Biorefineries operating continuously throughout the year will require a steady flow of feedstock. Switchgrass (Panicum virgatum L.) has been proposed as a dedicated energy crop. Harvest constitutes a major cost component of feedstock delivered cost. The number of harvest machines required to support a biorefinery can be reduced if the length of the harvest window is increased. Extending the harvest window to take advantage of reduction in harvest machinery investment costs has important biological consequences. Both harvestable biomass yield and fertilizer requirements may differ depending on time of harvest.

The objective of the research was to determine switchgrass yield response to N fertilizer for a single annual harvest in July and in October. Switchgrass was established in 2002 on a Kirkland silt loam soil near Stillwater, OK. Treatments in post-establishment years consisted of four N fertilization levels (34, 67, 134, and 269 kg ha^-1 yr^-1) and two harvest systems (once yr^-1 in October and twice yr^-1, July and October). Biomass yield from only the July harvest was used to estimate the July function.

The July harvest plateau yield of 9.77 Mg ha^-1 was achieved with an estimated annual N application of 90 kg ha^-1. The October harvest plateau yield of 12.31 Mg ha^-1 was achieved with an estimated annual N application of 71 kg ha^-1. Extending the harvest window by beginning harvest in July resulted in lower average harvestable yield and greater N requirement ha^-1. Additional research will be required to determine if using the July harvest from the plots harvested twice yr^-1 provides a reasonable proxy for a single annual harvest in July and to determine if the savings in harvest and storage costs would be sufficient to offset the added N and reduced average yield of an extended harvest season.