Kenneth P. Beamer, North Dakota State University, Fargo, ND, Stuart A. Weiss, University of the Virgin Islands, Kingshill, VI, Amanda J. Ashworth, USDA - United States Department of Agriculture, Fayetteville, AR, Fred L. Allen, Plant Sciences, University of Tennessee - Knoxville, Knoxville, TN and Patrick D Keyser, Center for Native Grasslands Management, Department of Forestry Wildlife and Fisheries, University of Tennessee, University of Tennessee, Knoxville, TN
Switchgrass (Panicum virgatum cv. Alamo) and guinea grass (Panicum maximum cv. Mombaza) have been proposed as sustainable alternatives to fossil fuels although still require non-renewable inputs, notably, inorganic-nitrogen (N). Further, climate change forecasts suggest southeastern USA may emulate more tropical or subtropical growing conditions. Objectives were to determine: i) effects of biochar (1 and 2 Mg ha-1), two intercropped legumes [sunn hemp (Crotalaria juncea cv. Tropic Sun: SH) and pigeon pea (Cajanus cajan cv. Mandarim: PP) intercrops] versus inorganic N [67 kg ha-1and 0 kg ha-1 (control)] on desired feedstock characteristics, yield, and soil characteristics; ii) tissue-nutrient levels over three harvest dates in a two-factor randomized block design; and, iii) switchgrass adaptation to more extreme (tropical) growing conditions. For both Panicum species, yields and feedstock characteristics were influenced by harvest timing (P<0.05), whereas soil amendments influenced these components to a lesser extent (P>0.05). However, results suggest pigeon pea and sunn hemp inter-crops, and biochar may supply analogous-N as synthetic fertilizers (P≤0.05). In general, initial harvests had the highest amounts of digestible 5 and 6 carbon sugars and N, P, and K tissue levels, whereas after more defoliation periods, higher ADF and NDF, suggesting greater digestibility. Switchgrass adaptation was moderate (5-30% weed cover) under the tropical environment, and growth should be sustainable under stochastic climates. Results suggest feedstock maturity stage can be manipulated for desired traits, whereas intercropping and amendments impact tissue composition to a lesser extent.