Corn stover is the above ground, non-grain portion of the corn plant (stalks, leaves, corn husks, and cobs). While corn grain is already used to produce ethanol and organic chemicals, the corn stover can also be collected and used for bioenergy and bioproducts. On average, around 71 million acres of corn are harvested each year in the U.S. and about 80% of production occurs in the Midwest.
The quantities of corn stover available depend on the quantities produced, minus the quantities that must remain on the field. Crop residues play a vital role in maintaining soil characteristics (e.g., soil organic matter and soil moisture), controlling erosion and chemical runoff, and ensuring the long-term productivity of the soil. Sufficient residue quantities must be left to maintain these functions.
The quantities of corn stover produced per acre are typically estimated by multiplying the grain yield by a stover-to-corn grain ratio, which is generally assumed to be 1:1. Most studies also assume a corn grain weight of 56 lbs/bu. Using these assumptions and the average harvested acres and yields from 2000-2004, 280.8 million dry tons of corn stover were produced annually.
The quantities of stover that must remain on the field to maintain soil characteristics depend on several factors. These include whether corn 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. Currently, about 80% of corn is produced in rotation with another crop, typically soybeans (75%), although other crops are also used. Tillage practices vary substantially by location; nationally, 62% of corn acres used conventional till operations, 18% used reduced till operations, and 20% employed no-till methods in 2004.
Residue cover plays a key role in limiting water and wind soil erosion. Crop residue decay returns carbon (organic matter) to the soil. Soil organic matter is crucial to the long term productivity of soil. Under existing corn production conditions (i.e., average corn acres and yields for 2000-2004, the existing mix of tillage practices, and a corn-soybean crop rotation), and leaving sufficient quantities of stover on the field to limit soil erosion and maintain soil organic matter, an estimated 57.6 million dry tons of corn stover could be available for bioenergy and bioproducts (figure 1).
Corn production is projected to increase over the next 10-20 years, which can result in greater quantities of stover production and availability for bioenergy and bioproducts. Estimated future stover quantities vary widely (from 90 million dry tons to 256.1 million dry tons), depending on the time frame examined, assumed acres, corn grain yields, management practices, and residue quantities that must remain on the field.
Costs of collecting corn stover are typically based on baling the stover, since baling readily fits into existing farm practices and can be used currently without major modifications. New approaches that simultaneously harvest and collect the grain and stover are under development, but not currently available. Estimated collection costs vary widely, depending on the year of the analysis, the assumed efficiency and type of equipment used, and the types of costs included in the analysis. However, most studies estimate that the majority of the corn stover that could be available, after accounting for quantities needed to maintain soil quality, can be collected at costs of less than $40/dt.