A quantitative review comparing the yield of switchgrass
in monocultures and mixtures in relation to climate and
management factors
D A N WA N G , D AV I D S . L E B A U E R and M I C H A E L C . D I E T Z E
Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
Abstract
Switchgrass (Panicum virgatum L.), a US Department of Energy model species, is widely
considered for US biomass energy production. While previous studies have demonstrated
the effect of climate and management factors on biomass yield and chemical characteristics
of switchgrass monocultures, information is lacking on the yield of switchgrass grown in
combination with other species for biomass energy. Therefore, the objective of this
quantitative review is to compare the effect of climate and management factors on the
yield of switchgrass monocultures, as well as on mixtures of switchgrass, and other species.
We examined all peer-reviewed articles describing productivity of switchgrass and extracted
dry matter yields, stand age, nitrogen fertilization (N), temperature (growing degree days),
and precipitation/irrigation. Switchgrass yield was greater when grown in monocultures
(10.9 t ha1, n5 324) than when grown in mixtures (4.4 t ha1, n585); yield in monocultures
was also greater than the total yield of all species in the mixtures (6.9 t ha1, n590). The
presence of legume species in mixtures increased switchgrass yield from 3.1 tha1 (n565)
to 8.9 t ha1 (n520). Total yield of switchgrass-dominated mixtures with legumes reached
9.9 tha1 (n525), which was not significantly different from the monoculture yield. The
results demonstrated the potential of switchgrass for use as a biomass energy crop in both
monocultures and mixtures across a wide geographic range. Monocultures, but not
mixtures, showed a significant positive response to N and precipitation. The response to
N for monocultures was consistent for newly established (stand ageo3 years) and mature
stands (stand age  3 years) and for lowland and upland ecotypes. In conclusion, these
results suggest that fertilization with N will increase yield in monocultures, but not
mixtures. For monocultures, N treatment need not be changed based on ecotype and stand
age; and for mixtures, legumes should be included as an alternative N source.
Keywords: mixture, monoculture, Panicum virgatum L., yield
Nomenclature:
CT5 culture type
ET5 ecotype
GDD5 growing degree days
MT5 mixture type
PR5 precipitation
SA5 stand age
Received 4 December 2009 and accepted 12 January 2010
Introduction
As a warm-season perennial grass native to North
America, switchgrass has been designated by the US
Department of Energy as a model bioenergy feedstock
because it can produce a high yield of biomass across a
wide geographic range (McLaughlin et al., 2006).
Switchgrass is attractive as a biofuel crop because it is
suitable for use on marginal, highly erodible, and
droughty soils; it has potential for sequestering large
Correspondence: Dan Wang, tel. 1 1 217 244 5548, e-mail:
dwng@illinois.edu
GCB Bioenergy (2010) 2, 16–25, doi: 10.1111/j.1757-1707.2010.01035.x
16 r 2010 Blackwell Publishing Ltd

amounts of atmospheric carbon; and it provides nesting
habitats for migratory animals (Sanderson et al., 1996;
Roth et al., 2005; McLaughlin et al., 2006). Water and
nitrogen (N) are the principal resources limiting pro-
ductivity in warm-season grass ecosystems (Elser et al.,
2007; Harpole et al., 2007), thus the potential of a grass-
land species to become a profitable bioenergy feedstock
requires efficient use of these resources.
The response of switchgrass yield to added N varies
widely, ranging from no response (Christian et al., 2002)
to a positive response (Brejda, 2000; Muir et al., 2001),
which could reflect differences in N availability, climate,
site history, or cultivars chosen (Parrish & Fike, 2005).
However, the replacement of the removed nutrients
from soils is an important issue for high-biomass pro-
ducing crops such as switchgrass. For example, harvest-
ing 11 t ha1 of switchgrass dry matter with 1.2% N
(equivalent to a crude protein concentration of 7.5%)
will remove about 130 kg of N ha1 yr1 (Mitchell et al.,
2008). Nitrogen removed as a percentage of N applied
can range fromo50% to 4100% (Parrish & Fike, 2005).
As stands age, N can be lost through biomass harvest-
ing and leaching. To replace this N, fertilizer use must
be optimized to balance the costs of fertilizer produc-
tion and application with the revenues generated from
improved yield. Biofuels possess the additional con-
straint that they need to produce more energy than they
consume, yet anthropogenic N comes with a substantial
cost in terms of energy consumed and CO2 emitted
(Adler, 2007). Optimizing switchgrass biomass yields
and maintaining quality stands requires proper N input
based on the needs of stands of different age, ecotype,
and species compositions.
In addition to planting bioenergy crops, other grass-
land-based resources such as conservation, marginal, or
abandoned lands could supply bioenergy feedstocks.
For example, land in the CRP (Conservation Reserve
Program) has been suggested as a potential resource for
biomass feedstock in the United States (Jewett & Sheaf-
fer, 1996; Lee et al., 2007; Mulkey et al., 2008). Tilman
et al. (2006) suggested that a low-input high-diversity
(LIHD) prairie system involving mixtures of native
grassland perennials can provide more usable energy,
greater environmental benefits, and less agrichemical
pollution per hectare than corn-ethanol or soybean-
biodiesel. In particular, using a diverse mixture of
native prairie species as biomass feedstocks may yield
greater net energy gains than monoculture energy crops
when converted into biofuels, while also providing
wildlife habitat and enriching degraded soils through
carbon sequestration and N fixation (Hill, 2007). The
strategies for achieving higher biomass in LIHD include
using N-fixing legumes as the primary source of N and
a diverse range of native prairie species to gain high
efficiency in exploiting resources. Grasses have bene-
fited from the addition of legumes when grown in
mixtures, with productivity equal to N-fertilized fields
(Crews & Peoples, 2004). Many of the yield benefits of
mixtures may be attributed to N transfer from the
legumes to the associated grasses. However, little in-
formation exists on switchgrass productivity when
grown in mixtures with temperate legumes.
While the yield potential of switchgrass in monocul-
tures has been tested by many on-farm studies and has
been modeled and reviewed extensively (McLaughlin
& Kszos, 2005; Sanderson et al., 2006; Schmer et al.,
2008), to date there have been no direct comparisons of
the yield of switchgrass monocultures and diverse
grassland mixtures. Furthermore, there are no reports
of how stand age and ecotype affect the yield of switch-
grass in response to N. Will monocultures and mixtures
respond to N treatment and environmental factors in
the same way? This study presents a new, updated
quantitative literature review of the yield of switchgrass
in order to address these following questions:
(1) How do the yields of switchgrass in monoculture
and mixtures compare?
(2) How do nitrogen addition and climate impact the
yield of switchgrass in monocultures and mixtures?
(3) How do ecotype and stand age affect the response
of monocultures yield to N?
Methods
Data collection and categorization
Peer-reviewed journal articles used in building the
database for this meta-analysis were obtained by
searching the Science Citation Index (SCI) of the Insti-
tute of Scientific Information. The list of articles ob-
tained was subsequently cross-checked with references
cited in a large number of review articles and books
with the aim of including all articles that have relevant
data for this meta-analysis. Articles published in Eng-
lish before the end of 2008 that met all of the following
criteria were included: (1) the study objective was for
biomass production, as opposed to only for forage
production; (2) for monoculture studies, articles con-
tained information on stand age (years), cultivar, eco-
type, site location, N fertilization level (kg ha1 yr1),
harvest date, and dry matter yield (t ha1 yr1); (3) for
mixture studies, articles contained information on site
location, N fertilization level, harvest date and dry
matter yield of switchgrass, and/or total yield of mix-
tures. Studies were classified as to whether they repre-
sented switchgrass monocultures or multispecies
mixtures that include switchgrass. For monocultures,
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based on the stand age, yield data were categorized into
newly established stands (o3 years) and mature stands
( 3 years). For mixtures, yield data was categorized
based on whether or not legume species were present in
the mixtures. Switchgrass-dominated mixtures were
also compared with experiments performed on restored
prairies without switchgrass present based on the
database compiled by LeBauer & Treseder (2008).
In total, 39 peer-reviewed articles on switchgrass
monocultures (Appendix A), eight on mixtures domi-
nated by switchgrass (Appendix B) and 25 on mixtures
without switchgrass (Appendix C) were included in
this analysis. We assumed studies conducted at differ-
ent sites, yields from different treatments (e.g. fertilizer
treatments), and different growing seasons were inde-
pendent. Graphical data were extracted from the arti-
cles using digitizing software (GETDATA GRAPH DIGITIZER
v. 2.22).
Variables
Yield of switchgrass in monocultures and mixtures was
evaluated for response to growing degree days (GDD),
precipitation, and N fertilizer. Above-ground biomass
of mixtures dominated by nonswitchgrass species was
also tested for response to N fertilizer. Daily maximum
and minimum temperature information for a given site
was extrapolated using LOCCLIM (v. 1.0 FAO, Rome,
Italy), which estimates local climate based on recorded
meteorological data. Annual precipitation, along with
any irrigation, was collected from articles. GDD were
calculated for a growing season with a base tempera-
ture of 10 1C. The growing season was defined by the
date of the last frost in the spring to the date of the first
frost in the autumn or date of harvest, which ever
occurred first (Heaton et al., 2004). When precipitation
information was not reported in an article, annual
precipitation for a given site was extrapolated using
the LOCCLIM program. Nitrogen fertilization values were
used as reported in all articles.
Data analysis
Data were sorted and tested for normality (PROC UNI-
VARIATE, SAS 9.1, SAS Institute, Cary, NC, USA).
Square-root transformed yield data were then analyzed
separately on monocultures and mixtures using mixed
models analysis of variance (PROC MIXED, SAS 9.1)
(Littell et al., 1999). The random effects in the mixed
model framework were used to account for aspects of
site-to-site variability that were not accounted for by the
fixed-effect covariates but which potentially caused
treatments within a site to not be independent (e.g. soil
types, soil micro fauna). Similarly, cultivar random
effects account for the differences within an ecotype
that could not be accounted for as fixed effects due to
limited and unbalanced replication. For monocultures,
a mixed model taking individual studies and cultivars
as random effects was conducted to test the fixed effects
of ecotype (ET; upland or lowland), stand age (SA;
newly established or mature stands), GDD, precipita-
tion (PR), N and the interactions (ETN and SAN)
on dry matter yield. Likewise, a mixed model taking
individual studies as a random effect was performed to
test the fixed effects of legume availability [mixture type
MT; with or without legume species], GDD, PR, N, and
the interaction of N and the presence of legumes on the
yield of switchgrass in mixtures. In order to compare
how yield of switchgrass in monoculture and mixtures
responded to N and climate, a weighted mixed model
was conducted to test the fixed effects of culture types
(CT; monoculture or mixture), N, PR, GDD, and their
interactions (CTPR, CTGDD, and CTN) on the
yield of switchgrass, with individual studies as random
effects. The yield data for monocultures used in this
weighted model was averaged across cultivars for a
given study and weighted by 1/(SD2). Tests of hypo-
theses were considered significant at P  0.05.
Results
Overall, switchgrass yield was twice as high in mono-
cultures compared with mixtures (10.9  5.5 vs.
4.4  4.5 t ha1, Po0.0001, Table 1). Among monocul-
tures, stand age affected switchgrass yield marginally
significantly (P5 0.067, Table 1). Among mixtures, the
presence of legumes increased both total yield and
switchgrass yield. Total and switchgrass yield in
switchgrass-dominated mixtures was 9.9  5.9 and
8.9  4.7 t ha1, respectively, when legumes were pre-
sent, and 5.7  4.0 and 3.1  3.2 t ha1, respectively, in
the absence of legumes. Mixtures without switchgrass
yielded 2.8  2.1 t ha1 (Table 1).
For monocultures, the variance components of the
random effect of individual studies and cultivars were
significantly different from 0, suggesting that individual
studies and cultivars did differ in their average yield
scores (Table 2). ET and SA significantly affected the
yield of switchgrass, with lowland ecotype and mature
stands having higher yield (Figs 1 and 2; Table 2).
Nitrogen and precipitation had significantly positive
effects on the yield (Table 2). The response to N for
monocultures was consistent for newly established
(stand age o3) and mature stands (stand age  3)
and for upland and lowland ecotypes (Figs 1 and 2;
NET and N SA in Table 2).
For mixtures, the variance component of the random
effect of individual study was not significantly different
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from 0, suggesting that individual studies did not differ
in their average yield scores (Table 3). Precipitation and
GDD had no significant effects on the yield of switch-
grass in mixtures (Table 3). Nitrogen had a marginally
significant effect on yield (P5 0.0594 in Table 3). The
response of mixtures to N was not affected by the
presence or absence of legumes (Fig. 3; NMT in Table
3). By contrast to switchgrass-dominated mixtures, total
yield in nonswitchgrass mixtures increased with the
rate of N addition (right panel in Fig. 4; r25 0.12,
Po0.0001, n5 314).
For the averaged yield across different cultivars for a
given study, the variance components of the random
effect of individual studies were significantly different
from 0, suggesting that individual studies did differ in
their average yield scores (Table 4). Monocultures had a
significantly higher yield than mixtures; precipitation
and N had significant positive effects on the weighted
yield of switchgrass in monocultures and mixtures
(Table 4). GDD had no significant effect on the yield
of switchgrass (Table 4). There was a significant differ-
ence for the yield of switchgrass in monocultures and
Table 1 Yield (t ha1) of switchgrass in monocultures and mixtures and total yield of switchgrass and nonswitchgrass dominated
mixtures
Culture type Estimated mean (t ha1) SD N P
Switchgrass yield in monocultures 10.9 5.0 324
Switchgrass yield in mixtures 4.4 4.5 85 o0.0001
Monocultures (stand age o3) 10.1 5.5 108
Monocultures (stand age  3) 11.2 4.8 216 0.0671
Switchgrass yield in mixtures (with legume) 8.9 5.2 20
Switchgrass yield in mixtures (without legume) 3.1 3.3 65 o0.0001
Total yield of mixtures (with switchgrass) 6.9 5.0 90
Total yield of mixtures (without switchgrass) 2.8 3.1 314 o0.0001
Total yield of switchgrass-dominated mixtures (with legume) 9.9 5.9 25
Total yield of switchgrass-dominated mixtures (without legume) 5.7 4.0 65 0.0023
Differences between two means were detected by t-test.
SD, standard deviation; N, number of observations.
Table 2 Fixed effects from mixed model of ecotype (ET;
upland or land), stand age (SA), growing degree days
(GDD), precipitation (PR), nitrogen (N), and the interaction
between N and SA and ET for switchgrass in monocultures on
dry matter yield, taking individual studies and cultivars as
random effects
Effect F-value Pr4F
ET 25.18 o0.0001
SA 33.51 o0.0001
GDD 2.51 0.1134
PR 27.78 o0.0001
N 5.49 0.0192
N SA 2.15 0.1430
NET 1.99 0.1582
For all the effects, the dfN (numerator degrees of freedom)5 1
and the dfD (denominator degrees of freedom)5 1812.
For the random effect of individual studies: Z5 3.38,
P5 0.0004.
For the random effect of cultivar: Z5 2.92, P5 0.0018.
Fig. 1 Response of the annual yield of upland and lowland
switchgrass in monocultures to nitrogen fertilizer. Solid lines
indicate least-squares linear regression; dashed lines represent
95% confidence limits.
Fig. 2 Response of the annual yield of newly established stands
(stand age o3) and mature stands (stand age  3) of switch-
grass monocultures to nitrogen fertilizer. Solid lines indicate
least-squares linear regression; dashed lines represent 95%
confidence limits.
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mixtures to response to GDD (Fig. 6; GDDCT in Table
4), but not to precipitation and N (Figs 4 and 5; PRCT
and NCT in Table 4).
Discussion
Switchgrass produced 6.4 t ha1 more biomass in mono-
cultures than in mixtures (Table 1) over a range of
growing conditions. Switchgrass in monocultures
yielded 10.9 t ha1 and no significant difference was
found across stand ages. This result is close to 10.3 t
ha1 reported by Heaton et al. (2004), even though
Heaton’s study only included mature stands. Mixtures
had an annual yield of 6.9 t ha1, of which switchgrass
accounted for 4.4 t ha1 on average. These results are
consistent with the study of Adler et al. (2005) in which
above-ground biomass averaged 6.6 t ha1 at 34 sites
surveyed across the northeast United States that in-
cluded Conservation Reserve Program (CRP; a land
set-aside program established by the USA Food Secur-
ity Act of 1985), wildlife habitat improvement program
(WHIP), mine reclamation, and other conservation
lands as a resource assessment for biomass production.
The results in this study were also comparable with the
4.4 t ha1 yield of mixtures grown with low inputs on
agriculturally degraded land (Tilman et al., 2006).
Monocultures had higher yield than mixtures, which
supports the view that changes in resource availability
Table 3 Fixed effects from mixed model of mixture type (MT;
with or without legume species), growing degree days (GDD),
precipitation (PR), nitrogen (N), and the interaction of NMT
on the yield of switchgrass in mixutes, taking individual
studies as random effects
Effect F-value Pr4F
MT 25.07 o0.0001
GDD 1.64 0.2182
PR 1.79 0.1844
N 3.67 0.0594
NMT 0.61 0.4382
For all the effects, the dfN5 1 and the dfD5 73.
For the random effect of individual studies: Z5 1.31,
P5 0.0952.
Fig. 3 Response of the annual yield of switchgrass in mixtures
with or without legume species, to nitrogen fertilizer. Solid lines
indicate least-squares linear regression; dashed lines represent
95% confidence limits.
Fig. 4 Response of the annual yield of switchgrass in mono-
cultures, mixtures with switchgrass (black dots), and mixtures
without switchgrass (gray dots) to nitrogen fertilizer. Solid lines
indicate least-squares linear regression; dashed lines represent
95% confidence limits.
Table 4 Fixed effects from weighted-mixed model of culture
types (CT; monoculture or mixture), nitrogen, precipitation
(PR), growing degree days (GDD), and their interactions on
the yield of switchgrass, taking individual studies as random
effects
Effect F value Pr4F
CT 11.07 0.0010
PR 15.69 o0.0001
N 22.39 o0.0001
GDD 2.06 0.1520
NCT 0.54 0.4623
PRCT 0.73 0.3922
GDDCT 4.83 0.0286
For all the effects, the dfN5 1 and the dfD5 402.
For the random effect of individual studies: Z5 3.92,
Po0.0001.
Fig. 5 Response of the annual yield of switchgrass in mono-
cultures and mixtures to annual precipitation. Solid lines indi-
cate least-squares linear regression; dashed lines represent 95%
confidence limits.
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are more important for productivity than changes in
diversity (Hooper et al., 2005; Spehn et al., 2005) (Fig 6).
Switchgrass yield was significantly higher in the
mixtures with legumes present compared with mix-
tures without legumes (Table 1). This supports the
findings of Blanchet et al. (1995) and Berdahl et al.
(2001) that legumes benefit the grass in a grass-legume
mixture if the legumes achieve an adequate stand
density. Total yield of the mixtures was significantly
higher when legumes were present (Table 1) and was
not significantly different from monoculture yield, sug-
gesting legumes can be incorporated into established
switchgrass without negatively impacting total dry
matter yields. Commercial fertilizer savings for biomass
systems using legumes deserves further study, because
legumes offer a more environmentally sound and sus-
tainable source of N to ecosystems (Crews & Peoples,
2004; Pretty, 2008). On average, about 72 kg N ha1 was
added to the soil by several winter legumes (Hargrove,
1986). The N2 fixation rate has also been reported for
red clover (Trifolium pratense L.) (373 kg N ha1), white
clover (Trifolium repens L.) (545 kg N ha1), and alfalfa
(Medicago sativa L.) (350 kg N ha1) (Carlsson & Huss-
Danell, 2003). Further research on the potential role of
legumes in sustainable biomass production is urgently
needed. Unresolved issues include when to harvest the
mixtures to get the highest yield while minimizing N
removal while promoting the establishment of the
temperate legumes in subsequent years and which
management strategies reduce competition between
warm-season grasses and temperate legumes.
Nitrogen limits net primary production (NPP) in
terrestrial ecosystems (Vitousek & Howarth, 1991; Le-
Bauer & Treseder, 2008), and our study specifically
demonstrates that N limits switchgrass production.
Use of N fertilizer must be optimized in biomass feed-
stock production to balance the economics, energy, and
environmental costs of fertilizer use with the resulting
gains in yield. This study shows that N addition
increases switchgrass yield in monocultures (Fig. 4).
Contrary to our expectation that N demand might
increase as stands age due to N removal from biomass
harvesting and leaching out of root zone, the response
of switchgrass yield in monocultures to N treatment did
not change as stands age (Fig. 2). However, switchgrass
yield in mixtures was not impacted by N addition, even
when legumes were absent (Figs 3 and 4). In contrast,
nonswitchgrass dominated mixtures responded to N
addition significantly (Fig. 4), which is consistent with
the study of LeBauer & Treseder (2008) in which tem-
perate grasslands NPP increased 53% after N addition.
The different response of switchgrass-dominated and
nonswitchgrass-dominated mixtures to N addition
might be due to the effect of end-of-season harvest for
biomass studies. As reported in Heaton et al. (2009),
switchgrass would have potentially removed 187 kg N
ha1 if harvested green, and as little as 5 kg N ha1 if
harvested in late winter. Warm-season grasses intern-
ally recycle N from the above-ground shoots to below-
ground, reducing N removal from the system, thereby
increasing the N-use efficiency (Parrish & Fike, 2005).
About 18% of the annual N demand of native prairie
species is supplied by internal reserves (McKendrick
et al., 1975). If the dead dry shoots are harvested after N
is reallocated belowground, internal cycling and storage
of N within switchgrass plant may contribute to its
conservative N use and reduce the need for additional
fertilizer (Beale & Long, 1997; Dubeux et al., 2007).
This analysis indicates switchgrass yield in monocul-
tures but not in mixtures was affected significantly by
water availability [Power40.99 by power analysis (Zar,
1999) for both monoculture and mixture; Fig. 5, Table 2].
Lee & Boe (2005) found that April and May precipita-
tion was a key indicator of biomass production for two
switchgrass cultivars in central South Dakota United
States. With mixed stands, however, individual species
within a mixture may respond differentially to moisture
received during different times in the growing season.
The lack of the response of the yield of switchgrass in
monocultures and mixtures to GDD was consistent with
the results reported in Heaton et al. (2004). The response
pattern suggests a broad optimal temperature range for
the yield of switchgrass. Though the yield of switch-
grass was not significantly affected by GDD, the phe-
nology of flowering time and nutrient translocation
might be affected (Sanderson, 1992). This phenology
change could also affect the yield.
In conclusion, this analysis demonstrates the poten-
tial of switchgrass for use as a biomass energy crop
across a wide geographic range. Nitrogen addition is
required for switchgrass growth in monocultures, and
N demand does not increase as switchgrass stands age.
Switchgrass and total yield in mixtures is comparable
Fig. 6 Response of the annual yield of switchgrass in mono-
cultures and mixtures to growing degree days (GDD). Solid lines
indicate least-squares linear regression, dashed lines represent
95% confidence limits.
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with other studies (Adler et al., 2005; Tilman et al., 2006),
which also confirmed the possibility of growing switch-
grass in mixtures as a biofuel feedstock in marginal
lands (Gonzalez-Hernandez et al., 2009). Switchgrass
yields significantly higher when legumes are present
in the mixtures, supporting the possibility of using
legumes as the N source. Legume-based agro-ecosys-
tems could maintain greater ecological integrity than
that of fertilizer-based systems (Crews & Peoples, 2004),
avoiding the competition for fertile soils with food
production and the possibility of ecosystem destruction.
Acknowledgements
This study was funded by Energy Bioenergy Institute (EBI). The
authors wish to thank Matthew Locus and Xiaohui Feng for their
critical reviews of the manuscript.
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