Biomass yield, energy values, and chemical
composition of hybrid poplars in short rotation
woody crop production and native perennial
grasses in Minnesota, USA

Diomides S. Zamora

a

,

*

, Gary J. Wyatt

b

, Kent G. Apostol

c

, Ulrike Tschirner

d

a

University of Minnesota Extension, 1530 Cleveland Ave, N, St. Paul, MN 55108, USA

b

University of Minnesota Extension, 1961 Premier Drive, Suite 110, Mankato, MN 56001, USA

c

Department of Natural Sciences, Emmanuel College, 181 Spring St, Franklin Springs, GA 30639, USA

d

Department of Bioproducts and Biosystems Engineering, 204 Kaufert Lab, 2004 Folwell Avenue St. Paul, MN 55108, USA

a r t i c l e i n f o

Article history:

Received 6 June 2012
Received in revised form
10 December 2012
Accepted 19 December 2012
Available online 25 January 2013

Keywords:

Conservation Reserve Programs
Hybrid poplars
Polycultures
Biomass
Cellulose
Lignin
Cellulosic ethanol

a b s t r a c t

The increasing cost of fossil fuels such as petroleum, and a desire to curtail greenhouse gas
emissions are driving the expansion of bioenergy. Plant biomass, including woody crops
and grasses, are potential energy sources. We examined the biomass production, chemical
composition, and energy content of selected hybrid poplar commercial clones such as NM6
(Populus nigra

Populus maximowiczii), D105 (Populus deltoides) and DN34 (P. deltoides P.

nigra) and native grasses and forbs established in polyculture systems in Minnesota, USA.
In our study, we found that at the end of the 13-year growing season, NM6 had significantly
greater (P

¼ 0.0122) biomass production than D105 and DN34 with a total biomass

production of 11.46 Mg ha

1

. The chemical composition (mass fraction % on dry basis) of

hybrid poplar clones generally contained 39% cellulose, 21% hemicellulose, 27% lignin, 1.3%
ash content, and about 17,900

e18,031 kJ kg

1

of dry wood. In contrast, after 7 years of

growth, biomass such as that from the 5 grass mixture (5G), produced the highest amount
of biomass (7.9 Mg ha

1

) and largest theoretical ethanol yield (425 L Mg

1

of dry biomass),

and contained mass fraction of 36% cellulose, 28% hemicellulose, 20% lignin, 6.04% ash
content and about 16,731 kJ kg

1

of dry grass. Our data also showed that the slash left on

site constituted a significant source of biomass (theoretically 375

e390 L of ethanol for every

megagram of biomass) that could be utilized for bioenergy. Seasonal timing of native
grasses harvest significantly affected ethanol yield (P

¼ 0.020) and energy content

(P

¼ 0.020). Strips of native grasses harvested in Spring 2009 that were allowed to re-grow

and harvested in Fall 2009 showed greater potential for ethanol production than those
harvested in Spring 2009. Thus, we suggest that hybrid poplars and native perennial
grasses offer promising potential as alternative sources of renewable energy. Results of our
study indicate that low-input high diversity systems can be utilized as an alternative
source of biomass for energy and it could facilitate commercial production of the crops.

ª 2012 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.:

þ1 612 626 9272.

E-mail address:

[e-mail]

(D.S. Zamora).

Available online at

www.sciencedirect.com

http://www.elsevier.com/locate/biombioe

b i o m a s s a n d b i o e n e r g y 4 9 ( 2 0 1 3 ) 2 2 2

e2 3 0

0961-9534/$

e see front matter ª 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.biombioe.2012.12.031

1.

Introduction

The interest and use of woody and grass-based feedstocks
for biofuel, bioenergy, and bioproducts, as a result of rising
fuel cost, are increasing because of the growing demand for
alternative energy sources. The United States of America
2007 Energy Independence and Security Act (EISA), invest-
ments in lignocellulosic biorefineries by the US

e Depart-

ment of Energy (DOE) and commercial entities, as well as the
proliferation of biomass energy-based markets, security and
policy drivers, have increased public interest in harvesting
non-grain biomass which include native grasses and woody
crops. This interest is promising because it is creating
investment and entrepreneurial opportunities in many rural
communities.

Biomass from grass monocultures, polycultures (mixture

of grasses/forbs) and short rotation woody crops (SRWCs)
such as hybrid poplars (Populus spp.) offer potential to achieve
the requirement of EISA of 2007, which mandates at least 60%
of renewable fuels be produced from cellulosic feedstocks.
Biomass energy producing companies in Minnesota need
information on productivity and energy content of potential
biomass feedstock suitable for their energy production
systems. Hybrid poplars are ideal feedstock for energy
because they could generate significant amount of biomass
and provide a plethora of ecological services including carbon
sequestration and wildlife habitat improvement. Similarly,
native perennial grasses such as switchgrass (Panicum virga-
tum L.) offer several conservation benefits compared to high-
intensity row crops, which is why they may be more suit-
able in some eco-regions and on some agricultural production
systems

[1]

. By virtue of their perennial nature, these crops

reduce the frequency of, and potential soil and environmental
degradation associated with annual tillage. Similarly, peren-
nials also capture solar radiation for a longer portion of the
year compared to annual species

[2]

. Switchgrass, a common

perennial grass being evaluated as a bioenergy feedstock, has
higher root density than annual crops (e.g. corn

e Zea mays L.)

or even alfalfa (Medicago sativa L.)

[3]

. These perennials are also

known to help stabilize soils, thus reducing erosion,
improving water quality, increasing and improving wildlife
habitat, and sequestering carbon

[3]

. Additionally, research on

the chemical composition of biomass has been focused on
individual forage and crop species such as switchgrass

[4]

with

very little published research on mixed native perennial
plants as a bioenergy feedstock.

Much research has been conducted regarding clonal

production of hybrid poplars as well as productivity assess-
ment of perennial grasses as affected by agronomic prac-
tices; however, there is insufficient information available on
the biomass production and energy conversion potential of
hybrid poplars and perennial grasses. Minnesota has a vast
tract of land planted with grasses and woody crops that are
enrolled in the federal Conservation Reserve Programs (CRP)
with about 12,000 ha of land being planted with SRWCs on
privately-owned lands in West-Central region of Minnesota.
Such native grassland areas and plantations have potential
biomass energy value. Though harvesting is not currently
permitted on most federal and state conservation lands,

collecting material from these areas could provide signifi-
cant amount of additional biomass

[5]

. If biomass from

grass/forbs polycultures were shown to be economically
feasible, landowners could maintain the perennial vegeta-
tion for biomass production after the CRP contract expires
and could provide alternative sources of income for the
landowners.

Our objective was to build on the works of Huang et al.

[6]

and Zalesny et al.

[7]

regarding the biomass productivity and

energy values of bioenergy crops. The data presented herein
are from two field studies investigating the biomass produc-
tion, chemical composition, and energy content of selected
hybrid poplar clones and native forbs and grasses established
in polyculture systems in Minnesota. Biomass in the form of
coarse woody (e.g. branches and tree tops), residues (or
sometimes called slash) left on site following traditional
woody timber harvests in Minnesota constitute a potentially
significant source of biomass that could be utilized for bio-
energy. Therefore, questions remain regarding whether
biomass removal from these areas would be a sustainable
practice given the potential impact on soil nutrient cycling
and other ecosystem functions. In the first study, we
measured the biomass production of various clones and
energy value of hybrid poplars. In the second study, we
compared the biomass and energy values between mono-
culture and mixture of grasses and forbs and examined how
seasonal time of harvest affected biomass yield and chemical
composition for energy production.

2.

Materials and methods

2.1.

Biomass from hybrid poplars

2.1.1.

Site description, clone selection and experimental

design

The study was conducted at the Herman Rosholt Farm in
Westport, Minnesota (45.7

N, 95.2

W). The soil topography

exhibited 0

e3% slopes where predominant soil type was

Estherville loom, which is characterized as sandy, mixed
mesic Typic Hapludolls. The climate of the area is mild during
summer and very cold during winter, characterized by an
annual average maximum temperature of 15.5

C and average

minimum temperature of

12

C, and annual average

precipitation of 627 mm. The mean precipitation during the
growing season from June to August, calculated over a period
of 13 years (1995

e2008) was 303 mm. The hybrid poplar clones

were planted in 1995 and 1997 as part of the research of the US
Forest Service in Rhinelander, Wisconsin, USA in partnership
with Pope Soil and Water Conservation District (SWCD) and
the WesMin Research and Conservation Development (RC&D)
in Alexandria, Minnesota. The projects had encompassed
planting different varieties of hybrid poplars to evaluate
growth, production, disease, drought tolerance, soil adapt-
ability, insect tolerance, survivability, ease of establishment,
and for parent material for crossbreeding. There were a total
of 2.82 ha of established poplar trees. In 1995 and 1997, 59 and
79 clones of hybrid poplars were planted respectively, as part
of the study, which were planted in Spring of both years. Both
hybrid poplar sites were irrigated. Trees were planted in

b i o m a s s a n d b i o e n e r g y 4 9 ( 2 0 1 3 ) 2 2 2

e2 3 0

223

randomized

block

design

planted

at

a

spacing

of

3.05 m

3.05 m, resulting in a stem area density of 1075 ha

1

[7]

. Clones were arranged in randomized block in all years to

minimize effects of any potential environmental gradient.
Border rows were established to reduce potential border
effects.

2.1.2.

Harvesting

In winter 2008, the 1995 and 1997 hybrid poplars plantings
were

harvested

using

logging

harvesting

equipment.

However, not all of the clones planted were subjected to
biomass assessment and energy content determination.
Rather, we focused our data collection on selected clones
such as NM6 (Populus nigra

P. maximowiczii), D105 (Populus

deltoides) and DN34 (P. deltoides

P. nigra), which are

preferred by landowners in Minnesota. Clones were chosen
from the breeding program of the Natural Resource Research
Institute (NRRI) at the University of Minnesota. Prior to
harvesting, height and diameter of these clones were
determined using forestry measuring devices such as
clinometers and diameter tape. Diameter at breast height
(DBH) was measured to the nearest 0.1 cm on these clones.
All diameter data were used to estimate woody biomass
yield from a model developed following destructive above-
ground harvest of multiple hybrid poplar genotypes across
numerous sites

[8]

. This model was most recently used by

Zalesny and Zalesny

[9]

, Netzer et al.

[10]

, and Rie-

menschneider et al.

[11]

to determine the biomass produc-

tion of hybrid poplar in Westport, Minnesota.

Woody biomass

¼

6

:16 2:23 DBH

þ 0:353 DBH

2

(1)

Individual-tree biomass data from Ref.

[1]

were multiplied

by the stocking rate of 1075 ha

1

, and their product was

divided by the age of the trees to calculate biomass per unit
land area per year.

Base, middle, and slash (composed of tree branches and

tops) biomass were 30%, 50%, and 20% of total tree biomass,
respectively. The woody biomass from the following tree parts
were sampled to determine its energy content; hence, a total
of three samples per tree.

2.2.

Biomass from grass monoculture and polycultures

2.2.1.

Site, species description, and experimental design

Four established (polyculture native/forbs perennial grasses)
Conservation Reserve Program (CRP) sites located in the
Minnesota river watershed encompassing Watonwan and
Cottonwood counties in Southwestern, Minnesota were
selected for the study (

Table 1

). Each site, which varied in

species composition, was designated as i) monoculture of
switchgrass, MS, ii) mix of four grasses with little forbs, 4G

þ F,

iii) mix of 5 grasses and 13 forbs, 5G

þ 13F, and iv) 5 grasses

mix, 5G. Prior to converting into CRP using native mix grasses,
each site was under row crop production systems.

Table 1

shows planting application rate of each CRP site including
the year of CRP establishment. Topography from each site
ranges from 1 to 3%. Soil characteristics of each site vary. The
dominant soil in MS site is Dassel fine sandy loam which is
characterized as coarse-loamy, mixed, superactive, mesic

Typic Endoaquolls, with 0

e1% slope. Soil in 4G þ F site is

dominated by Kingston silty clayloam, which is characterized
as fine-silty, mixed superactive, mesic Aquatic Hapludolls.
Clarion loam series dominates soil condition in the 5G

þ 13F

site which is characterized as fine-loamy, mixed, superactive,
mesic Typic Hapludolls. The dominant soil in 5G is Estherville
sandy loam which is characterized as sandy, mixed Typic
Hapludolls. The annual average minimum and maximum
temperature is

7

C and 13

C, respectively, with an average

annual precipitation of 650 mm.

2.2.2.

Harvesting

Six strips in each site, which were considered as replicates,
were established with a total of 24 strips for the entire study.
Each strip (2.43 m

121.9 m) was mowed using disk mower

and harvested grasses were then baled 4 h after harvesting.
Treatments included 1) harvested in May 2009 (Spring
Harvest), 2) strips harvested in May 2009 were allowed to re-
grow and harvested in Fall 2009 (Fall Regrowth), and 3)
delayed harvest until Fall 2009 (Fall Harvest). Harvested
biomass was determined for each time of harvest and
compared with each other. Baled materials were weighed to
determine biomass (Mg ha

1

). Prior to baling, four grass

samples were collected in each strip. Samples were system-
atically collected in the strip such that samples were collected
from 15.2, 45.7, 76.2 and 106.7 m away from the start of the
strip. Each sample was placed in a bag separately for compo-
sition analysis.

3.

Energy content, chemical composition and

ethanol yield determination

Wood and grass samples were sent to the Kaufert Wood
Science Laboratory at the University of Minnesota for
composition analysis. Prior to chemical analysis, wood and
grassland samples were grounded to pass through 40 mesh
based on specification of the National Energy Research
Laboratory (NREL) test procedure

[12]

. Samples were

analyzed for cellulose, hemicellulose, and lignin content.
Klason lignin content was measured in accordance with the
method described in TAPPI (T222om-23). For this analysis,
ground samples were hydrolyzed in a water bath for 60 min
at 30

C using H

2

SO

4

with mass fraction of 72%. The samples

were diluted with deionized water and heated in an auto-
clave at 120

C for 60 min. After cooling to room tempera-

ture, the acid-insoluble lignin was filtered off (Klason lignin),
dried at 105

C and weighed. Carbohydrates were deter-

mined by subjecting the samples to acid hydrolysis using
Klason lignin methodology, removal of acid-insoluble lignin
through filtration, and determination of sugar concentra-
tions in the filtrate using the HPLC method described in the
Laboratory Analytical Procedure NREL LAB method # 002

[12]

.

The HPLC used was a Waters system equipped with a Bio-
Rad De-ashing cartridge in line with a VARIAN MetaCarb
87P analytical column. The operating pressure was 3720 kPa
at 0.3 mL min

1

of flow rate and the column temperature

was 80

C.

Ash content was determined using TAPPI standard test

procedure TAPPI T 211, combustion at 525

C. Further,

b i o m a s s a n d b i o e n e r g y 4 9 ( 2 0 1 3 ) 2 2 2

e2 3 0

224

theoretical ethanol yield (L Mg

1

of dry biomass) was

calculated using the NREL online calculator (

http://www1.

eere.energy.gov/biomass/ethanol_yield_calculator.html

).

Values of each sugar component such as glucan, xylan,
galactan, arabinan and mannan were plugged in the online
calculator. The potential ethanol yield was calculated from
the sum of C6 carbohydrates (mannose, galactose, and
glucose) and C5 cell wall carbohydrate xylose and arabinose

[13]

. Ethanol values were then recorded. Gross energy

values were estimated using a calculative method proposed
by Jung et al.

[14]

.

4.

Data analyses

Independent yearly data for hybrid poplar were analyzed
using analysis of variance (ANOVA), within the framework of
randomized block design (SAS Institute, Cary, NC) with a clone
main effect. In each analysis, main effects were tested for
significance using appropriate error terms. If significant
treatment effects were revealed at

a ¼ 0.05, the Duncan

procedure was used for mean separation. ANOVA was also
used to analyze data for the second independent study.

5.

Results and discussion

5.1.

Hybrid poplar

5.1.1.

Biomass production and chemical composition

The amount of biomass produced between clones planted in
1995 (P

¼ 0.04) and 1997 (P ¼ 0.0122) differed significantly. At

the end of the 13-year growing season for the 1995 planting,
the total amount of biomass production of the NM6 clone was
11.46 Mg ha

1

, which is almost similar to the biomass

production of the same clone in previous studies

[7]

. The D105

clone showed significantly (P

¼ 0.04) lower (28%) biomass than

NM6 (

Fig. 1

A). A similar trend was observed on the produc-

tivity of hybrid poplar clones planted in 1997, with signifi-
cantly greater (P

¼ 0.0122) biomass observed for NM6 than for

DN34. At the end of the 11-year growing period, NM6 exhibited
total biomass production of 7.5 Mg ha

1

, while DN34 was at

Table 1

e Description, date establishment, species planted and management of the Conservation Reserve Program (CRP)

sites in Southwestern, Minnesota, USA for biomass production and energy content assessment and evaluation.

Site

Description

Date planted

Species

Seeding rate

(kg ha

1

)

Management

MS

Monoculture
switchgrass

May 20, 2002

Switchgrass (Panicum virgatum L.)

1.10

Burning once

4G

þ F

Mix of four grasses
with little forbs

June 1, 2002

Switchgrass (P. virgatum L.)

0.37

Clipping and

Big bluestem (Andropogon gerardii Vitman)

0.73

Burning once

Indiangrass (Sorghastrum nutans L.)

0.27

Slender wheatgrass (Elymus trachycaulus
(Link) Gould ex Shinners)

0.10

Forbs

0.10

Goldenrod (Solidago spp.)
Gray headed coneflower

(Ratibida pinnata (Vent.) Barnh.)

5G

þ 13

Mix of 5 grasses
and 13 forbs

June 1, 2002

Indiangrass (Sorghastrum nutans L.)

0.39

None

Big bluestem (Andropogon gerardii Vitman)

0.55

Sideoats grama (Bouteloua curtipendula
(Michx.) Torr.)

0.10

Little bluestem (Schizachyrium scoparium
(Michx.) Nash)

0.38

Switchgrass (P. virgatum L.)

0.10

Forbs

1.46

Goldenrod (Solidago spp.)
Gray headed coneflower

(Ratibida pinnata (Vent.) Barnh.)

Golden alexander (Zizia aurea L.)
Yarrow (Achillea millefolium L.)
Bee balm (Monarda fistulosa L.)

5G

5 grasses mix

May 25, 2002

Switchgrass (P. virgatum L.)

0.37

Weed control

Big bluestem (Andropogon gerardii Vitman)

0.54

Indiangrass (Sorghastrum nutans L.)

0.37

Sideoats grama (Bouteloua
curtipendula (Michx.) Torr.)

0.18

Canada wild rye (Elymus canadensis L.)

0.37

Forbs

1.46

Goldenrod (Solidago spp.)
Golden alexander (Zizia aurea L.)
Yarrow (A. millefolium L.)
Blue veravain (Verbana hastata L.)

b i o m a s s a n d b i o e n e r g y 4 9 ( 2 0 1 3 ) 2 2 2

e2 3 0

225

only 5.5 Mg ha

1

. The biomass yield obtained in this present

study is consistent with the observations reported in the Lake
States with 7.8 Mg ha

1

and 11.76 Mg ha

1

[15]

and 8.4 Mg ha

1

in the Upper Peninsula of Michigan for over 10 years

[16]

and

elsewhere

[17,18]

. The higher growth rate of clone NM6

compared with DN34 and D105 may be related to its particu-
larly rapid early growth and its exceptional early rooting

[19,20]

.

Cellulose and hemicellulose content of both clones (D105

and NM6) were not significantly different. As expected the
larger proportion of carbohydrates consisted of cellulose, with
a mean of 39% (mass fraction % on dry basis) in both clones,
whereas hemicellulose had a mean value of 21% (

Fig. 1

B).

Lignin, on the other hand, was significantly different
(P

¼ 0.0208) between D105 and NM6, with NM6 having 8%

times higher lignin content than D105. Cellulose and hemi-
cellulose (together known as holocellulose) which are
composed entirely of sugar units have a relatively low heat

content because of their high level of oxidation while lignin
has a lower degree of oxidation and considerably higher heat
of combustion

[21]

.

5.1.2.

Ethanol yield, energy content, and ash content

Theoretical mean ethanol production for hybrid poplar was
394 L Mg

1

of dry biomass (

Fig. 2

A). There was no variation in

theoretical ethanol yield for hybrid poplar in 1995 but NM6
had greater ethanol production than DN34 in 1997 (

Fig. 2

A).

This result indicates that hybrid poplar could be harvested as
early as the end of the 12-year growing season, or even earlier,
if the sole purpose of planting is for sugar based biofuels. As
observed in other studies, hybrid poplar clones such as NM6
seem particularly promising for achieving high yields in
a short (5

e10 years) time period

[22

e24]

.

There was a significant variation in theoretical ethanol

production among tree components in 1995 (P

¼ 0.001) and

1997 (P

¼ 0.001) with the base (tree trunk) and the slash

(composed of branches and tree tops) having the greatest and
least theoretical ethanol yield, respectively (

Fig. 2

B). However,

evaluation of the calculated total energy content of the clones

0

100

200

300

400

500

D105

NM6

DN34

NM6

1995

1997

Ethanol

Y

ield on dry

Biomass

(L M

g

-1

)

Hybrid Poplar Clones

0

100

200

300

400

500

Base

Middle

Slash

Base

Middle

Slash

1995

1997

Eth

a

nol

Y

ield o

n

Dry

Bio

m

ass

(LM

g

-1

)

Tree Parts

B

A

Fig. 2

e Theoretical ethanol yield (A) that could be

generated based on the amount of biomass produced of
commonly used hybrid poplar clones in Western
Minnesota, USA, and the theoretical ethanol yield that
could be produced based on tree parts in each year of
planting across clones (B). Means with the same letter
within the year are not significantly different at

a [ 0.05

level of significance.

0

2

4

6

8

10

12

14

D105

NM6

DN34

NM6

1995

1997

Biomass (M

g

ha

-1

)

Hybrid Poplar Clones

0

5

10

15

20

25

30

35

40

45

Cellulose

Hemicellulose

Lignin

Chemical composition

(mass fraction %

on

dry

basis)

D105

NM6

B

A

Fig. 1

e Biomass production (A) of commonly used hybrid

poplar clones in Minnesota planted in 1995 and 1997, and
the chemical composition (B) of hybrid poplar clones in
Minnesota planted in 1995 in Westport, Minnesota, USA.
For graph A, means with the same letter within a specific
year are not significantly different at

a [ 0.05 level of

significance. For graph B, means with the same letter
within chemical composition category are not significantly
different at

a [ 0.05 level of significance.

b i o m a s s a n d b i o e n e r g y 4 9 ( 2 0 1 3 ) 2 2 2

e2 3 0

226

planted in 1995 and 1997 showed no variations among them.
Energy content for NM6 and DN 34 was 18,124

289 kJ kg

1

of

wood and 18,035

103 kJ kg

1

of wood, respectively in 1995

and was 18,031

81 kJ kg

1

of wood (NM6) and

17,900

206 kJ kg

1

of wood (D105), respectively, in 1997.

Similarly, ash contents between and across hybrid poplar
clones were not significantly different in both years, with
1.35% and 1.18% of dry weight (mass fraction) for 1995 and
1997, respectively. These values are within the range of ash
content of hybrid poplar reported by Tharakan et al.

[25]

who

evaluated energy feedstock characteristics of willow and
hybrid poplar clones at harvest age.

When conventional forest harvesting operations are used,

there is a large amount of slash (residues), which includes the
tree tops and small branches, left on site because these
materials are currently uneconomical to recover. These
materials comprised of 20% or more of the total biomass per
tree

[17]

. Our data showed that at least 375

e390 L of ethanol

for every megagram of biomass could be generated from these
residues (

Fig. 2

B). This constitutes a significant source of

biomass that could be utilized for bioenergy. On the other
hand, from an environmental perspective, leaving logging
residues on sites are particularly important in sustaining
multiple trophic level food web interactions, nutrient cycling,
and soil sustainability

[26

e28]

. Future research should address

the quantities of nutrients potentially removed with the
residues, their potential replacement with fertilizer, and the
effects of frequent removals over long time periods.

5.2.

Native grasses

5.2.1.

Biomass production

The amount of biomass production among CRP sites also
varied significantly (

Fig. 3

). 5G had the highest amount of

biomass produced (7.9 Mg ha

1

), which is 73% higher

compared to the amount of biomass produced in 4G

þ F Mix

(2.1 Mg ha

1

). Cardinale et al.

[29]

observed that mixtures of

species produce an average of 1.7 times more biomass than
monocultures.

5.2.2.

Ethanol yield, energy and ash content, and

chemical composition

Fig. 4

A shows that 5G can theoretically produce the largest

amount of ethanol per unit biomass, followed by mono-
culture, 4G

þ F, and 5G þ 13F Mix. The ranking of CRP sites in

order of decreasing total energy value (content) was:
5G

> MS > 4G þ F > 5G þ 13F Mix. An energy value as high as

16,731 kJ kg

1

of biomass was obtained for the 5G mix after

seven years of growth (

Fig. 4

B). The amount of holocellulose,

and with that the theoretical ethanol yield, are highest in 5G
and lowest in the 5G

þ 13F (

Fig. 4

A and B). Both cellulose and

hemicellulose are the source of the fermentable sugars used
in biological conversion processes generating liquid fuels,
such as cellulosic ethanol. Since lignin content significantly
inhibits release of fermentable sugars in biomass, grass with
less lignin has greater potential for energy production of bio-
fuels from biomass sugars. In addition to direct relationship
between the amount of biomass produced and theoretical
ethanol yield, differences in species composition among sites
explains the variations in ethanol production as also noted by
Valentas et al.

[30]

who found and indicated that CRP areas

dominated with C4 species such as switchgrass but with some

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

Monoculture

Switchgrass

4G + F Mix

5G + 13 F Mix

5G mix

Biomass (M

g

ha

-1

)

CRP Site

Fig. 3

e Grass biomass production of different

Conservation Reserve Program sites in Southwestern,
Minnesota, USA that were evaluated for the study. Each
site differed in species composition. Means with the same
letter are not significantly different at

a [ 0.05 level of

significance.

0

100

200

300

400

500

Monoculture

Switchgrass

4G + F Mix

5G + 13 F Mix

5G Mix

Ethanol

Y

ield on

dry

Biomass

(L M

g

-1

)

CRP Site

3000

6000

9000

12000

15000

18000

Monoculture

Switchgrass

4G + F Mix

5G + 13 F Mix

5G Mix

Ener

gy

Value

of

dry

biomass

(kJ

kg

-1

)

CRP Site

A

B

Fig. 4

e Theoretical ethanol yield (A) and energy value/

content (B) of biomass harvested from Conservation
Reserve Program (CRP) sites in Southwestern, Minnesota,
USA with varying species composition. In each graph (A or
B), means with the same letter are not significantly
different at

a [ 0.05 level of significance.

b i o m a s s a n d b i o e n e r g y 4 9 ( 2 0 1 3 ) 2 2 2

e2 3 0

227

forbs (similar to the 5G site of our study) produced more
ethanol per ton of biomass primary because of the higher
photosynthetic pathway of the species capable of producing
more biomass. Forbs species with N-fixing capability could
help enhance C4 species to produce more biomass.

Fig. 5

A shows the order of ash content starting with the

highest percentage of ash: 4G

þ F mix > 5G þ 13F

Mix

> monoculture > 5G Mix. The ash content of these

grasses is within the range reported by Jenkins et al.

[3]

who

evaluated the different properties of biomass suitable for
energy including switchgrass and SRWCs. High ash content
of a plant part makes it less desirable as fuel. It has been
reported that reduction in ash concentration is related to
increased biofuel quantity in switchgrass

[30,31]

. A similar

trend was observed in terms of chemical composition of
grassland biomass. Across our CRP sites, about 38% (mass
fraction on dry basis) of the chemical composition of grass-
land biomass is made up of cellulose (

Fig. 5

B). Unlike hybrid

poplar, the lignin content of grassland biomass is much lower
compared to hemicellulose.

The high biomass yield and energy content observed in the

5G is partly explained by the high site fertility in terms of

NO

3

1

availability. A NO

3

1

as high as 2.4 mg kg

1

was re-

ported in 5G mix and as low as 0.6 mg kg

1

of NO

3

1

in 4G

þ F

mix. Similar to our study, Tilman et al.

[32]

observed a signif-

icant increase in energy per hectare for polyculture crops as
compared to monoculture switchgrass. Contrary to our find-
ings, Adler et al.

[33]

reported that biomass production

increased as the number of plant species decreased in CRP and
other conservation grasslands. These results have implica-
tions for developing alternative (low-input high diversity
systems) uses of the CRP lands. Mixed prairie systems provide
ecological services that monoculture crops do not. They are
usually planted to restore biodiversity and improve soil
structure and maintenance. Plant diversity has been sug-
gested as a way to maximize sustainable biomass production
of prairies for biofuel production

[32]

.

5.2.3.

Seasonal time of native grasses harvest

Native grasses could be a good source of biomass for ethanol
production

[30,34,35]

. Biomass production in the Spring

harvest was 4.03 Mg ha

1

, compared with 5.02 Mg ha

1

and

6.75 Mg ha

1

for those Regrowth and Fall harvests, respec-

tively (P

¼ 0.0001) (

Fig. 6

A). Our study showed that timing of

harvest significantly altered biomass production (P

¼ 0.001)

theoretical ethanol yield (P

¼ 0.0203) (

Fig. 6

B), and energy

0

2

4

6

8

10

12

14

Monoculture

Switchgrass

4G + F Mix

5G + 13 F Mix

5G Mix

A

sh Content

(mass fraction %

on

dry

basis)

CRP Site

0

10

20

30

40

Monoculture

Switchgrass

4G + F Mix

5G + 13 F Mix

5G Mix

Chemical Comp

osition

(mass fraction %

on

dry

basis)

Lignin

Cellulose

Hemicellulose

CRP Site

A

B

Fig. 5

e Ash content (A) and chemical composition

(cellulose, hemicellulose, and lignin) (B) of grass biomass
harvested from the Conservation Reserve Program (CRP)
sites in Southwestern, Minnesota, USA with varying
species composition. For graph A, means with the same
letter are not significantly different at

a [ 0.05 level of

significance.

0

1

2

3

4

5

6

7

8

Spring

Regrowth

Fall

Biomass (M

g

ha

-1

)

Time of Harvest

100

200

300

400

500

Spring

Regrowth

Fall

Ethanol

Y

ield on dry

Biomass

(L M

g

-1

)

Time of Harvest

A

B

Fig. 6

e Grass biomass production (A) based on time of

harvest of the 5G CRP site used for the study and their
associated theoretical ethanol yield (B). In each graph,
means with the same letter are not significantly different
at

a [ 0.05 level of significance.

b i o m a s s a n d b i o e n e r g y 4 9 ( 2 0 1 3 ) 2 2 2

e2 3 0

228

content (P

¼ 0.020). Both theoretical ethanol yield and energy

content in Regrowth harvest were 7% and 9% higher, respec-
tively, compared with the Spring harvest, while theoretical
ethanol yield and energy content were similar between
Regrowth and Fall harvests. Strips of native grasses harvested
in Spring 2009 that were allowed to re-grow and harvested in
Fall 2009 (Regrowth) showed greater potential for ethanol
production than those harvested in Spring 2009 alone.
However, combining biomass harvested in Spring and
Regrowth (harvested in Fall) would have a higher biomass
yield than Fall harvest alone (

Fig. 6

A). Further, we hypothe-

sized that harvesting grass biomass in Fall would produce
more energy than in Spring due to higher quality of feedstock
as observed in difference of energy content between Spring
and Regrowth harvests. Changes in mineral concentrations
occur in plants after Fall, and most of these minerals are
retranslocated to the belowground biomass. Therefore, timing
of biomass harvest for energy is necessary

[30]

. Harvesting

grasses twice a year would provide a significant amount of
biomass for energy. However, caution should be observed in
terms of its impacts on wildlife or on the land management
scenarios of the landowners.

6.

Conclusion

Our study showed that biomass from hybrid poplars in short
rotation woody crop production and native perennial areas
could significantly contribute to the required amount of
biomass materials for production facilities for cellulosic
ethanol in Minnesota. The biomass production of 11 Mg ha

1

at the end of 13-year growing cycle of hybrid poplar could
translate to approximately 3.85

e4.92 m

3

ha

1

of ethanol with

a total HHV (higher heating value) energy content of
192 GJ ha

1

. Biomass production from grassland area is

7

e8 Mg ha

1

as it was found in our study, due to the

comparatively high content of fermentable sugars such
biomass could translate to ethanol production of approxi-
mately 1.7

e2.1 m

3

of ethanol per hectare.

Our study also showed that biomass production in grass-

land areas could be improved if such area is harvested at least
once per year. About half of the amount of biomass could be
added to the overall biomass production in CRP areas resulting
in greater amount of liters of ethanol produced if harvesting or
mowing is employed. However, care should be given full
consideration before harvesting the area. Management goal or
long term commitment of the landowners to conservation
should be given full consideration before harvesting prior to
operational practice. Results of our study would be useful for
biomass-using facilities in Minnesota as they seek innovative
and sustainable use of biomass feedstock such as energy
contents. These findings could serve as tools in decision-
making of these facilities as to what feedstock is to be
employed that is compatible to their operation.

Acknowledgments

The authors are grateful to the Three Rivers RC&D Council for
the financial support provided to this study through the

Productive Conservation Working Lands (PWCL) project. They
are also grateful for the work of Peter Gillitzer, Dean Schmidt,
and Ernie Schmitt for their assistance in the data collection.

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