Harvest Timing and N Application Rate Effects on Switchgrass Biomass, Leaf and Stem Chemical Composition and Ethanol Yield.

Biomass chemical composition dynamics can influence the potential dual purpose use of switchgrass. This 2-yr research conducted at the Brown Loam Branch Experiment Station, Raymond, MS on a Loring silt loam soil during the 2011 and 2012 growing seasons quantified the effects of harvest time (HT) and N application rates on primary and aftermath harvests forage nutritive value, feedstock chemical composition, and ethanol yield of switchgrass. Treatments were all factorial combinations of four N application rates (0, 40, 80 and 120 kg ha^-1) and six monthly harvest times (May to October) in a RCBD with treatments allocated in a split-plot arrangement. In the primary harvest, ADF, NDF, lignin, cellulose and hemicellulose concentrations in the total aboveground biomass, stem and leaves increased from the May to October HT in both years while in the aftermath, fiber concentration decreased from May to August HT with stems components accumulating greater fiber concentrations than leaf. Crude protein and in vitro dry matter digestibility decreased with plant maturity and were greatest in the leaf component. Similar to fiber concentration, the concentrations of major sugars increased with maturity and were greatest in the most mature stand in the aftermath. Total ethanol yield was greatest in the least mature stands, however, total ethanol production increased with maturity and is largely a result of biomass yield. Nitrogen rate did not influence the forage nutritive value or feedstock composition in either the primary or aftermath harvests. These results indicate that a two-harvest system (early season and end-of-season) has potential for both forage and feedstock use, however, dual use will depend on the economics of forage vs. feedstock use.