Harvest Frequency and Nitrogen Rate Effects On Switchgrass Biomass Production and Nutrient Removal.

Dual-purpose switchgrass (Panicum virgatum L.) use requires frequent harvesting which can influence the nutrient balance of production systems.   This study conducted from 2008 to 2011 at Raymond, MS on a Loring silt loam quantified the yield, agronomic traits, and nutrient removal responses of ‘Alamo’ switchgrass to two N rates (80 and 160 kg N ha^-1) and four harvest frequencies (HF; 1, 2, 3, and 6 harvests annually).  The experiment was a 4 × 2 factorial in a randomized complete block design with three replications.  There was a harvest frequency (HF) effect (P < 0.0001) and an N rate × year interaction (P = 0.0094) on dry matter yield (DMY).   Across HF, DMY in 2008 and 2009 was 17.7 vs. 15.1 Mg ha^-1 at the higher compared to the lower N rate while 2010 and 2011, DMY averaged 17.3 Mg ha^-1.  A single fall harvest (24.0 Mg ha^-1) or two harvests annually (20.5 Mg ha^-1) produced the greatest total DMY.  As HF increased to three (14.5 Mg ha^-1) and six (10.3 Mg ha^-1) harvests, DMY decreased.  There was a HF effect on tissue concentration of all elements, nutrient removal, and agronomic traits (P < 0.05).  Nutrient concentration generally increased with HF while removal rates decreased with HF, driven by reduced DMY.  Plant height decreased with increased HF with subsequent reduction with increasing age of stand. Tiller density remained relatively constant throughout the growing season for the single end of season harvest; however, increasing HF increased tiller density after the first harvest in multiple HF.  The results indicate that less frequent harvest of Alamo switchgrass produced the greatest DMY but harvest management decisions likely will depend upon the potential for minimizing nutrient removal from the system, desired feedstock quality, and the economics of forage versus feedstock use.