Intermediate energy crops (IEC)

Technical articles - Carbon impact, Environment & biodiversity

Soil regeneration

CIVE

An intermediate crop with an energy purpose (IEC) is a crop planted and harvested between two main crops in a crop rotation. IEC are harvested to be used as input in an agricultural methanization unit.

The whole challenge of IEC is to achieve better productivity at a controlled cost, without impacting the previous and following crops.

Description

IEC are crops positioned between two main food crops. They belong to the family of multi-service intermediate crops (CIMS). Their management is often similar to that of cover crops. The place of these intermediate crops in the rotation is the primary factor for choice and success. Many species can be used as IEC : vetch, oat, phacelia, fodder pea, rye, clover, mustard, etc.

Two types of IEC are possible : winter IEC and summer IEC. They can be grown alone or in association such as a cereal with a legume. Experiments are underway with new species, including beets for example, allowing yields of 20 to 25 t/ha of dry matter or more with the sowing date (May sowing) producing a tonnage comparable to maize and exceeding it if the farmer also harvests the leaf.

The objective of IEC is twofold : provision of ecosystem services and biomass production for non-food uses. Their cost must be as low as possible to remain competitive against other resources.

Harvesting should ideally align with the recommended dry matter rates for ensiling the species in place (for example 30 to 35% DM in maize, > 28% in sorghum). Once these stages are reached, waiting is unnecessary : the production gain would be small for a high risk of lodging. Under these conditions, silo storage allows good preservation of IEC and their methanogenic potential. For winter IEC, dry matter rates at harvest are often much lower. Juice recovery systems from silos should be planned.

The choice of species and variety of IEC must be made according to the expected biomass production and their adaptation to climatic conditions (risk of frost, drought tolerance).

Winter IEC

They are sown in late summer or early autumn and harvested in early spring before a summer food crop.

Varietal choice should favor earliness, biomass productivity, tolerance to some pests, diseases or viruses (JNO…) or frost resistance depending on local conditions. The introduction of legumes limits biomass production but contributes to the nitrogen autonomy of farms via the return of methanization digestates to the soil. A threshold of 20% legumes seems appropriate (common vetch, hairy vetch, faba bean…). Species choice will depend on pedoclimatic conditions to avoid dominance of one of the two species in the mix. In all cases, simple mixtures (2 species) are preferred.

Winter IEC must be harvested late to obtain an interesting yield but also early enough to sow the next crop.

These IEC have the advantage of being little sensitive to water supply in most situations. However, they can impact the following summer crop with a partly consumed water reserve at sowing. This impact will depend on the year's climatic conditions and the soil's useful reserve level.

In no-till or simplified technique. Undercover sowing can also be considered.

Summer IEC

They are sown in summer after a winter crop (early harvest) and harvested in early autumn. The earlier the summer IEC sowing, the better the productivity.

Varietal choice must focus on species productive on a short cycle (90 days). Indeed, summer IEC have a tight schedule very dependent on the previous crop. Thus, barley, pea, rapeseed, maize, sorghum, and sunflower are good choices when the schedule allows, while millet can be interesting in tighter schedules. For summer IEC, there is a priori no interest in mixing species. Species choice must also consider pest risks and weed control issues related to the previous crop (hard-to-manage regrowth).

If the IEC lasts long, it tends to replace a food crop.

These IEC have development strongly dependent on water supply with limited productivity in case of water shortage. Their short cycle limits their impact on the following crop, and their positioning before winter soil recharge causes little competition for water reserves.

In simplified sowing. Undercover sowing can also be considered.

Benefits of IEC

• Limitation of nitrate leaching
• Limitation of soil erosion
• Soil structuring
• Limitation of many weeds development (competition for resources)
• Increase of carbon storage in soil
• Control of certain diseases of the main crop, if the association of the two crops is favorable
• Preservation of bee populations, in case of nectar-producing plants
• Limitation of phytopathogen development of main crops by breaking their life cycle
• Increase of the annual biomass production of a field
• They act as organic fertilizers if buried, or nitrogen fertilizers if the digestate is then returned to the soil at the same place after methanization

IEC vs CIPAN

IEC present as many or even more advantages than Intermediate Nitrate Trap Crops (CIPAN) :

• The biomass returned to the soil at harvest of a winter IEC is equivalent to the biomass produced by CIPAN destroyed at the end of winter.

• Root biomass also plays a positive role on soil organic status.

• Ensiling IEC allows storing the equivalent of nearly 3 years of biomass production. IEC fulfill their cover role just like a CIPAN while returning more carbon to the soil.

Limitations of IEC

Depending on the type of crop, soil, and the period chosen by the farmer to plant their IEC :

• Additional working time for the farmer
• Sometimes increased risk of soil compaction due to this additional harvest
• Water consumption by evapotranspiration (potentially problematic if the crop is grown in dry season and water is scarce)

Methanization of IEC

The current outlet for IEC is methanization, but valorization is only beginning. In the future, IEC could feed industrial units at the heart of the bioeconomy : cellulosic ethanol units or biorefineries focused on extracting high value-added molecules.

IEC are an interesting substrate in methanization thanks to their high methanogenic potential, ranging between 100 and 300 Nm³CH₄/tDM (normal cubic meter of methane per ton of dry matter), depending on the species used, while limiting the use of dedicated energy crops.

Like any plant (except woody plants), the methanogenic potential of IEC is much higher than that of livestock effluents. However, they cost much more to produce.

To be profitable for energy production, IEC must generate a significant amount of biomass per hectare to offset production, harvesting, preparation, and transport costs.

For the methanizer to cover all costs related to this crop (operational + fixed costs allocated), a minimum yield of 4 tDM/ha is considered necessary. The challenge is therefore to work on species and varieties according to the pedoclimatic context of the area to reach and exceed this threshold stably. The challenge is therefore to work on species and varieties according to the pedoclimatic context of the area to reach and exceed this threshold stably.

Different fast to fairly fast-growing species can theoretically be used such as :

Energy potential of different crops and covers, .

	Nm3 CH4/tFM	Nm3 CH4/tDM
Maize	302	280
BMR forage sorghum	275	250
Forage sorghum	300	275
Oat/Phacelia/Sunflower/Vetch/Radish	245	210
Millet/Nyjer/Sunflower/Vetch	260	240
Millet/Clover	250	210
Oat/Vetch/Clover	255	225
Millet/Clover	280	250
Millet/Nyjer/Lentil	250	210
Ryegrass/Clover	290	255
Rye/Vetch/Clover	287	255
Oat/Triticale/Rye/Pea/Vetch	302	275
Silphium		155 to 210
Italian ryegrass	390
Millet	260
Sunflower	300

Methanogenic potentials of covers range between 210 and 280 Nm³ CH4/tDM. Covers with the best potentials are : Sorghums, Maize, and all long-cycle covers. These values are obtained under very favorable laboratory production conditions. Thus, the potential of covers may vary depending on : the methanization technology used, the harvesting and conservation method of covers employed, the storage time of the material, etc.

Obviously, production costs of each crop, suitability to each plot context, etc., must also be considered... A fertilized crop will have a higher energy potential, but since nitrogen is produced from natural gas, the final interest can be debated...!

The choice of species or species planted in association (e.g., "méteil" composed of cereals and legumes) is made according to the pedoclimatic context and the previous crop and production period.

Appendices

See the following crops :

• Oat
• Rapeseed
• Faba bean
• Legumes
• Maize
• Millet
• Mustard
• Barley
• Phacelia
• Pea
• Sunflower
• Triticale
• Rye
• Forage sorghum
• Vetch

Sources

• Intermediate crops - Arvalis
• IEC : and your fields also become energy sources - Arvalis
• What is an intermediate crop with energy purpose? - Ministry of Agriculture and Food
• IEC - Chamber of Agriculture of Normandy
• Wikipedia page on IEC