Effect of harvest timing on the quality of switchgrass for biofuel: Changes in lignocellulose, carbon and potential energy production

Bioenergy is important in the United States to improve energy security and environmental integrity. There is a strong need to increase production of biofuels to cope with rising energy costs and the risk of climate change related to fossil fuel combustion. In 1991, use of switchgrass for bioenergy was recommended by the U.S. DOE. Switchgrass is a promising feedstock due to its high productivity, low input requirements and positive environmental effects. To produce ethanol from switchgrass, high levels of cellulose and low levels of lignin are ideal for biochemical conversion while higher lignin and energy content are desirable for thermochemical conversion. These values change, however, during the growth of the plant. Determination of the optimum harvest date is therefore important to provide the best feedstock. The objective of this study was to identify this optimum window of harvest dates by harvesting different switchgrass plants at multiple times from a one-cut system during the 2011 and 2012 growing seasons. A total of 13 and 8 harvests of lowland switchgrass were taken at different dates in a one-cut system in 2011 and 2012, respectively. Samples were analyzed for lignocellulosic content, total carbon and energy that would affect the overall production and efficiency of ethanol from switchgrass. Generally, cellulose and hemicellulose concentrations increased through July and then remained relatively constant while the lignin concentrations continued to increase during the entire growth period studied (June–November). Carbon concentration decreased significantly followed by a significant increase in the mid/late growing season of 2011 and 2012. A period of drought in the beginning of 2012 affected cellulose, hemicellulose and carbon concentrations most likely causing these changes to be delayed by approximately one month. Energy content for switchgrass increased significantly, remained relatively similar from August and was unaffected by drought. It was concluded that switchgrass could be harvested earlier than the current recommended time, which is first killing frost, based on the requirements of conversion method selected. However, harvesting might be delayed in case of drought based on its severity