Corn stover feedstock trials to support predictive modeling

To be sustainable, feedstock harvest must neither degrade soil, water, or air resources nor negatively impact productivity or subsequent crop yields. Simulation modeling will help guide the development of sustainable feedstock production practices, but not without field validation. This paper introduces field research being conducted in six states to support Sun Grant Regional Partnership modeling. Our objectives are to (1) provide a fundamental understanding of limiting factor(s) affecting corn (Zea mays L.) stover harvest, (2) develop tools (e.g., equations, models, etc.) that account for those factors, and (3) create a multivariant analysis framework to combine models for all limiting factors. Sun Grant modelers will use this information to improve regional estimates of feedstock availability. A minimum data set, including soil organic carbon (SOC), total N, pH, bulk density (BD), and soil‐test phosphorus (P), and potassium (K) concentrations, is being collected. Stover yield for three treatments (0%, 50%, and 90% removal) and concentrations of N, P, and K in the harvested stover are being quantified to assess the impact of stover harvest on soil resources. Grain yield at a moisture content of 155 g kg^?1 averaged 9.71 Mg ha^?1, matching the 2008 national average. Stover dry matter harvest rates ranged from 0 to 7 Mg ha^?1. Harvesting stover increased N–P–K removal by an average of 42, 5, and 45 kg ha^?1 compared with harvesting only grain. Replacing those three nutrients would cost $53.68 ha^?1 based on 2009 fertilizer prices. This first‐year data and that collected in subsequent years is being used to develop a residue management tool that will ultimately link multiple feedstock supplies together in a landscape vision to help develop a comprehensive carbon management plan, quantify corn stover harvest effects on soil quality, and predict regional variability in feedstock supplies.     

Nutrient Removal as a Function of Corn Stover Cutting Height and Cob Harvest

One-pass harvest equipment has been developed to collect corn (Zea mays L.) grain, stover, and cobs that can be used as bioenergy feedstock. Nutrients removed in these feedstocks have soil fertility implication and affect feedstock quality. The study objectives were to quantify nutrient concentrations and potential removal as a function of cutting height, plant organ, and physiological stage. Plant samples were collected in 10-cm increments at seven diverse geographic locations at two maturities and analyzed for multiple elements. At grain harvest, nutrient concentration averaged 5.5 g?N kg^?1, 0.5 g?P kg^?1, and 6.2 g?K kg^?1 in cobs, 7.5 g?N kg^?1, 1.2 g?P kg^?1, and 8.7 g?K kg^?1 in the above-ear stover fraction, and 6.4 g?N kg^?1, 1.0 g?P kg^?1, and 10.7 g?K kg^?1 in the below-ear stover fraction (stover fractions exclude cobs). The average collective cost to replace N, P, and K was $11.66 Mg^?1 for cobs, $17.59 Mg^?1 for above-ear stover, and $18.11 Mg^?1 for below-ear stover. If 3 Mg ha^?1 of above-ear stover fraction plus 1 Mg of cobs are harvested, an average N, P, and K replacement cost was estimated at $64 ha^?1. Collecting cobs or above-ear stover fraction may provide a higher quality feedstock while removing fewer nutrients compared to whole stover removal. This information will enable producers to balance soil fertility by adjusting fertilizer rates and to sustain soil quality by predicting C removal for different harvest scenarios. It also provides elemental information to the bioenergy industry.     

Crop Residue Mass Needed to Maintain Soil Organic Carbon Levels: Can It Be Determined?

Corn’s (Zea mays L.) stover is a potential nonfood, herbaceous bioenergy feedstock. A vital aspect of utilizing stover for bioenergy production is to establish sustainable harvest criteria that avoid exacerbating soil erosion or degrading soil organic carbon (SOC) levels. Our goal is to empirically estimate the minimum residue return rate required to sustain SOC levels at numerous locations and to identify which macroscale factors affect empirical estimates. Minimum residue return rate is conceptually useful, but only if the study is of long enough duration and a relationship between the rate of residue returned and the change in SOC can be measured. About one third of the Corn Stover Regional Partnership team (Team) sites met these criteria with a minimum residue return rate of 3.9?±?2.18 Mg stover ha^?1 yr^?1, n?=?6. Based on the Team and published corn-based data (n?=?35), minimum residue return rate was 6.38?±?2.19 Mg stover ha^?1 yr^?1, while including data from other cropping systems (n?=?49), the rate averaged 5.74?±?2.36 Mg residue ha^?1 yr^?1. In broad general terms, keeping about 6 Mg residue ha^?1 yr^?1 maybe a useful generic rate as a point of discussion; however, these analyses refute that a generic rate represents a universal target on which to base harvest recommendations at a given site. Empirical data are needed to calibrate, validate, and refine process-based models so that valid sustainable harvest rate guidelines are provided to producers, industry, and action agencies.     

Do Yield and Quality of Big Bluestem and Switchgrass Feedstock Decline over Winter?

Switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerdardii Vitman) are potential perennial bioenergy feedstocks. Feedstock storage limitations, labor constraints for harvest, and environmental benefits provided by perennials are rationales for developing localized perennial feedstock as an alternative or in conjunction with annual feedstocks (i.e., crop residues). Little information is available on yield, mineral, and thermochemical properties of native species as related to harvest time. The study’s objectives were to compare the feedstock quantity and quality between grasses harvested in the fall or the following spring. It was hypothesized that biomass yield may decline, but translocation and/or leaching of minerals from the feedstock would improve feedstock quality. Feedstock yield did not differ by crop, harvest time, or their interactions. Both grasses averaged 6.0 Mg ha^?1 (fall) and 5.4 Mg ha^?1 (spring) with similar high heating value (17.7 MJ kg^?1). The K/(Ca?+?Mg) ratio, used as a quality indicator declined to below a 0.5 threshold, but energy yield (Megajoule per kilogram) decreased 13 % by delaying harvest until spring. Only once during the four study-years were conditions ideal for early spring harvest, in contrast during another spring, very muddy conditions resulted in excessive soil contamination. Early spring harvest may be hampered by late snow, lodging, and muddy conditions that may delay or prevent harvest, and result in soil contamination of the feedstock. However, reducing slagging/fouling potential and the mass of mineral nutrients removed from the field without a dramatic loss in biomass or caloric content are reasons to delay harvest until spring.     

Vertical Distribution of Corn Stover Dry Mass Grown at Several US Locations

Corn (Zea mays L.) stover was identified as a renewable non-food agricultural feedstock for production of liquid fuels, biopower, and other bioproducts, but it is also needed for erosion control, carbon sequestration, and nutrient cycling. To help balance these multiple demands, our objectives were to (1) determine height distribution of corn stover biomass, (2) quantify the percentage of stover that is corn cob, and (3) develop a general relationship between plant harvest height and stover remaining in the field for a broad range of growing conditions, soil types, and hybrids in different regions. Plant height, dry grain, stover, and cob yield data were collected at eight US locations. Overall, stover yield increased about 0.85 Mg ha^-1 and cob yield increased about 0.10 Mg ha^-1 for each 1.0 Mg ha^-1 increase in dry grain yield. At grain harvest, the stover-to-grain ratio ranged from 0.64 to 0.96 and cob-to-grain ratio ranged from 0.11 to 0.19. A strong nearly 1:1 linear (r ²?=?0.93) relationship between the relative cutting height and relative biomass remaining in the field was observed across all sites. These data were requested by the US Department of Agriculture-Natural Resource Conservation Service to help improve version 2 of the Revised Universal Soil Loss Equation (RUSLE2) and Wind Erosion Prediction System and better estimate corn stover harvest rates based on cutting height or selective organ harvest (e.g., grain and cob only). This information will improve the capacity of RUSLE2 and similar models to predict the erosion risk associated with harvesting corn residues.     

Assessing the Soil Carbon,Biomass Production,and Nitrous Oxide Emission Impact of Corn Stover Management for Bioenergy Feedstock Production Using DAYCENT

Harvesting crop residue needs to be managed to protect agroecosystem health and productivity. DAYCENT, a process-based modeling tool, may be suited to accommodate region-specific factors and provide regional predictions for a broad array of agroecosystem impacts associated with corn stover harvest. Grain yield, soil C, and N₂O emission data collected at Corn Stover Regional Partnership experimental sites were used to test DAYCENT performance modeling the impacts of corn stover removal. DAYCENT estimations of stover yields were correlated and reasonably accurate (adjusted r ²?=?0.53, slope?=?1.18, p?<<?0.001, intercept?=?0.36, p?=?0.11). Measured and simulated average grain yields across sites did not differ as a function of residue removal, but the model tended to underestimate average measured grain yields. Modeled and measured soil organic carbon (SOC) change for all sites were correlated (adjusted r ²?=?0.54, p?<<?0.001), but DAYCENT overestimated SOC loss with conventional tillage. Simulated and measured SOC change did not vary by residue removal rate. DAYCENT simulated annual N₂O flux more accurately at low rates (≤2-kg N₂O-N ha^?1 year^?1) but underestimated when emission rates were >3-kg N₂O-N ha^?1 year^?1. Overall, DAYCENT performed well at simulating stover yields and low N₂O emission rates, reasonably well when simulating the effects of management practices on average grain yields and SOC change, and poorly when estimating high N₂O emissions. These biases should be considered when DAYCENT is used as a decision support tool for recommending sustainable corn stover removal practices to advance bioenergy industry based on corn stover feedstock material.     

Modeling Water Quality Impacts of Cellulosic Biofuel Production from Corn Silage

The growing interest in the use of alternative biomass products for fuel production requires a thorough understanding of the environmental impacts associated with the production of these bioenergy crops. Corn silage is a potential bioenergy feedstock; however, water quality implications for its utilization as a biofeedstock are not understood. The objective of this work was to evaluate water quality impacts associated with corn silage production. The GLEAMS-NAPRA model was used to quantify runoff, percolation, erosion, nitrate-nitrogen, total phosphorus, and pesticide losses attributed to the production of corn silage with and without winter cover crops for two tillage options (conventional tillage and no till) on three Indiana soils. Results revealed that corn silage would generate greater annual surface runoff (1 to 6 mm) and percolation (1 to 20 mm) compared with corn grain and grain plus stover cropping systems. Silage/winter cereal rye cover crop reduced annual surface runoff and percolation and was strongly influenced by increases in evapotranspiration, when compared with continuous silage production. Silage managed with winter cereal rye cover crop influenced water quality by reducing annual nitrate losses with runoff from a low of 14 % to a high of 27 %, with relatively no effect because of tillage management. No-till practice on silage system produced significantly greater phosphorus losses (7.46 to 18.07 kg/ha) in comparison to silage/cereal rye, corn grain, and grain plus stover harvest (p?<?0.05). For every 1,000 l of ethanol produced from corn silage, erosion losses ranged from 0.07 to 0.95 t/ha for conventional tillage practices and from 0.06 to 0.83 t/ha for no-till practices. The feasibility of cropping systems such as corn silage/cereal rye could contribute to large-scale biomass production but should be further investigated.     

Multilocation Corn Stover Harvest Effects on Crop Yields and Nutrient Removal

Corn (Zea mays L.) stover was identified as an important feedstock for cellulosic bioenergy production because of the extensive area upon which the crop is already grown. This report summarizes 239 site-years of field research examining effects of zero, moderate, and high stover removal rates at 36 sites in seven different states. Grain and stover yields from all sites as well as N, P, and K removal from 28 sites are summarized for nine longitude and six latitude bands, two tillage practices (conventional vs no tillage), two stover-harvest methods (machine vs calculated), and two crop rotations {continuous corn (maize) vs corn/soybean [Glycine max (L.) Merr.]}. Mean grain yields ranged from 5.0 to 12.0 Mg ha^?1 (80 to 192 bu ac^?1). Harvesting an average of 3.9 or 7.2 Mg ha^?1 (1.7 or 3.2 tons ac^?1) of the corn stover resulted in a slight increase in grain yield at 57 and 51 % of the sites, respectively. Average no-till grain yields were significantly lower than with conventional tillage when stover was not harvested, but not when it was collected. Plant samples collected between physiological maturity and combine harvest showed that compared to not harvesting stover, N, P, and K removal was increased by 24, 2.7, and 31 kg ha^?1, respectively, with moderate (3.9 Mg ha^?1) harvest and by 47, 5.5, and 62 kg ha^?1, respectively, with high (7.2 Mg ha^?1) removal. This data will be useful for verifying simulation models and available corn stover feedstock projections, but is too variable for planning site-specific stover harvest.     

Evaluating Local Crop Residue Biomass Supply: Economic and Environmental Impacts

The increasing interest in energy production from biomass requires a better understanding of potential local production and environmental impacts. This information is needed by local producers, biomass industry, and other stakeholders, and for larger scale analyses. This study models biomass production decisions at the field level using a case example of a biomass gasification facility constructed at the University of Minnesota??Morris (UMM). This institutional-scale application has an anticipated feedstock demand of about 8,000?Mg?year^?1. The model includes spatial impacts due to sub-field variation in soil characteristics and transportation costs. Results show that the amount of biomass producers could profitably supply within a 32.2-km radius of UMM increases as plant-gate biomass price increases from $59 to $84?Mg^?1, with 588,000?Mg annual biomass supply at $84?Mg^?1. Results also show that the most profitable tillage and crop rotation practices shift in response to increasing biomass price with producers shifting from a corn-soybean rotation toward continuous corn. While biomass harvest is conducive to increased soil erosion rates and reduced soil organic carbon levels, changes in crop production practices are shown to at least partially offset these impacts. Transportation costs tend to concentrate and intensify biomass production near the biomass facility, which also tends to concentrate environmental impacts near the facility.     

Crop Residue Harvest Economics: An Iowa and North Dakota Case Study

Rigorous economic analyses are crucial for the successful launch of lignocellulosic bioenergy facilities in 2014 and beyond. Our objectives are to (1) introduce readers to a query tool developed to use data downloaded from the Agricultural Research Service (ARS) REAPnet for constructing enterprise budgets and (2) demonstrate the use of the query tool with REAPnet data from two field research sites (Ames, IA, and Mandan, ND) for evaluating short-term economic performance of various biofuel feedstock production strategies. Our results for both sites showed that short-term (<3 years) impacts on grain profitability were lower at lower average annual crop residue removal rates. However, it will be important to monitor longer term changes to see if grain profitability declines over time and if biomass harvest degrades soil resources. Analyses for Iowa showed short-term breakeven field-edge biomass prices of $26–$42 Mg^?1 among the most efficient strategies, while results for North Dakota showed breakeven prices of $54–$73 Mg^?1. We suggest that development of the data query tool is important because it helps illustrate several different soil and crop management strategies that could be used to provide sustainable feedstock supplies.     

The Feasibility of Producing Adequate Feedstock for Year-Round Cellulosic Ethanol Production in an Intensive Agricultural Fuelshed

To date, cellulosic ethanol production has not been commercialized in the United States. However, government mandates aimed at increasing second-generation biofuel production could spur exploratory development in the cellulosic ethanol industry. We conducted an in-depth analysis of the fuelshed surrounding a starch-based ethanol plant near York, Nebraska that has the potential for cellulosic ethanol production. To assess the feasibility of supplying adequate biomass for year-round cellulosic ethanol production from residual maize (Zea mays) stover and bioenergy switchgrass (Panicum virgatum) within a 40-km road network service area of the existing ethanol plant, we identified ～14,000 ha of marginally productive cropland within the service area suitable for conversion from annual rowcrops to switchgrass and ～132,000 ha of maize-enrolled cropland from which maize stover could be collected. Annual maize stover and switchgrass biomass supplies within the 40-km service area could range between 429,000 and 752,000 metric tons (mT). Approximately 140–250 million liters (l) of cellulosic ethanol could be produced, rivaling the current 208 million l annual starch-based ethanol production capacity of the plant. We conclude that sufficient quantities of biomass could be produced from maize stover and switchgrass near the plant to support year-round cellulosic ethanol production at current feedstock yields, sustainable removal rates and bioconversion efficiencies. Modifying existing starch-based ethanol plants in intensive agricultural fuelsheds could increase ethanol output, return marginally productive cropland to perennial vegetation, and remove maize stover from productive cropland to meet feedstock demand.     

Impact of Harvest Equipment on Ash Variability of Baled Corn Stover Biomass for Bioenergy

Cost-effective conversion of agricultural residues for renewable energy hinges not only on the material’s quality but also the biorefinery’s ability to reliably measure quality specifications. The ash content of biomass is one such specification, influencing pretreatment and disposal costs for the conversion facility and the overall value of a delivered lot of biomass. The biomass harvest process represents a primary pathway for accumulation of soil-derived ash within baled material. In this work, the influence of five collection techniques on the total ash content and variability of ash content within baled corn stover in southwest Kansas is discussed. The equipment tested included a mower for cutting the corn stover stubble, a basket rake, wheel rake, or shred flail to gather the stover, and a mixed or uniform in-feed baler for final collection. The results showed mean ash content to range from 11.5 to 28.2 % depending on operational choice. Resulting impacts on feedstock costs for a biochemical conversion process range from $5.38 to $22.30 Mg^?1 based on the loss of convertible dry matter and ash disposal costs. Collection techniques that minimized soil contact (shred flail or nonmowed stubble) were shown to prevent excessive ash contamination, whereas more aggressive techniques (mowing and use of a wheel rake) caused greater soil disturbance and entrainment within the final baled material. Material sampling and testing were shown to become more difficult as within-bale ash variability increased, creating uncertainty around feedstock quality and the associated costs of ash mitigation.     

Yield and Nitrogen Removal of Bioenergy Grasses as Influenced by Nitrogen Rate and Harvest Management in the Coastal Plain Region of North Carolina

The agronomic performances of giant miscanthus (Miscanthus x giganteus) and switchgrass (Panicum virgatum L.) grown as bioenergy grasses are still unclear in North Carolina, due to a relatively short period of introduction. The objectives of the study were to compare the biomass yield and annual N removal of perennial bioenergy grasses and the commonly grown coastal bermudagrass [Cynodon dactylon (L.) Pers.], and to determine the optimum N rates and harvest practices for switchgrass and miscanthus. A 4-year field trial of the grasses under five annual harvest frequencies (May/Oct, June/Oct, July/Oct, Aug/Oct, and October only) and five annual N rates (0, 67,134, 202, and 268 kg N ha^?1) was established at a research farm in Eastern North Carolina in 2011. Across harvest treatments and N rates, greatest biomass was achieved in the second growth year for both miscanthus (19.0 Mg ha^?1) and switchgrass (15.9 Mg ha^?1). Grasses demonstrated no N response until the second or the third year after crop establishment. Miscanthus reached a yield plateau with a N rate of 134 kg ha^?1 since achieving plant maturity in 2013, whereas switchgrass demonstrated an increasing fertilizer N response from 134 kg N ha^?1 in the third growth year (2014) to 268 kg N ha^?1 in the fourth growth year (2015). The two-cut harvest system is not recommended for bioenergy biomass production in this region because it does not improve biomass yield and increased N removal leads to additional costs.     

Effect of Potassium and Nitrogen Fertilizer on Switchgrass Productivity and Nutrient Removal Rates under Two Harvest Systems on a Low Potassium Soil

Biomass demand for energy will lead to utilization of marginal, low fertility soil. Application of fertilizer to such soil may increase switchgrass (Panicum virgatum L.) biomass production. In this three-way factorial field experiment, biomass yield response to potassium (K) fertilizer (0 and 68 kg?K?ha^?1) on nitrogen (N)-sufficient and N-deficient switchgrass (0 and 135 kg?N?ha^?1) was evaluated under two harvest systems. Harvest system included harvesting once per year after frost (December) and twice per year in summer (July) at boot stage and subsequent regrowth after frost. Under the one-cut system, there was no response to N or K only (13.4 Mg?ha^?1) compared to no fertilizer (12.4 Mg?ha^?1). Switchgrass receiving both N and K (14.6 Mg?ha^?1) produced 18 % greater dry matter (DM) yield compared to no fertilizer check. Under the two-cut harvest system, N only (16.0 Mg?ha^?1) or K only (14.1 Mg?ha^?1) fertilizer produced similar DM to no fertilizer (15.1 Mg?ha^?1). Switchgrass receiving both N and K in the two-cut system (19.2 Mg?ha^?1) produced the greatest (P?<?0.05) DM yield, which was 32 % greater than switchgrass receiving both N and K in the one-cut system. Nutrient removal (biomass?×?nutrient concentration) was greatest in plots receiving both N and K, and the two-cut system had greater nutrient removal than the one-cut system. Based on these results, harvesting only once during winter months reduces nutrient removal in harvested biomass and requires less inorganic fertilizer for sustained yields from year to year compared to two-cut system.     

Efficient Methods of Estimating Switchgrass Biomass Supplies

Switchgrass (Panicum virgatum L.) is being developed as a biofuel feedstock for the United States. Efficient and accurate methods to estimate switchgrass biomass feedstock supply within a production area will be required by biorefineries. Our main objective was to determine the effectiveness of indirect methods for estimating biomass yields and composition of switchgrass fields. Indirect measurements were conducted in eastern Nebraska from 2003 to 2007 in which switchgrass biomass yields were manipulated using three nitrogen rates (0 kg N ha^-1, 60 kg N ha^-1, and 120 kg N ha^-1) and two harvest periods (August and post-killing frost). A modified Robel pole was used to determine visual obstruction, elongated leaf height, and canopy height measurements. Prediction models from the study showed that elongated leaf height, visual obstruction, and canopy height measurements accounted for >?91%, >?90%, and >?82% of the variation in switchgrass biomass, respectively. Regression slopes were similar by cultivar (“Cave-in-Rock” and “Trailblazer”), harvest period, and across years indicating that a single model is applicable for determining biomass feedstock supply within a region, assuming similar harvesting methods. Sample numbers required to receive the same level of precision were as follows: elongated leaf height<canopy height<visual obstruction. Twenty to 30 elongated leaf height measurements in a field could predict switchgrass biomass yield within 10% of the mean with 95% confidence. Visual obstruction is recommended on switchgrass fields with low to variable stand densities while elongated leaf height measurements would be recommended on switchgrass fields with high, uniform stand densities. Incorporating an ocular device with a Robel pole provided reasonable frequency estimates of switchgrass, broadleaf weeds, and grassy weeds at the field scale.     

Development of Sustainable Corn Stover Harvest Strategies for Cellulosic Ethanol Production

To prepare for a 2014 launch of commercial scale cellulosic ethanol production from corn/maize (Zea mays L.) stover, POET-DSM near Emmetsburg, IA has been working with farmers, researchers, and equipment dealers through “Project Liberty” on harvest, transportation, and storage logistics of corn stover for the past several years. Our objective was to evaluate seven stover harvest strategies within a 50-ha (125 acres) site on very deep, moderately well to poorly drained Mollisols, developed in calcareous glacial till. The treatments included the following: conventional grain harvest (no stover harvest), grain plus a second-pass rake and bale stover harvest, and single-pass grain plus cob-only biomass, grain plus vegetative material other than grain [(MOG) consisting of cobs, husks, and upper plant parts], grain plus all vegetative material from the ear shank upward (high cut), and all vegetative material above a 10 cm stubble height (low cut), with a John Deere 9750 STS combine, and grain plus direct baling of MOG with an AgCo harvesting system. Average grain yields were 11.4, 10.1, 9.7, and 9.5 Mg ha^?1 for 2008, 2009, 2010, and 2011, respectively. Average stover harvest ranged from 0 to 5.6 Mg ha^?1 and increased N, P, and K removal by an average of 11, 1.6, and 15 kg Mg^?1, respectively. Grain yield in 2009 showed a significant positive response to higher 2008 stover removal rates, but grain yield was not increased in 2010 or 2011 due to prior-year stover harvest. High field losses caused the direct-bale treatment to have significantly lower grain yield in 2011 because the AgCo system could not pick up the severely lodged crop. We conclude that decreases in grain yield across the 4 years were due more to seasonal weather patterns, spatial variability, and not rotating crops than to stover harvest.     

Bagasse Silage from Sweet Pearl Millet and Sweet Sorghum as Influenced by Harvest Dates and Delays between Biomass Chopping and Pressing

Bagasse remaining after extracting the juice from crop biomass for ethanol production could be preserved as silage and used in animal feedstock, but the nutritive and conservation attributes of bagasse silage from sweet sorghum (Sorghum bicolor (L.) Moench) and sweet pearl millet (Pennisetum glaucum (L.) R.Br) are not well known. We evaluated the nutritive and conservation attributes of silages made with the bagasse of two species (sweet pearl millet and sweet sorghum) harvested on two dates (August and September) at two sites in Québec (Canada) and ensiled after four delays between biomass chopping and pressing (0.5, 2, 4, and 6 h). Bagasse silages made in laboratory silos were considered well preserved (pH?≤?4.0, NH₃-N?<?100 g kg^?1 total N, lactate?>?30 g kg^?1 DM, no propionic and butyric acids) regardless of species, harvest date, or delay between biomass chopping and pressing. Sweet pearl millet and sweet sorghum bagasse silages had similar total N concentration, in vitro true digestibility of dry matter (IVTD), and in vitro neutral detergent fiber digestibility (NDFD). Bagasse silage made from biomass harvested in August rather than in September had a 4 % greater concentration of total N, a 4 % greater IVTD, and a 8 % greater NDFD. The delay between biomass chopping and pressing did not affect the nutritive and conservation attributes of silages. Juice extraction from the biomass of sweet pearl millet and sweet sorghum did not impair attributes of good silage fermentation but it reduced its nutritive value.     

Investigating the Efficacy of Integrating Energy Crops into Non-Profitable Subfields in Iowa

Within-field spatial variability reduces growers’ return on investment and overall productivity while potentially increasing negative environmental impacts through increased soil erosion, nutrient runoff, and leaching. The hypothesis that integrating energy crops into non-profitable segments of agricultural fields could potentially increase grain yield and biomass feedstock production was tested in this study using a statewide analysis of predominantly corn- and soy-producing counties in Iowa. Basic and rigorous controls on permissible soil and soil-carbon losses were imposed on harvest of crop residues to enhance year-to-year sustainability of crop and residue production. Additional criteria limiting harvesting costs and focus on large-area subfields for biomass production were imposed to reduce the impacts of energy crop integration on grain production. Model simulations were conducted using 4 years (2013–2016) of soil, weather, crop yield, and management practice data on all counties in Iowa. Miscanthus (Miscanthus x giganteus), switchgrass (Panicum virgatum), and crop-residue-based bioenergy feedstock systems were evaluated as biomass. Average energy crop and plant residue harvesting efficiencies were estimated at 50 and 60%, respectively. Because of higher potential yields, average logistics costs for miscanthus-based biomass production were 15 and 23% lower than switchgrass-based and crop residue-based biomass productions, respectively, under basic sustainability controls, and 17 and 26% lower under rigorous sustainability controls. Subfield shape, size, area, and harvest equipment size were the dominant factors influencing harvesting cost and efficiency suggesting that in areas where subfields are predominantly profitable or harvesting efficiencies low, other options such as prairie strips, buffer zones around fields, and riparian areas should be investigated for more profitable biomass production and sustainable farming systems.     

Biomass production potential of grasslands in the oak savanna region of Minnesota, USA

Biomass harvested from grasslands formerly used for forage production or set aside for conservation has been identified as a potential source of bioenergy feedstocks. Our objective was to characterize yield and chemical composition of biomass harvested from existing grasslands in the oak savanna region of Minnesota and to determine whether aggregated soil properties and grassland type influence biomass yield and feedstock properties. The influence of soil type and dominant functional plant groups on biomass yield, theoretical ethanol yield, and mineral, ash, and lignocellulosic concentration was measured on biomass harvested from 32 grassland sites. Soils with high productivity ratings, as measured by the Minnesota Crop Productivity Index, produced 36 % more biomass than lower quality soils. Grasslands dominated by warm-season species produced 18 % more biomass than those dominated by cool-season species, when measured after senescence during the late-fall harvest time. Biomass harvested from sites dominated by cool-season grasses had higher N, Mg and Cl concentrations than those dominated by warm-season grasses, suggesting that such grasslands could have lower efficiency in thermochemical conversion processes and that repeated harvesting from such grasslands could remove nutrients from the systems. In addition, glucose and xylose concentrations were slightly higher in biomass from sites dominated by warm-season grasses, which resulted in an estimated additional 12 L?Mg^?1 of ethanol over those dominated by cool-season grasses. Combined with the greater yields, warm-season grasslands could produce an additional 376 L?ha^?1 year^?1.     

Switchgrass Biomass and Nitrogen Yield with Over-Seeded Cool-season Forages in the Southern Great Plains

In dry climates with long, hot summers and freezing winters, such as that of the southern Great Plains of North America, switchgrass (Panicum virgatum L.) has proven potential as a cellulosic bioenergy feedstock. This trial looked at dry matter (DM) and N yield dynamics of switchgrass overseeded with cool-season legumes and rye (Secale cereale L.), compared to switchgrass fertilized with 0, 56 and 112 kg N ha^-1 yr^-1 at an infertile and a fertile location. Optimal N fertilizer rate on switchgrass was 56 kg N ha^-1 at the infertile location. Legume yield was greater in the first season after planting, compared to subsequent years where annual legumes were allowed to reseed and alfalfa (Medicago sativa L.) was allowed to grow. This suggests that the reseeding model for annual legumes will not work in switchgrass swards grown for biomass unless soil seed banks are built up for more than one year, and that overseeding with alfalfa may have to be repeated in subsequent years to build up plant populations. Overseeding rye and legumes generally did not suppress or enhance switchgrass biomass production compared to unfertilized switchgrass. However, cumulative spring and fall biomass yields were generally greater due to winter and spring legume production, which could be beneficial for grazing or soil conservation systems, but not necessarily for once-yearly late autumn harvest biofuel production systems.