Irrigated Switchgrass Genotype Reponses to Nitrogen for Cellulosic Feedstock in the Pacific Northwest.

Irrigated Switchgrass Genotype Reponses to Nitrogen for Cellulosic Feedstock in the Pacific Northwest. S.C. Fransen*, H.P Collins and R.A. Boydston. Washington State University and USDA-ARS, Prosser, WA.

A five year switchgrass (Panicum virgatum) field study was conducted on an Quincy sand near the Columbia River in the Pacific Northwest. Two upland, Cave-In-Rock and Shawnee, and one lowland, Kanlow switchgrasses were planted and no fertilizer applied in May2004 and harvested for biomass that fall. From 2005 through 2009 grasses were harvested in July and October for biomass with two nitrogen rates (112 and 224 kg N ha^-1) were split applied in the spring and after first biomass harvest. Phosphorus, Potassium and Sulfur were applied yearly according to soil test results. Grasses were irrigated under solid set with approximately 67 cm of water applied during the growing season. Establishment year biomass averaged 3.1 Mg DM ha^-1. Biomass yields ranged from 15.9 to 24.5 Mg DM ha^-1 over the next five years. Cave-In-Rock averaged 11.2 and 6.1 Mg DM ha^-1, Shawnee averaged 11.5 and 7.9 Mg DM ha^-1, and Kanlow 12.5 and 9.7 Mg DM ha^-1 for first and second biomass harvests over the five years, resp. Response to N was not significant for first biomass harvest  but the 224 kg N rate produced more biomass for the second harvest. Switchgrass can be grown for high cellulosic biomass yields under irrigation in the West. Yields increase with time then appear to plateau and stabilize. There is opportunity to increase biomass yields further through genotype selection and N management for the second harvest in the fall.