Wheat is the major small grain crop produced in the U.S., but several other grain crops such as barley, oat, rye, rice, and grain sorghum are also produced. The straw (stems and leaves) of these crops could be used for bioenergy and bioproducts. Barley, oat, and rye production have been declining in recent years, with production in 2006 being only about half of what was produced in 2000. Rice production is limited (about 3 million acres), but highly concentrated - with production occurring within just a few counties in six states (Arkansas, California, Louisiana, Mississippi, Missouri, and Texas). About 7 million acres of grain sorghum are harvested annually, and production occurs mainly in the southern U.S.
In the absence of data on residue yields, the quantities of grain straw produced per acre are estimated by multiplying the grain yield by a residue-to-grain ratio. Most studies assume a residue-to-grain ratio of 1.5:1 for barley, 1.4:1 for oat, 1.5:1 for rye, 1.5:1 for rice, and 1:1 for sorghum; grain weights are assumed to be 48 lbs/bu for barley, 32 lbs/bu for oat, 56 lbs/bu for rye, and 55 lbs/bu for sorghum (Brown, 2003; Heid, 1984; Larson, 1997a, 1997b). Using these assumptions, and the 2005 harvested acres and grain yields, an estimated 37 million dry tons of straw were produced by these five crops (table 1).
Not all of the straw that is produced can be removed. 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 quantities of straw must be left on the field to maintain these functions. The quantities that must remain depend on several factors, including 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 to produce the crops (particularly tillage operations), the physical characteristics of the soil (e.g. soil type and erodibility), field characteristics (e.g. slope), and climate.
At present, for these crops, there are few analyses that rigorously examine the quantities of residues that must remain to maintain soil quality. After accounting for soil needs, it is unlikely that sufficient quantities of rye and oat straw could be available to justify collection. Rice straw quantities are highly concentrated in a few counties in just six states, and for these regions, could be a viable biomass resource.