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Abstract
Switchgrass (Panicum virgatum L.) was managed as a bioenergy crop in field trials on marginal cropland on 10 farms in Nebraska, South Dakota, and North Dakota. The fields were established in 2000 and 2001 with cultivars 'Trailblazer', 'Cave-in-Rock', 'Shawnee', and 'Sunburst'. Agricultural inputs, switchgrass stand frequency, weed stand frequency, quadrat yields, baled yields, indirect yield and plant compositions methods were recorded annually for each field. Annual baled yields from established fields averaged 5.2 -11.1 Mg·ha-1 with a resulting average estimated net energy yield of 60 GJ·ha-1·y -1. Switchgrass produced 540% more renewable than nonrenewable energy consumed. Estimated average greenhouse gas (GHG) emissions from cellulosic ethanol derived from switchgrass were 94% lower than GHG emissions from gasoline. Temporal stability for switchgrass stands increased on quadrat sites with higher stand frequencies and temporal stability decreased on quadrat sites with lower stand frequencies within the majority of fields. Temporal stability for yield was lower in areas with higher yields than the field means while temporal stability increased in areas that had lower than average field means. Topographic effects on switchgrass stand frequency and yield were largely insignificant.
A secondary study was conducted in eastern Nebraska from 2003 to 2007 at three nitrogen rates (0 kg N ha-1, 60 kg N ha-1, and 120 kg N ha-1), using cultivars Cave-in-Rock and Trailblazer and at two harvesting periods. A modified Robel-pole was used to determine visual obstruction, elongated leaf height, and canopy height measurements. Prediction models from the secondary study using elongated leaf height, visual obstruction, and canopy height measurements accounted for >91%, >90%, and >82% of the variation in switchgrass biomass, respectively. There was a linear relationship between indirect measurements and harvested switchgrass field yields, but more variation occurred at the field-scale sites. Incorporating an ocular device with a Robel pole provided a reasonable estimate of the frequency of grassy weeds, broadleaf weeds, and switchgrass.
