Harvesting biomass for bioenergy removes large quantities of nutrients from the system (Vogel et al. 2002; Fike et al. 2006a, b; Mitchell et al. 2008; Guretzky et al. 2011). Typically, N is the most limiting nutrient for switchgrass production and is the most expensive annual input. Consequently, reducing N removal from the switchgrass systems has a positive effect on the economic and environmental sustainability. In switchgrass production systems, N removal is a function of biomass yield and N concentration, with biomass N concentration increasing as N fertilization rates increase (Vogel et al. 2002). In general, harvesting 10 Mg ha-1 of switchgrass DM with whole-plant N concentration of 1% will remove 100 kg of N ha-1, whereas if harvest is delayed until after senescence, N concentration can decline to 0.6% or less, resulting in the removal of only about 60 kg of N ha-1 (Mitchell and Schmer 2012). From a producer’s perspective, this 40 kg of N ha-1 reduction in N removal may be an acceptable trade-off for the yield losses associated with delaying harvest.

In a multi-environment study evaluating numerous N rates and harvest dates, biomass yield was optimized when switchgrass was harvested at the boot to post-anthesis stage and fertilized with 120 kg N ha-1 (Vogel et al. 2002). At this harvest date and fertility level, the amount of N removed at harvest was similar to the amount of N applied. Soil NO3-N did not increase throughout the study (Vogel et al. 2002), suggesting N losses from the system (and consequent environmental impact of such management) would be minimal.

Information on total nutrient removal in switchgrass production systems is lacking. Collins et al. (2008) reported irrigated switchgrass in the Pacific Northwest yielded from 14.5 to 20.4 Mg dry matter ha-1 y-1 and each kg of N produced 83 kg of biomass. Macronutrient export averaged 214 kg N ha-1, 40 kg P ha-1, 350 kg K ha-1, 15 kg S ha-1, 60 kg Ca ha-1, 38 kg Mg ha-1, and 6 kg Fe ha-1. Averaged across cultivars, switchgrass removed less than 1 kg ha-1 of B, Mn, Cu, and Zn. In the eastern USA, delaying harvest until spring reduced ash content and leached nutrients from the vegetation (Adler et al. 2006). Although management of all nutrients in the system is important, N is the most expensive, has the greatest potential for environmental contamination, and has the greatest influence on life cycle assessment (Mitchell and Schmer 2012). Consequently, given the interaction of N rate and harvest date, it is important to only replace the N needed for the production system to prevent over-fertilization and soil N accumulation.