Wheat is the principal food grain crop produced in the U.S. Small quantities of wheat grain are used currently to produce ethanol and organic chemicals, and these uses could potentially expand in the future. However, at present, the main interest in wheat as a bioenergy and bioproduct resource is the straw. Wheat straw is the above ground, non-grain portion of the wheat plant (stems, leaves, and chaff).
On average, around 50 million acres of wheat are harvested each year, with production concentrated in the upper Midwest, Plains States, and Pacific Northwest. About 2/3 of the wheat produced in the U.S. is winter wheat (wheat planted in the fall and harvested the next year) and about 28% is spring wheat (wheat planted in the spring and harvested the same year).
The quantities of wheat straw 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 wheat straw produced per acre are typically estimated by multiplying the grain yield by a residue-to-grain ratio. Most studies assume a residue-to-grain ratio of 1.7:1.0 for winter wheat and 1.3:1.0 for spring wheat. Most studies also assume a wheat grain weight of 60 lbs/bu. Using these assumptions and the average harvested acres and yields from 2000-2004, 93.1 million dry tons of wheat straw were produced annually.
The quantities of residues that must remain on the field to maintain soil characteristics depend on several factors. These include whether wheat 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 25% of wheat is produced in a continuous cropping system, about 22% of wheat acres are produced in a wheat-fallow rotation (particularly in areas where rainfall is a limiting constraint), and the remaining acres are produced in rotation with another crop such as soybeans, other small grains, and corn. Tillage practices vary substantially by location; nationally, 68% of wheat acres used conventional till operations, 21% used reduced till operations, and 10% used no-till practices in 2000.
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 the soil. Under typical current wheat production conditions (i.e., average wheat acres and yields for 2000-2004, the existing mix of tillage practices, and a continuous wheat rotation), and leaving sufficient quantities of straw on the field to limit soil erosion and maintain soil organic matter, an estimated 4.62 million dry tons of wheat straw could be available for bioenergy and bioproducts (figure 1).
Wheat production is projected to increase modestly over the next 10-20 years, which could result in greater quantities of straw production and availability for bioenergy and bioproducts. Estimated future straw quantities vary widely (from 5.7 million dry tons to 51.8 million dry tons), depending on the time frame examined, assumed acres, wheat grain yields, management practices, and residue quantities that must remain on the field.
Costs of collecting wheat straw are typically based on baling the straw, 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 straw are under development. 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 wheat straw that could be available, after accounting for quantities needed to maintain soil quality, can be collected at costs of less than $40/dt.