Cover Crops: real agronomic tools

1 - What are cover crops used for?

Cover crops recycle fertilizing elements.

The objective of a cover is to reorganize potentially leachable nutrients (N, P, K) into plant material.
In this way, they limit the fertility “leaks” from our production systems.
The recycling process allows collecting elements that descend into the section to reorganize vegetation on the surface.
In this context, we can observe a “recycling of fertilizing elements” process.

In the diagrams below, we can observe that the level of nitrogen residue is globally 4 to 7 times higher for bare soil or a late-planted cover than a summer cover planted from the harvest time.

Source : Nitrawal, Melles trial (Belgium), 2008

An important point here is the relationship between biomass produced and nitrogen captured. A synthesis carried out by the Nord-Pas-de-Calais Chamber of Agriculture, based on 14 years of tests with nearly 300 different cover crops, showed that, on average, the covers could capture 26 units of nitrogen per ton of dry matter (DM) produced. In this framework, the more a cover crop is developed, the more nitrogen it will capture.

Besides recycling fertility, some cover crops containing a dominant leguminous plant can capture atmospheric nitrogenthanks to a rhizobia symbiosis. These legumes will first capture nitrogen available in the soil. Thanks to their link relationship with rhizobia, they will also capture atmospheric nitrogen and add it to the system.

Cover crops are useful for managing weeds.

Of course, they cannot replace a weed killer and will be ineffective on weeds that are already present before sowing. However, they can be formidable competitors in terms of space, food resources and sometimes by developing processes such as allelopathy (explained below).

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First lever: aim for a quick start of the cover crop with strong covering power to compete with light. Everyone has already observed the low level of weeds under a developed crop cover with high biomass.

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Second lever: create a competition based on nutrients.As most weeds are nitrophilous, they will be placed in unfavorable conditions if their access to this resource is restricted. By establishing a high carbonated cover crop (choice of species or late destruction) with a high C/N, we can “empty” the upper limit of available nitrogen.
Note that the cover crop will also compete against weeds on the water resource.

The objective of the strategies described above is to place weeds in poor growing conditions. Thus, it is common to observe in a high-performing cover crop, often deformed, poorly developed seedlings that will eventually die.

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Consider allelopathy as an attractive lever.

This barbaric term designates all the direct and indirect biochemical interactions of certain plants on surrounding plants. In concrete terms, some plants release phytotoxic compounds. These compounds limit the germinative power of their neighbours and induce malformations leading to their death.

Some cover crops are known to have these mechanisms:

•  Poaceae (rye, oats, wheat)
• The Fabaceae(sweet clover, some varieties of alfalfa, red clover),
• The Alliaceae,
• The Polygonaceae (buckwheat)
• But it is mainly the Brassicaceae (mustards, camelina, wild turnips, Chinese radish…) that seem to possess the strongest allelopathic activity.

Globally, for optimal management of the weed flora, we advise combining several routes.

Cover crops structures the soil

For proper operation, the floor must be vertically organized.

This can be provided directly by the roots of the cover crops. Indeed, they allow this reorganization by proposing taproots (with high penetration capacity) during periods of the year favorable to deeply structuring thanks to humid and loose soil (winter period).

The cover also serves as a food resource for the soil’s macro and microorganisms, which have a role in constructing the ground.

Firstly, earthworms, mainly anecics ones, feed on organic matter on the surface, digest in the depths, and reject part of their droppings in their burrows and the rest on the surface, forming in worm castings.

Earthworms develop vertical caves, 80% of which will be explored by roots that find nutrients and a great way to descend into the soil effortlessly.

Microorganisms, attracted by the roots of the cover crops, are also involved in the proper structuring of the soil. Their production of organic glues, such as glomalin, will make it possible to preserve the earth aggregates organized by the cover crop’s roots and earthworms. These glomalin productions are concentrated mainly around the roots because the microorganisms feed on the root’s exudates.

But they can also be found around earthworm burrows, where microorganisms are drawn to the droppings on which they feed.

Cover crops promote soil life

To maintain a strong biological activity in the soils, it is ESSENTIAL that food resources stay available continuously throughout the year.

The most limiting factor for the earthworm population is not phytosanitary products but the lack of food, such as plant debris, on the ground.

Soil that is overly clean due to the burial of crop residues is thus highly detrimental to the development of earthworm populations.

Therefore, the cover crops are an essential tool to provide a varied and continuous food resource for the soil’s life. The cover becomes a resource once it is destroyed and spread over the ground. This is when the soil organisms start to degrade the material. One of the effects of this degradation will be to reduce the C/N of the ingested cover crop (generally between 20 for a young legume cover crop and more than 100 for a late grain or brassica crop) to a C/N between 8 and 10.

But beware, the higher the C/N of the cover crop, the more nitrogen soil biology will have to find to degrade this material. You’ll have to manage the destruction of your cover crops according to your objectives.
For example, it is possible to produce a carbonaceous cover before maize with destroyed forest rye before sowing by rolling. The objective is then to come and sow directly or with a strip-till to conserve this biomass on the surface and offer these food resources to the telluric organisms throughout the maize crop cycle.

It will also be necessary to use localized fertilizer application.

The cover crop will finally create a favorable microclimate at ground level. In summer, the cover crops make it possible to reduce the temperature of the ground, thus preserving the biological activity whose optimal temperature is around 20°C. In autumn, the covers form a blanket that acts as an insulator, delaying the decrease of this activity due to cold weather.

The cover crops provide an ecosystem service (pollination, lowland fauna, erosion control, water management, communication, etc..).

In addition to the advantages mentioned above, cover crops offer a variety of additional benefits.

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 They can offer pre-winter and/or early spring blooms to help pollinators gather resources to get through the winter and help them easily find food resources to restart the season. In this context, they can also be a significant honey resource and, for some, a source of additional income.

Overall, agricultural development has led to a decrease in small plains’ fauna by reducing the attractiveness of these territories with the removal of refuge areas and food resources.

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Cover crops are an excellent solution to this problem as they provide an ideal refuge from predators and are rich in insects that make up most of the food of the lowland fauna.

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Cover crops also play a significant role in limiting erosion. When combined with simplified working techniques, they allow the increase of organic matter on the surface and the number of residues, which slows down the speed of water and limits erosion.
Because of their power to structure the soil, either directly by their roots or by stimulating soil biology, cover crops also allow water to infiltrate the soil. When combined with soil conservation practices, they contribute to the increase of soil organic matter, thus increasing the useful reserve. Note that a single gram of humus is capable of retaining ten times its weight in water. In this context, soils have an increased water reserve and a better resistance to droughts.

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Finally, in a world where communication with the public has become indispensable, cover crops play the role of ambassador for an agriculture that is reinventing itself.

2 - How does one choose a cover crop?

The choice of cover crops varies according to individual needs and objectives. Nevertheless, we can generalize different points:

• A mixture of types is required. A minimum of three types can be indicated. For different reasons, species may have different response levels to climatic conditions so, on the same mix sown each year, the cover crop will look different every year depending on the conditions.
• Which destruction method to considered? One of the most economical is freezing. However, depending on the region, not everyone can benefit from this method.
• Another particularly effective method is the annually choice of a cover crop and its destruction by rolling after flowering. In fact, after this stage, the plant has completed its reproductive cycle and will not be able to do it again.
• Keep in mind the primary culture in order to avoid creating a cover crop that will consists mainly of host plants for diseases or pests. This has less impact in a highly diversified cover crop.
• Make the most of aerial and underground strata for high biomass production.

For “nutrient recycling” aspects, one should favor relatively aggressive species on available fertility such as radish, mustard, cabbage, phacelia, diploid oats or rye.
Regarding “weed control” considerations, one can opt for rapid establishment cover crops in dry conditions, with high covering power such as mustard, vetch, fox tail millet or phacelia.
To play on an allopathic effect you can choose rye, flax, camelina, barley, lopsided oat, sorghum or fox tail millet.
For the production of a carbonaceous cover crop, one can choose combinations with mustard, forest rye or forage sorghum.
For a soil structuring objective, one can turn to pivot-rooted species such as sunflower, Chinese radish, mustard, fodder rapeseed, broad bean.

3 - What is the best cover crop before corn?

Corn is an extraordinary crop due to its high biomass production but also a calamity for soil life due to the lack of food resources during its cycle.
An attempt will therefore be made to achieve a substantial cover crop in order to bring material to be digested for the soil biology throughout the corn cycle.
For corn, we will also seek a good soil structuring both in depth to allow the corn roots to go down in the profile, and on the surface to guarantee a good seed soil contact.
The nitrogen requirements for corn are primarily before the first 10 leaves. Low C/N cover should be preferred unless a nitrogen-based weed management approach with fertilizer localization is required.

In practical terms, we recommend a long intercropping after cereals, either with sowing as soon as harvest time permits, or sowing in autumn.
With a cover crop planted early (from the previous harvest), one can foresee a destruction after flowering during December/January by rolling (ideally under frost) and/or by grazing.

With a later cover crop, it will be preferable to destroy the crop in the month before seeding the corn or after seeding with chemical destruction to obtain as much material as possible.

Different methods can be used to prepare the sowing according to the conditions, soil types and the level of soil functioning:

• For less prepared soils, the use of strip-till is recommended. This can be done a first time with the tine in late summer/early autumn in dry conditions. Then a second time depending on the soil type to warm up the seedling line in the spring without the tine, in wet conditions.
• Direct seeding can also be done depending on the level of advancement of the soil.

For the choice of cover crop type, we recommend a multi-species cover with the objective of maximum biomass production with a C/N around 60 in order to combine decomposition time and limitation of nitrogen depletion.

A cover crop with

• a crop with broad beans, Chinese radish, sunflower, mustard and fox tail millet to structure the soil in depth
• phacelia and clover to loosen the surface soil
• pea and vetch to reduce the C/N of the cover crop
• finally flax, camelina and sorghum to try to have an effect on weed emergence and maximize the ground impact of the cover crop.

4 - How does one sow covers crops?

The first factor in order for a cover crop to be successful depends on the weather, just as any agricultural production.  Nevertheless, it is possible to put all the chances on its side to make a successful cover.
The sowing is probably the most important point. Indeed, poor sowing conditions lead to significant collection losses, especially in dry conditions. To do this, the soil/seed contact needs to be enhanced without disturbing the soil too much so as not to increase weed growth and dry out the surface. For this, the best solution is a seeder with fine tines (12 mm wide maximum) for cover crops sowing in dry conditions.

The sowing must be carried out immediately after the harvest if the conditions allow it or as of the first rains in August. It is necessary to maximize the number of seeds per m². For this, small seeds such as phacelia, flax, camelina or clover must be included in the mixture.
The second point is the fertilization of the cover crop. Like any direct seeded crop, localized fertilization is a key factor of success. A contribution of only 30 units allows significant production gains.

5 - Further information

Cover crop grazing

Grazing is an efficient and cost-effective means of destroying cover crops. In addition, it allows a quick return of the recycled elements through the cover. Once the cover crop has been digested by the animals, the fertilizing elements become available.

This method accelerates the recycling cycle of the elements. In this way, the cover crops can be directly enhanced by the next crop.

In addition, this method makes it possible to produce a non-negligible meat product at a lower cost by making savings on the destruction of covers crops. This method is also called soft and does not damage the fauna of the lowland. Finally, it makes it possible to feed a great diversity of insects thanks to the animals’ excrements.

Cover crops as a bioreactor

Finally, the plant cover can be used as a bioreactor to inoculate the soil with beneficial microorganisms, such as the installation of a mycorrhizal fungus network, which will be used for the next crop.

Setting up a mycorrhizal network takes some time and its energy can be high for a small crop such as corn. The cover crop could then play the role of a reactor, allowing the network to be installed.

Once this solid foundation is installed in the ground, the next crop will simply need to be plugged in to receive all the benefits.

The mycorrhizal network can withstand the destruction of the cover crop and connect to the next crop, provided it has not been disturbed by ploughing.