The main energy crops are short-rotation woody crops, herbaceous woody crops, grasses, starch crops, sugar crops, forage crops, and oilseed crops. Energy crops are fast-growing, genetically improved trees and grasses grown under sustainable conditions for harvest at 1 to 10 years of age.

Agricultural residues, grasses, algae, kelps, lichens, and mosses are also impor­tant biomass feedstocks in the world. Algae can grow practically wherever there is sufficient sunshine. Some algae can grow in saline water. The most significant feature of algal oil is its yield and, hence, its biodiesel yield. According to some estimates, the yield (per acre) of oil from algae is over 200 times the yield from the best-performing plant/vegetable oils (Sheehan et al. 1998). Microalgae are the fastest growing photosynthesizing organisms. They can complete an entire growing cycle every few days. Approximately 46 tons of oil/ha/year can be produced from diatom algae. Different algae species produce different amounts of oil. Some algae produce up to 50% oil by weight.

Commercial energy crops are typically densely planted, high-yielding crop spe­cies where the energy crops are burnt to generate power. Woody crops such as wil­low and poplar are widely utilized, and tropical grasses such as miscanthus and pennisetum purperium (both known as elephant grass) are receiving more attention from emerging energy crop companies.

Genetic research into dedicated energy crops is still at a very early stage. Cur­rent research is focused on mapping gene sequences and identifying key locations where modifying genetic code could provide significant benefits. Modern biotech­nology can also be used for increasing yields and modifying plant characteris­tics to enhance their conversion into fuels. Genetic engineering may result in en­ergy crops that have a higher percentage of cellulose or hemicellulose and lower lignin content for increasing alcohol production yield, as well as a greater ability to take up carbon in their root systems. Crops could also be modified to produce large quantities of the enzymes that are necessary for feedstock conversion into ethanol. Oilseed crops could be bioengineered to become the source of bio-based lubricants and esterified fatty acids, which are the main ingredient in biodiesel (UN 2006).

Genetic modification of fuel-dedicated crops may raise fears linked to perceived threats of agrobiotechnology to plant life and health, to the conservation of bio­diversity, and to the environment at large. The environmental, sustainability, and public-perception aspects of genetically modified energy crop plantations should be carefully evaluated before widespread production starts (UN 2006).