Grass-Legume Intercrops to Produce Biomass for Bioenergy.

Including (multi)annual energy crops (e.g., triticale, fescue…) within crop successions allows to maintain the production of food and feed. However, they often require higher N fertilization compared to perennial energy crops (e.g., miscanthus, switchgrass…), leading to higher N losses and energy costs. Intercropping (multi)annual energy crops with legumes might offer the opportunity to reduce the use of N fertilizer while maintaining yields, thus improving the environmental and energetic performances, as it has been already observed for grain and forage production.

An experiment was set up in Versailles (Paris Basin, France) for three years (2010-2013) to assess the agronomic, environmental and energetic performances of various grass-legume intercrops (compared to the corresponding sole crops) including annual winter crops (triticale, forage pea, vetch and red clover) and multi-annual crops (tall fescue, orchard grass, alfalfa). Two N fertilizer treatments were applied on sole grasses and grass-legume intercrops (without and with, half rate on intercrops compared to sole grasses), no N on sole legumes. Agronomic performances were measured (biomass production and quality) as well as the N₂O emissions on non-fertilized treatments (static chambers) and the nitrate leaching risk (soil mineral N content after harvest). The energetic cost of production was calculated (INDIGO®).

Intercrops, even without N fertilization, showed the highest yields. The yields of fertilized sole grass were also high, but they received two times more N fertilizer compared to fertilized intercrops. N fertilization did not influence the total yield of intercrops but significantly increased the proportion of grass in the mixture. Nitrate leaching risk was reduced in intercrops compared to sole legumes, and no difference was observed between intercrops and sole grasses. N₂O emissions were similar between non-fertilized treatments. The energetic cost to produce one ton of biomass was lower for intercrops compared to sole legumes or sole grasses, and lower for non-fertilized treatments compared to fertilized ones. In terms of quality, intercrop biomass might be better adapted for bioenergy production (biochemical or thermochemical process) than sole legume (accounting for ash, lignin, cellulose, hemicellulose, N content).

These results show the interest of grass-legumes intercrops as feedstock for bioenergy production, regarding agronomic, environmental and energetic performances.