The principal oilseed crop grown in the U.S. is soybean. Over the past seven years, nearly 73 million acres were harvested annually and average national yields ranged between 33.9 and 43 bu/ac (USDA-NASS). It is most frequently produced in rotation with corn, but also in rotation with winter wheat in the Plains states and sometimes double cropped with wheat in the South. Over 92% of soybean acres are in rotation with another crop (USDA-ERS, 2006). Production of soybean uses conservation tillage practices to a greater extent than any other crop. Around 33% of soybean acres are planted using no-till operations and another 24% use reduced till operations. The remainder (43%) use more intensive tillage practices (CTIC, 2000). Besides soybean, other oilseed crops are produced. From years 2000-2006, the average harvested acres of sunflower, safflower, flaxseed, and canola were 2.25 million, 186,000, 651,000, and 1.18 million respectively (USDA-NASS).
Cotton is not typically thought of as an oilseed crop, although cottonseed oil is recovered. The residues produced from cotton production could be available for bioenergy and bioproduct use. About 13 million acres (range of 12.0 to 13.83 million between years 2000-2005) of cotton are harvested annually in the U.S., with average national yields ranging between 632 and 855 lbs/ac (USDA-NASS). Production occurs in the Southern part of the U.S. (figure 1). Cotton is produced in a continuous cropping system more than any other major crop (60% of acres) (USDA-ERS, 2006). About 8% of cotton acres are planted using no-till practices and 4% of the acres use reduced till practices. The remaining acres (88% of total) utilize intensive tillage operations (CTIC, 2000).
Few studies estimate oilseed residue quantities that might be available for bioenergy use. Most analysts assume that no soybean residues are available because residue quantities produced are generally low and degrade rapidly in the field. After accounting for quantities needed to maintain soil quality and reduce erosion, the amounts that can be collected are usually too small to be economically viable.
Perlack et al. (2005) assume, as do most studies, that under current conditions no soybean residues are available, but estimate that under a moderate crop yield assumption (average yield of 44.8 bu/ac and 71.4 million harvested acres), 12.7 million dry tons could be available by mid-century. Under a high crop yield scenario (48.7 bu/ac and 63.4 million harvested acres), an estimated 47.9 million dry tons of soybean residue could be available. The availability of soybean residue is due to higher yields and an assumption that new soybean varieties with a higher residue-to-grain ratio than current varieties (2.0, as compared to 1.5) will be developed. Perlack et al. (2005) also estimate that 5.5 million dry tons of cotton residues could be available by mid-century under a moderate yield increase assumption (12.3 million acres and a residue yield of 1.1 dt/ac) and that 8.9 million dry tons could be available by mid-century under a high yield increase assumption (12.3 million acres with a residue yield of 1.2 dt/ac). No economic analysis was conducted. Analyses of other oilseed residues is not available.