Destroying a Vegetation Cover
Ideally, the destruction of the cover would be done at the time of sowing, to maximize the effect of the cover on the soil and biodiversity. The later the cover is destroyed, the more aerial biomass it will produce and the more carbon and nitrogen it will fix. Livestock farmers are well aware of this phenomenon. In spring, each day gained before harvest increases the production of fodder. Of course, the same applies to cover crops.
Choice of date
Before choosing the method of cover destruction, the intervention date must be determined. This is a compromise between two objectives:
- Allowing enough time for the cover to fully play its role: trapping nitrates, fixation of nitrogen by legumes, soil protection, ...
- Avoiding a depressive effect on the following crop: by preserving water and nitrogen availability without hindering its establishment.
The date of cover destruction is essential and aims to align the period of high mineralization with the absorption period of the following crop.
According to species
The destruction date varies depending on whether the species are legumes, grasses, or crucifers. It is necessary to consider the seed setting of the species: species like fodder radish, which set seed quickly after establishment, must be destroyed in autumn. Conversely, if the cover is composed of legumes, with a lower C/N ratio and a longer development cycle, destruction can be delayed because mineralization can occur more rapidly.
According to desired objectives
The more advanced the cover stage, the higher the lignin content and the slower the nutrient release:
- If the goal is to enrich the soil with organic matter, the cover can be left to lignify to achieve an adequate C/N ratio.
- If the goal is to provide nitrogen to the following crop, destruction should wait until flowering.
- Finally, if the goal is to trap nutrients and avoid nitrogen leaching (for environmental and agronomic benefits or to comply with regulations), destruction should wait until nitrogen deficiency symptoms appear on the cover (signaling that all soil nitrogen has been trapped). To limit nitrate leaching, the cover must be maintained during October and November (drainage period). Leaving it longer is unnecessary for this objective as the cover’s efficiency in trapping nitrates decreases over time.
- If the goal is to maximize root effect: destroying the cover at sowing maximizes root effects, increases root exudates, and helps preserve endomycorrhizae as long as possible. Their survival in soil without roots is estimated at only a few weeks.
- If the goal is to address a water issue:
- As long as the cover is alive, it continues to “pump water”. This phenomenon is well known by livestock farmers, accustomed to ensiling ryegrass in spring, for example. In drying situations, a living cover risks excessively drying the soil profile and harming the following crop. In such cases, anticipating destruction is important to avoid this.
- Conversely, this phenomenon can be used to allow spring cereal sowing on fields that have difficulty drying out. In such cases, sowing into a living cover is the best way to dry the profile earlier and maximize soil bearing capacity.
To avoid difficulties at destruction and allow the cover to decompose and provide part of the trapped nitrogen, the nitrogen needs of the following crop must be considered: early nitrogen needs require early cover destruction (mid-November). For crops with later nitrogen needs, destruction can occur later. This also aligns with nitrate directives in most departments.
According to nitrogen return
The period just before flowering is when the plant is most concentrated in nutrients, with short-term effects:
- Supply of labile carbon (high nutritional or energetic value matter, unprotected and easily degradable by microorganisms) to better feed the soil;
- “Priming effect” : over-mineralization of soil organic matter after fresh organic matter input;
- Development of a bacterial pool in the soil.
The period after flowering marks the transfer of nutrients to the plant’s reproductive organs and the increase in the cover’s C/N ratio, with medium to long-term effects:
- Development of a fungi pool in the soil: lignins feed fungi, promoting organic matter stabilization.
- Storage of stable organic matter: improving soil structural stability and system resilience.
Most stable organic matter comes from roots and exudates. It is therefore essential for an agricultural system to have both labile and stable organic matter.
Indicator: the C/N ratio
The C/N ratio is an indicator that assesses the aptitude of organic matter to decompose more or less rapidly in the soil: the higher the C/N ratio, the slower the mineralization dynamics.
The trial was conducted on three types of covers:
- Legume in green (C/N = 12).
- Legume + crucifer in brown (C/N = 19).
- Crucifer in orange (C/N = 23).
It can be seen that the higher the C/N ratio, the slower the nitrogen is mineralized.
Common vetch returns more nitrogen and faster than the other two.
Methods for calculating nitrogen returns
For a faba bean cover
- Returns: C/N = 13; production of 3 tDM/ha with 42% C ⇒ 1260 kg of C/ha; 1260/13 = 96 kg N/ha returned.
- Needs: Soil C/N is between 8 and 12. When organic matter input falls outside this range, microorganisms will seek nitrogen (or carbon, more rarely) in the soil if lacking in the cover. Bacteria consume about 30% of the returned carbon: 1260*30% = 378 kg C/ha ingested by soil microorganisms. Using a ratio to return to a C/N of 10, this results in a nitrogen need of 37.8 kg N/ha to find.
- Positive balance: 96 - 37.8 = 58 kg N/ha.
For an oat cover
- Returns: C/N = 43; production of 3.6 tDM/ha; 33% C ⇒ 1188 kg of C/ha ⇒ 1188/43 = 27 kg N/ha returned.
- Needs: Bacteria consume about 30% of the returned carbon ⇒ 1188*30% = 365.5 kg C/ha ingested by soil microorganisms ⇒ Using a ratio to return to a C/N of 10, this results in a nitrogen need of 35 kg N/ha to find.
- Negative balance: 27 - 35 = -8 kg N/ha.
According to destruction stage
Agronomically, destruction just before flowering is optimal for good legume development.
The following graph, from Francis Rayns’ work, shows the impact of three different destruction dates for a winter vetch cover on nitrogen fluxes in the soil:
- The red curve represents the “early destruction” modality on March 26. It shows an increase compared to the control in nitrogen release. However, it remains below the other two modalities.
- The green curve represents the “late destruction” modality on May 2: a real gain in mineral nitrogen is observed compared to the other two modalities. Nitrogen release from the legume is optimal and the return on investment in terms of nitrogen nutrition for the following crop is better.
The approach to cover destruction must always remain pragmatic, but these figures show that for the same initial cover, the return in terms of nutrient release is different depending on the destruction date.
According to the following crop
Spring crop
The cover must be destroyed about two months before sowing the following crop, to allow residue decomposition time and avoid depressive effects on the following crop.
Autumn crop
It is possible to destroy the cover just before sowing the following crop without depressive effects: cereals have lower nitrogen needs in autumn. The autumn and winter period allows nitrogen from the cover to mineralize and the soil water reserve to recharge before cereal stem elongation.
According to soil type
The cover decomposes more or less quickly depending on soil type. For example, in a clay-limestone soil, grasses and cereals decompose very poorly after winter. Soil type also affects field operations: heavy soil often requires early cover destruction because drying is slow, while a light, well-structured, and well-drained soil tolerates later destruction.
The table below gives some recommendations to choose the ideal moment for cover destruction depending on soil type, following crop, and whether ploughing is used.
| Soil type | Following crop | ||
|---|---|---|---|
| Winter crop | Spring crop | Maize, sunflower | |
| Healthy loam, chalk, sand | Just before sowing | From mid-November to mid-December | From November to February (early March at the latest) |
| Clayey loam, clay-limestone soil | - Ploughing: from
15/11. - No-till: between 15/11 and early February | ||
| Clay soil | - No-till: Just before sowing
- Ploughing: Anticipate destruction and ploughing date |
- No-till: 15/11
- Ploughing: Anticipate destruction and ploughing date |
- No-till: mid-November to mid-December
- Ploughing: Anticipate destruction and ploughing date |
Table 1: Recommended cover destruction dates according to soil and following crop - Arvalis.
Refer to the nitrate directive of your department to determine from which date cover destruction is authorized!
In general, a well-developed cover is easier to destroy than a small cover, despite a higher risk of clogging for some tools.
Choice of technique
Rolling, mowing, ploughing, tillage, or even frost, there are multiple destruction techniques and several factors must be considered to choose the cover destruction method.
Available equipment
The species (or species) of the cover crop must of course be chosen according to the available equipment.
In case of no-till
Species that can be destroyed by means other than ploughing must be chosen, such as frost-sensitive species.
Soil bearing capacity
If fields are inaccessible during winter due to non-bearing soils, species that can be destroyed by frost or mechanically in spring should be favored. Despite these recommendations, the cover must be monitored and ready for intervention. This applies to frost-sensitive species: if there is no frost before December 31, mechanical destruction must be considered. Also, if around mid-November a species is flowering, mechanical intervention is necessary to prevent seed setting.
Different destruction methods
Chemical
The chemical destruction method of cover crops in interculture is used in conservation agriculture to disturb the soil as little as possible.
This strategy must also consider weed issues on each field (especially those with a history of grassy weeds) and the approval status of products, which has evolved, notably for glyphosate after sowing.
In a report published on October 9, 2020, ANSES revealed the results of its comparative evaluation of alternatives to glyphosate in interculture. It concluded that this molecule remains authorized in three situations:
- in no-till before winter and spring crops, at a dose of 1080 g/ha/year,
- after summer/autumn ploughing before spring crops in hydromorphic soils only, at a dose of 1080 g/ha/year,
- within the framework of “regulated control” (presence of thistle, ragweed…) and some cases of regulated pest control (presence of quarantine bacteria or nematodes on potato regrowth, for example), at a dose of 2880 g/ha/year.
In situations where it remains authorized (no-till or summer/autumn ploughing in hydromorphic soils), glyphosate remains useful: easy to implement, high work rates, relatively independent of soil moisture with little impact on soil structure in wet autumns.
Many species are sensitive to this herbicide, especially grasses (cover crops, regrowth, or weeds). It is also possible to combine it with 2,4-D to improve efficacy on dicotyledon covers. However, the use of this product entails a waiting period before sowing certain crops.
Currently, the nitrate directive may limit chemical destruction options in some departments, especially on regularly ploughed fields. Check with your prefecture.
In situations where glyphosate will be banned, or where the 1080 g/ha/year dose may be challenged (notably on perennials or annuals developed before sowing a spring crop), cover species choice can adapt by favoring species easy to destroy mechanically or by frost. Grass covers (especially rye) seem less suited to these techniques than others, such as mustard or phacelia. Weed management or regrowth will also remain a problem, especially if the cover could not exert its competitive power.
Effect of some herbicides on alfalfa, bird's-foot trefoil, and clover
Mechanical
Ploughing
In case of ploughing, it is possible to take advantage of soil inversion to destroy the cover without additional passes. In this case, cover destruction does not generate extra costs compared to usual practice without cover.
However, some complications may arise. After burial, plants can regrow via their reserve organs. This is especially true for radish or rapeseed. Clogging can also occur with very tall covers like mustard. To avoid this, three options can be considered:
- Mow the cover before ploughing.
- Choose a species that grows less tall: mustard can be replaced by phacelia, for example. The extra cost of phacelia seeds can be “amortized” by saving mowing.
- Flatten the cover to facilitate burial. Various techniques exist: rollers in front of the tractor, bars, or chains. With these systems, it is preferable to remove the harrows to facilitate vegetation flow. This results in a less aesthetic plough but more agronomically relevant. Some residues are pressed on the plough furrow side instead of being fully buried at the bottom.
Mowing
Mowing cover crops is a very common and easy to implement technique. The mower allows better mineral availability, provided the cover does not have too high a C/N ratio, it reduces vegetation volume, which facilitates subsequent tillage. Since the mower does not touch the soil, only soil bearing capacity must be considered to avoid compaction. Another feature of this technique is leaving 100% of residues on the surface for good soil protection. However, mowing is not recommended on grass covers since they can regrow after cutting.
Some new “chopper” rollers (heavy, solid, equipped with protruding blades and operated at high speed) have an action mode close to mowers by chopping stems of fragile plants (mustard, phacelia…).
Advantages:
- Priming effect on organic matter decomposition.
- Choice of seeder.
Disadvantages:
- Low work rate;
- Cost: €40/ha;
- Higher fuel consumption;
- Combined: requires lifting power and front PTO.
Stubble cultivation
Using a stubble cultivation tool can allow destroying a cover crop while preparing the seedbed for the next crop. This strategy combines relatively favorable cost and work rate.
There are many tools of this type: independent disc stubble cultivators, rolling hoes, cultivators with two or three rows of tines... Even if the work is slightly rough, it will refine over the winter under the influence of the climate. It will also help the soil to dry more quickly on the surface in spring.
- With tine tools, the cover crop will be more or less buried depending on the working depth. Clogging phenomena (vetch) can occur with very developed covers. Preliminary shredding may then be necessary.
- ⚠️ Beware of clogging (vetch);
- ⚠️ Works poorly on grasses;
- ⚠️ Works on crucifers if they are not strongly developed.
- Independent disc stubble cultivators allow mulching of cover crops. Tools with large discs are well suited, but adapting equipment such as a roller knife improves the efficiency of tools with small discs. This equipment is now available from most manufacturers.
- 👍 They chop organic matter more finely;
- 👍 Interesting on young grasses;
- ⚠️ Difficult to achieve homogeneous work on developed grasses.
- Rolling hoes are also very comfortable in cover crops, even with dense vegetation.
To avoid any unpleasant surprises, it is necessary to ensure working under good drying conditions. These good conditions may only appear late in winter (or not at all in case of a mild and humid winter), so it may be interesting to consider other destruction methods less dependent on soil moisture, such as shredding if the cover is sensitive to this destruction method. This climatic uncertainty shows us how important it is to adapt practices by starting with a choice of cover crop species sensitive to different mechanical destruction methods, in order to keep several options available.
Advantages:
- Soil warming and mineralization.
Disadvantages:
- Mineralization: loss of soil organic matter;
- Impact on soil structure: beware of conditions for reworking (risk of smoothing).
- Cost: about €30/ha including traction and labor.
Rolling
Rolling cover crops at subzero temperatures can work in winter: the injuries caused by the roller amplify the effects of frost on plants. It allows keeping the soil covered with residues. However, it can cause soil compaction under tractor wheels, especially if the soil is not frozen under the cover. Rolling performed on light frosts is quite effective on many frost-sensitive species, especially if they are well developed.
Conversely, less frost-sensitive covers (grasses weeds or regrowth of wheat) are little affected by rolling.
In the absence of frost, rolling results are disappointing on almost all species, even with a "rolo faca". Only some very sensitive species, such as phacelia, will be destroyed.
Quite fast and inexpensive, rolling on frost remains constraining in terms of work organization: you must be available in the mornings or nights when it will freeze. Frost may also appear somewhat late, often in January or February. It is therefore not well suited for early spring crop establishment.
The roller-crimper (FACA - DALBO and SACHO)
It allows laying down and injuring a cover crop without disturbing the soil to avoid re-germination.
Advantages:
- Works well on headed grasses.
- High work rate.
- Affordable cost: €10/ha.
- Strong coherence in a no-till / strip till approach as it allows keeping the soil covered.
Disadvantages:
- Weight: between 4 and 6 tons. Can impact the soil if cover is not dense.
- Grasses at tillering stage or not yet sufficiently grown can regrow (alternative intervention for these species).
The chopping roller
The chopping roller allows cutting the cover crop into small pieces. Advantages:
- Accelerates the degradation of organic matter, thus faster nutrient release.
- High work rate.
- Cost: €42/h (excluding traction): < €10/ha
Disadvantage:
- Grasses can regrow from secondary tillers (less common with FACA rollers which pinch).
The cover crop
For good efficiency, it must be wide and heavy with many discs.
Specialized tools
Other tools intended for cover crop destruction exist, such as:
- The Glypho Mulch.
- The disc weeder
Frost
Letting the cover crop freeze is undoubtedly the ideal solution.
However, the occurrence of frost at sufficiently low temperatures is uncertain and sometimes late compared to destruction date objectives. This option is therefore more relevant in regions with a continental climate, such as northeastern France or mountainous areas. It is possible to predict the chances of success depending on the local climate, desired destruction date, and cover crop species sown. The more developed the cover, the more sensitive it is to frost.
Warning: weed infestation of the plot must also be considered. Many weeds or wheat regrowth are quite frost-resistant. Very frost-sensitive covers will disappear at the first light frosts and stop competition with weeds, thus allowing the plot to green up during winter.
Grazing
A technique that is particularly developing for sheep. Covers are sown immediately after harvest and can be grazed two months later. "Provided there are species adapted to animal grazing, the quality of cover crops meets their needs without concentrate or forage input," explains Idele.
Cover crop destruction to be adapted case by case
Autumn destruction
In the case of short intercrops, cover crop destruction (summer species) must take place before sowing winter crops. Possibility to shred or crimp to prevent seed setting of the cover. Depending on cover development, soil structure, and moisture, destruction can be done by rolling (+ herbicide?) for no-till or a soil working tool (stubble cultivator…) to prepare the seedbed at the same time. For those wishing to plow clays before winter, keep the cover as long as possible and do not bury it at the bottom of the furrow.
Winter destruction
As soon as the temperature drops below 0, species such as sorghum, sunflower, buckwheat,... will freeze. Their decomposition will then begin and increase in spring. For other species, rolling on frost (amplifies the effect of cold), notably with a roller faca, is a good way to get rid of the cover. The optimum is to roll very early in the morning when temperatures are lowest. The effectiveness will be greater if the cover is well developed. However, beware of grasses which are little sensitive to rolling.
Spring destruction
Several tools allow cover destruction in spring. Soil condition will indicate the optimal date: do not keep the cover alive too late in dry weather, whereas it can pump excess water in very rainy weather.
- Stubble cultivators: allow destroying the cover and starting to prepare the seedbed, even if preparation will still be rough (possibility to refine with a powered tool).
- Goosefoot-type tools: perform scalping of the cover but can smooth the soil under some conditions, requiring reworking.
- Rototiller (horizontal axis cultivator): a powered tool that gives rather satisfactory results (destruction and preparation in one pass) but with slower work rate. Similarly, the rotary harrow can also be considered.
- Plowing: can be considered for destruction but the cover's effect on soil structure during winter will be lost.
- Shredder: in case of overly developed covers, it can create a mulch that facilitates the passage of another soil preparation tool.
- Strip-till: can be an intermediate solution with soil working only on a band about 20cm around the seed line. A roller is mounted in front to lay down the cover. This could be a good compromise between soil cover and mineralization/warming on the row.
Destruction for no-till sowing
Cover crop destruction is also possible without any soil working. In sufficiently developed vegetation, direct sowing can be envisaged with appropriate equipment and settings. Passing a roller in the sowing direction will then create a mulch cover allowing partial avoidance of weed emergence. Depending on the crop sown, species present in the cover, and weed pressure, a treatment with glyphosate and/or 2,4-D can be applied.
Advantages and disadvantages of different techniques
Each destruction method has its advantages and disadvantages. To avoid degrading soil structure, cover destruction, whatever the technique, must be done under optimal conditions. The table below gives the main advantages and disadvantages of destruction techniques. (Costs communicated were established in 2018).
| Destruction technique | Advantages | Disadvantages | Average cost/ha/intervention |
| Plowing |
|
|
€65 - 70 |
| Shredding |
|
|
€50 - 55 |
| Stubble cultivation |
|
|
€20 - 25 |
| Frost |
|
|
0 € |
| Rolling + frost |
|
|
€20 - 25 |
Effectiveness of different tools depending on the cover in place
| Cover in place | Plowing | Shredding | Stubble cultivation | Rolling
(on frost) |
Frost | Chemical |
|---|---|---|---|---|---|---|
| Nyjer, Buckwheat | ++++ | +++ | +++ | ++++ | ++++ | +++ |
| White mustard | ++++ | ++++ | ++++ | +++ | +++ | +++ |
| Forage radish | +++ | ++ | ++ | ++ | ++ | ++ |
| Phacelia | ++++ | +++ | +++ | ++++ | +++ | +++ |
| Rye, Ryegrass | +++ | + | ++ | + | + | +++ |
| Headed oat | +++ | ++ | +++ | ++ | +++ | ++++ |
| Lentil, Vetch, Faba bean | ++++ | depending on
height |
+++ | +++ | +++ | +++ |
Regulatory note
https://www.lesillon.info/2019/02/11/4199-les-regles-destruction-couverts-vegetaux.html
Further reading
- Cover crops
- Choosing a cover crop
- Sowing a cover crop
- Successful cover crop
- Rolling cover crops during the intercrop period
- Permanent cover crops
Sources
- Choosing the most appropriate cover crop destruction technique - Arvalis-infos
- Cover crop destruction: Everyone has their technique! - Terre-net
- Choosing and succeeding with your cover crop during the intercrop in organic farming - ITAB
- What technical itineraries for my cover crops? - Tarn Chamber of Agriculture
- How to choose your destruction date? - AgroLeague
- How to choose your destruction method? - AgroLeague
- What is the best time to destroy a cover before sowing a spring crop?- AgroLeague
Appendices
Contribue à
