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bioweb.sungrant.org » Technical » Biomass Resources » Agricultural Resources » Crop Residues » Wheat Straw

Wheat Straw	Printer Friendly

Wheat is the principal food grain crop produced in the U.S. On average, around 50 million acres of wheat are harvested each year (range of 45.8 to 53.1 million acres between 2000 and 2005). Production is concentrated in the upper Midwest, Plains States, and Pacific Northwest, but some production occurs in most states. 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 remaining wheat acres are planted to specialty varieties, such as Durum wheat. Average national yields of winter wheat are 43 bu/ac and have ranged between 38.2 and 46.7 bu/ac over the last seven years. National average spring wheat yields have ranged between 29.1 and 43.2 bu/ac between 2000 and 2006 (USDA-NASS). Wheat yield variation by geographic region can be substantially higher than annual average national variations.   Small quantities of wheat grain are used currently to produce ethanol and organic chemicals, and these uses could potentially expand in the future (Sparks, 2002). However, at present, the principal 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).   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.   In the absence of data on residue yields, the quantities of wheat straw produced per acre are estimated by multiplying the wheat grain yield by a residue-to-wheat grain ratio (i.e., harvest index). Most studies assume a residue-to-grain ratio of 1.7:1.0 for winter wheat and 1.3:1.0 for spring wheat (Brown, 2003; Heid, 1984; Larson, 1997a, 1997b). Additionally most, but not all, studies assume a wheat grain weight of 60 lbs/bu.   The quantities of residues that must remain on the field to maintain soil characteristics depend on a number of factors including 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.    Generally, for crops that produce substantial quantities of residues, more residues can be removed under continuous cropping systems than if the crop is produced in rotation with other crops. However, continuous cropping systems often have greater pest and disease vulnerabilities. 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 (e.g. barley and oats), and corn (Kim and Quimby, 2003; USDA-ERS, 2006).   Available wheat straw quantities are greater with the use of less intensive tillage practices such a no-till. According to the Conservation Tillage Information Center (2000), about 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.  Nelson (2002, 2003, 2004), using the Revised Universal Soil Loss Equation (RUSLE) for rain erosion (Renard, 1996) and the Wind Equation (WEQ) (Skidmore, 1970, 1979, 1980) for wind erosion, developed a methodology to estimate straw quantities needed to keep water and wind erosion below the tolerable soil loss level (T), which is the maximum rate of soil erosion that will not lead to prolonged soil deterioration and/or loss of productivity. Quantities that must remain were estimated for each crop rotation and tillage practice combination for every soil type in all counties where wheat is grown.   Crop residue decay returns carbon (organic matter) to the soil. Soil organic matter is crucial to the long term productivity of soil, as it affects soil processes such as cation exchange, aggregate stability, water holding capacity, and soil microbial activity. A national assessment of the amounts of straw needed to maintain soil organic matter (similar to the erosion analysis) is not available. Other factors (e.g. soil moisture, microbial activity, impacts on long term productivity, and equipment constraints) have not been systematically evaluated, but could affect the quantities of removable residues.   The estimated quantities of wheat straw available for bioenergy and bioproducts depends critically on the assumed wheat acres, wheat yields, crop rotation, tillage practice, and quantities that must remain on the soil. Based on average harvested acres and yields from 2000-2004, 93.1 million dry tons of wheat straw were produced annually (Walsh, 2006) (figure 1). Nelson (2004), using average acres and yields from 1997 to 2001, estimated 34 to 390 million dry tons of wheat straw could be available in the Midwest if every cropland acre (not just wheat acres) was in the specified rotation (continuous or rotated with other grains), and tillage (no-till, conventional, reduced till) combination assuming sufficient straw quantities remained to control for wind and water erosion at the tolerable soil loss level.   Both Perlack et al. (2005) and Walsh (2006) use Nelson’s erosion analysis to estimate available wheat straw quantities. Perlack et al. estimated that 11 million dry tons of wheat straw are currently available for bioenergy use, based on 48.8 million harvested acres of wheat with an average yield of 40.1 bu/ac, a continuous wheat rotation, current mix of tillage practices, and controlling for soil erosion. Walsh (2006) estimates that 59.9 million dry tons of wheat straw could be available (assuming average acres and yields for 2000-2004), if all acres are in a continuous wheat rotation and use no-till operations, and erosion constraints only are accounted for, but that only 4.62 million dry tons of wheat straw are available if all wheat acres are in a continuous wheat rotation and use the current mixture of tillage practices, and both erosion and soil organic matter constraints are accounted for (figure 2). Soil organic matter constraints are based roughly on the Residue Equivalent Value quantities (the quantities of residues that must be returned to soil annually to maintain a constant level of organic matter) as estimated by the Soil Conditioning Index model developed by USDA (Lightle, 1997, 1999).                                   Few studies that estimate regional or national wheat straw quantities include an economic cost component. Those that do, generally assume that the wheat straw is baled, as this method involves off-the-shelf technology and uses widely available equipment. Baling readily fits into the existing infrastructure and can be used now without major modifications. However, baling involves multiple passes over the field following grain harvest and can be problematic in areas with short collection periods (due to weather conditions which limit field access) and may increase contamination of the straw with dirt. New approaches that simultaneously harvest and collect the wheat grain and straw are under development or recently available (INL, 2002; McLeod, 2006). These approaches are expected to reduce collection costs.   Kerstetter (2001), using average wheat and barley acres and yields for 1995-1999, estimated that 11.02 million dry tons of wheat and barley straw are potentially available in WA, OR, ID, and MT. The analysis assumed that 3000 lbs/ac of straw was left to maintain soil characteristics. The researchers estimated the cost of mowing and baling to be $32/ac, the fertilizer replacement cost was $3.00/dry ton, and the cost of storing the straw was $7.00/dry ton.   Gallagher (2003a; 2003b), using 1997 wheat acres, wheat yields, and input costs, estimated that 25.5 million dry tons of wheat straw could be available for bioenergy uses at prices of between $14/dt and $42/dt, depending on region. The analysis assumed that sufficient straw quantities were left to provide 30% coverage of the field surface (715 lbs/ac for spring and winter wheat in continuous cropping systems or in rotation with other crops; 1020 lbs/ac for wheat produced in a wheat-fallow rotation). Acres included were those for which the erosion level was below the tolerance level when these amounts of residues were left. The analysis was based on representative soil types by major crop production region. Price included the cost of chopping and baling the straw, on-farm hauling of bales, and a fertilizer replacement cost of $4.99/dt of straw removed. Use of wheat straw for bedding and forage was accounted for.   Rooney (1998) estimated that no wheat straw could be available for bioenergy and bioproducts, based on 1996 acres and yields. The analysis assumed that 65% of the generated wheat straw needed to remain on the field and eliminated quantities (because of costs) for areas where, after leaving 65% of the straw, the quantities available for collection were less than 1 dt/ac.    Projected quantities of wheat straw that could be available in the future are estimated by multiplying projected wheat grain yields by a residue-to-grain ratio. Most analyses assume the current residue-to-grain ratio remains unchanged as grain yields increase, which given the lack of data, is a reasonable assumption. However, it should be noted that some breeding approaches (e.g., the development of semi-dwarf or dwarf varieties), or development of varieties with high residue and high grain yields, might alter the ratio leading to lower or higher quantities of straw. Future grain yield projections are typically based on a combination of genetic improvements, more efficient use of inputs (e.g. fertilizers and herbicides), and improvements in the equipment used to produce and harvest wheat straw (FAPRI, 2007; USDA, 2007; RCA III Symposium, 1997). The removal of wheat straw alters the assumed management practices and increases the uncertainty of applying future grain yield projections to estimate future straw quantities.   A workshop of crop and livestock experts (RCA III Symposium, 1997) projected national average wheat grain yields of 69 bu/ac by 2030 and 80 bu/ac by 2050 (compared to the 1990-1992 yields of 38 bu/ac). Projected yields are a result of combined genetic, management and equipment improvements. The USDA projects average national yields of 45.2 bu/ac and 49.7 million harvested acres by 2016 (USDA-OCE, 2007). The Food and Agricultural Policy Institute (FAPRI, 2007) projects average national wheat grain yields of 44.9 bu/ac and 48.7 million harvested acres by 2016.   Perlack et al.  (2005) estimated that by mid-century, 34.8 million dry tons could be available under moderate wheat grain yield increases (48.1 bu/ac) and 51.8 million dry tons under high wheat grain yield increases (55.8 bu/ac). The moderate scenario assumed 52.3 million acres and the high grain yield scenario assumed 47.25 million acres. Both scenarios assumed that all acres are in a continuous wheat rotation, all acres use no-till practices, and quantities of residues needed to maintain erosion at the tolerable soil loss level are left on the field. No economic analysis was conducted.   Walsh (2006) estimated county wheat straw supply curves (i.e., quantities of straw available by price) for the years 2005, 2010, 2015, 2020, and 2025. The analysis uses a dynamic model of the U.S. agricultural sector (POLYSYS; De La Torre Ugarte, 2000) to estimate wheat acres over time. The analysis assumed a continuous wheat rotation and that over the time period evaluated, wheat grain yields increased from 42.7 bu/ac (in 2006) to 50.5 bu/ac; acres using reduced and no-till practices increased from 40 to 75%; and collection costs decreased to 75% of current round baling costs. The analysis controlled for water and wind soil erosion at the tolerable soil loss level (using Nelson’s work) and included a rough estimate of the quantities needed to maintain soil organic matter at existing levels, based roughly on the Residue Equivalent Values in the Soil Conditioning Index (Lightle, 1997, 1999). Costs include fertilizer replacement, collection, and movement of bales to the edge of the field, but not transportation to a user facility. Costs differ regionally as a result of regional differences in input costs. The estimated county supply curves were aggregated to obtain national wheat straw supplies. Estimated national wheat supplies for select prices are shown in table 1.

References
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