Cereal grains (wheat, barley, oats, sorghum and rice) are widely grown in the United States and wheat straw constituted 20-25% of potential 2012 U. S. biofuel feedstocks (Table 8.1). Agronomic considerations for determining supplies of wheat straw that can be harvested sustainably include: (1) annual wheat straw yield and its stability; (2) straw harvesting efficiencies; (3) crop rotation and tillage practices for assessing soil conservation and sustainability factors; (4) nutrient removal and fertilizer replacement values; (5) site- specific field evaluations including economic factors that inform decision support systems; and (6) competing economic uses for harvested cereal straw. Addressing these issues has been the focus of several recent research efforts including the Sun Grant partnership [32, 33], the U. S. Pacific Northwest, the Climate Friendly Farming™ project [34], and the USDA Solutions To Economic and Environmental Problems (STEEP) grant program [35].

In the United States, the amount of wheat straw potentially available for use as a biofuel feedstock was assessed through the Sun Grant partnership where the team used USDA — NASS county level grain yield data from 1999 through 2008 [32]. Grain yield data were combined with the harvest index (HI), the ratio of grain yield to total aboveground biomass (grain plus straw) at harvest, to estimate straw yields. The HI of wheat, however, is not a constant value [32], with reported values ranging from 0.20 to 0.70 with an average across locations and years of 0.44. This average is greater than the historic HI value of 0.375 commonly used for winter wheat [19], presumably because newer grain varieties are more efficient and produce less straw per unit of grain than older varieties. The HI data have important implications for estimating the amount of straw produced based on grain yield because an increase in HI from 0.375 to 0.44 results in a 24% reduction in estimated wheat straw yield. Consequently, generating straw yield maps for the United States based on grain yield can only be considered as a first step toward evaluating straw feedstocks for the purpose of siting biofuel plants. In addition to overall production, understanding the year-to-year stability of straw yield is also an important consideration for assessing feedstock supplies. Karow [32] noted that significant annual fluctuations in wheat straw stocks could occur where some areas with high average straw yields also had years with no or limited wheat straw yield.

Overall straw yield serves as a starting point for quantifying available biofuel feedstock that can be sustainably harvested. Factors such as straw harvesting efficiencies, residues (straw) required for controlling wind and water erosion, and for maintaining soil produc­tivity then reduce the amount of straw that can be harvested without impairing the soil resource base. Current straw harvesting efficiencies (e. g. straw baling) are near 50% [7]; however, technological advances could increase residue harvesting efficiencies to around 75% [36]. It is more difficult to assess the multitude of crop rotation and soil tillage factors that influence how harvesting crop residues will affect soil conservation and other agroe­cosystem services. In many cases, conservation needs that depend on leaving adequate cereal residues in the field will be more limiting than current harvesting efficiencies.

In developing estimates for straw feedstocks that could be sustainably harvested, Kerstet — ter and Lyons [37] estimated that dry straw inputs of 3.4-5.6 Mg ha-1 yr-1 are required for conservation purposes in the western United States, whereas others [38] reported 4.5 Mg residue ha-1 yr-1 were needed. These numbers are similar to the 4-5 Mg residue ha-1 yr-1 reported [39] to be required for maintaining soil organic matter in dryland cropping sys­tems near Pendleton, OR. Assuming a harvest index of 0.4, wheat grain yields of 2.0-3.3 Mg ha-1 yr-1 (3.0-5.0 Mg ha-1 yr-1 of wheat straw) would be needed to supply straw for conservation needs and harvestable straw estimates would need to be based on grain yields that exceed this threshold. An important point to realize in these calculations is that the quantities of residue required for conservation needs are on an annual basis. In many dryland scenarios, however, continuous wheat is seldom grown and crop rotations often include a fallow year when no crop or crop residues are produced [4]), or where other crops such as peas (Pisum sativum) or lentils (Lens culinaris) that produce far less residue than wheat are grown [14]. Thus, crop residue production must be quantified for an entire rota­tion in order to assess the average annual residue returns on a rotational basis. Therefore, in a two-year, wheat-fallow rotation, wheat will need to produce grain yields of 4.0-6.6

Mg ha-1, twice that reported [37,38] to meet conservation needs. Unfortunately, many estimates of wheat straw availability have assumed continuous wheat [37,38]) production when assessing conservation needs. This has resulted in “sustainable harvest estimates” for wheat straw that are greatly inflated when compared to the actual amount available with other rotations. Accurate estimates of the wheat residue quantities returned to soil are in themselves insufficient to assess sustainable residue harvest, due to the important influence of other key factors such as crop rotation and tillage practice.

Evaluating the impact of straw harvest on important soil quality indicators such as SOC, aggregation, or erosion requires long-term research, since annual changes are generally very small and can be temporally dynamic. In recognition of this need, the Sun Grant partnership organized a symposium at the 2009 International American Society of Agronomy (ASA) meetings entitled “Residue Removal and Soil Quality — Findings from Long-Term Research Plots.” Presentations at this symposium examined residue removal impacts in the context of various management practices including crop rotation, tillage, applied fertilizer and irrigation. The articles developed from this symposium were subsequently published in the Agronomy Journal (Huggins et al. [33]). The series includes results from long-term studies in Europe, Canada, Australia, and the United States. Key points included an assessment [40] that reviewed long-term studies from Europe, Australia, and Canada and cautioned against annual removal of straw because of the potential decrease in SOC. Due to the site-specific nature of residue harvest, they recommended that straw removal studies be coupled to areas where residue harvest is actually being considered and to not extrapolate using data from other areas.

Near Pendleton, OR [41], it was concluded from long-term dryland cropping system studies that residue removal in this predominantly wheat-fallow area will increase SOC depletion and that residue harvest will only be sustainable if wheat-fallow was replaced with continuous cropping and no-tillage. Nafziger and Dunker [42] reported on the long­term SOC trends under different crop rotation and fertilizer treatments at the University of Illinois Morrow Plots and emphasized the importance of adequate nutrient levels for maintaining SOC. Long-term plots at the University of Missouri Sanborn Field showed that the amount of field residues returned was positively related to SOC (Miles and Brown, 2011 [43]). Gollany et al. (2011) [44] evaluated five long-term field experiments in North America with the CQESTR model and concluded that increasing soil carbon inputs through manure additions and/or crop intensification as well as reducing tillage were important strategies for mitigating residue harvest impacts on SOC. Finally, in irrigated systems, Tarkalson et al. (2011) [30] reported that SOC either increased or remained constant when wheat residues were removed and hypothesized that belowground biomass production was important for maintaining or increasing SOC under irrigation. They also pointed out that irrigated cropping systems in the Pacific Northwest and elsewhere tend to be diversified with crops such as alfalfa (Medicago sativa), potato (Solan spp.), and sugarbeet (Beta vulgaris) in addition to wheat and corn, and that very little data on residue removal effects on SOC is available for those situations.

In combination, these papers conclude that under dryland or rainfed conditions, residue harvest will negatively impact soil organic matter and associated soil properties; how­ever, harvest effects will be situation-dependent. Consequently, assessing residue harvest must be placed in a farming systems context that includes an evaluation of economic and environmental trade-offs specific for a given farm and location. Future challenges include the development of science-based, site-specific decision aids that enable growers to make economically sound and environmentally sustainable choices regarding residue harvest.

In 2009, USDA-ARS and land grant scientists in the Pacific Northwest established long­term field studies from a combination of current and new field locations to assess economic impacts of residue removal as well as effects on soil properties, soil-borne disease and crop performance [35]. Specific objectives of the project funded through theUSDA Solutions To Economic and Environmental Problems (STEEP) grant program are to: (1) establish or use existing long-term field sites and assess impacts of wheat residue removal by mechanical harvest and burning on economic returns and subsequent crop performance; (2) assess environmental impacts (soil carbon sequestration, nutrient cycling, soil erosion) of residue removal by mechanical harvest and burning on established sites; and (3) develop field — scale and regional assessments of economic and environmental trade-offs associated with harvesting or burning crop residues.

Preliminary STEEP research from the Washington State University (WSU) Cook Agron­omy Farm (CAF) estimated that the potential site-specific (37-ha field) lignocellulosic ethanol production from winter wheat residues would range from 813 to 1767 l ha-1 and average 1356 l ha-1; thus, indicating that targeted harvesting of crop residues would be an important consideration. Harvesting only winter wheat residues, in a three-year rotation with spring wheat and spring peas (Pisum sativum), reduced residual carbon inputs to levels below that required to maintain SOC under conventional tillage practices. This occurred as a function of both residue removal and inclusion of the low residue producing spring pea crop in rotation with wheat. Harvesting winter wheat residues under conventional tillage resulted in negative Soil Conditioning Indices (SCI) throughout the field. In contrast, SCIs under no-till were positive despite residue harvesting. Increased nutrient removal is also a consideration associated with harvesting crop residues for any use. In the STEEP study, the estimated value of N, P, K, and sulfur (S) removed in harvested wheat residue was $13.71 Mg-1. In high residue producing areas of the field, the estimated value of harvested residue in fertilizer replacement dollars exceeded $25 ha-1. Based on the potential SOC impact and increased nutrient cost, we concluded that substantial trade-offs exist in har­vesting wheat straw for biofuel and that trade-offs should be evaluated on a site-specific basis. Furthermore, support practices such as crop rotation, reduced tillage and site-specific nutrient management need to be considered if residue harvest is to be a sustainable option (Huggins and Kruger, 2010 [14]).

Potential impacts of crop residue removal on SOC were also simulated for different tillage and rotation scenarios in the Pacific Northwest using the CropSyst model [45]. Preliminary outcomes show that harvesting winter wheat residue at the lowest simulated removal rate (50%) resulted in SOC losses over a 30-year simulation (Figure 8.7). Harvesting less than 50% of the residue was not considered to be practical or a cost-effective use of producer time and equipment. Use of continuous no-till practices, however, partially compensated for the effects of winter wheat residue removal on SOC.

From an economic perspective dryland wheat growers typically receive from $3 to $5 Mg-1 in the Pacific Northwest, from custom operators who harvest the majority of the straw that is exported from this region. Traditionally, the primary motive for growers to sell residue is to reduce post-harvest tillage operations, thus reducing their total operating costs in high-yielding areas by $35-60 ha-1 depending on tillage practices. However, growers have expressed concerns over long-term impacts of continual straw removal. Once the field

studies and model simulations are more complete, we will estimate long-term economic impacts using partial enterprise budgets including nutrient replacement costs over time.

Sun Grant researchers are also evaluating existing straw markets to identify areas of potential residue harvest [32]. Existing markets for straw can be useful for identifying where straw is readily and reliably available. Identifying these potential markets is also important because they may significantly influence straw prices in a future biofuel market. With this background, the next steps in the DOE Sun Grant project are to identify those areas in the United States where sustained residue harvest seems feasible and to characterize those areas by determining: (1) What makes residue harvest possible in these areas? (2) Are these conditions likely to continue in the future? (3) If the area is irrigated, is the water source stable and will electricity costs affect production? (4) Are alternative markets already in place for harvested residues and, if so, at what cost would residues need to be purchased for biofuel use to be competitive? These and other questions need to be addressed as we think about residue harvest for biofuel use and the design of needed research and decision support systems for a residue-based biofuel system [33]).