Straw is a co-product of cereal crops such as wheat, barley, oats and rye. It consists of the stems left after harvest, and can either be left on the ground to enrich soil organic matter, or collected and put to various uses. Managing straw is therefore an agronomic, environmental, energy-related and economic issue.
Straw is collected at harvest time. The process involves three steps: harvesting (cutting), threshing (separating the grain), and cleaning (removing impurities). After harvest, part of the straw remains on the ground as stubble, while the rest may be shredded, ploughed in, or baled (round or rectangular bales) for storage or transport. A particular fraction, chaff (fine straw), is made up of fine debris (glumes, husks, weed seeds) and is an increasingly valued resource.
Leaving straw on the ground helps preserve soil structure, chemical fertility and biological life. Exporting straw must therefore be carefully weighed so as not to compromise the agronomic balance. Straw can also be shallowly incorporated to form humus. It is likewise used as mulch in horticulture or for crop protection, sometimes with allelopathic effects depending on the species (oats, rye).
Straw is a key input in livestock farming, notably as bedding for cattle, sheep, pigs and horses. It absorbs animal waste and contributes to manure production. In periods of drought it can also serve as animal feed, although its low nutritional value means it must be combined with other protein and sugar sources. Arvalis has developed a calculator to facilitate straw/manure exchanges between grain growers and livestock farmers.
The use of straw in construction is growing steadily. Used mainly as standardised bales (37 x 47 x 80–120 cm, density 80–120 kg/m³), it serves as thermal insulation or as a load-bearing element. Several techniques exist: load-bearing straw walls, timber-frame infill, prefabricated panels, external insulation. A 100 m² house requires around 500 bales, i.e. 10 tonnes of straw, grown on 2.5 hectares.
Straw can be used as a fuel (with a calorific value of 4 kWh/kg), or converted into biogas, biochar or pellets. In France, the total stock of crop residues is estimated at 50 Mt/year, but only a fraction (1 to 1.5 Mtoe) is theoretically mobilisable for energy, the rest being dedicated to agricultural uses.
Two reference frameworks exist for estimating straw resources:
For example, in Bourgogne-Franche-Comté, the additional available volume is estimated at 147,000 tonnes per year, which represents a potential of 14,700 houses of 100 m². This is equivalent to 1% of the region's housing stock.
Chaff, long neglected, is now recognised for its agronomic and energy potential. Its use is becoming more structured, thanks to falling recovery costs and agronomic benefits (weed reduction). It represents around 10 to 14% of the dry matter produced for wheat.
The profitability of the straw supply chain depends heavily on market prices, transport costs and local uses. In periods of strong demand or rising bale prices, collection becomes widespread. Conversely, large volumes may be abandoned for lack of outlets.
Data from INSEE, Agreste, the agricultural census (RA), and tools such as ELBA and the BDNI database make it possible to model straw requirements for livestock farming and to assess mobilisable resources by crop type or territory. These databases are essential for planning innovative and sustainable uses of the resource.
Straw is a multifunctional resource at the crossroads of many issues: agronomy, livestock farming, sustainable housing and energy. Making the most of it requires a good understanding of flows, farming practices, logistical constraints and local uses. The structuring of the supply chain – particularly around construction and energy – will depend on the ability to organise the resource while respecting agro-ecological balances.