Grasses and legumes in mixture: an energy intercropping system intended for anaerobic digestion

Batch testing of biomethanization was conducted on organic matrices from a mixture of grasses (triticale and barley) and legumes (field bean - Vicia faba L. var. minor). These tests were performed in mesophylic conditions (35°C) on previously chopped and ensiled biomass. The three crops have been cultivated in Southern Italy (Puglia region), both as single-crops and intercropping between triticale or barley and field bean, in different mixture combinations, during the season 2009-2010. Emphasis was placed on the determination of the chemical composition of feedstock from the three single species and their different intercropping ratios, mowed at two subsequent stages (milk and dough development of grasses), also assessing their consequent biogas and methane potential yields after silage. Seven overall treatments have been compared: the three species in monoculture (triticale, barley and field bean, respectively); two mixtures between triticale and field bean (with triticale at 70 and 50%, respectively); two mixtures between barley and field bean (again with barley at 70 and 50%, respectively). Immediately after cutting and for the next 90 days, biomass samples were closed into plastic mini-silos, each having a 5-L capacity, in order to simulate the silage process. Thereafter, the batch testing was performed and biogas and methane production have been determined, with respect to the main chemical characteristics of the chopped and ensiled biomass samples, able to affect biogas and methane yield. Considering the single-crop treatments the highest biogas per hectare production has been found with respect to triticale (8737.1 nm3 ha-1) and barley (8837.6 nm3 ha-1), at the first and second harvesting stage, respectively. Concerning grass-legume intercropping, the highest biogas yield (8635.0 nm3 ha-1) was observed with reference to the 70:30 mixing ratio, specifically on barley mowed at the milk development stage. The methane content in the biogas ranged from 61.1% (on a 50:50 mixing ratio of barley at the milk stage) to 53.7% (on a barley monoculture at the dough stage).