[5][6][7] Whole crops such as maize, Sudan grass, millet, white sweet clover, and many others can be made into silage and then converted into biogas.
[3] Anaerobic digesters or biogas plants can be directly supplemented with energy crops once they have been ensiled into silage.
The fastest-growing sector of German biofarming has been in the area of "Renewable Energy Crops" on nearly 500,000 ha (1,200,000 acres) of land (2006).
It is estimated that the energy yield presently of bioenergy crops converted via silage to methane is about 2 GWh/km2 (1.8×1010 BTU/sq mi) annually.
In Europe and especially Germany, however, this rapid growth has occurred only with substantial government support, as in the German bonus system for renewable energy.
There has been a preoccupation with cellulosic bioethanol in America as the agricultural structure supporting biomethane is absent in many regions, with no credits or bonus system in place.
[citation needed] Consequently, a lot of private money and investor hopes are being pinned on marketable and patentable innovations in enzyme hydrolysis and similar processes.
Bioethanol also refers to the technology of using principally corn (maize seed) to make ethanol directly through fermentation.
There are many benefits associated with the use of biofuels such as reduced greenhouse gas emissions, lower cost than fossil fuels, renewability, etc.
Plants best suited for future bioenergy feedstocks should be fast growing, high yielding, and require very little energy inputs for growth and harvest etc.
It represents a cheaper alternative to fossil fuels while being extremely diverse in the species of plants that can be used for energy production.
But issues regarding cost (more expensive than other renewable energy sources), efficiency and space required to maintain production need to be considered and improved upon to allow for the use of biofuels to be commonly adopted.
In addition to absorbing CO2 in its above-ground tissue, biomass crops also sequester carbon below ground, in roots and soil.
Soil organic carbon has been observed to be greater below switchgrass crops than under cultivated cropland, especially at depths below 30 cm (12 in).