Sustainable Intercropping of Switchgrass and Hybrid Poplar for Bioenergy Production.

Switchgrass (Panicum virgatum L.) is a perennial warm-season grass identified as a model species for bioenergy feedstock. Limited land resource is a problem for biomass production; therefore research efforts are necessary to improve land and water use efficiency of switchgrass production system. The Boardman Tree Farm (BTF), located in Boardman, OR, is the largest hybrid poplar production in North America. While growing hybrid poplar trees at BTF, open ground may be utilized for herbaceous crop production. A three-year field study was conducted to evaluate the influence of hybrid poplar trees on switchgrass architecture, biomass production, and forage quality. Three cultivars of switchgrass (Kanlow, Blackwell, and Trailblazer) were planted in alleys of hybrid poplar trees (Clone: OP367 or PC4) in 2011, and monoculture plots were seeded to switchgrass and poplars. Switchgrass architecture was quantified by tiller samples and leaf area index (LAI), while biomass was harvested twice a year (July and October), dried, and ground to evaluate forage quality by Near infrared spectroscopy (NIRS). Land Equivalent Ratio (LER) was estimated to determine land use efficiency. Intercropping of switchgrass with hybrid poplar in the second production year has shown declining yield of switchgrass from 10 to 7 Mg DM ha^-1 compared to the increasing trend for monoculture grass from 15 to 17 Mg DM ha^-1 from 2012 to 2013. Tree yield increased in the intercropping plot (15 Mg DM ha^-1) compared to tree only plots (13 Mg DM ha^-1), due probably to N fertilizer added to the intercropping plots and/or synergistic effect from intercropping with switchgrass. Despite the yield reduction of switchgrass, LER remains higher (> 1.5) in intercropping plots than monoculture plots, indicating that the intercropping system provided higher total biomass per unit area than monoculture plots. Forage quality, switchgrass architecture, and 2014 data will be included in the presentation.