Perennial grasses
Perennial grasses are widely used as fodder crops and have in former times significantly contributed to the energy supply on farms. Since the mid-1980s there has been increasing interest in the use of perennial grasses as energy crops through a number of modern energy conversion routes.

The characteristics which make perennial rhizomatous grasses (PRG) attractive for biomass production are their high yield potentials, the high lignin and cellulose contents of their biomass and their generally anticipated positive environmental impact. Because the need for soil tillage in perennial grasses is limited to the year in which the crops are established the risk of soil erosion is significantly lower than in annual crops and soil carbon contents increase. The rhizome system of perennial rhizomatous grasses allows them to recycle and store nutrients. This results in very efficient use of nutrients and low demand for fertilizers. Since few natural pests occur they may also be produced with little or no pesticide use. Studies on flora and fauna showed that perennial rhizomatous grasses increase the abundance and activity of different species, especially birds, mammals and insects. Perennial rhizomatous grasses can therefore contribute to the ecological value of agriculture and function as landscape elements.

The table below gives an overview on perennial grasses tested as energy crops in Europe and the reported yields.

Source: Lewandowski et al., 2002

The choice of the appropriate location is the most important factor driving the biomass yields of the grasses. Miscanthus (Miscanthus spp.), switchgrass (Panicum virgatum), reed canarygrass (Phalaris arundinacea) and giant reed (Arundo donax) are particularly interesting for the following reasons:

•     their high biomass yield potential
•     the concentration of the yield in one harvest, and delayed harvest is possible
•     their persistence and yield stability
•     their efficient use of resources and low input demand
•     the benefits of their rhizome systems.

Many of the tested C3 grasses shown in the above have a high potential, but high yields are only obtained with multiple cutting systems and high nitrogen input. A delayed harvest of these grasses is not possible due to lodging. The four grasses mentioned above are characterized by concentrating the yield in one harvest. Furthermore a late harvest, i.e. after winter in early spring, can be performed. A late harvest is the most important mean to optimize the combustion quality of biomass from these grasses because over winter the biomass can dry out to water contents of 20% and a significant reduction of combustion relevant components like chloride, potassium, nitrogen and others occurs.

Switchgrass is native to North America where it occurs naturally from 55°N latitude to central Mexico. It is a tall C4 grass. It does well on a wide range of soil types and is drought tolerant.

Switchgrass yields by region

 
The C4 grass miscanthus has its origin in East Asia. Most trials in Europe have been performed with the genotype Miscanthus x giganteus. Due to limited frost hardiness of the rhizomes M. x giganteus can only be grown in areas where soil temperatures do not drop below –3.5°C. New, more winter hard miscanthus genotypes were bred which however have a lower biomass production potential than M. x giganteus.

Miscanthus yields by region

One of the main barriers for the production of perennial rhizomatous grasses for bioenergy is the high biomass production costs. These can in future be reduced by:

•     The development of more cost effective and safe establishment methods
•     Mechanisation of establishment and harvest of PRG
•     Breeding of varieties for biomass production and adapted to all areas of Europe, especially dry areas
•     Further development of the crop management system for PRG
•     Biomass quality management
•     Quantification of ecological benefits, integration into multiple land use systems.

Energy crops for liquid biofuels production

Rapeseed, sunflower or soybean are used for biodiesel production, and sugar cane, wheat or sweet sorghum for bioethanol production.

Sweet sorghum is a particularly interesting and promising crop. Its main characteristics are:

•     High productivity (25 to 45 dry ton biomass/ha) producing significant amounts of starch (grains), sugar and lignocellulosics material
•     Annual, C4 plant with a high photosynthetic efficiency
•     Suitable to different climatic zones (from tropical to temperate, as long as accumulated temperature reaches 2,600-4,500°C) and soil conditions (pH 5.0-8.5)
•     Drought resistance and low irrigation requirements (1/3 of the needs of sugar cane)
•     Low fertilizer and pesticides requirements
•     Large number of cultivars available (>400): conventional genetic improvement could make possible an yearly 1% yield increase.

Sweet sorghum field and panicle

Average productivity of sweet sorghum

Sweet sorghum vs. sugar cane for energy content (toe/1000 t cane)

For more information about the interesting potential of sweet sorghum as a versatile crop, please read the Sweet sorghum leaflet prepared under the LAMNET Project.
For more information about energy crops

Links:
EECI Network http://www.eeci.net
NewCROPTM http://www.hort.purdue.edu/newcrop/default.html