Agricultural crop residues are the above ground, non-grain portion of agricultural crops (e.g., stalks, leaves, grain chaff, corn husks and cobs). For some crops, most notably corn and soybean, the grain and oilseed are already used to produce bioenergy and bioproducts. However, the residues from most grains (corn, wheat, barley, oat, rye, grain sorghum, and rice) could also be collected and used for bioenergy and bioproducts. Oilseed crops (soybean, sunflower, canola, safflower, and flaxseed) tend to produce fewer residues than grain crops, but limited oilseed residues could also be available. Residues from other miscellaneous crops (such as cotton, grass seed production, orchard and vineyard pruning) could also be sources of biomass.
The quantities of agricultural crop residues that can be available depend on the quantities produced minus the quantities that must remain on the field to maintain soil characteristics (e.g., soil organic matter and soil moisture), control soil erosion and chemical runoff, and ensure long-term productivity of the soil. Sufficient residue quantities must be left to maintain these benefits and required quantities depend on a number of factors, such as the crop produced, whether it is produced in a continuous cropping system or in rotation with other crops, the timing and type of management practices used (particularly tillage operations), the physical characteristics of the soil (soil type and erodibility), field characteristics (slope), and climate.
Corn residues (stover) and wheat straw have been more extensively evaluated than other crop residues. Their estimated available quantities and costs differ substantially by study, depending on assumptions regarding corn and wheat grain yields, acres of production, quantities of residue that must remain to maintain soil quality, fertilizer replacement costs, collection technologies and costs, and time frame of the analysis. For example, estimated corn stover quantities range from 50 million dry tons (2005; $50/dry ton collection and fertilizer replacement costs) to 256 million dry tons (mid-century).
Agricultural crop residue collection costs are generally based on baling in most studies as this method has been demonstrated at a commercial scale, involves off-the-shelf technology, and uses widely available equipment. However, baling can involve multiple passes over the field and can be expensive. Limited access to fields (due to weather) and increased contamination of the residue with dirt are additional concerns associated with baling. New approaches relying on the simultaneous harvest and collection of grain and residues are being investigated.