Switchgrass (Panicum virgatum L.) may have value as forage and a bioenergy feedstock. Our objective was to evaluate how harvest system and N fertilizer rates affected biomass yield and nutrient composition of young stands of switchgrass (cv. Alamo) in the southern Great Plains, USA. Nitrogen fertilization increased biomass yields from 10.4, 10.8, and 12.2 Mg ha −1 at 0 kg Nha −1 to 13.7, 14.6, and 21.0 Mg ha −1 at 225 kg Nha −1 when harvested after seed set (October), after frost (December), and twice per year after boot stage (July) and frost, respectively. Nutrient concentrations and removal were generally twice as great when biomass was harvested twice versus once per year. Precipitation strongly affected biomass yields across the two years of these experiments. When late-summer precipitation is available to support regrowth in this environment, harvesting switchgrass twice per year will result in greater biomass yields. Harvesting twice per year, however, will increase fertilization requirements and reduce feedstock biomass quality. Switchgrass harvested during mid-summer after boot stage was of poor forage quality. To have value as a dual-purpose forage and bioenergy feedstock, switchgrass would need to be utilized during spring to early summer while in a vegetative stage.

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Increasing desire for renewable energy sources has increased research on biomass energy crops in marginal areas with low potential for food and fiber crop production. In this study, experiments were established on low phosphorus (P) soils in southern Oklahoma, USA to determine switchgrass biomass yield, nutrient concentrations, and nutrient removal responses to P and nitrogen (N) fertilizer application. Four P rates (0, 15, 30, and 45 kg Pha −1 ) and two N fertilizer rates (0 and 135 kg Nha −1 ) were evaluated at two locations (Ardmore and Waurika) for 3 years. While P fertilization had no effect on yield at Ardmore, application of 45 kg Pha −1 increased yield at Waurika by 17% from 10.5 to 12.3 Mg ha −1 . Across P fertilizer rates, N fertilizer application increased yields every year at both locations. In Ardmore, non-N-fertilized switchgrass produced 3.9, 6.7, and 8.8 Mg ha −1 , and N-fertilized produced 6.6, 15.7, and 16.6 Mg ha −1 in 2008, 2009, and 2010, respectively. At Waurika, corresponding yields were 7.9, 8.4, and 12.2 Mg ha −1 and 10.0, 12.1, and 15.9 Mg ha −1 . Applying 45 kg Pha −1 increased biomass N, and P concentration and N, P, potassium, and magnesium removal at both locations. Increased removal of nutrients with N fertilization was due to both increased biomass and biomass nutrient concentrations. In soils of generally low fertility and low plant available P, application of P fertilizer at 45 kg Pha −1 was beneficial for increasing biomass yields. Addition of N fertilizer improves stand establishment and biomass production on low P sites.

Biomass demand for energy will lead to utilization of marginal, low fertility soil. Application of fertilizer to such soil may increase switchgrass (Panicum virgatum L.) biomass production. In this three-way factorial field experiment, biomass yield response to potassium (K) fertilizer (0 and 68 kgK ha −1) on nitrogen (N)-sufficient and N-deficient switchgrass (0 and 135 kgNha −1) was evaluated under two harvest systems. Harvest system included harvesting once per year after frost (December) and twice per year in summer (July) at boot stage and subsequent regrowth after frost. Under the one-cut system, there was no response to N or K only (13.4 Mgha −1) compared to no fertilizer (12.4 Mgha −1). Switchgrass receiving both N and K (14.6 Mgha −1) produced 18 % greater dry matter (DM) yield compared to no fertilizer check. Under the two-cut harvest system, N only (16.0 Mgha −1) or K only (14.1 Mgha −1) fertilizer produced similar DM to no fertilizer (15.1 Mgha −1). Switchgrass receiving both N and K in the two-cut system (19.2 Mgha −1) produced the greatest (P< 0.05) DM yield, which was 32 % greater than switchgrass receiving both N and K in the one-cut system. Nutrient removal (biomass×nutrient concentration) was greatest in plots receiving both N and K, and the two-cut system had greater nutrient removal than the one-cut system. Based on these results, harvesting only once during winter months reduces nutrient removal in harvested biomass and requires less inorganic fertilizer for sustained yields from year to year compared to two-cut system.

Sweet sorghum (Sorghum bicolor [L.] Moench) accumulates fermentable sugars in the stem and is increasingly being studied as a potential source of feedstock for bioethanol production. The objective of this study was to evaluate biomass and grain yield in five sweet sorghum cultivars (Dale, M81E, Sugar Drip, Della and Keller) and to determine quality of extractable juice and grain. Randomized complete block experiments were performed in the summer of 2009, 2010, and 2011. Leaf dry weight varied with year and cultivar and averaged 6177 kg·ha −1 . Fresh stem weight ranged from 21 to 54 Mg·ha −1 with a mean across years and cultivars of 32.9 Mg·ha −1 . Variations in stem weight were correlated with extractable juice volumes that ranged from 10 to 24 m 3 ·ha −1 . Juice Brix values fell within a narrow range (14% -19%) across years and cultivars with an average of 15.6%. In all production years, theoretical sugar and ethanol yield were always numerically higher for Keller and M81E. Grain yield was lowest in Keller (90 kg·ha −1 ), but ranged from 400 to 1300 kg·ha −1 in other cultivars with a mean of 584 kg·ha −1 across years. However, Keller had the highest starch content with a lower proportion of resistant starch in the grain. Except for Keller, the cultivars tested are potential sources of both fermentable sugars and grain.