|| Thomason, W. E., Raun, W. R., Johnson, G. V., Taliaferro, C. M., Freeman, K. W., Wynn, K. J., Mullen, R. W.
||Switchgrass (Panicum virgatum L.) is currently being evaluated as a raw material for producing fuel, chemicals, and electricity, Switchgrass biomass is bound by the growing environment that includes fertility. More information is needed on sustainable switchgrass production as influenced by nitrogen fertility and harvest management. Field experiments were initiated at Chickasha and Perkins, OK in 1996 and 1998, respectively to evaluate switchgrass response to applied nitrogen (N) at rates of 0, 112, 224, 448, and 896 kg ha(-1). In addition, harvest frequency and time of N application were evaluated. Yield maximums and the greatest N, potassium (K), phosphorus (P), and sulfur (S) uptake values were achieved with 448 kg N ha(-1) applied all in April and harvested three times. In fact, harvest frequency was the most important factor affecting yields over the course of these studies with average dry matter yields of 16.3, 14.7, and 12.9 Mg ha(-1) yr(-1) for three, two, and one harvest yr(-1), respectively. No significant change in soil organic carbon was detected over time. Although dry matter yields were found to decline with time, total N uptake did not. Forage N concentration was found to be greater in later years, thus increasing production costs. While yields were highest (18.0 Mg ha(-1)) with 448 kg N ha(-1) applied all in April and three harvests, applying 0 N and harvesting three times produced almost as much total biomass (16.9 Mg ha(-1)). This limited response to N is possibly explained by the evolution of switchgrass under low N conditions. Increasing forage concentrations of K, magnesium (Mg), P. and S were noted with increasing yields, indicating a potential for response to these nutrients.