Maintaining cereal stubble during intercrop to preserve biodiversity when sowing cover crops
Cereal fields are preferred habitats for species dependent on large-scale crops such as grey partridges, corncrakes, skylarks, or yellowhammers. Cereal stubbles are privileged sites for the wintering of many bird species[1].
Objectives
In recent years, early stubble cultivation to establish CIPAN has caused significant habitat loss for birds, preventing them from completing their breeding cycle[2]. To address this, conserving standing stubble during the sowing of green manure appears to be an appropriate solution[3]. Green manures (CIPAN) are essential to meet objectives of soil nutrient capture, carbon storage, soil structure improvement, increase in soil organic matter content[4], and also to provide shelter and food resources for wildlife in autumn.
Expected benefits
Conserving stubble allows the maintenance of farmland birds within fields. They can be considered as beneficial organisms with a potential impact on the seed bank present on the soil surface[5] (see figure n°1). Intercrop covers play a role in nutrient storage and protecting the soil surface against erosion or climatic extremes. (See Soil cover fact sheet).
An adult skylark consumes on average 6g of seeds per day. Over a year, a pair of skylarks can consume about 3.2 kg of weed seeds during their 9 months presence in France. However, if cereals are stubble-cultivated too early, the skylarks' presence time will be almost halved and weed seed consumption will also decrease. Source: Powolny T. (2012)
Expert opinion
Harvesting induces a rapid habitat change for the small wildlife living there. Maintaining cereal stubble limits the impact of this phenomenon by preserving a favorable site for the life cycle of many species.» David Granger, Project Manager for agriculture, wildlife and game damage at the French Office for Biodiversity.
Methodology and how to implement this practice on my farm?
To reconcile these different objectives, alternatives to post-stubble sowing exist: sowing into the cereal before harvest and direct seeding into stubble. Green manures composed of multiple species are recommended because a single species cannot meet all objectives. Complex diversified green manures are generally more productive than simple mixtures. In a complex mixture, there will always be some species that manage to develop regardless of climatic conditions.
A cover favorable to biodiversity must be sufficiently:
- Covering to create shelter during winter and compete with weeds
- Permeable so that small wildlife can move easily[6].
The Agrifaune program has tested and validated a number of covers allowing to reconcile agronomy, economy, and wildlife. Click here to access these covers.
Broadcast sowing before harvest
There are two possible sowing periods for broadcast sowing of green manure.
At the end of winter, before the cereal restarts
Broadcast sowing can be done with a fertilizer spreader or a slug pellet distributor. To ensure good emergence, a pass with a spring tine harrow can be done additionally. Clovers are best suited to this implantation method.
Sown at 10kg/ha, they will germinate at the end of winter and be "suppressed" by the wheat until harvest. After harvest, the established clover will benefit from light and residual moisture to grow and develop. This technique allows obtaining a vegetative cover regardless of summer precipitation levels.
Cost of clover sown by broadcast at 10 kg/ha: about €60/ha.
Advice
This practice is not recommended in fields with a problem of dicotyledonous weeds such as thistle. Indeed, herbicide would destroy the cover before harvest.
In the last month before harvest
Either a few days before harvest or during rain in the preceding weeks.
Broadcast sowing before harvest must be done with a fertilizer spreader using sprayer tracks. The advantage is to take advantage of the last rainfall before harvest and residual surface moisture to place seeds in optimal germination conditions. In some pedoclimatic contexts, the soil surface is too dry at harvest time, making sowing ineffective. It is then preferable to anticipate sowing and take advantage of the last rainfall during May/June.
For broadcast sowing before harvest, it is recommended to sow small seeds that can easily germinate such as oats, buckwheat, millet, or rapeseed.
Beware of the persistence of herbicides used on cereal vegetation, as this could hinder cover development.
For more information on plants that can easily be broadcast sown under the combine harvester, you can consult the Arvalis cover crop fact sheets or the GIEE MAGELLAN ACACIA tool (Help for Choosing and Adapting Intercrop Covers in Crop Rotations).
The complex aspect of this practice will be the homogeneous distribution of seeds on the soil surface, especially for small seeds like clover. The seeder will have no trouble distributing large seeds, but lighter ones will struggle to be projected over the desired distance. To compensate for these physical constraints, it is recommended to "stick" small seeds to large ones with a mixture of flour and powdered glucose.
Advice
Seed sticking recipe for 1 ha for a mixture sown at 110 kg/ha:
Use a concrete mixer to make the mixture.
- 100kg large seeds / 2 liters of water / 10 kg glucose - Mix
- 6 liters of water to create the glue - Mix
- 10 kg small seeds - Mix
- 14 kg flour to dry the mixture and prevent clumping.
For more information, watch the video by GIEE MAGELLAN:
Broadcast sowing has the advantage of a very high work rate (10 ha/hour) with very low cost. Moreover, the work is done during a quiet period. However, this technique requires rain after spreading or sowing to achieve good emergence.
Direct sowing into stubble
To successfully establish a green manure in stubble (stubble left in place after harvest), direct seeding with a tine seeder gives very good results. The small tines of the seeder (about 1 cm wide) open a clear furrow to promote seed-soil contact. For this type of sowing, disc seeders are not recommended as they tend to accumulate residues at the bottom of the furrow. These residues dry it out, greatly limiting germination potential. To reduce drying, it is recommended to leave tall straw (40 cm)[7]. This will improve sowing success.
It is recommended to sow immediately after combine harvesting and at the latest within three days following harvest. This sowing technique is interesting because it preserves residual moisture still present at harvest while ensuring optimal seed placement. It is also cost-effective provided an adapted seeder is available. Moreover, this method allows sowing a greater diversity of species than broadcast sowing.
Destruction of green manures
The least impactful method for wildlife is frost. But as frost periods are becoming rarer, relying on this method is difficult.
Grazing of the intercrop is an interesting option but is not applicable everywhere because the presence of a ruminant herd is necessary on or near the farm.
For shallow stubble cultivation and mechanical destruction methods, implementation of scaring techniques should be considered as these covers could become deadly traps for small wildlife.
Finally, if no other alternative is possible, chemical weed control remains authorized.
Limits
When fields were harvested under difficult conditions during the previous harvest (compaction and wheel tracks), stubble cultivation then allows leveling the field. In fields heavily impacted by weeds, false seedbeds can help limit the seed bank. However, soil work will limit its useful reserve available: evaporation of 4 mm of water per day, thus compromising the success of green manure sowing[8]. In the case of organic matter burial, these practices are limited.
Note
To optimize carbon sequestration in the soil, it is recommended to maintain green manures in vegetation for 6 to 8 months. The result of maintaining these covers for a long time is the sequestration of 126 kg of C/ha/year[9]. It also creates shelter and cover zones in winter for both sedentary species (grey partridges, hares, etc.) and migratory species (skylark, corncrake, etc.). For the composition of the chosen mixture, care must be taken to favor rather late plants because early plants lignify more quickly. Lignin can cause nitrogen depletion for the following crop.
To go further
You can consult the Magellan guide.
Sources
- ↑ Donald P.F. ; Evans A.D. ;1994. Habitat selection by Corn buntings Milaria calandra in winter. Bird Study, 41 : 199-210
- ↑ 2 Millot F. et al. ; 2017. Poor reproduction of grey partridges in recent years: What role do cereal harvests play? Faune Sauvage n°317, P89-92
- ↑ Eraud C. ;2002. Ecology of the Skylark Alauda arvensis in Cultivated Environments, Ecological Characteristics of the Habitat and Conservation Perspectives, Thesis of the Ecole Pratique des Hautes Etudes, ONCFS, Ministry of National Education, Research and Technology, p168
- ↑ Labreuche J. et al. ; 2011. Catch crops Impacts and management, Arvalis Plant Institute, p231
- ↑ Stoate C. et al ; 2017. Field of the future, 25 years of Allerton project – A winning blueprint for farming, wildlife and the environment, GWCT Allerton Project, 36p.
- ↑ Heckenbenner B. et al. ; 2011. CIPAN: when the regulatory tool becomes an agronomic and wildlife asset, Faune sauvage, n°291, p11-19.
- ↑ Thomas F. ; 2017
- ↑ Hatfield L. et al. ; 2001. Managing Soils to Achieve Greater Water Use Efficiency: A Review, Publications from USDA-ARS / UNL Faculty. 1341
- ↑ Pellerin S. et al. ; 2019. Storing carbon in French soils, what potential regarding the 4 per 1000 objective and at what cost? Study report summary, INRA, 114P
Annexes
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