"Feedstock" is a general term for the plant matter used to make fuel. Agricultural wastes, trees, grasses, corn, wood wastes and residues, aquatic plants, animal wastes and municipal wastes are examples of biomass feedstocks.
Agricultural wastes include corn stalks, sugar cane wastes and rice hulls. Natural Resources Defense Council (NRDC) advocates increased government investment in the use of these wastes for making ethanol motor fuel. "Tapping agricultural wastes and other renewable feedstocks to produce fuel has tremendous potential to reduce U.S. oil dependence," according to an NRDC report.
Net environmental benefits are generally thought to be greater when crop wastes are used as a feedstock for fuel, compared to corn and other crops. Since crop wastes are the by-product of existing agricultural activity, use of these wastes does not contribute to pesticide and fertilizer pollution, soil erosion or water use.
Though use of crop wastes to generate energy causes air pollution, these wastes might otherwise be landfilled or subject to open burning. Open burning, in particular, results in worse pollution than when energy is generated in a process using effective pollution controls.
In its biomass guidance document the Sierra Club favors returning crop wastes to the soil ("for soil health, tilth, fertility, and nurturing the organisms populating the below ground ecosystem") rather than using the wastes as fuel. Meanwhile, the document acknowledges that in many cases farmers burn off these wastes, "posing a health threat to nearby residents."
The term "energy crops" is often used to refer to trees and grasses that offer greater environmental and energy benefits compared with food crops like corn. Corn is the crop most often used as a feedstock for ethanol at present.
Some trees -- such as poplar, maple, black locust, willow, sycamore, sweetgum and eucalyptus -- will grow back after being cut off close to the ground, allowing them to be harvested every three to eight years for 20 or 30 years before replanting. Some native grasses can be harvested for up to 10 years before replanting.
Food crops, like corn, must be replanted every year and require much closer management and greater use of fertilizers, pesticides and energy, compared with trees and grasses.
Ethanol can be made from either cellulose or carbohydrates. Trees and grasses are a source of cellulose while corn is used as a source of carbohydrates. According to a paper by the Oak Ridge National Laboratory, "more cellulose can be produced per unit land area than carbohydrates. Therefore, cellulose-based ethanol production is a more efficient use of land."
Farming of corn is a relatively energy-intensive process, but ethanol from corn still yields 34 percent more energy than the total amount required to farm the corn and make the ethanol.
(Source: U.S. Department of Agriculture - PDF)
Other crops have a much higher energy yield: ethanol made from "energy crops" -- grasses and trees -- yields 4-5 times as much energy as needed to farm the crops and make the ethanol.
The energy yield from biomass is highest when energy crops are used to generate electricity in a power plant, with energy yields perhaps 10 times greater than total energy inputs.
In contrast to food crops that pull nutrients from the soil, energy crops actually improve soil quality. Prairie grasses, with their deep roots, build up topsoil, putting nitrogen and other nutrients into the ground. Since they are replanted only every 10 years, there is minimal plowing that causes soil to erode.
Energy crops can create better wildlife habitat than food crops. Since they are native plants, they attract a greater variety of birds and small mammals than modern industrial food-producing farms. They improve the habitat for fish by increasing water quality in nearby streams and ponds. And since they have a wider window of time to be harvested, energy crop harvests can be timed to avoid critical nesting or breeding seasons.
Compared to undisturbed natural habitat, energy crops are not as good for supporting biodiversity. But energy crop farms can be managed so as to be much closer to the natural world than industrial food- producing farms. Nonetheless, the environmental benefits of biomass hinge on whether energy crops are managed with sustainable agricultural practices. Just like food crops, they can be mishandled, with productivity increased by greater chemical inputs.
From the Natural Resources Defense Council website: "Prospects for lowering costs on and expanding ethanol production are limited due to the high level of inputs required to produce agricultural crops (e.g., fertilizer, pesticides, tractor fuel) and the resulting high cost and substantial environmental impact. Biomass energy crops, if grown in bulk, however, could be a profitable alternative for farmers, complementing, not competing with, existing crops and thus providing an additional source of income for the agricultural industry. A substantial amount of agricultural land exists which is marginal for conventional crop production but which can be brought into productive use to grow energy crops (since perennial herbaceous and woody energy crops can be selected which provide advantages such as erosion protection or drought tolerance)."