Management of switchgrass for bioenergy and forage share some commonalities, of particular interest in bioenergy crop production is: (1) rapid establishment of switchgrass to generate harvestable biomass in the seeding year, (2) highly efficient management of soil and fertilizer N to minimize external energy inputs, and (3) harvest management to maximize yields of lignocellulose. Bioenergy cropping may entail management for multiple services in addition to biomass yield including soil C sequestration, wildlife habitat, landscape management, and water quality protection. Management is a critical factor especially as land classified as marginal or idle land will be emphasized for bioenergy

Biological N2 fixation (BNF) via legume intercrops can provide an alternative to inorganic N fertilizer and thereby minimize the economic and environmental costs of bioenergy feedstock and forage production. The objectives of this study were to (i) verify switchgrass (Panicum virgatum L.) as a non‐N2–fixing reference plant for distinguishing between soil‐ and atmosphere‐derived N; (ii) determine BNF levels via the 15N enrichment method for one cool‐season legume (red clover [Trifolium pratense L.]) and three warm‐season legumes ([partridge pea [Chamaecrista fasciculate Michx.], sunn hemp [Crotalaria juncea L.], and pigeonpea [Cajanus cajan (L.) Millsp.]) when intercropped into switchgrass and a near relative, guineagrass (Panicum maximum L.); and (iii) evaluate the validity of the N‐difference method by comparing it against the 15N enrichment technique in humid temperate and semiarid tropical climates. The results revealed little difference in the N assimilation rates of legume and non‐N2–fixing reference plants, suggesting that switchgrass is an appropriate reference species. Annual fixation for red clover was greatest, followed by partridge pea, and lastly sunn hemp in temperate systems (87, 84, and 35 kg ha−1, respectively), all of which tended to supply greater amounts of N in subsequent seasons. Considerably greater BNF occurred in tropical intercrop systems (exceeding 240 kg ha−1). Consequently, the BNF of selected legumes approximates or exceeds recommended N fertilizer rates (67 kg N ha−1) in both humid temperate and semiarid tropical pasture and feedstock systems. The N‐difference method did not provide estimates for BNF that were comparable to 15N‐enrichment‐derived values (P > 0.05).

Second-generation feedstocks such as switchgrass (Panicum virgatum L.) have been proposed as sustainable alternatives to fossil fuels, although they still require nonrenewable inputs, notably, inorganic N. Th erefore, our objectives were to determine (i) the eff ects of biochar (1 and 2 Mg ha -1 ), three intercropped legumes [red clover (Trifolium pratense L.), partridgepea (Chamaecrista fasciculata [Michx.] Greene), and sunn hemp (Crotalaria juncea L.)] vs. inorganic N [67 kg ha -1 and 0 kg ha -1 (control)] on desired feedstock characteristics, yield, and soil characteristics; and (ii) feedstock alterations and tissue-nutrient levels for postsenescence (November) and overwintering (February) harvests in a twofactor randomized block design. Overwintering harvests increased P and K remobilization, ethanol yield, fermentable sugars, and in-fi eld dry-down (P £ 0.05), although yield losses occurred (22%). November harvests had greater tissue N and fermentable substrates, leading to greater soil nutrient removals. Consequently, harvests manipulated the desired feedstock traits, whereas soil amendments had little eff ect on feedstock characteristics. Th erefore, the results suggest that legume intercrops (partridgepea) and biochar may supply analogous N to synthetic fertilizers (P £ 0.05), thereby displacing inorganic N without altering feedstock quality. However, for inorganic N alternatives to be competitive on a break-even cost basis, greater biomass yields need to be obtained under these management practices.

Across much of the eastern United States, tall fescue [TF; Schedonorus arundinaceus (Schreb.) Dumort.], a cool-season (i.e., C 3 ) perennial grass, is the primary forage for pasture systems, thereby leaving producers vulnerable to reduced summer forage production and drought. Warm-season (i.e., C 4 ) forages can complement existing production systems by supplementing summer forage production and drought resiliency. Therefore, our objective was to compare five, C 4 forage options in a grazing trial: switchgrass (SW; Panicum virgatum L.), eastern gamagrass (EG; Tripsacum dactyloides L.), a big bluestem (Andropogon gerardii Vitman) and indiangrass (Sorghastrum nutans L. Nash) blend (BBI), bermudagrass (BG; Cynodon dactylon L. Pers), and crabgrass (CG; Digitaria sanguinalis L. Scop.). Research was conducted 2014-2016 at two locations in Tennessee. Weaned beef heifers (237-242 kg initial weight) grazed 1.2-ha pastures with three replications per species and location. Average daily gains (kg d −1 ) (0.62 [BBI], 0.41 [BG], 0.44 [CG], 0.42 [EG], 0.51 [SW]), grazing days (d ha −1 ) (412 [BBI], 459 [BG], 455 [CG], 664 [EG], 617 [SW]), and total gain (kg ha −1 ) (259 [BBI], 186 [BG], 200 [CG], 276 [EG], 315 [SW]) all varied among forages (P < .001). Similarly, forage nutritive values differed (P < .001)among forages: season-long crude protein ranged from 94 (BG) to 115 (CG and EG) g kg −1 , neutral detergent fiber (NDF), 601 (CG)-680 (SW) g kg −1 , and acid detergent fiber (ADF) 379 (BG)-417 (EG) g kg −1 . These forage options should be evaluated in the context of TF pastures to establish a broader understanding of their contribution within an overall forage system.

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