Piptatherum miliaceum: a native perennial grass species for bioenergy

The range of possible plant species for biomass purposes is limited in Mediterranean environments, under rainfed regime, where summer drought imposes the use of tolerant species. Piptatherum miliaceum (L.) Coss. (smilo grass) is a perennial grass of Mediterranean origin, native in Sardinian pastures. It has a good palatability for ruminants in the early stage of growth, whereas the senescent stems and leaves are less palatable but substantial quantities of matter are available at the end of growth season. The introduction of plants for energy production from biomass in Southern Italy may take advantage of the availability of huge amounts of smilo grass biomass at the beginning of summer. Smilo grass can be directly established from seed and the possibility to grow it as dual purpose (forage and bioenergy) crop may be more acceptable to farmers than the cultivation of rhizomatous species, especially giant reed (Arundo donax L.), considered as an invasive plant. In North Sardinia, Italy, a study aimed at evaluating the suitability and potential of smilo grass as plant for bioenergy purposes was started in 2011. Eleven populations of smilo grass, previously collected from Sardinian sites differing for soil type and microclimates, are being evaluated in a nursery field. Conventional forage species (tall fescue (Festuca arundinaceae and cocksfoot (Dactilys glomerata L.)) and species for bioenergy such as cardoon (Cynara cardunculus L.), giant reed, miscanthus (Miscanthus x giganteus), and switchgrass (Panicum virgatum L.) grown close to the nursery field, are used as references. Several traits are being investigated: plant phenology and harvesting habit (attitude to lodging), fresh and dry matter yield, biomass partitioning, and its qualitative characteristics (hemicellulose, cellulose, lignin content, ultimate and proximate analyses, ash composition, and calorific value). The dry matter yield of P. miliaceum ranged from 1200 to 1900 g plant-1, while the moisture content of its biomass ranged from 38 to 46%. The aboveground biomass was allocated mainly in tillers (55-62%), then in leaves (17-24%) and panicles (17-27%). On average, the yield per plant of smilo grass was twice higher than that of tall fescue, the most productive forage species. Moreover, the performances of smilo grass were also interesting in comparison with cardoon and miscanthus. Ash mean content was 5.2 to 7.8% in tillers, 6.9 to 9.4% in panicles, and 16 to 19.7% in leaves. Hemicellulose (24-27% in leaves, 26-30% in tillers and 32-36% in panicles), cellulose (26-33% in panicles, 36-41% in tillers, and 37-41% in leaves ), lignin (8-12% in leaves, 10-15% in panicles, 12-15% in tillers). Calorific value ranged from 16 to 17 MJ kg-1 in leaves and tillers, respectively. The ultimate analysis revealed that C and H contents were on average 49 and 6.2%, respectively, whereas N content ranged from 1.4 to 3.6 % in tillers and leaves, respectively. Our preliminary results point out that smilo grass is able to produce high amounts of DM showing in meantime a wide range of variability that allows the selection of promising genotypes for bioenergy well suited to rainfed Mediterranean conditions.