Changes in Nutrient Characteristics of Switchgrass for Bioenergy

Switchgrass (Panicum virgatum L.) was identified as a "model" bioenergy crop by the USDOE in 1991. Since then, there has been little research to understand changes in switchgrass quality during the growing season or the effect that drought may have on these quality characteristics. This study determined changes in total N, P, K, S, Ca, Mg, moisture concentration, and dry weight yield through intensive sampling during the growing season. Subsamples from a one-cut system were harvested on 13 different dates in 2011 and eight different dates in 2012 between late May and early November. Low rainfall in late spring and early summer 2012 created drought conditions that led to some differences between years. In general, the measured variables decreased with time, but dry weight yield increased in both years and peaked around late August. In 2011, a significant rise in N between late July and mid-August may have been caused by reproductive growth and/or organic matter mineralization, although this has not been documented elsewhere. Differences between normal (2011) and drought (2012) conditions included lower biomass production, reduced N concentrations, increased S and P concentrations in late August, and increased plant moisture in late August and October under drought conditions. Years with drought conditions may allow more staggered harvesting because nutrient ranges are smaller. This will allow a more continuous supply of feedstock to the biorefinery. These data and future economic analyses will allow better decision-making opportunities for producers.