in summer cover crops and benefits, AgroLeague

Sources

• Cover crop datasheets - Arvalis-infos
• Decision support tool Arvalis : Choix-des-couverts.arvalis-infos.fr
• Wikipedia page on cover crops
• Making the right cover crop choices
• Arvalis: Some key elements to succeed with your cover crop
• Choosing and succeeding with your cover crop during the intercrop period in organic farming - ITAB
• Which mixtures for cover crops?, AgroLeague