Estimating Standing Crop and Botanical Composition of Switchgrass Swards Using Indirect Methods.

Switchgrass is being evaluated as a potential biofuel feedstock for the United States.  Efficient and accurate methods to determine switchgrass biomass yields will be important to quantify feedstock levels for optimal cellulosic biofuel refinery production. 
The objective of this study was to determine the effectiveness of visual obstruction and plant height measurements in measuring switchgrass biomass yields.  The study was conducted in eastern Nebraska from 2003 to 2007 under three nitrogen rates (0 kg N ha^-1, 60 kg N ha^-1, and 120 kg N ha^-1), using two cultivars (‘Cave-in-Rock’, and ‘Trailblazer’), and at two harvesting periods (boot stage and post-killing frost) and on switchgrass fields in Nebraska, South Dakota, and North Dakota from 2003 to 2005.  A modified Robel-pole was used to determine visual obstruction, elongated leaf height, and canopy height measurements.  Prediction models from the switchgrass nitrogen study using elongated leaf height, visual obstruction, and canopy height measurements accounted for >91%, >90%, and >82% of the variation in switchgrass biomass yield, respectively.  Regression slopes were not significantly different by cultivars or by harvesting periods suggesting a single model is applicable for determining biomass yield in a region with differing cultivars and harvesting periods assuming similar harvesting methods and growth stages.  There was also a linear relationship between indirect measurements and harvested switchgrass field yields, but there were was considerable more variation than the switchgrass nitrogen study because of different cutting heights, differences between measurement date and harvesting date, and lodging effects. Visual obstruction is recommended on switchgrass fields with low to variable stand densities while elongated leaf height measurements would be recommended on switchgrass fields with high stand densities and with high biomass yields.  Incorporating an ocular device with a Robel pole provided a reasonable functional species group estimate on switchgrass fields.