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In the Mediterranean rainfed systems, perennial warm-season grasses are profitable crops for the production of herbage as forage or feedstock for bioenergy purposes. During summer, when the production of cool-season crops is scarce, warm-season grasses can improve the productivity and stability of forage cropping systems. In Italy, switchgrass (Panicum virgatum L.) can be cultivated for herbage production or as energy crop. The objective of this work was evaluating if relay intercropping with cool-season legumes could be suited to convert a mature stand of switchgrass from energy to dual, energy and forage, production, together with improving the productivity and the quality of the harvestable biomass. All these things considered, a field experiment was carried out in Central Italy, on mature stands of two switchgrass varieties, Alamo and Blackwell, overseeded with two legumes: sulla (Hedysarum coronarium L.) and crimson clover (Trifolium incarnatum L.). The intercropping system was compared with fertilized and un-fertilized pure switchgrass stands. After two years of study, data showed that the intercropping increased the total above ground biomass (AGB) productivity. In the second year, the increase in total AGB production for switchgrass mixtures compared with the pure stands was greater for sulla, a biennial legume, than crimson clover. Introduction Future cropping systems have to meet several goals simultaneously: i) improving productivity of croplands and grasslands in order to produce food for increasing world population (Lutz et al., 2001); ii) reducing global environmental impact; iii) enhancing land-use efficiency; iv) mitigating climate changes through the reduction of greenhouse gases (GHG) emissions; and v) setting up adaptation strategies to reduce the vulnerability of crops by a sustainable intensification (Campbell et al., 2014; Godfray and Garnett, 2014; Pretty and Bharucha, 2014; Wezel et al., 2014). Several authors reported that the introduction of perennial crops, through the conversion from croplands to grasslands, can contribute to increase the sustainability of agricultural production (Glover et al., 2010, 2012). In fact, perennial crops can provide several ecosystem services such as: i) maintaining soil fertility; ii) increasing the soil carbon stock potential compared to annual crops (Glover et al., 2010; Monti, 2012); iii) enhancing soil protection by all-year-round vegetation cover and contrasting soil erosion (Durán Zuazo and Rodríguez Pleguezuelo, 2008; Vallebona et al., 2016); iv) improving biodiversity in farmland and guaranteeing higher resilience of the agro-ecosystem (Peyraud et al., 2014). In the Mediterranean, perennial warm-season grasses are profitable crops for the production of herbage as forage or feedstock for bioenergy utilization (Monti et al., 2012). In addition, in summer dry period when the conventional production, as pasture or fodder, is poor, warm-season grasses can improve the productivity and the stability of Mediterranean forage cropping systems (Gherbin et al., 2007). Among perennial warm-season grasses, switchgrass (Panicum virgatum L.), a prairie species, native of North America and originally domesticated for pasture production, was recognized for its ability of accumulating large amount of biomass even under drought conditions (Monti et al., 2012). Starting from the 90s, owing its high yield potential, switchgrass has been introduced in Europe as promising crop for bioenergy sector (Parrish et al., 2005). In the last decade, several studies highlighted the suitability of both lowland and upland ecotypes to Mediterranean environments (Monti et al., 2008, 2012; Alexopoulou et al., 2015; Nassi o Di Nasso et al., 2015). Furthermore, the literature evidenced that: i) switchgrass productivity decreases after the third year of cultivation (Alexopoulou et al., 2015); ii) multi-harvest systems (more than one cut per year) negatively affect the plantation life span (Monti et al., 2008); iii) switchgrass yields are affected by nitrogen (N) fertilization (Nassi o Di Nasso et al., 2015). Concerning the response of this crop to N fertilization, Ashworth et al. (2015b) indicated that sustainable production levels (evaluated using an life cycle assessment approach) could be achieved with low N fertilization rate (67 kg ha–1), since higher amount of N decreases the efficiency of the Correspondence: Giorgio Ragaglini, Institute of Life Sciences, Sant’Anna School of Advanced Studies, Via Santa Cecilia 3, 56127,

italian journal of agronomy

The overseeding of two cool-season legumes (Hedysarum coronarium L. and Trifolium incarnatum L.) on switchgrass (Panicum virgatum L.) mature stands increased biomass productivity