a quantitative review comparing the yields of two candidate c4 perennial biomass crops in relation...

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Biomass and Bioenergy 27 (2004) 21–30

A quantitative review comparing the yields of twocandidate C4 perennial biomass crops in relation to

nitrogen, temperature and water

Emily Heatona;b, Tom Voigtc, Stephen P. Longa;b;∗

aDepartment of Crop Sciences, University of Illinois, 190 Edward R. Madigan Laboratory, 1201 West Gregory Drive,Urbana, IL 61801-3838, USA

bDepartment of Plant Biology, University of Illinois, 190 Edward R. Madigan Laboratory, 1201 West Gregory Drive,Urbana, IL 61801-3838, USA

cDepartment of Natural Resources and Environmental Sciences, University of Illinois, S-410 Turner Hall,1102 S. Goodwin, Urbana, IL 61801, USA

Received 29 May 2003; received in revised form 15 October 2003; accepted 24 October 2003

Abstract

C4 herbaceous rhizomatous perennials have many positive attributes as potential and actual biomass energy crops onagricultural land. Two species from this group have become widely planted. Miscanthus × giganteus has been extensivelystudied and planted in Europe, and Panicum virgatum in N. America. To date, side-by-side comparisons of mature standsof these crops have not been reported in the peer-reviewed literature. We examined all peer-reviewed articles describingproductivity of these species and extracted dry matter yields, nitrogen fertilization (N), temperature (growing degree days)and precipitation/irrigation. Only yields reported 3 or more years after planting were included. Despite being on di9erentcontinents, trials spanned similar temperature, N and water ranges. Miscanthus × giganteus (97 observations) yieldedan average 22 Mg ha−1 compared to 10 Mg ha−1 for P. virgatum (77 observations). Both crops showed a signi:cantpositive response to water and N, but not to temperature. Miscanthus × giganteus yielded signi:cantly more biomass thanP. virgatum across the range of all three variables. There were di9erences between the species in their apparent responsesto these variables. Miscanthus × giganteus showed a stronger response to water, while P. virgatum showed a signi:cantlystronger response to nitrogen. Since energetic viability, and pro:tability, of biomass crops hinges critically on high outputsof biomass energy for low inputs of money and fossil fuels, these results suggest that M. × giganteus holds greater promisefor biomass energy cropping than does P. virgatum across the range of trial conditions undertaken to date.? 2003 Elsevier Ltd. All rights reserved.

Keywords: Miscanthus × giganteus; Panicum virgatum; Switchgrass; Productivity; Biofuel; Energy crops; Renewable energy; Biomassenergy

∗ Corresponding author. Tel.: +1-217-333-2487; fax: +1-217-244-7563.

E-mail address: [email protected] (S.P. Long).

1. Introduction

In the United States (US) and the European Union(EU), biomass energy research has investigated bothwoody and herbaceous crops grown for energy. These

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22 E. Heaton et al. / Biomass and Bioenergy 27 (2004) 21–30

biomass energy crops have included tree species(Salix, Populus spp.), traditional row crops (Zeamays, Triticum spp.) and perennial rhizomatousgrasses (PRGs) (Arundo, Panicum, Miscanthus).Of these, the last group has the most characteristicsfavorable to biomass energy production [1–3].

An ideal biomass crop is characterized by attributesthat allow it to provide high-energy output for littleinput. A biomass crop must require only low invest-ments of energy (fossil fuels) and money if it is to beenvironmentally and commercially viable while stillproviding clean, cost-e9ective fuel [4,5]. PRGs, espe-cially those using the C4 photosynthetic pathway, typ-ically use nutrients, water, and solar radiation moreeHciently than other plants [6]. The perennial rhizomesystem allows nutrients to be cycled seasonally be-tween the above- and below ground portions of theplant, thus minimizing external additions of fertilizer[7]. If the crop is harvested after senescence, and as-sociated nutrient translocation, has occurred, the re-sultant fuel will have a low mineral content and there-fore release little pollution when combusted [8]. As aperennial, the crop requires only one planting and re-lated tillage, reducing costs and fossil fuel use, as wellas soil erosion [9–11]. Such physiological aspects al-low PRGs to provide clean fuel and more dry matterper unit input than can other potential biomass cropoptions [3,12]. Finally, the crop would ideally use theC4 photosynthetic pathway. Biomass crops are essen-tially used to collect and store solar energy for later re-lease, and for all but the coldest environments, C4 pho-tosynthesis is an important factor in maximizing theeHciency of conversion of intercepted solar radiationinto stored biomass energy (radiation use eHciency,or RUE) [6]. By concentrating carbon dioxide aroundRubisco, C4 plants largely eliminate photorespiration[13]. This allows them to use more of the availablelight energy as well as convert it to stored energy moreeHciently than plants with the C3 pathway [14]. Thegain in RUE conferred by C4 photosynthesis can beas much as 40% over conversion rates in C3 plants[15].

Over the past few decades, C4-PRGs have been in-vestigated on both sides of the Atlantic. US researchhas focused on the native prairie grass, switchgrass(P. virgatum L.) [16,17,19], while EU programs haveconsidered Miscanthus × giganteus, a hybrid speciesfrom Asia [18]. Both of these grasses have additional

Table 1Characteristics favoring the use of perennial rhizomatous grassesas energy crops and additional attributes of P. virgatum and M.× giganteus

Characteristic C3 perennial P. M. ×rhizomatous virgatum giganteusgrasses

Recycles nutrients + + +annuallyClean burning fuel + + +High water use + + +eHciencyHigh nutrient use + + +eHciencyLong canopy duration + + +C4 photosynthesis + +High radiation use + +eHciencyUses existing equipment + + +Alternative use as forage + +High yielding + ++Winter standing +Sterile (non-invasive) +

characteristics that favor their use as biomass cropsover other PRGs (Table 1).

Though P. virgatum has traditionally been of in-terest as a warm-season forage crop, the US Depart-ment of Energy (DOE) chose to examine it as a modelspecies for biomass energy production [19]. These tri-als have led to the development of high yielding cul-tivars such as “Alamo” that can achieve annual yieldsof 20 Mg ha−1 [20]. P. virgatum responds stronglyto nitrogen (N) fertilizer, and is often drought toler-ant [21–23]. It can e9ectively sequester carbon in thesoil, and provides excellent cover for wildlife [24–26].As a native species, its use as a biomass crop is con-sidered more environmentally acceptable than the in-troduction of an exotic species for the same purpose.This, however, is somewhat undermined by the factthat the improved cultivars may have a very di9erentgenetic make up than local populations and, as fertileopen-pollinating crops, could disrupt native popula-tions [27].

By contrast, M. × giganteus has been investigatedin several countries throughout the EU, largely as partof the Miscanthus Productivity Network. The mainobjective of this network was to generate informationon the potential ofM. × giganteus as a non-food crop

E. Heaton et al. / Biomass and Bioenergy 27 (2004) 21–30 23

in the EU [18]. Among other results, projects withinthe network found M. × giganteus yields can reach40 Mg ha−1 under good growing conditions [28]. Thecrop can photosynthesize well at low temperatures[29], and attain high yields with little N fertilizer [30].Like P. virgatum it has been shown to be a good toolfor carbon sequestration and soil quality improvement,as well as wildlife cover [31,32]. Though an exoticspecies, M. × giganteus is a sterile triploid hybrid ofM. sinensis andM. sacchari;orus and is incapable ofproducing viable pollen or spreading by seed [33]. Ithas the disadvantage that planting must be from rhi-zome fragments or young plants, but the advantagethat it cannot become invasive nor transfer genes towild populations.

While agronomic development programs have beenestablished to explore the biomass crop potential ofP. virgatum in the US, and M. × giganteus in theEU, to date there have been no direct comparisonsof mature stands of these two crops reported in thepeer-reviewed literature. How might these crops com-pare when grown side by side? Are some environ-ments more favorable to P. virgatum production, andothers to M. × giganteus? This study addresses thefollowing questions:

(1) How do the yields of P. virgatum and M. ×giganteus compare?

(2) What are the major factors inNuencing M. × gi-ganteus and P. virgatum yields?

A quantitative review of all reported yields in thepeer-reviewed literature for these crops was under-taken to address these questions.

2. Methods

Peer-reviewed journal articles were surveyedusing the Science Citation Index ExpandedTM

(Thomson-ISI, Philadelphia, PA, USA) and Silver-Platter (Ovid Technologies, New York, NY, USA)electronic databases. P. virgatum is the subject ofmany articles on forage production and quality, butthe cultural practices between forage and biomasscrop production can be very di9erent. For this reason,only studies of P. virgatum produced with practicestypical of those used in biomass crop production were

included. Several varieties or experimental lines ofP. virgatum investigated for biomass production wereincluded and yields were averaged across variety.Since no improved varieties of M. × giganteus havebeen developed, and the species is by nature clonal,averaging M. × giganteus data were not necessary.Articles used in the quantitative review contained thefollowing information: stand age (years), variety orcultivar, site location (latitude), nitrogen fertilizationlevel (kg ha−1 yr−1), harvest date, and end of seasonabove ground dry matter yield (Mg ha−1 yr−1). Sinceyields ofM. × giganteus and P. virgatum are variablein the early years after planting, only yields reportedfor mature stands (3 yr or older) were considered.

Of 136 peer-reviewed articles on M. × giganteusand P. virgatum, only 21 articles, representing 174observations, met the above criteria (Appendix A).Parameter values from di9erent treatments (e.g. fer-tilizer treatments) were assumed to be independent,following meta-analytic methods [34–36]. Data wereextracted from these articles using digitizing softwareprograms (Grafula 3 v.2.10, Wesik SoftHaus, St. Pe-tersburg, Russian Federation).

2.1. Variables

Yields of P. virgatum and M. × giganteus wereevaluated for response to growing degree days (GDD),precipitation and N fertilizer, as described below.

GDD were calculated for a growing season witha base temperature of 10◦C used for both species[37,38]. The growing season was de:ned by the dateof the last frost in the spring to the date of :rst frostin the autumn or date of harvest, which ever occurred:rst. Daily maximum and minimum temperature in-formation for a given site was extrapolated using Loc-Clim (v.1.0 FAO Rome, Italy), a computer programthat estimates local climate based on recorded meteo-rological data [39].

When reported, actual growing season precipitation(along with any irrigation) was used. When precipi-tation information was not present in an article, dailyprecipitation for a given site was extrapolated usingthe LocClim program. Nitrogen fertilization values(kg ha−1 yr−1) were used as reported in all articles.

Biomass production of M. × giganteus and P.virgatum usually involves a single harvest after veg-etative growth has ceased in the autumn. The crop


Days after Sept. 1

0 50 100 150

Dry

Matter

(Mg h

a )

-1

0

10

20

30

40M. x giganteus slope = -0.07P. virgatum slope = -0.01

Fig. 1. Linear regression of M. × giganteus (•) and P. virgatum(©) yield loss with delayed harvest date. September 1 is anapproximate date of maximum annual biomass.

is either harvested immediately, to obtain maximumbiomass yields, or it is allowed to dry down in the:eld. The latter option saves on drying and storagecosts, but reduces harvestable yield due to leaf dropand weathering [40,41] and many studies vary harvestdate to investigate this interaction. The yields of bothcrops as reported in the reviewed articles were linearlyregressed with the reported harvest dates to estimateyield loss with delayed harvest. Aided by graphicalinterpretation of this data, September 1 was chosento represent the date by which maximum biomasshad been achieved in both M. × giganteus andP. virgatum (Fig. 1). The regression equation for eachcrop was then used to scale the actual yield values toSeptember 1 values, and factor out the e9ect of har-vest date. The scaled yields thus represented mature,maximum autumn yields prior to any leaf drop, al-lowing the comparison of yields on a common basis.P. virgatum yields decreased an average of 0:01 Mgfor each day harvest was delayed after September 1,while M. × giganteus yields decreased an averageof 0:07 Mg day−1 (Fig. 1). Miscanthus × giganteusyield losses may be exacerbated, since the crop is of-ten harvested in late winter or early spring [41], while

P. virgatum is typically harvested within a fewmonths of maximum biomass yields.

2.2. Analysis

Data were sorted and reviewed for normality ofboth original data and residuals (PROC UNIVARI-ATE, SAS Institute Cary, NC, USA). Data were thenanalyzed using a mixed model analysis of variance,essentially regressing the dependent variable againsteach independent variable. Signi:cance of variablesand least-squared means were determined using theF statistic (� = 0:05) (PROC MIXED, SAS InstituteCary, NC, USA). Both main e9ects and species byvariable interactions were investigated; additionally,the yield response to each variable was analyzed sep-arately within each species to understand the basisunderlying the interactions. Higher order interactionswere not investigated due to the unstructured natureof the data (i.e., without a uni:ed experimental designamong all studies reviewed, these interactions are se-riously confounded and interpretation is speculative).

3. Results

Overall, M. × giganteus was found to yield 12 Mgmore biomass per ha than P. virgatum (p¿ 0:004),representing a two-fold di9erence, with means of22:4(±4:1) and 10:3(±0:7) Mg ha−1, respectively(Table 2). Comparison of the raw yield data suggeststhis relationship is consistent over each variable ofinterest (Fig. 2). An overall analysis of variance indi-cated that only precipitation (p¡ 0:0001) and N fer-tilizer (p=0:0003) seem to inNuence yield. GDD didnot signi:cantly a9ect yield in either crop (Table 3).However, when crop means were calculated withineach variable (e.g. Fig. 2), M. × giganteus did, on

Table 2Yield (Mg ha−1) of M. × giganteus and P. virgatum (least-squared means)

Crop Estimated mean Standard error P

M. × giganteus 22.4 4.1 —P. virgatum 10.3 0.7 —Di9erence of means 12.1 4.1 0.0040


Growing Degree Days

0 1000 2000 3000 4000

Yie

ld (

Mg

ha-1

)

0

10

20

30

40

Nitrogen (kg/ha)0 50 100 150 200

Yie

ld (

Mg

ha-1

)

0

10

20

30

40

April - September Precipitation (mm)

0 300 450 600 750

Yie

ld (

Mg

ha-1

)

0

10

20

30

40

(a)

(b)

(c)

Fig. 2. Response of the annual yield of M. × giganteus (•)and P. virgatum (©) to (a) nitrogen fertilizer; (b) seasonal pre-cipitation; (c) thermal time. Solid lines indicate least-squares lin-ear regression by crop, dashed lines represent 95% con:dencelimits.

Table 3Tests of the :xed e9ects of precipitation, growing degree days,nitrogen and interactions with species (P. virgatum, M. × gigan-teus) on dry matter yield

Variable DF F P

Precipitation 166 39.97 ¡ 0:0001Precipitation × species 166 11.11 0.0011GDD 166 1.67 0.1982GDD × species 166 1.66 0.1992Nitrogen 166 13.94 0.0003Nitrogen × species 166 0.57 0.4523

average, yield signi:cantly (p¡ 0:0001) more thanP. virgatum.

A signi:cant interaction between crop type and pre-cipitation (p=0:001) implied thatM. × giganteus andP. virgatum respond di9erently to precipitation. Toexamine this further, the yield response of each cropto each variable was investigated singly (Table 4).The highly signi:cant overall e9ect of precipitation(p¡ 0:0001) seemed to be largely driven by the re-sponse ofM. × giganteus yield to precipitation.M. ×giganteus yields appear to be more strongly inNuencedby precipitation (p¡ 0:0001) than do those of P. vir-gatum (p= 0:01). Conversely, the signi:cant overallresponse to N (p = 0:0003) seems to be a result ofthe yield response of P. virgatum to N (p= 0:0001),sinceM. × giganteus yields were not signi:cantly in-Nuenced by N at the 5% level (p = 0:08). GDD didnot signi:cantly inNuence either crop.

4. Discussion

Results of this analysis indicate that:

(1) Miscanthus× giganteus produces more biomassper unit area and per unit input than doesP. virgatum; and

(2) Miscanthus× giganteus yields are most stronglyinNuenced by water, while those of P. virgatumare more strongly controlled by N; GDD did notsigni:cantly a9ect yield in either crop.

Miscanthus × giganteus, on average, produced12 Mg ha−1 more biomass thanP. virgatum (Table 2)when considered over a range of growing conditions,


Table 4Tests of the :xed e9ects of precipitation, growing degree days and nitrogen by species

M. × giganteus P. virgatum

Variable DF F P DF F PPrecipitation 93 34.93 ¡ 0:0001 73 6.55 0.0126GDD 93 1.59 0.2101 73 0.04 0.8487Nitrogen 93 3.15 0.0792 73 16.40 0.0001

representing more than a two-fold di9erence in yield.Moreover, it appears that M. × giganteus will pro-vide more biomass per unit input (e.g. water, N) thanP. virgatum (Fig. 2). As noted earlier, maximal pro-duction of biomass from minimal inputs is requiredfor a biomass crop to be both economically and en-ergetically viable. While establishment costs mayvary between M. × giganteus and P. virgatum, man-agement of stands (i.e. herbicide, fertilizer, harvestregime) is similar, and over the long run, given thesame market, the crop that produces more biomassper unit input is likely to be the more pro:table. Forexample, given seed costs of $27/kg for P. virgatum(Seedland, Inc., Wellborn, FL USA) and $0.04 perM. × giganteus rhizome [42], and a seeding rateof 7 kg pure live seed/ha [43] and 10,000 plants/ha[11] for each respective crop, plant material costscome to $207/ha for P. virgatum and $402/ha forM. × giganteus, a $195 di9erence. If a modest cropvalue of $40/Mg is assumed (Bullard [11] suggested$56/Mg farm gate price), then with the yields foundin this analysis, mature crops of P. virgatum andM. × giganteus would gross $412/ha and $896/ha,respectively, a $484 di9erence. Even if additionalcosts incurred in M. × giganteus establishment, e.g.additional labor in rhizome planting, the yield dif-ference between the two crops should translate toa pro:t di9erence favoring M. × giganteus within3 yr of planting, as mature yields of both crops arereached.

This analysis indicates M. × giganteus and P. vir-gatum use inputs di9erently, as evidenced by the vary-ing inNuence of water and N on yield (Table 4). Al-though M. × giganteus produces more biomass overthe range of variables examined here, its yield is morea9ected by water availability than is that of P. vir-gatum, and less a9ected by N fertilization. In areaswith ample rainfall but concern over N contamina-

tion of water supplies, it may be better for a biomassproducer to include M. × giganteus in the farmingsystem, while conversely, in arid areas without con-tamination concerns a producer may see greater yieldsgrowing P. virgatum with adequate N fertilization.Though GDD did not signi:cantly a9ect yield in ei-ther crop, accumulated temperature is necessary forNowering in both crops [44,45], and if limiting, couldnegatively a9ect yield.

It is notable that M. × giganteus is essentially anunimproved crop. One clone, taken from an ornamen-tal collection in Denmark, has been used in the ma-jority of EU trials [18,46,47]. Results presented herelikely represent a test of one genotype over a range ofconditions. Though M. × giganteus has arisen morethan once over time, as a naturally occurring sterilehybrid, genetic variation and recombination is limited.By contrast, P. virgatum can be either tetraploid oroctaploid, and similar cytotypes interbreed easily [27].Therefore, wide genetic variability occurs within thespecies naturally. Breeding of improved P. virgatumvarieties for forage has been done since the early partof the last century and has provided the basis for im-proved biomass lines through an extensive breedinge9ort [48]. The goal of one of these breeding projects,established in collaboration with the University of Ne-braska in 1990, was to achieve P. virgatum yields of22 Mg ha−1 in the Midwest—the same yield M. ×giganteus averaged in this study. Since unimprovedM. × giganteus already achieves the targeted yield ofP. virgatum, it is reasonable to expect that if breedingand improvement e9orts similar to those undertakenwith P. virgatumwere to be implemented forM. × gi-ganteus, yields ofM. × giganteusmight increase dra-matically, and nutrient use might be made even moreeHcient. Initial work on selection of drought-resistantMiscanthus species has indicated considerable vari-ation in plant response to drought stress within the


genus, suggesting cultivars could be bred that are bet-ter adapted low or unevenly spaced precipitation [49].Given the hybrid’s sterility, anM. × giganteus breed-ing program would have to rely on unconventionaltechniques, but it is possible with genetic engineeringand rehybridization of the parents species,M. sinensisand M. sacchari;orus, to make new M. × giganteushybrids [33].

In conclusion,M. × giganteus and P. virgatum areperennial rhizomatous grass species with great poten-tial for use as biomass energy crops across a widerange of growing conditions. This statistical analy-sis of yields reported in peer-reviewed journal articlesindicates M. × giganteus yields signi:cantly morebiomass than P. virgatum over a range of growingconditions, and suggests variation in nutrient use be-tween crops. Since there are no side-by-side :eld tri-als comparing mature stands of these crops reportedin the peer-reviewed literature, information presentedhere will aid biomass producers in making crop selec-tion decisions. Overall, M. × giganteus appears to bethe more promising of the two crops, yielding twiceas much as P. virgatum and achieving higher yieldsper unit input, thus allowing for higher pro:ts givensimilar market conditions.

Acknowledgements

The Illinois Council on Food and Agriculture Re-search substantially funded this work under projectILLU-15-0271. The authors wish to thank Pat B. Mor-gan, Shawna L. Naidu, Carl J. Bernacchi, Charles P.Chen, Richard J. Webster, Xinguang Zhu and Victo-ria E. Wittig for critical reviews of the manuscript,and M. Katherine Ciccodicola and Janel K. Woods fortheir help in gathering articles.

Appendix A.

Further reading

[1] Beale CV and Long SP. Can perennial C4 grassesattain high eHciencies of radiant energy conver-sion in cool climates? Plant-Cell-and-Environ-ment 1995, 18(6), 641–650.

[2] Beale CV and Long SP. Seasonal dynamics ofnutrient accumulation and partitioning in the

perennial C4-grasses Miscanthus × giganteusand Spartina cynosuroides. Biomass and Bioen-ergy 1997, 12(6), 419–428.

[3] Beale CV, Morison JI and Long SP. Water useeHciency of C4 perennial grasses in a temper-ate climate. Agricultural and Forest Meteorology1999, 96(1–3), 103–115.

[4] Beuch S, Boelcke B and Belau L. E9ect of the or-ganic residues of Miscanthus × giganteus on thesoil organic matter level of arable soils. Journalof Agronomy and Crop Science 2000, 184(2),111–119.

[5] Ercoli L, Mariotti M, Masoni A and Bonari E.E9ect of irrigation and nitrogen fertilization onbiomass yield and eHciency of energy use in cropproduction of Miscanthus. Field Crops Research1999, 63(1), 3–11.

[6] Himken M, Lammel J, Neukirchen D, Czypi-onka KU and Olfs HW. Cultivation of Miscant-hus under west European conditions: Seasonalchanges in dry matter production, nutrient uptakeand remobilization. Plant and Soil 1997, 189(1),117–126.

[7] Jorgensen U. Genotypic variation in dry mat-ter accumulation and content of N, K and Cl inMiscanthus in Denmark. Biomass & Bioenergy1997, 12(3), 155–169.

[8] Lewandowski I and Kicherer A. Combustionquality of biomass: Practical relevance and ex-periments to modify the biomass quality ofMiscanthus × giganteus. European Journal ofAgronomy 1997, 6(3-4), 163–177.

[9] Schwarz H, Liebhard P, Ehrendorfer K andRuckenbauer P. The e9ect of fertilization onyield and quality of Miscanthus sinensis ‘gigan-teus”. Industrial Crops and Products 1994, 2,153–159.

[10] Schwarz H. Miscanthus sinensis ‘giganteus’ pro-duction on several sites in Austria. Biomass &Bioenergy 1994, 5, 413–419.

[11] Van Der Werf HMG, Meijer WJM, MathijssenEWJM and Darwinkel A. Potential dry matterproduction of Miscanthus sinensis in the Nether-lands. Industrial Crops and Products 1993, 1,203–210.

[12] Muir JP, Sanderson MA, Ocumpaugh WR,Jones RM and Reed RL. Biomass produc-tion of ‘Alamo’ switchgrass in response to


nitrogen, phosphorus, and row spacing. Agron-omy Journal 2001, 93(4), 896–901.

[13] Ma Z, Wood CW and Bransby DI. Impact of rowspacing, nitrogen rate, and time on carbon par-titioning of switchgrass. Biomass & Bioenergy2001, 20(6), 413–419.

[14] Reynolds JH, Walker CL and Kirchner MJ. Ni-trogen removal in switchgrass biomass under twoharvest systems. Biomass & Bioenergy 2000,19(5), 281–286.

[15] Sanderson MA, Read JC and Reed RL. Harvestmanagement of switchgrass for biomass feed-stock and forage production. Agronomy Journal1999, 91(1), 5–10.

[16] Madakadze IC, Stewart KA, Peterson PR, Coul-man BE and Smith DL. Cutting frequencyand nitrogen fertilization e9ects on yield andnitrogen concentration of switchgrass in ashort season area. Crop Science 1999, 39(2),552–557.

[17] Madakadze IC, Stewart K, Peterson PR, Coul-man BE, Samson R and Smith DL. Light in-terception, use-eHciency and energy yield ofswitchgrass (Panicum virgatum L.) grown in ashort season area. Biomass & Bioenergy 1998,15(6), 475–482.

[18] Madakadze IC, Coulman BE, Peterson P,Stewart KA, Samson R and Smith DL. Leafarea development, light interception, andyield among switchgrass populations in ashort-season area. Crop Science 1998, 38(3),827–834.

[19] Madakadze IC, Coulman BE, Mcelroy a R, Stew-art KA and Smith DL. Evaluation of selectedwarm-season grasses for biomass production inareas with a short growing season. BioresourceTechnology 1998, 65(1–2), 1–12.

[20] Clifton-Brown JC, Lewandowski I, Andersson B,Basch G, Christian DG, Kjeldsen JB, JorgensenU, Mortensen JV, Riche aB, Schwarz KU, TayebiK and Teixeira F. Performance of 15 Miscant-hus genotypes at :ve sites in Europe. AgronomyJournal 2001, 93(5), 1013–1019.

[21] Clifton-Brown JC and Lewandowski I. ScreeningMiscanthus genotypes in :eld trials to optimisebiomass yield and quality in southern Germany.European Journal of Agronomy 2002, 16(2),97–110.

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a quantitative review comparing the yields of two candidate c4 perennial biomass crops in relation...

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