University of Nebraska - Lincoln

DigitalCommons@University of Nebraska - Lincoln

Agronomy & Horticulture -- Faculty Publications Agronomy and Horticulture Department

1-1-2012

Switchgrass Harvest and Storage

Robert B. Mitchell

USDA-ARS, University of Nebraska - Lincoln, rmitchell4@unl.edu

Marty R. Schmer

USDA-ARS, University of Nebraska - Lincoln, marty.schmer@ars.usda.gov

Follow this and additional works at: http://digitalcommons.unl.edu/agronomyfacpub

Part of the Plant Sciences Commons

This Article is brought to you for free and open access by the Agronomy and Horticulture Department at DigitalCommons@University of Nebraska -

Lincoln. It has been accepted for inclusion in Agronomy & Horticulture -- Faculty Publications by an authorized administrator of

DigitalCommons@University of Nebraska - Lincoln.

Mitchell, Robert B. and Schmer, Marty R., "Switchgrass Harvest and Storage" (2012). Agronomy & Horticulture -- Faculty Publications.

Paper 548.

http://digitalcommons.unl.edu/agronomyfacpub/548
Chapter 5

Switchgrass Harvest and Storage

Rob Mitchell and Marty Schmer

Abstract The feedstock characteristics of the conversion platform will influence

the optimal harvest and post harvest management practices for switchgrass.

However, many of the harvest management practices are tied to plant phenology

and will be similar across platforms. Proper harvest and storage of switchgrass will

help provide a consistent and high-quality feedstock to the biorefinery. Bioenergyspecific switchgrass strains are high-yielding and in most cases can be harvested

and baled with commercially available haying equipment. Many options are

available for packaging switchgrass for storage and transportation, but large round

bales or large rectangular bales are the most readily available and are in use on

farms. Large round bales tend to have less storage losses than large rectangular

bales when stored outside, but rectangular bales tend to be easier to handle and

load a truck for transport without road width restrictions. Although there is limited

large-scale experience with harvesting and storing switchgrass for bioenergy,

extensive research, as well as a history of harvesting hay crops for livestock in

many agroecoregions, makes harvesting and preserving switchgrass for bioenergy

feasible at the landscape scale.

R. Mitchell (&)

Grain, Forage, and Bioenergy Research Unit, USDA-ARS,

135 Keim Hall, University of Nebraska, Lincoln, NE 68583, USA

e-mail: Rob.mitchell@ars.usda.gov

M. Schmer

Agroecosystem Management Research Unit, USDA-ARS,

131 Keim Hall, University of Nebraska, Lincoln, NE 68583, USA

e-mail: Marty.schmer@ars.usda.gov

A. Monti (ed.), Switchgrass, Green Energy and Technology,

DOI: 10.1007/978-1-4471-2903-5_5, Springer-Verlag London 2012

113

Mitchell & Schmer in Green Energy and Technology, 2012, Switchgrass, p. 113-127
5.1 Introduction

Switchgrass is not a new crop and switchgrass research is not a new phenomenon.

The USDA location in Lincoln, Nebraska, USA has been conducting switchgrass

research continually since 1936. Although the first 50 years of research focused

on switchgrass for livestock and conservation, the research since 1990 within

USDA-ARS, numerous universities, and more recently private industry, has

emphasized bioenergy [1, 2]; see also Chap. 1). Although there is limited

large-scale experience with harvesting and storing switchgrass for bioenergy, more

than 20 years of bioenergy research from small plots to on-farm trials provides

experience and critical insights.

Switchgrass is native to the North American tallgrass prairie and is broadly

adapted to habitats east of the Rocky Mountains and south of 55N latitude [3].

Switchgrass plants are generally caespitose or with short rhizomes and reproduce

both sexually and asexually. Switchgrass has two primary ecotypes (upland and

lowland) and two primary ploidy levels (tetraploid and octoploid) [2]. Switchgrass

genotypes are largely self-incompatible and seed production results from crosspollination by wind [1]. Switchgrass generally grows 1–3 m tall depending on

location and genetic background and can develop extensive root systems to a depth

of 3 m [1]. The aboveground growth and root structure makes switchgrass

well-suited for dual use as a biomass crop and vegetative filter strips which have

removed 47–76% of the total reactive P in surface runoff water in areas treated

with manure [4].

Morphology and phenological development are important to understand when

managing switchgrass for bioenergy. The growth form of both the caespitose and

rhizomatous plants is erect with leaf blade length ranging from 10 to 60 cm

depending on genotype, environment, and location within the plant [2]. Switchgrass

plants tend to be less prone to lodging than other warm-season grasses. Switchgrass

is photoperiod sensitive and requires shortening day length for floral induction,

which helps explain why switchgrass morphology is strongly correlated to day of the

year (DOY) and growing degree days (GDD) [5]. Switchgrass has a determinate

growth habit where most vegetative growth terminates with inflorescence development [5, 6], which has implications for regrowth following harvest. Following

floral induction, tillers advance to the seed ripening stages, growth stops, and tiller

senescence occurs. In switchgrass swards in eastern Nebraska, there were no

vegetative tillers present by DOY 196 and 100% of the tillers had elevated apical

meristems [7]. Any regrowth following a harvest at or after this stage will occur only

from retillering. In eastern Nebraska, sufficient regrowth to warrant a second harvest

after a killing frost occurs about one year out of four [8]. For a more complete review

of the morphology and tiller dynamics of warm-season grass swards, see Mitchell

and Moser [9].

Canopy architecture affects the physiology of growing plants and compositional

characteristics of harvested biomass [10] and breeding for increased biomass

and digestibility changed the canopy architecture of switchgrass [11]. Canopy

114 R. Mitchell and M. Schmer
architectural traits such as tiller density, phenology, and leaf area index (LAI)

are in a continual state of flux and functions of tiller morphology and the growth

stage distribution of tillers within the tiller population [11, 12]. In Trailblazer

switchgrass, there was an inverse relationship between advancing phenology and

tiller density, with tiller density declining by an average of 9.4 tillers m-2 d-1 and

an average tiller density of 1,525 tillers m-2 during the 2 year study [7]. Quantifying the phenology of tiller populations provides information for understanding

these architectural changes in the grass sward. For example, switchgrass phenology advanced linearly with DOY and GDD across six environments in Nebraska

and Kansas [5]. The predictability of switchgrass development in response to DOY

and GDD indicates switchgrass management recommendations for adapted cultivars may be made based on DOY within a region [5]. Switchgrass LAI increased

as phenology advanced and varied across years with maximum LAI ranging from

4.9 to 7.7, with at least 95% of the variation in LAI explained by DOY [7]. If the

selected conversion platform targets feedstock material harvested after senescence,

there will be less variability in the phenologic stage of the swards at harvest and

may provide a more uniform product to the biorefinery. However, the morphological status during the growing season will have implications for other management decisions.

5.2 Harvest Management

The bioenergy conversion platform likely will determine the optimal harvest and

post harvest management practices for switchgrass [2]. However, many of the

harvest management practices will be similar for all conversion platforms. Many

agroecoregions in the US have a history of harvesting and preserving hay for

livestock, so making adjustments to harvesting for bioenergy production will be an

easy transition. Due to the extensive research conducted on switchgrass, best

management practices and extension guidelines have been developed for many

regions [8, 13, 14]. High-yielding switchgrass fields ([12 Mg ha-1

) can be harvested and baled with commercially available haying equipment, but some

important items must be considered [8]. For example, self-propelled swathers with

rotary heads (disc mowers) will be required to optimize efficiency and handle the

volume of material harvested from switchgrass bioenergy production fields [8].

Cutting height is easily adjusted and in most cases will be 10–15 cm, which keeps

the windrows elevated above the soil surface to facilitate air movement and more

rapid drying to less than 20% moisture content prior to baling [2]. After harvest,

switchgrass can be packaged for storage and transportation in large round bales or

large rectangular bales [2, 8]. Large round bales tend to have less storage losses

than large rectangular bales when stored outside, but rectangular bales tend to be

easier to handle and load a truck for transport without road width restrictions [8].

These technologies are in use on farms to harvest and package forages for livestock and are discussed in more detail in later sections.

5 Switchgrass Harvest and Storage 115
5.2.1 Timing and Frequency

Maximum biomass yield with high lignocellulose content is the primary objective of

most herbaceous bioenergy feedstock harvests [8, 15]. Depending on ecoregion,

switchgrass biomass can be maximized with a one-cut or multi-harvest system

[13, 16–18]. Most research supports a single annual harvest for optimizing biomass

and energy inputs, as well as maintaining stands. For example, Sanderson et al. [17]

concluded a single harvest near DOY 260 maximized biomass yield in the southcentral USA. In most rainfed environments of the Great Plains and Midwest USA,

maximum first-cut yields and long-term stand maintenance can be achieved by

harvesting switchgrass once during the growing season to a 10-cm stubble height

when panicles are fully emerged to the post-anthesis stage, near DOY 215 [8, 18,

19]. However, harvesting after frost minimizes nutrient removal, especially N [13].

With upland ecotypes, plant material senesces rapidly and is completely dormant

within 7 days of killing frosts. However, lowland ecotypes grown in northern latitudes senesce and enter dormancy slowly after exposure to killing frost. This difference in response by ecotype is illustrated by upland and lowland plants harvested

27 days after the first killing frost and exposed to low temperatures of less than 0C

on 17 of the 27 days. The completely dormant material is Shawnee, whereas the

material with green stem bases is a lowland strain selected from Kanlow (Fig. 5.1).

This delay in entering dormancy may be one explanation for the winter injury

susceptibility of lowland ecotypes. However, harvest strategies for upland and

lowland ecotypes have not been compared in agro-ecoregions where both ecotypes

occur, so harvest strategies may vary [8]. Proper harvest timing, cutting height and

maintaining adequate N fertility are important management practices required to

maximize yield and ensure persistent switchgrass stands [2, 8]. As previously

mentioned, time of harvest research generally indicates a single harvest at postanthesis maximizes yield, but harvesting after a killing frost ensures stand persistence and productivity, especially during drought [2, 8]. For example, Vogel et al.

[18] reported switchgrass biomass increases up to anthesis, then decreases by

10–20% until killed by frost. This fits well with recommendations by Mitchell et al.

[8] who reported switchgrass should not be harvested within 6 weeks of the first

killing frost or below a 10-cm stubble height to ensure carbohydrate translocation to

the plant crowns for setting new tiller buds and maintaining stand productivity.

Wullschleger et al. [20] compiled a database comprised of switchgrass biomass

production studies conducted at 39 field sites in 17 states which supported the single

harvest for bioenergy. They reported the switchgrass mean biomass yield across all

locations was 8.7 ± 4.2 Mg ha-1 for upland cultivars and 12.9 ± 5.9 Mg ha-1 for

lowland cultivars and the yield difference between ecotypes was significant.

Additionally, they reported that there was no evidence that small plots biased

switchgrass yield when compared to field-scale sites and stressed the importance of

single harvest systems for biomass energy.

Several studies throughout the Great Plains and Midwest have evaluated

switchgrass harvest management. Phenologic stage at first harvest did not affect

116 R. Mitchell and M. Schmer
switchgrass persistence, but regrowth potential decreased as first harvest was

delayed to later stages of development and later DOY [21]. Harvesting switchgrass

two or three times each year resulted in the greatest stand reductions [22].

Switchgrass harvested once at anthesis in Nebraska and Iowa had greater biomass

than areas harvested twice [18]. Biomass was maximized with a single harvest

during anthesis and yields ranged from 10.5 to 12.6 Mg ha-1 yr-1 with no stand

reduction [18]. In Texas, Sanderson et al. [17] harvested several switchgrass

strains once or twice per growing season from multiple environments and concluded that a single harvest in autumn maximized biomass and maintained stands.

In general, delaying harvest until after a killing frost reduces yield, but

ensures stand productivity and persistence, especially during drought, and reduces

N fertilizer requirements for the following year by about 30% [2, 8]. Post-frost

harvests allow N and other nutrients to be mobilized into roots for storage during

winter and use for new growth the following spring, but will reduce the amount

of snow captured during winter and will limit winter wildlife habitat value [8].

Harvesting after a killing frost is a logical management decision for thermo-chemical

conversion platforms and biopower because N, Ca, and other plant nutrients that

function as contaminants in the thermo-chemical process are minimized in the plant

tissue. Another alternative harvest time is to leave switchgrass standing in the field

over winter and harvest the following spring [23]. Delaying harvest until spring

reduced yield by 20–40% compared with harvesting in autumn after a killing frost, but

had no effect on gasification energy yield [23]. Yield losses associated with delaying

harvest until spring may be acceptable if wildlife cover during winter is critical [23].

These studies from a broad geographic range in the USA support a single

annual harvest will maximize biomass and maintain stand persistence, but harvest

timing needs to be considered for optimizing biofuel production. Additionally, the

conversion process is an important consideration when determining the optimum

harvest date. With good harvest and fertility management, productive stands can

be maintained indefinitely and certainly for more than 10 years [8].

Fig. 5.1 Upland and lowland ecotypes of switchgrass enter dormancy at different rates when

grown in the same environment. Both photographs were taken following field-scale switchgrass

harvest in eastern Nebraska on 15 November, 2011. Notice that the upland cultivar a ‘Shawnee’

is completely dormant, whereas the lowland experimental strain b still has green stem bases

(photos by Rob Mitchell)

5 Switchgrass Harvest and Storage 117
5.2.2 Nutrient Removal

Harvesting biomass, whether crop residue or dedicated herbaceous perennial

feedstocks such as switchgrass, removes large quantities of nutrients from the

system [19]. In most agro-ecoregions, nitrogen (N) is the most limiting nutrient

for switchgrass production and is the most expensive annual production input.

Consequently, reducing N removal from the switchgrass production system

has a positive effect on the economic and environmental sustainability of the

system. For example, harvesting 10 Mg ha-1 of switchgrass DM with whole-plant

N concentration of 1% will remove 100 kg of N ha-1

, whereas if harvest is delayed

until after senescence, N concentration can decline to 0.6%, resulting in the removal

of only 60 kg of N ha-1

. Depending on conversion platform and a predictable

harvest window in autumn or winter, this 40 kg of N ha-1 reduction in N removal

may be an acceptable trade-off for the yield losses associated with delaying harvest.

Collins et al. [24] reported the average yield for three switchgrass cultivars

irrigated in the Pacific Northwest ranged from 14.5 to 20.4 Mg dry matter ha-1

yr-1. They reported each kg of N produced 83 kg of biomass and the macronutrient export averaged 214 kg N ha-1

, 40 kg P ha-1, 350 kg K ha-1, 15 kg S

ha-1

, 60 kg Ca ha-1, 38 kg Mg ha-1, and 6 kg Fe ha-1. Averaged across cultivars, switchgrass removed less than 1 kg ha-1 of B, Mn, Cu, and Zn. Additionally,

delaying harvest until spring reduced ash content and leached nutrients from the

vegetation [23]. Although management of all nutrients in the system is important,

N is the most expensive, has the greatest potential for environmental contamination, and has the greatest influence on life cycle assessment.

Nitrogen removal in switchgrass production systems is a function of biomass

yield and N concentration, with biomass N concentration increasing as N

fertilization rates increase [18]. In a multi-environment study evaluating numerous

N rates and harvest dates, biomass was optimized when switchgrass was harvested

at the boot to post-anthesis stage and fertilized with 120 kg N ha-1 [18]. At this

harvest date and fertility level, the amount of N removed at harvest was similar to

the amount of N applied, and soil NO3-N did not increase throughout the study

[18]. Consequently, it is important to consider the interaction of N rate and harvest

date to only replace the N needed for the production system to prevent overfertilization and soil N accumulation.

5.2.3 Soil Carbon

As mentioned previously, switchgrass has an extensive perennial root system

which protects soil from erosion and sequesters carbon (C) in the soil profile [25].

Soil organic carbon (SOC) typically increases rapidly when annual cropland is

converted to switchgrass [26, 27]. The amount of C sequestered depends on the

climate, soil type, original soil C content, time, and placement depth of C [28, 29].

118 R. Mitchell and M. Schmer
For example, switchgrass grown and managed for bioenergy on three marginally

productive cropland sites in Nebraska resulted in an average SOC increase of

2.9 Mg C ha-1 yr-1 in the top 1.2 m of soil in just 5 years [30]. In South Dakota,

switchgrass grown in former cropland enrolled in CRP stored SOC at a rate of

2.4–4.0 Mg ha-1 yr-1 at the 0–90 cm depth [31]. McLaughlin et al. [32] reported

an average of 1.7 Mg C ha-1 yr-1 sequestered in the Southeast U.S. on switchgrass experimental plots. Soil carbon levels on low-input switchgrass fields have

been shown to increase over time, across soil depths, and are higher than adjacent

cropland fields in the Northern Plains [25]. A similar result was found between

switchgrass and a corn (Zea mays L.)-soybean (Glycine max (L.) Merr.)-alfalfa

(Medicago sativa L.) rotational system in Iowa [33]. Switchgrass managed for

bioenergy on multiple soil types in the Northern Plains was carbon-negative,

sequestering 4.42 Mg C ha-1 yr-1 into the soil profile [34]. In the Southeast

U.S.A., an estimated 0.17–0.21 Mg C ha-1 yr-1 was sequestered on switchgrass

plots, managed as a bioenergy crop, based on SOC that was near steady state [35].

Nitrogen applications on switchgrass plots did not alter root C storage when

compared with non-fertilized plots in a 2 year study [36]. However, fertilization of

grasslands increased the amount of C sequestered by 0.30 Mg ha-1 yr-1 on 42

studies throughout the world [28]. Microbial biomass carbon increased after

establishment of switchgrass and carbon mineralization increased by 112 and

254% at depths of 0–0.15 m and 0.15–0.30 m, respectively [36]. Soil organic C

increased at rates ranging from 1.7 to 10.1 Mg C ha-1 yr-1 after switchgrass

establishment throughout North America [31, 34, 35, 37].

5.3 Storage Management

Substantial amounts of switchgrass biomass will need to be safely stored on a

year-around basis to supply a cellulosic biorefinery. Cellulosic biorefineries in the

U.S. are expected to keep only a 72 h feedstock inventory with the remaining

feedstock inventory at the edge of field or at satellite storage facilities [38].

At present, there is uncertainty on the overall capacity of cellulosic biorefineries

but techno-economic models have evaluated refinery sizes ranging from 535 to

8,000 dry Mg feedstock per day [39–41]. Offsite storage management will be

critical to maintain desirable composition characteristics and to ensure feedstock

access under variable weather conditions. Storage infrastructure requirements will

need to be cost effective, maintain desirable quality characteristics depending on

conversion technology, provide an aerobically stable environment, and have

flexible delivery schedules depending on regional weather factors [42].

Storage requirements and management will be dependent on how switchgrass is

harvested. In the near-term switchgrass will be harvested and baled using commercial hay equipment. Self-propelled mower/conditioners (swather) with rotary

heads are effective in harvesting high-yielding ([12 Mg ha-1

) switchgrass fields

(U.S. [43]). The conditioner component on a swather accelerates switchgrass

5 Switchgrass Harvest and Storage 119
drying by crushing plant stems but not altering plant structure and consolidates the

switchgrass into a windrow [38]. After harvest, the baling step bundles switchgrass

into a more condensed form to ease handling, transport and storage. Variable

chamber round balers or rectangular balers will likely be used to consolidate and

bundle switchgrass. Round balers will typically make a bale that is 1.2–1.8 m in

diameter and 1.2–1.8 m in length. Large rectangular bale size ranges from 0.9 to

1.2 m in height and width and 1.8–2.4 m in length. Round balers and large

rectangular balers require switchgrass moisture levels to be B18 and B16%,

respectively, at time of baling to reduce storage losses. Bale moisture content in

excess of these respective values may result in composition degradation or

spontaneous combustion. Field drying prior to baling is required to meet safe

moisture levels for baling which may be hindered depending on the region and

harvest date. Balers can be modified to spray preservatives (e.g. propionic acid)

onto hay limiting microbial growth and removing excess moisture for hay with

20–25% moisture content [44].

The density of a round bale or a large rectangular bale will vary depending on

harvest period with anthesis harvest bales having a greater density than post-killing

frost harvest bales. There are advantages and disadvantages for the round baling or

large rectangular baling methods but both are capable of processing switchgrass

and are commercially available to producers. The round baler is one-fourth to

one-third the capital cost as a large rectangular baler [45] but the field capacity of a

round baler is lower because the baler needs to be stopped to wrap and release the

bale. Large rectangular balers continuously bales without the need for stopping

and is estimated to cost less per unit of harvested area [46]. Smaller bioenergy

producers may opt for the round baler methods because of the lower capital costs

or may outsource harvest and baling to custom harvesting enterprises that are

equipped with large rectangular balers. Rectangular bales need to be removed from

the field soon after baling and protected from precipitation events because the flat

surface of the bale does not shed water and resultant DM losses can be large [44].

Commercially available self-propelled or pull-type round or rectangular bale

stacking equipment collect bales within the field and are able to place bales at the

edge of field for short-term or long-term storage until feedstock delivery to a

biorefinery. These stacking systems significantly lower energy use and increase

field capacity efficiency when compared with a single bale loader system.

Switchgrass round bales have less storage losses than large rectangular bales when

stored outside as they are less prone to water penetration especially when net

wrapped [38]. Net wrapped round bales had 60–70% lower DM losses when

compared with round bales tied with plastic twine [47]. Rectangular bales tend to

be easier to handle and load a truck for transport without road width restrictions.

The time required to load bales onto semi trailers is double for round bales than it

is for rectangular bales [38]. Unless cellulosic biorefineries stipulate a certain

baling method or alternative harvest method, both baling methods will likely occur

for a given region.

Consolidation methods other than baling may be implemented in regions where

weather conditions or existing infrastructure enterprises allow for alternative

120 R. Mitchell and M. Schmer
harvesting scenarios [19, 48–51]. Wet storage methods have been proposed for

switchgrass in regions where drying conditions for baling operations are not

possible because of high relative humidity and increased chance of a precipitation

event after harvest [50]. Switchgrass harvested using wet storage methods include

either a swather harvest and then chopped using a self-propelled forage harvester

with a windrow pickup or directly cut with a self-propelled forage harvester with

an attached rotary head that blows the material into adjacent semi bulk trailers.

Moisture content for switchgrass at time of pickup under wet storage methods

are[40%. Advantages to wet storage methods include reduced harvest costs, lower

DM losses during storage, improved switchgrass cell wall recovery during enzymatic

hydrolysis and lower potential risk of fire during storage [50]. Disadvantages for the

wet storage method include higher equipment and storage structure costs than a

conventional baling system [44]. The wet storage method was found to be more

expensive than other collection and storage methods for cellulosic refinery sizes

greater than 1,500 Mg switchgrass per day because of the high cost of the ensiling pit

and transportation of wet material by truck [51].

Regions where silage harvesting is common would likely have increased participation in storing switchgrass under wet conditions. Field chopping using a forage

harvester can be done at moisture levels similar to baling in less humid regions. Field

chopping has an added advantage to baling in that particle size is much smaller which

may eliminate a preprocessing step at the biorefinery [19]. Estimated delivery costs

for chopped switchgrass biomass are less than for a conventional baling system [51].

Chopped biomass requires specific storage areas either at farm site or at a satellite

storage facility. Chopped biomass has the lowest bulk density and densification may

be an issue for long-term storing and transporting the material [19]. Southeastern

U.S. researchers have proposed increased densification of chopped switchgrass by

using modulizing technology developed for the cotton industry [48, 49].

A loafer stacker system has also been proposed as a cost effective method to

collect switchgrass for biomass production [52]. The loafing system is similar to

the field chopping system (dry storage) with the exception that instead of blowing

switchgrass material into a semi trailer the loaf stacker picks up switchgrass from

the windrow and makes a biomass stack approximately 2.4 m wide, 6 m long, and

3.6 m high [51]. The roof of the loafer stacker has a dome shape which creates a

biomass stack that resembles a bread loaf and is designed to shed water. Field

capacity of a loafer stacker is lower than either conventional baling system or a

forage chopper. Once the loafer stacker is full, the operator needs to immediately

transport the biomass stack to the edge of field or use specialized trailers to

transport the biomass stack after harvest. Biomass stacks are also susceptible to

large biomass loss in regions with significant wind velocities if placed perpendicular to prominent wind direction.

The U.S. Department of Energy has proposed a uniform-stacking feedstock

supply design that can pre-process switchgrass and other cellulosic materials

regardless of collection method for use in a large-scale cellulosic biorefinery

(C4,535 Mg d-1

) which would increase regional and producer flexibility to harvest and collect switchgrass [38].

5 Switchgrass Harvest and Storage 121
5.3.1 Desirable Storage Characteristics

The ideal storage management procedures are to preserve switchgrass so that it

enters and leaves the storage phase in an unaltered state [53]. Key factors in

minimizing storage loss for bales are to ensure low moisture levels prior to storage

and protection from moisture during the storage phase. Low relative humidity and

low ambient temperatures during storage also reduce DM loss and composition

degradation. Maintaining low biological activity during storage to reduce microbial growth and subsequent storage loss is also important.

Switchgrass with higher levels of N or with increased soluble sugars have

increased potential for microbial growth and degradation during bale storage [38].

Harvest dates determine overall N and soluble sugar content in switchgrass [54].

Storage conditions that reduce the potential for spreading crop diseases, low rodent

populations, and mold spore formation are also desirable [38].

5.3.2 Storage Platforms

Although there is limited research on switchgrass storage platforms for specific

bioenergy purposes, there is significant storage research on forages that offer

insights into the advantages and disadvantage of different storage options.

Near-term storage strategies include placing bales outside on well-drained surfaces

(i.e. gravel, crushed rock), tarping, bale wrapping in plastic and indoor placement.

Optimal storage platforms are dependent on expected bale storage losses and

projected storage costs to offset these losses. For example, enclosed buildings are

the most expensive storage platform but also ensure the greatest switchgrass value

and lowest storage loss [55]. Proper storage of wet material or ensiling has been

well documented for a number of feedstocks including switchgrass [38, 44, 56].

Pre-processing steps such as pelletizing or briquetting switchgrass provides

decreased storage losses and decreased transportation costs [57]. Estimated capital

costs for a pellet mill or briquettor, however, potentially offset any near-term

savings in storage or transportation costs [41].

5.3.3 Storage Losses

Limited research has been conducted on DM losses during switchgrass storage

with most research evaluating storage loss using the baling method. In Texas, DM

losses for large, round bales ranged from 1 to 5%, with larger losses occurring with

drier material [58]. Switchgrass round bales stored for 6–12 months inside had

0–2% DM losses, whereas bales stored outside lost 5–13% of the original bale

weight [58]. In southeastern U.S., round bales with higher initial moisture content

122 R. Mitchell and M. Schmer
and longer storage times caused increased DM loss when stored outside [59].

In Indiana, switchgrass round bales wrapped in twine had 13% DM loss on sod but

bales stored on crushed rock had 5% DM loss after six months [60]. Switchgrass

round bales stored outside on either sod or gravel showed similar DM losses

12 months after baling in Texas [58]. Estimated DM storage losses in excess of

16% are required to cover the initial cost of storage sites using crushed rock for

improved drainage [38]. In southern Europe, switchgrass round and rectangular

bales showed minimal storage loss and no visible microbial activity when stored

under a sheltered roof [61]. Storage loss was found to be greater for tarped large

rectangular bales than for tarped round bales and that delivery costs increased with

larger storage times due to increased storage losses [62]. Tarped and untarped

large rectangular bales had DM losses of 7% and up to 25%, respectively, six

months after harvest in Nebraska [8]. Water and temperature together determines

microbial damage for storage systems with regions having high relative humidity

and having temperatures results in increased storage degradation on portions of

biomass in direct contact with air [38]. In general, biomass stored dry should be

kept at moisture levels below 15% to prevent biomass degradation by filamentous

fungi and bacteria [63]. Additional physical factors that cause storage loses include

wind erosion or handling losses, moisture partitioning, bulk settling, and dust

accumulation [38].

5.3.4 Changes in Composition During Storage

Composition changes during storage will likely be more unfavorable for biochemical conversion than either thermo-chemical conversion technology or direct

combustion for electrical generation. Switchgrass round bales stored unprotected

outside lost up to 11% of ethanol extractables, which could significantly reduce

conversion to ethanol [64]. Biomass quality heterogeneity will occur within bales

with portions of the bale showing no signs of degradation while other portions

having significant spoilage or composition degradation. Round bales can be

segmented to four portions based on the potential for deterioration [65].

Approximately 33% of a round bale circumference contacts the ground after

settling which can absorb moisture and result in spoilage [38]. The first round bale

portion is where the round bale contacts the ground up to 15 cm. A transitional

area above this portion (15–30 cm) can also be degraded depending on moisture

conditions and length of storage. Sanderson et al. [58] noted that switchgrass round

bales stored on sod had a large, black layer where the bale was in contact with sod

whereas round bales stored on gravel did not have this layer of spoilage indicating

outside storage method will influence composition heterogeneity of bales. The

third portion of the round bale is the outer 15 cm of the round bale not in contact

with the ground. This portion can also have compositional changes depending on

weather factors, length of storage, and wrapping methods (i.e. plastic twine, plastic

net-wrap). The final portion of the round bale is the core which is the least likely to

5 Switchgrass Harvest and Storage 123
have compositional changes during storage. Biomass degradation from weather

and microbial activity can be as high as 42% by volume for a round bale [65].

For large rectangular bales, moisture can penetrate at the top of the bale or can be

absorbed at the bottom of the bale. A large rectangular bale is comprised of a

number of layers of switchgrass compressed together. Water channeling can occur

within these layers causing heterogeneous spoilage ([38]; Fig. 5.2).

Large rectangular bales are typically stacked so water channels between layers

can cause biomass degradation to adjacent bales as well. Chopped switchgrass in a

dry form will have the most compositional changes around the outer surface layer

(0–0.8 m) with the interior portions unaltered. The amount of compositional

changes by volume from dry chopped switchgrass piles is a result of the overall

stack size. Sulfuric acid pretreatment on switchgrass stored under wet conditions

inhibited microbial activity and resulted in 7% higher ethanol conversion efficiency than untreated switchgrass [50].

5.4 Conclusions

This brief overview has scratched the surface of switchgrass harvest and storage

management. Proper harvest and storage management is paramount to providing a

consistent and high-quality feedstock to the biorefinery. Although the bioenergy

conversion platform will guide the switchgrass harvest and post harvest

management practices, proper handling will ensure optimum biofuel recovery.

Continued research on the effects of harvest and storage management on feedstock

characteristics is critical as landscape scale deployment of switchgrass for

bioenergy moves forward. Important areas for continued research include the

effects of compositional changes during storage on biofuel production; harvest

timing effects on ecosystem services, especially SOC sequestration, wildlife, and

Fig. 5.2 Large rectangular

bales are susceptible to

spoiling on the top and

bottom of the bale if not

stored properly. This bale

was cut in half to expose the

spoilage on the bale interior

(photo by Rob Mitchell)

124 R. Mitchell and M. Schmer
pollinator habitat, and GHG emissions and mitigation; and long-term research

evaluating harvesting effects on macronutrient and micronutrient removal, as well

as developing strategies for maintaining soil nutrient status.

References

1. Vogel KP (2004) Switchgrass. In: Moser LE, Burson BL, Sollenberger LE (eds)

Warm-season (C4) Grasses. ASA-CSSA-SSSA, Madison

2. Vogel KP, Sarath G, Saathoff A, Mitchell R (2011) Switchgrass. In: Halford N, Karp A (eds)

Energy Crops. The Royal Society of Chemistry, Cambridge

3. Stubbendieck JL, Hatch SL, Butterfield CH (1997) North American Range Plants, 5th edn.

University of Nebraska Press, Lincoln

4. Sanderson MA, Jones RM, McFarland MJ, Stroup J, Reed RL, Muir JP (2001) Nutrient

movement and removal in a switchgrass biomass-filter strip system treated with dairy

manure. J Environ Qual 30:210–216

5. Mitchell RB, Moore KJ, Moser LE, Fritz JO, Redfearn DD (1997) Predicting developmental

morphology in switchgrass and big bluestem. Agron J 89:827–832

6. Dahl BE, Hyder DN (1977) Developmental morphology and management implications.

In: Sosebee RE (ed) Rangeland plant physiology. Society for Range Management, Denver,

pp 257–290

7. Mitchell RB, Moser LE, Moore KJ, Redfearn DD (1998) Tiller demographics and leaf area

index of four perennial pasture grasses. Agron J 90:47–53

8. Mitchell RB, Vogel KP, Schmer MR, Pennington D (2010) Switchgrass for biofuel production.

http://www.extension.org/pages/Switchgrass_for_Biofuel_Production. Accessed 30 Nov 2011

9. Mitchell RB, Moser LE (2000) Developmental morphology and tiller dynamics of warmseason grass swards. pp 47–64. In: K.J. Moore and B.E. Anderson (eds), Native warm-season

grasses: research trends and issues. CSSA Spec. Publ. 28, CSSA/ASA, Madison

10. Nelson CJ, Moser LE (1994) Plant factors affecting forage quality. In: Fahey GC Jr et al (eds)

Forage quality, evaluation, and utilization. ASA/CSSA/SSSA, Madison, pp 115–154

11. Redfearn DD, Moore K, Vogel K, Waller S, Mitchell R (1997) Canopy architecture and

morphology of switchgrass populations differing in forage yield. Agron J 89:262–269

12. Moore KJ, Moser LE (1995) Quantifying developmental morphology of perennial grasses.

Crop Sci 35:37–43

13. Hancock DW (2009) The management and use of switchgrass in Georgia. Georgia

Cooperative Extension Bulletin 1358

14. Wolf DD, Fiske DA (2009) Planting and managing switchgrass for forage, wildlife, and

conservation. Virginia Cooperative Extension Bulletin, 418-013

15. Hohenstein WG, Wright LL (1994) Biomass energy production in the United States: An

overview. Biomass Bioenerg 6:161–173

16. Monti A, Bezzi G, Pritoni G, Venturi G (2008) Long-term productivity of lowland and

upland switchgrass cytotypes as affected by cutting frequency. Bioresour Technol 99:

7425–7432

17. Sanderson MA, Read JC, Reed RL (1999) Harvest management of switchgrass for biomass

feedstock and forage production. Agron J 91:5–10

18. Vogel KP, Brejda JJ, Walters DT, Buxton DR (2002) Switchgrass biomass production in the

Midwest USA: harvest and nitrogen management. Agron J 94:413–420

19. Mitchell RB, Vogel KP, Sarath G (2008) Managing and enhancing switchgrass as a

bioenergy feedstock. Biofuel Bioprod Bior 2:530–539

20. Wullschleger SD, Davis EB, Borsuk ME, Gunderson CA, Lynd LR (2010) Biomass

production in switchgrass across the United States: database description and determinants of

yield. Crop Sci 102:1158–1168

5 Switchgrass Harvest and Storage 125
21. Anderson BE, Matches AG (1983) Forage yield, quality, and persistence of switchgrass and

caucasian bluestem. Agron J 75:119–124

22. Newell LC, Keim FD (1947) Effects of mowing frequency on the yield and protein content of

several grasses grown in pure stands, Nebr Agric Exp Stn Bull 150

23. Adler PR, Sanderson MA, Boateng AA, Weimer PJ, Jung H-JG (2006) Biomass yield and

biofuel quality of switchgrass harvested in fall or spring. Agron J 98:1518–1525

24. Collins HP, Fransen S, Hang A, Boydston RA, Kruger C (2008) Biomass production and

nutrient removal by switchgrass (Panicum virgatum) under irrigation. In: ASA-CSSA-SSSA

international annual meetings abstract, Houston on CD

25. Liebig MA, Johnson HA, Hanson JD, Frank AB (2005) Soil carbon under switchgrass stands

and cultivated cropland. Biomass Bioenerg 28:347–354

26. Mitchell RB, Vogel KP, Uden DR (2012) The feasibility of switchgrass for biofuel

production. Biofuels 3:47–59

27. Schmer MR, Liebig MA, Vogel KP, Mitchell RB (2011) Field-scale soil property changes

under switchgrass managed for bioenergy. GCB Bioenergy 3:439–449

28. Conant RT, Paustian K, Elliot ET (2001) Grassland management and conversion into

grassland: Effects on soil carbon. Ecol Applic 11:343–355

29. Monti A, Barbanti L, Zatta A, Zegada-Lizarazu W (2011) The contribution of switchgrass in

reducing GHG emissions. GCB Bioenergy doi: 10.1111/j.1757-1707.2011.01142.x

30. Liebig MA, Vogel KP, Schmer MR, Mitchell RB (2008) Soil carbon storage by switchgrass

grown for bioenergy. Bioenergy Res 1:215–222

31. Lee DK, Owens VN, Doolittle JJ (2007) Switchgrass and soil carbon sequestration response

to ammonium nitrate, manure, and harvest frequency on Conservation Reserve Program land.

Agron J 99:462–468

32. McLaughlin SB, De La Torre Ugarte DG et al (2002) High-value renewable energy from

prairie grasses. Environ Sci Tech 36:2122–2129

33. Al-Kaisi MM, Yin X, Licht MA (2005) Soil carbon and nitrogen changes as influenced by

tillage and cropping systems in some Iowa soils. Agric Ecosyst Environ 105:635–647

34. Frank AB, Berdahl JD, Hanson JD, Liebig MA, Johnson HA (2004) Biomass and carbon

partitioning in switchgrass. Crop Sci 44:1391–1396

35. Garten CT, Wullschleger SD (2000) Soil carbon dynamics beneath switchgrass as indicated

by stable isotope analysis. J Environ Qual 29:645–653

36. Ma Z, Wood CW, Bransby DI (2001) Impact of row spacing, nitrogen rate, and time on

carbon partitioning of switchgrass. Biomass Bioenerg 20:413–419

37. Zan CS, Fyles JW, Girouard P, Samson RA (2001) Carbon sequestration in perennial bioenergy,

annual corn and uncultivated systems in southern Quebec. Agric Ecosys Environ 86:135–144

38. Hess JR, Kenney KL, Ovard LP, Searcy EM, Wright CT (2009) Uniform-format bioenergy

feedstock supply system: a commodity-scale design to produce and infrastructure-compatible

bulk solid from lignocellulosic biomass. Idaho National Laboratory, Idaho Falls

39. Aden A, Ruth M, Sheehan J, Ibsen K, Majdeski H, Galvez A (2002) Lignocellulosic biomass

to ethanol process design and economics utilizing co-current dilute acid prehydrolysis and

enzymatic hydrolysis for corn stover. National Renewable Energy Laboratory, Golden

40. Laser M, Jin H, Jayawardhana K, Lynd LR (2009) Coproduction of ethanol and power from

switchgrass. Biofuel Bioprod Bior 3:195–218

41. Yu T, Larson JA, English BC, Cho S (2011) Evaluating the economics of incorporating

preprocessing facilities in the biomass supply logistics with an application in east Tennessee

The Southeastern Sun Gran Center Final Report

42. Inman D, Nagle N, Jacobson J, Searcy E, Ray AE (2010) Feedstock handling and processing

effects on biochemical conversion to biofuels. Biofuel Bioprod Bior 4:562–573

43. Doe US (2011) US Billion-ton update: Biomass supply for bioenergy and bioproducts

industry. Oak Ridge National Laboratory, Oak Ridge

44. Collins M, Owens VN (2003) Preservation offorage as hay and silage. In: Barnes RF, Nelson CJ,

Collins M, Moore KJ (eds) Forages: an introduction to grassland agriculture, 6th edn. Iowa State

Press, Ames

126 R. Mitchell and M. Schmer
45. Turhollow A, Downing M, Butler J (1998) Forage harvests and transportation costs.

Oak Ridge National Laboratory, Oak Ridge

46. Lazarus W, Selley R (2005) Farm machinery economic costs estimates for 2005. University

of Minnesota Extension Service, Minneapolis

47. Shinners KJ, Boettcher GC (2006) Drying, harvesting and storage characteristics of perennial

grasses as biomass feedstocks. ASABE Annual International Meeting, Minneapolis

48. Bransby DI, Smith HA, Taylor CR, Duffy PA (2005) Switchgrass budget model: An

interactive budget model for producing and delivering switchgrass to a bioprocessing plant.

Ind Biotech 2:122–125

49. Cundiff JS, Fike JH, Parrish DJ, Alwang J (2009) Logistic constraints in developing

dedicated large-scale bioenergy systems in the Southeastern United States. J Environ Eng

135:1086

50. Digman M, Shinners K, Muck R, Dien B (2010a) Full-scale on-farm pretreatment of

perennial grasses with dilute acid for fuel ethanol production. Bioenerg Res 3:335–341

51. Kumar A, Sokhansanj S (2007) Switchgrass (Panicum vigratum L.) delivery to a biorefinery

using integrated biomass supply analysis and logistics (IBSAL) model. Bioresour Technol

98:1033–1044

52. Sokhansanj S, Sudhagar M, Turhollow A, Kumar A, Bransby D, Lynd L, Laser M (2009)

Larg-scale production, harvest, and logistics of switchgrass (Panicum virgatum L.)-current

technology and envisioning a mature technology. Biofuel Bioprod Bior 3:124–141

53. Hess JR, Wright CT, Kenney KL (2007) Cellulosic biomass feedstocks and logistics for

ethanol production. Biofuel Bioprod Bior 1:181–190

54. Dien B, Jung H, Vogel K, Casler M, Lamb J, Iten L, Mitchell R, Sarath G (2006) Chemical

composition and response to dilute-acid pretreatment and enzymatic saccharification of

alfalfa, reed canary grass, and switchgrass. Biomass Bioenerg 30:880–891

55. Duffy M (2007) Estimated costs for production, storage and transportation of switchgrass.

PM 2042. Iowa State University Extension, Ames

56. Digman MF, Shinners KJ, Muck RE, Dien BS (2010b) Pilot-scale on-farm pretreatment of

perennial grasses with dilute acid and alkali for fuel ethanol production. T Asabe 53:

1007–1014

57. Tumuluru JS, Wright CT, Hess JR, Kenney KL (2011) A review of biomass densification

systems to develop uniform feedstock commodities for bioenergy application. Biofuel

Bioprod Bior 5:683–707

58. Sanderson MA, Egg RP, Wiselogel AE (1997) Biomass losses during harvest and storage of

switchgrass. Biomass Bioenerg 12:107–114

59. Cundiff JS, Marsh LS (1995) Effects of ambient environment on the storage of switchgrass

for biomass to ethanol and thermochemical fuels. National Renewable Energy Laboratory,

Golden

60. Johnson KD, Cherney JH, Greene DK, Valence IJ (1991) Evaluation of switchgrass and

sorghum biomass potential. Oak Ridge National Laboratory, Oak Ridge

61. Monti A, Fazio S, Venturi G (2009) The discrepancy between plot and field yields: Harvest

and storage losses of switchgrass. Biomass Bioenerg 33:841–847

62. Larson JA, Mooney DF, English BC, Tyler DD (2010) Cost analysis of alternative harvest

and storage methods for switchgrass in the southeastern. In: U.S. Southern agricultural

economics association annual meeting, Orlando

63. Rotz CA (2003) How to maintain forage quality during harvest and storage. Adv Dairy

Technol 15:227–239

64. Wiselogel AE, Agblevor FA, Johnson DK, Deutch S, Fennell JA, Sanderson MA (1996)

Compositional changes during storage of large round switchgrass bales. Bioresour Technol

56:103–109

65. Rider AR, Batchelor D, McMurphy W (1979) Effects of long-term outside storage on round

bales. In: Am Soc Agric Eng, St. Joseph, MI, ASAE Paper No. 79–1538

5 Switchgrass Harvest and Storage 127