Biomass Feasibility Project Final Report - Xcel Energy

Dedicated Energy Crops
Crops raised for the sole purpose of producing energy in the forms of electricity and/or heat.
Over the past several decades, much research has looked into the possibility of using dedicated
crops to produce energy, particularly short-rotation woody crops like hybrid poplar and willow
and herbaceous crops like switchgrass, reed canary grass, and miscanthus. The research led by
David Tilman at the University of Minnesota has brought attention also to the potential
advantages of perennial prairie grasses.
Although BioPET does not include dedicated energy crops, we include them in this written report
because of their great potential. Among the benefits expected from using perennial crops for
energy are high biomass accumulation and low tillage, soil erosion, and herbicide applications.
Since these crops regenerate after harvest, they do not need to be reestablished for a number
of years. But they are not low input crops. With the exception of diverse prairie grasses (see
below), the continual harvest of biomass from perennial biomass plots requires nutrient
replacement at levels similar to annual row crops (Brummer, Burras, Duffy, and Moore, 2002).
Dedicated energy crops may end up costing more than agricultural or forest residues because
they must cover the full costs of land and the establishment and maintenance of the stand. In
contrast, residues like corn stover or logging wastes are byproducts of primary products like corn
grain or roundwood that carry the costs of establishment, maintenance, land rents, and harvest.
Usually, the price of residues is based only on the added costs of handling them. But, on the
other hand, the cost of establishing and maintaining prairie grasses should not be high. Their
economic outcomes will have to await their planting in substantial acreages.
Herbaceous crops. Much of the research in herbaceous cellulosic feedstocks like switchgrass,
reed canary grass, and miscanthus has focused on perennial monocultures. But it is also possible
to seed switchgrass stands with legumes such as alfalfa. This offers the farmer the benefits of
reduced nitrogen applications and the use the first cut from the field as forage and the second
cut as fuel.
Switchgrass. Switchgrass is perhaps the best studied perennial grass in the US – it has even been
mentioned in a State of the Union address. A switchgrass stand successfully established in year
one can be harvested in the fall of year two and for several years thereafter. Switchgrass stands
are expected to last for ten years before reestablishment, but some may remain productive
much longer. Yields vary widely depending on soils and rainfall. They tend to be lower in the dry
high plains states than they are in the moist southeastern states. In Iowa yields range from 1 to
5.25 tons per acre, with an average of roughly 2.5 tons per acre. A detailed study of switchgrass
production costs in southern Iowa estimates a cost of $70/ton with a yield of 4 tons/acre
(Brummer et al., 2002).
Diverse prairie grasses. In recent years ecologists have found increasing evidence that
ecological diversity creates greater biomass yield and stability than a monoculture does. In a
recent publication, ecologists at the University of Minnesota summarized the results of a longterm
study of biomass yields and carbon sequestration on plots of degraded land containing
1,2,4,8 or 16 different perennial grassland species. They found that the most diverse plots
produced 238% more bio-energy than monoculture prairie grasses and claimed to sequester 4.4
MG h-1 year-1 in the soil and roots, compared to 0.14 MG ha-1 year-1 for monoculture crops.
The researchers estimate, from these figures, that energy derived from such diverse plots will be
carbon negative (ie. that the plants will store more carbon in the soil each year than the fossil
Identifying Effective Biomass Strategies: Page 23
Quantifying Minnesota’s Resources and Evaluating Future Opportunities