Dual-Use Bioenergy-Livestock Feed Potential of Giant Miscanthus, Giant Reed, and Miscane ()
2US Department of Agriculture, Agricultural Research Service, Poultry Production and Products Safety Research, Fayetteville, AR, USA.
3Grassland Soil and Water Research Laboratory, USDA ARS, Temple, TX, USA.
4West Virginia University, Plant & Soil Sciences, Morgantown, WV, USA.
5Division of Plant Sciences, University of Missouri, Columbia, MO, USA.
6New Energy Farms, Leamington, Canada.
Cite this paper
Compared with their first-generation biofuel counterparts, second-generation biomass crops require fewer inputs, produce greater energy on a landmass basis, reduce greenhouse gas emissions, and do not directly compete for arable land for food production   . As much as 21.3 million ha of existing agricultural land in the US has been projected for conversion to perennial grass feedstocks  , therefore, second-generation feedstock crops may indirectly compete for hay and grazing landmass. Therefore, in order to transform the forage-livestock industry towards second-generation biofuels without affecting grassland resources for animal operations, identifying dual-use perennial forage feedstocks is key.
Small commercial farms, those with gross cash farm income from $10,000 to $249,999, account for 36% of all farms and 22% of total agricultural production in the US, but they are challenged to remain profitable  . A new market for grassland agriculture is emerging because the Energy Independence and Security Act will require 36 billion gallons of renewable fuels by 2022  . To meet this goal, an estimated 1 billion Mg∙yr−1 dry lignocellulosic biomass would be converted to renewable, liquid biofuels  .
There are approximately 746,000 small farms in the US  , and many of those could benefit from an integrated, dual-use, bioenergy-livestock production system. Considering, bioenergy-livestock feeds could produce sustainable yields of combustible energy (high thermal output) or be fed to livestock    . Producers that adopt this multifunctional practice could make fairly short-term management decisions to shift between commodities (bioenergy or livestock feed) based on markets, thereby minimizing risk and maximizing production efficiency  . For switchgrass (Panicum virgatum L.) and other perennial forage species, usage is primarily driven by cattle (Bos L. spp.) weight gains or hay sales, but bioenergy could provide a secondary market option  . It is less clear; however, if perennial grasses traditionally considered dedicated bioenergy feedstocks offer a secondary market as livestock feed.
Commercially, sugarcane (Saccharum L. spp.) leaves are mechanically separated from stems during harvest, a convenient technology, whereby leaf tissue could be diverted to bioenergy or livestock feed uses  as has been done with maize (Zea mays L.)-stover   . The fibrous, lignocellulosic components of plant cell walls (cellulose and hemicellulose) can produce substantial amounts of 5 and 6 carbon sugars   . For instance,  found that whole-plant (leaf and stem) conversion of giant miscanthus (Miscanthus × giganteus J.M. Greef & Deuter ex Hodkinson & Renvoize), giant reed (Arundo donax L.), and sugarcane yields ranges from 0.29 to 0.33 g glucose g−1 dry mass, or is about 0.1 g bioethanol g−1 dry mass.
Plant tissue can differ in various feedstock quality attributes including concentrations of primary metabolites, fiber, gross energy density, and digestibility; which can impact their downstream value and use. Some types of sugarcane, i.e. energy cane, differs from commercial sugarcane by having high stalk fiber concentrations  and some [Saccharum sp. × Miscanthus sp. (miscane)] reportedly have lower leaf ash and N concentrations than switchgrass and giant miscanthus  , which are desirable feedstock attributes. Further, harvest timing influences tissue composition, as delayed harvests allow leaf nutrients to translocate to stems or rhizomes, or are sloughed off as leaf litter  .
Limited information is available on the potential of giant miscanthus and giant reed to serve as livestock feed. Giant reed is invasive in many riparian areas    , but could serve as an animal fodder or bioenergy resource during its eradication  . Fresh, whole-chopped sugarcane is similar to other roughage sources, like cottonseed (Gossypium hirsutum)-hulls, when fed to cattle  . Sugarcane has adequate in vitro dry matter digestibility (IVDMD) for livestock  , and IVDMD of miscane can exceed that of giant miscanthus and switchgrass in Florida  . Similarly, adequate IVDMD concentration has been reported for giant reed leaves in Georgia  , while that of giant miscanthus and giant reed stems are quite low, even for C4 grasses    .
Given the history of cultivation and use of perennial grasses  , an existing knowledge base on cultural practices, availability of harvesting technology, and familiarity with livestock production, small farmers might transition to a dual- use system if given a bioenergy market option. Thus, research is needed on chemical composition of leaf and stem tissue of perennial grasses that might be used in dual-use, bioenergy-livestock feed practices, as concentrations of primary plant metabolites in leaf and stem tissues could affect their ultimate utilization. The objective of this study was therefore to compare feedstock quality of leaf and stem tissues of dedicated bioenergy feedstocks (giant miscanthus, giant reed, and miscane) when grown with or without supplemental irrigation on an upland site.
2. Materials and Methods
2.1. Experimental Description and Management
Giant miscanthus, miscane, and giant reed were grown on an upland Leadvale silt loam soil (fine-silty, siliceous, thermic Typic Fragiudult) with a fragipan at 0.4 to 0.6 m depth near Booneville, Arkansas (35.08˚N, 93.98˚W). In the fall of 2006, giant miscanthus rhizomes (proprietary clone Q4264, Biomass Industrial Crops, Ltd., United Kingdom), axillary internode buds of miscane (sugarcane clone US84-1028; F1 hybrid of Saccharum hybrid × Miscanthus spp. bred and selected at US Department of Agriculture, Agricultural Research Service, Canal Point, Florida), and giant reed (originally obtained from a riparian area along the Little River near Temple, Texas) were transplanted in the greenhouse. Further information on the selected varieties can be found in  . On 29 March 2007, greenhouse-grown clones of the three species were planted as split plots in four replications under either rainfed or irrigated whole plots. Split-plots consisted of 5 clones of each species arranged in a single row and spaced 1 m apart. Rows of split-plots were spaced 2.5 m apart and were randomly arranged within main plots. Plants of the respective species were planted at the ends of each split-plot and two rows of sugarcane were planted between main plots.
Weed control was conducted annually in May with applications of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] at 1.12 kg ai ha−1, and was supplemented by manual weeding as needed. During spring green-up, all plots were fertilized with 80 kg N ha−1 in the form of ammonium nitrate (NH4NO3) and 60 kg P ha−1 (P2O5), and 100 kg N ha−1 (NH4NO3) was applied in both 2008 and 2009. In irrigated whole plots, precipitation was supplemented during the growing season by additions of 719, 580, and 690 mm irrigations during 2007, 2008, and 2009, respectively. To provide irrigation, soaker hoses were installed for each irrigated row and water was applied three times per week except when a rainfall event exceeding 12 mm occurred.
2.2. Biomass Sampling and Tissue Analyses
A representative 2-stalk (giant reed and miscane) or 4-stalk (giant miscanthus) sample was taken 0.15 m above soil surface on plant-cane (establishment year) (PC, 29 October 2007), first-ratoon [or subsequent shoots sprouting from plant base (FR, 27 October 2008)], and second-ratoon (SR, 19 November 2009) crops prior to the first frost. Samples were separated (split-split plot) into leaf lamina (leaf) and stem plus leaf sheath tissue components (stem), and dried at 60˚C. Plant residue tissue samples were ground to 1 mm particle size on a Wiley mill (Arthur Thomas Co., Philadelphia, Pennsylvania) and stored at −20˚C until laboratory analysis. Tissue sample N analyses were conducted by combustion (Vario Macro CN, Elementar Americas, Inc., Mt. Laurel, New Jersey). The concentration of IVDMD was determined using the procedure of  modified for the Ankom Daisy II fiber analyzer #F200 (Ankom Technology, Macedon, New York). Source of fluid for IVDMD analysis was a ruminally-cannulated steer (Bos taurus L.) adapted for 10 d to a basal diet of cracked maize and common bermudagrass (Cynodon dactylon L.) hay (15:85 ratio of grain:hay, w:w), and allowed ad libitum access to fresh water.
Cellulose, hemicellulose, and acid detergent lignin (ADL) concentrations were determined by mass loss during sequential treatment of samples in acid detergent solution (72% v:v H2SO4), and combustion at 550˚C following methods described by Ankom Technology for the Filter Bag Method. Specifically, bags were agitated every 30 mins by manually pushing a 2 L beaker up and down. After 3 hrs, samples were removed and rinsed with water to remove acid. Rinses were repeated until pH was neutral, then samples were rinsed with acetone for 2 - 3 mins and then air dried. Thereafter, drying was completed in a batch oven at 105˚C for 2 hrs, then placed in a desiccator until cooled to ambient temperature and re-weighed. Acid detergent lignin is acid insoluble residue minus ash  . Cellulose was calculated as  and hemicellulose was calculated as  .
[Acid detergent fiber (ADF) ? ADL] = cellulose. (1)
[Neutral detergent fiber (NDF) ? ADF] = hemicellulose. (2)
For analysis of total nonstructural carbohydrates (TNC), samples were further ground to 0.5 mm particle size on an Udy cyclone sample mill (Seedburo Equip- ment Co., Chicago, Illinois). Total nonstructural carbohydrate concentrations were determined using an automated hydrolysis method  . Dry samples were combusted at 550˚C to measure mineral ash residue.
Gross calorific value (combustible energy, MJ∙kg−1) was measured using a bomb calorimeter (IKA C2000, IKA Works; Wilmington, North Carolina). The procedure used 10 ignitions of C6H5COOH standard according to ASTM International  . Briefly, a 0.50-g sample was pelletized, weighed to 5 decimal places, and combusted in purified O2 atmosphere. To account for HNO3 formed during combustion, the inside of the vessel and the crucible and components were rinsed into a beaker with H2O purified by a Labconco Water Pro PS (Labconco Corp, Kansas City, Missouri) 4-stage purification system (18.1 MΩ∙cm−1 resistivity). Two drops of methyl red titration indicator were added to the beaker and the solution was titrated with 0.035 M Na2CO3 solution.
2.3. Statistical Analyses
Analysis of variance tests of feedstock quality characteristics were performed using a mixed linear model procedure, Proc Mixed in SAS v.9.2   , with irrigation treatment (whole-plot), species (split-plot), tissue component (split- split plot), year, and their interactions as fixed effects, and replication as a random effect. Species within replication and year was the repeated measure with a first-order autoregressive covariance structure and restricted maximum likelihood estimation method   . When main effect differences were found, pair-wise post hoc comparisons were performed using Least Square Means using the Tukey Honest Significant Difference test (HSD) at a Type I error rate of 5%.
3. Results and Discussion
Overall results revealed significant effects for year, plant residue component (leaf and stem), and species for all traits examined (Table 1). Irrigation main effects impacted (P < 0.05) TNC, ADL, cellulose, and energy density but not IVDMD, hemicellulose, ash, and N. Because a large number of two and three-way interaction effects were significant, results herein are focused on significant three-way interactions involving species as follows: year × tissue × species for IVDMD, ADL, and N; and tissue × species × irrigation for TNC, cellulose, and gross energy density. Since these three-way interactions were not significant for hemicellulose and ash, tissue × species interactions are highlighted herein for these two traits.
Across the three years, IVDMD for the three species ranged from 330 to 672
Table 1. Probability values from analysis of variance of in vitro dry matter digestibility (IVDMD), total nonstructural carbohydrates (TNC), acid detergent lignin (ADL), hemicellulose (Hemi), cellulose (Cell), ash, nitrogen (N), and energy in response to year (Y), residue tissue component (T), species (S), and irrigation (I) fixed effects.
g∙kg−1 for leaf tissue and 193 to 547 g∙kg−1 for stem residue (Table 2). In general, average leaf IVDMD (539 g∙kg−1) was greater than stem IVDMD concentrations (386 g∙kg−1). Specifically, giant reed had greater leaf IVDMD (>642 g∙kg−1) than giant miscanthus and miscane, and miscane had greatest stem IVDMD concentrations (>428 g∙kg−1) among all species (Table 2). Giant reed leaf and stem IVDMD concentrations were stable across years, whereas giant miscanthus stem IVDMD concentration decreased with every season. Giant miscanthus also had lower ratoon leaf (<495 g∙kg−1) and ratoon stem IVDMD concentrations (<303 g∙kg−1) than other species.
In vitro dry matter digestibility is a function of soluble cellular components including TNC and crude protein (CP) as well as variable amounts of cell wall constituents (CP, cellulose, and hemicellulose) that are readily degraded by rumen microorganisms  . Thus, high stem IVDMD of miscane may have been due to greater soluble sugars and a less lignified parenchyma than stems of other species. Greater concentrations of stem IVDMD in miscane than giant miscanthus and switchgrass is consistent with previous reports  . While primarily a livestock feed attribute, IVDMD concentration has been indirectly linked to methane production  and positively associated with bioethanol production  . Nonetheless, despite its high stem IVDMD, miscane productivity is low at this latitude because of poor ratooning  .
Table 2. Concentrations of in vitro dry matter digestibility (IVDMD), acid detergent lignin (ADL), and nitrogen (N) as affected by the crop × tissue × species interaction.
aPC = plant-cane; FR = first-ratoon; SR = second-ratoon; bVariable means within an irrigation treatment followed by a common letter do not differ significantly at α = 0.05 using Tukey’s HSD.
Giant miscanthus leaf IVDMD concentration was comparable to that of single-cut per year, whole-plant switch grass in West Virginia  . Giant reed leaf and stem IVDMD compared favorably to the 541 g∙kg−1 and <290 g∙kg−1, respectively, reported for giant reed by  . Similarly, miscane leaf (520 g∙kg−1) and stem (507 g∙kg−1) IVDMD observed in this study are consistent with an absence of tissue differences reported by  in Florida. However, the observed IVDMD of giant miscanthus leaves and stems, and giant reed stems, would be among the lowest in a wide range of reported values (280 to 740 g∙kg−1) for C4 grasses   .
Like many traditional livestock feeds, green-harvested tissues could be fed fresh-chopped or ensiled   . Steer weight gain was adequate when fresh- chopped sugarcane was fed at 30% - 40% of the total diet, but performance decreased when the dietary proportion was >60%  . This suggests that sugarcane is similar to other roughage sources (e.g. cottonseed hulls) when fed in steer fattening diets, but it should not be a major component of the diet. Ensiling chopped sugarcane decreases total digestible nutrient concentration, due to conversion of sugar to alcohols, therefore, silage is better for maintenance rather than for increasing stocker cattle weights  . Incubation of sugarcane bagasse with white-rot fungi (i.e. Lentinula edodes, Ceriporiopsis subvermispora) that selectively degrade lignin may substantially increase IVDMD  , although the effect of such biological degradation on livestock intake was not assessed and the practicality of such technology to improve livestock feed quality is unknown.
3.2. Acid Detergent Lignin
Giant miscanthus and giant reed had greater leaf ADL concentrations (>91.5 g∙kg−1) than miscane (<80.2 g∙kg−1) and any other feedstock source in this study (Table 2). Similarly, giant miscanthus stem ADL (>86.1 g∙kg−1) was consistently greater than that of miscane (<69.0 g∙kg−1). In contrast, giant reed stem ADL concentration was only greater (P < 0.05) than that of miscane in PC but not in ratoon crops. Averaged across treatments, leaf tissue (97.3 g∙kg−1) had significantly greater ADL than stem tissue (77.0 g∙kg−1). Leaf and stem ADL did not change (P ≥ 0.05) over the course of three seasons, except for giant miscanthus leaves, which became significantly more lignified. Concentrations of ADL were within the range (20 to 115 g∙kg−1) previously reported for various C4 grasses  . Reference  reported giant reed leaves (<41 g∙kg−1) had less ADL than stems (90 g∙kg−1), whereas ADL observed in this study averaged 109 g∙kg−1 for leaves and 76.5 g∙kg−1 for stems across all seasons.
Lignin is the major non-polysaccharide component of plant cell walls, reduces digestibility of polysaccharides through cross-linkages, and is virtually indigestible by rumen microorganisms   . Lignin is high in combustible energy, similar to that of cellulose  , and the values we found should not cause excessive tar production during gasification  .
3.3. Nitrogen Concentration
Giant reed had greater (P < 0.05) leaf N concentrations (16.1 to 27.6 g∙kg−1) than the other two species (4.4 to 17.5 g∙kg−1) (Table 2). Second ratoon giant miscanthus had the lowest leaf N (4.4 g∙kg−1) and stem N (1.5 g∙kg−1) concentrations among species and crops. Leaf and stem N concentrations were lower in ratoon than PC for giant miscanthus and giant reed, but not miscane. Second ratoon leaf and stem N concentrations were also lower than PC for each species, suggesting a decrease in N uptake in ratoon crops and greater N-use efficiency with crop age. This was particularly evident for giant miscanthus considering its relatively high yields  . Decreases in N concentration in giant miscanthus from PC to ratoon crops were also previously observed  .
Across diverse species, bioenergy feedstocks can be highly variable in N concentrations, ranging from 0.5 [Sequoia sempervirens (Lamb. ex D. Don) Endl. wood] to 30.4 g∙kg−1 (Triticum aestivum L. dust)  . Tissue-N is usually considered an anti-quality attribute in combustible feedstocks, as concentrations of 10 g∙kg−1 or greater can necessitate NOx removal at thermochemical conversion facilities  . On that basis alone, leaf residue would be less preferred as a combustible feedstock than stem tissue, except for SR giant miscanthus leaves. In practice, however, while coal contains more N (159 g∙kg−1) than switchgrass (8.7 g∙kg−1), blending coal with a second-generation feedstock is an approach to reduce total NOx emissions from utility boilers compared to coal-firing alone  . A similar approach might be taken with high-N bioenergy feedstocks for combustion.
Conversely, plant proteins are a beneficial attribute in livestock feed, and are often expressed as CP  . Data presented herein are consistent with previous findings  in that leaves of mature plants have substantially greater CP concentration than stems.
Conflicts of Interest
The authors declare no conflicts of interest.
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