Predicted harvest time effects on switchgrass moisture content, nutrient concentration, yield, and profitability

Production costs change with harvest date of switchgrass (Panicum virgatum L.) as a result of nutrient recycling and changes in yield of this perennial crop. This study examines the range of cost of production from an early, yield-maximizing harvest date to a late winter harvest date at low moisture and low nutrient concentration using different harvest systems as dictated by the moisture content of the standing crop. Harvest systems with a field-drying interval and multiple harvest passes were compared to a single-pass harvest when moisture content had naturally declined to storage-safe conditions or when artificial drying at the plant would be required. Results showed that the single-pass harvest requiring artificial drying was either i) as costly or more so than declines in yield observed with letting the standing crop dry to 20% moisture in the field; or ii) not economically viable in comparison to multi-pass harvest with a field drying interval at higher yield. Sites where yield losses due to harvest delays were small showed promise for the single-pass harvest at storage-safe moisture, as nutrient replacement costs with greater nutrient recycling and harvest cost savings with a single pass offset yield losses with delayed harvest. Extending the harvest season had different producer cost ramifications amongst environments and led to large changes in nutrient concentrations in harvested biomass. This may be problematic for biorefineries seeking stable nutrient content in feedstock.