Long-term productivity of lowland and upland switchgrass cytotypes as affected by cutting frequency

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Bioresource Technology 99 (2008) 7425–7432

Review

Long-term productivity of lowland and upland switchgrass cytotypes as affected by cutting frequency A. Monti *, G. Bezzi, G. Pritoni, G. Venturi Department of Agroenvironmental Science and Technologies, University of Bologna, Viale Fanin 44, 40127 Bologna, Italy Received 8 October 2007; received in revised form 15 February 2008; accepted 21 February 2008 Available online 2 April 2008

Abstract A considerable number of studies has been conducted on switchgrass (Panicum virgatum L.) as a bioresource for energy over the last few years. Nonetheless, some important issues concerning the agro-technique are still open. This research examines the long-term total dry matter yield (TDM) and ash content of two lowland (L) and two upland (U) switchgrass cytotypes, as affected by one or two-cut system, under southern EU climatic conditions (44°330 N). Overall, L produced higher TDM than U (on average 14.9 and 11.7 Mg ha1, respectively); two-cut system allowed to produce higher biomass yields (especially in U) than single harvest during the two first years, but it also drastically reduced plant vigour and productivity of all cytotypes in the following two years. Moreover, under two-cut system almost total seasonal biomass derived from the early harvest, while the second cut slightly contributed to the total seasonal biomass, nor it appeared to offset the additional harvest costs. Biomass quality was also significantly affected by cutting frequency, with twocut system leading to a considerably higher ash content of biomass. Therefore, it is perceived that two-cut system is not worthwhile with U and L cytotypes as bioresource for energy production under southern EU conditions. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Harvest; Biomass; Ash content; Energy

1. Introduction Much attention has been recently paid in switchgrass as potential bioenergy crop. The main reason behind this interest is the ability of switchgrass to produce high biomass yields under low input techniques (Vogel et al., 2002). This characteristic is due to the high resistance of switchgrass to pests and diseases as well as high tolerance to severe water stress conditions, salinity and marginal soils (McLauglin and Kszos, 2005). Moreover, with respect to other perennial energy crops (e.g. giant reed and miscanthus), switchgrass has a number of significant advantages that make this crop very attractive to farmers. For example, the equipment needed for growing, harvesting and storage is very familiar to farmers; also, switchgrass, being seedable, is much more economic to be planted than other *

Corresponding author. Tel.: +39 051 2096653; fax: +39 051 2096241. E-mail address: [email protected] (A. Monti).

0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.02.034

perennial crops being propagated by rhizomes (e.g. miscanthus and giant reed). Nonetheless, switchgrass pays a significant lower biomass yield to other competing annual and perennial energy crops (Lewandowski et al., 2003; Monti et al., 2005); for example, fibre sorghum or giant reed can produce up to twofold or threefold biomass yields than switchgrass under similar environmental conditions (Lewandowski et al., 2003). As a result, whether or not switchgrass will be the winner crop for energy will mostly depend on technical advances toward achieving a higher productivity. In this view, the research on switchgrass has been intense over the last decades, covering genotype screening (Taliaferro, 2002; Monti et al., 2004), establishment technique (Elbersen et al., 1999; Monti et al., 2001; McLauglin and Kszos, 2005), nitrogen and water management (McLauglin and Kszos, 2005 and references therein), harvest time (Venturi et al., 2004) and environmental and economic impacts (Fazio et al., 2007; Monti et al., 2007).

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Overall, it resulted that nitrogen and water are major factors affecting biomass yield, but they also radically contribute to energy consumption, environmental charge and management costs (McLauglin and Kszos, 2005; Monti et al., 2007). As such, Parrish et al. (2003) pointed out that, in a single-cut system, 50 kg ha1 year1 of N are adequate to maintain satisfactory switchgrass yields, though Muir et al. (2001) reported that the optimal nitrogen dose is more than twofold. These results were also corroborated by other findings (McLauglin and Kszos, 2005 for a review). Water is also a major determinant of switchgrass yield (McLaughlin et al., 1997), however, a widespread opinion is that current levels of water consumption by agriculture is not sustainable into the next future as more water resources will be needed for human, municipal, industrial needs, or eventually for food crops (Condon et al., 2004; Ciais et al., 2005). Therefore improving productivity of energy crops by increasing water supply seems generally not practicable. Another possible way to improve switchgrass productivity, that has not probably received adequate attention, is the optimization of cut management. Switchgrass is conventionally harvested once per year after the killing-frost, however repeated harvests over the growing season could increase the total biomass yield by exploiting the re-growth capacity of the crop, providing that additional biomass yields will offset the extra harvest costs (Keeney and De Luca, 1992; Vogel et al., 2002). At our knowledge, no experiences on the effect of cutting frequency on switchgrass yield have been conducted in Europe so far. Also, the US studies mostly refer to forage yields, that require different quality characteristics of the feedstocks than those needed for biofuels (Stroup et al., 2003; Sarath et al., 2007). Nonetheless, researches specifically addressed to energy end-use showed counteracting results, with genotypes reaching the highest yields under either cutting systems (Madakadze et al., 1999; Vogel et al., 2002; Thomason et al., 2004; Fike et al., 2006a). Again, a number of studies did not distinguish between upland and lowland cytotypes (Madakadze et al., 1999; Sanderson et al., 1999; Vogel et al., 2002), two main switchgrass groups being expected to respond very differently to harvest management because of their diverse morphological characteristics (Porter, 1966), ploidy level, habitat preference, input requirements (Stroup et al., 2003) and cycle length (Hultquist et al., 1996). In short, upland cytotypes are characterized by a shorter growth cycle, faster growth and higher photosynthetic rates than lowland types (Wullschleger et al., 1996), which allow to conclude the primary period of vegetative growth early in the summer. In a recent study Fike et al. (2006a) found upland cytotypes to produce 38% more dry biomass when cut twice per year, while the yield increase of lowland cyto types was trivial. The simultaneous presence of upland and lowland types having succeeding harvest times in the same farm might be strategic to allow a regular round-year biomass supply to conversion plants. Moreover, a higher

harvest flexibility could help farmers in reducing risks associated with fluctuations of crop and bioenergy markets, as well as weather patterns. Alike productivity, the quality of biomass can be significantly affected by cut management. For example, comparing single and double-cut systems, Reynolds et al. (2000) found that the nitrogen content of summer-harvested biomass was about threefold than in late autumn-harvested biomass. Again, in red canary grass, a very similar crop to switchgrass, alkali and chlorine, which are released during combustion causing fouling and corrosion problems in boilers, declined by a factor of 2–6 by delaying the harvest after the killing-frost (Burvall, 1997). Moreover, ash fusion temperature increased by leaf loss and leaching of alkali from 1070 °C to 1400 °C (Burvall, 1997). This was likely due to the incomplete mineral translocation from the above ground organs to rhizomes during summertime, as well as the higher leaf component of still green biomass (Sanderson et al., 1999, 1996). The calorific value of biomass was also found to decrease by 0.2 MJ kg1 with every 10 g kg1 increase in ash content (Jenkins et al., 1998). Finally, ash content can also drastically change among cytotypes. For example, comparing 18 switchgrass cytotypes (Monti et al., 2005) showed the ash content to be typically higher in upland than lowland cytotypes and negatively related to biomass yield. Therefore, the objectives of this study were to assess not only the long-term biomass yield of upland and lowland cytotypes of switchgrass in response to a different harvest management, but also to determine the quality of raw materials.

2. Methods 2.1. Management and measurements The field trial was conducted at the experimental farm of the University of Bologna (Lat. 44°250 , Long. 11°280 , 80 m a.s.l.) in the period 2002–2006. According to soil taxonomy (USDA, 1999) soil was classified as Udifluventic Haplustepts fine silty, mixed, superactive, mesic. Soil physical– chemical analysis resulted in pH 7.7; 42%, 34% and 24% of sand, silt and clay contents, respectively; very rich in exchangeable K (115 mg kg1); 1.6% organic matter and 0.03 and 1.5 Mpa of field capacity and wilting point, respectively. Rainfall and air temperature were registered hourly by an automatic weather station near the field (Fig. 1). Biomass productivity and quality of switchgrass (Panicum virgatum L.) were investigated as affected by cytotype and cutting frequency. Plots were arranged in a split-plot design with four blocks. Cytotype was the main factor, while cut system the sub-factor; year was considered as a random factor. The four cytotypes were: Trailblazer and Shawnee, both upland (U); Alamo and SL 93-3, both lowland (L) (Casler et al., 2004). The two-cut systems were:

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Fig. 1. Cumulative monthly rainfall (vertical black bars) during the growing season (April–October) of the period 2002–2006. Average local rainfall is indicated as blank bars. Vertical arrows represent the harvests: 1C and 2C indicate 1st (early July) and 2nd (February) harvest under the two-cut system; SC means one-cut system with biomass harvested once a year after killing-frost (February).

harvesting once per year (SC) on February; harvesting twice per year, with harvests taking place during the first week of July (1C), and in wintertime (2C), concurrently with SC. At the occurrence of 1C, U were from boot stage to early anthesis, which correspond to specific index scores from 3.1 to 3.3 (Moore et al., 1991). At the same time, L were at intermediate stem elongation phase, that is ca. 2.5 index score. In order to strengthen the establishment, in 2002 all cytotypes were harvested once after killing-frost (February 2003) while cutting management was first imposed in the year after establishment (2003), namely first productive year. Switchgrass was sown in May 2002 by a precision smallplot drill (Vignoli s.r.l.) at a seeding rate of ca. 4 kg ha1 which corresponds to about 800 pure live seeds (PLS) m2, 20 cm row-spaced into a firmly and packed seedbed. Before sowing, 44 kg ha1 of P (triple super phosphate 46%) and 40 kg ha1 of N (urea 46%) were distributed. A dose of 100 kg ha1 year1 of N was distributed annually soon

after sprouting out of new tillers. Because of favourable weather conditions irrigation was never required. Biomass was harvested and removed by sickle-bar mower at a 5 cm stubble height. Yield determinations (i.e. total fresh and dry biomass) were made in the middle of each plot on 6.6 m2 (2 m  3.3 m) sampling area. A representative sub-sample of about 200 g of whole biomass was hand-collected from the windrows, dried at 105 °C until constant weight into a forced-air oven, and then weighted to determine the moisture content at harvest time. Tillers density was measured on two linear meter per plot during the two first years and after the last harvest (February 2006), and the plant height was measured by averaging 20 tiller heights for each replication up to the last ligulate leaf. Ash content of the whole biomass was determined on sub-samples of 3 g combusted into an electrically heated muffle furnace at 550 °C for 15 h (ISO 1575:1987). Before combustion, biomass was ground in a hammer mill to pass a 1 mm screen.

Table 1 Coefficients used for comparative analysis by independent contrasts P i

Contrasts

Treatments L-SC

L-1C

L-2C

U-SC

U-1C

U-2C

U vs. L SC vs. (1C + 2C) SC vs. 1C 1C vs. 2C (U vs. L)  (1C vs. 2C) (U vs. L)  (SC vs. (1C + 2C)) (U vs. L)  (SC vs. 1C)

1 2 1 0 0 2 1

1 1 1 1 1 1 1

1 1 0 1 1 1 0

1 2 1 0 0 2 1

1 1 1 1 1 1 1

1 1 0 1 1 1 0

0 0 0 0 0 0 0

U and L, upland and lowland cytotypes; SC = one-cut system (harvested once a year); 1C and 2C = 1st and 2nd harvest under a two-cut system (harvested twice a year).

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2.2. Statistical analysis

3.2. Biometry and productivity

All data were subjected to the analysis of variance (ANOVA) according to a split-plot design (Systat 10.2, Systat software inc., San Jose, USA). The choice of the statistical model took into account the multi-year data, i.e. the constraint relating to monoculture involving plot randomisation only in the first year (Satterthwaite, 1946). To test the significance of factors and interactions, the independent linear contrasts method was applied, which enable to separate the interaction effects thus to answer specific questions about treatments (Snedecor and Cochran, 1980). Specific questions and coefficients of the linear contrasts are shown in Table 1. As for the interactions, the coefficients were obtained by multiplying the corresponding coefficients of the main effects.

Because of significant second order interaction between cytotype, cut system and year, data are presented by individual years. Also, since Alamo and SL 93-3 performed very similarly, as well as Trailblazer and Shawnee, the four cytotypes were pooled into two major groups, lowland (L, Alamo and SL 93-3) and upland (U, Trailblazer and Shawnee) which will be therefore compared later on. Overall, plant height did not change appreciably during the years under the one-cut system, while it progressively decreased under the two-cut system, especially as concern L cytotypes at early harvest time (Fig. 2). Productivity (total dry biomass) ranged from 10.5 to 13.1 Mg ha1 in U cytotypes, and from 11.4 to 19.1 Mg ha1 in L. Total biomass yield was from 9.3 to 21.0 Mg ha1 under the two-cut system, and from 9.2 to 17.2 Mg ha1 under onecut system (Fig. 3). In the establishment year, TDM was significantly higher in L (12.2 Mg ha1) than U (6.8 Mg ha1). In 2003, TDM differed between cytotypes and cut systems. Overall, L were more productive than U (9.1 and 7.0 Mg ha1, respectively), while two-cut system lead to an overall higher TDM than SC (13.0 and 11.3 Mg ha1, respectively) (Fig. 3). The two switchgrass groups had an opposed response to cutting frequency, which was also statistically significant (P 6 0.01) from the linear contrasts analysis (Table 2). Specifically, in U the first cut (1C) resulted in a significant higher TDM than SC (11.6 and 9.2 Mg ha1, respectively), while in L, 1.5 Mg ha1 more biomass was achieved in SC with respect to 1C. Noteworthy, the first harvest under two-cut system accounted for

3. Results 3.1. Weather data Summer (June–August) average air temperature was about 4.5 °C warmer in 2003 (27.7 °C) than 2004 and 2005, while 2006 showed intermediate values (25.2 °C) very close to the local long-term average temperature (24.5 °C). Within the same period, rainfall (Fig. 1) was also clearly lower in 2003 (only 45 L m2) than following three years (147, 180, 131 L m2, respectively) and long-term local average rainfall (141 L m2). As for the total seasonal rainfall (April–October), 2003 and 2006 registered the lowest amount (332 and 352 L m2) while 2005 the highest (708 L m2), almost 120 L m2 higher than 2004 (Fig. 1).

Fig. 2. Plant height of upland (U) and lowland (L) cytotypes at the last ligulate leaf at harvest time during the period 2003–2006. 1C and 2C indicate 1st (early July) and 2nd (February) harvest under the two-cut system; SC means one-cut system with biomass harvested once a year after killing-frost (February). Vertical bars are the mean standard errors.

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Fig. 3. Total dry matter (TDM) of upland (U) and lowland (L) cytotypes during the period 2003–2006. 1C and 2C indicate first (on July) and second (on February) cut under a two-cut system; SC means biomass harvested once in February. Different letters mean statistical differences on total biomass yield between two harvest managements (i.e. one- vs. two-cut system).

Table 2 Results of the independent linear contrast analysis on total dry matter yield (TDM) and ash content (% dry basis) Source of variation

DF

P 2003

U vs. L SC vs. (1C + 2C) SC vs. 1C 1C vs. 2C (U vs. L)  (1C vs. 2C) (U vs. L)  (SC vs. (1C + 2C)) (U vs. L)  (SC vs. 1C) MSE

1 2 2 2 2 2 2 36

2004

2005

2006

TDM

Ash

TDM

Ash

TDM

Ash

TDM

Ash

<0.01 <0.01 0.82 <0.01 0.50 0.08 0.04

0.82 <0.01 <0.01 0.98 0.71 <0.01 0.96

<0.01 <0.01 0.31 <0.01 0.74 0.03 0.03

<0.01 <0.01 <0.01 0.05 0.02 <0.01 0.42

<0.01 <0.01 <0.01 <0.01 0.10 0.07 0.01

<0.01 0.02 0.03 0.14 <0.01 0.02 0.46

0.23 <0.01 <0.01 <0.01 0.02 0.18 1.00

0.02 0.01 0.18 <0.01 0.10 0.01 0.93

U and L = upland and lowland cytotypes; SC = one-cut system (harvested once a year); 1C and 2C = 1st and 2nd harvest under a two-cut system (harvested twice a year); MSE, mean square error; DF, degree of freedom; P, probability of wrongly rejecting the null hypothesis if it is in fact true.

98% and 84% of total seasonal yield in U and L, respectively (Fig. 3). As expected, in 2004, the biomass production further increased both in L (+3.6 Mg ha1) and U (+1.7 Mg ha1), likely due to plant development. Again, L produced a significant higher (+46%) TDM than U (Fig. 3). The interaction ‘cytotypes  cut system’ was also significant and reflected that of 2003 (Table 2). Specifically, TDM was always higher under two-cut system with respect to SC, but this was significantly more marked in U (+70%). 1C contributed 88% toward TDM in U, while in L it accounted for three-fourths of the total seasonal yield. The fourth year (2005) confirmed the higher productivity of L (+17%), but this was less clear than previous 3 years. In addition, the effects of cut frequency was radically

different to that observed in two previous years (Fig. 3): in L, two-cut system appeared no longer worthwhile, which was explained by a progressive decrease of TDM in 1C that lead to a considerably lower biomass yield than SC. In agreement with previous years, in U, 2C slightly contributed to the total seasonal productivity (12%), while it accounted for 29% in L (Fig. 3). In 2006, TDM was not significantly different between the two cytotype groups (Table 2). In agreement with 2005, SC clearly exceeded (about +50%) the two-cut systems in term of TDM yield. This was mostly due to the fall in TDM yield of 1C, which resulted less than the half than SC (14.5 and 6.5 Mg ha1, respectively) (Fig. 3). Noteworthy, 1C contributed 81% and 59% toward TDM in U and L, respectively.

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Table 3 Ash content of whole biomass (% dry basis) of upland (U) and lowland (L) cytotypes Cut system

2003

2004

2005

2006

U

L

Mean

U

L

Mean

U

L

Mean

U

L

Mean

SC 1C 2C 1C + 2C Mean

3.3 5.9 6.3 5.9 5.4

3.3 6.2 5.7 6.2 5.4

3.3 6.1 6.0 6.1 5.4

3.8 5.7 6.7 5.7 5.5

3.2 5.8 4.4 5.8 4.8

3.5 5.8 5.6 5.8 5.1

4.8 5.6 8.6 5.6 6.2

3.1 5.3 4.6 5.4 4.6

4.0 5.5 6.6 5.5 5.4

4.8 5.5 8.9 5.5 6.2

4.4 5.4 6.8 5.4 5.5

4.6 5.5 7.9 5.5 5.8

SC, single annual cut; 1C and 2C, first and second cut in a two-cut system.

3.3. Ash content In the establishment year (2002), ash content was not significant different between cytotypes and it averaged 3.2% on total dry biomass. In 2003, ash content did not significantly change between cytotypes, though it was significantly affected (P 6 0.01) by cut management: harvesting once per year allowed to almost halve the ash content (Table 3), an effect that was recurrent also in the following years. Overall, ash content ranged from 3.1% to 8.9% (Table 3). Biomass harvested once per year (SC) generally showed a clearly lower ash content than 1C or 2C. Moreover, ash content was similar in L and U under 1C and SC, while it was always significantly higher in U than L in the re-growth biomass (2C). Furthermore, cut management always significantly interacted with cytotype. This interaction was explained by the different behaviour of the two cytotypes in accumulating minerals in re-growth biomass (2C): in comparison to 1C, U showed a remarkable increase (up to 62% in 2005) in ash content in 2C; in contrast, the re-growth biomass of L resulted in a decreasing ash content in 3 years out of four (Table 3). The only exception was the last year when both cytotypes showed an increase in ash content from 1C to 2C, nonetheless this was again much more remarkable in U (+3.4%) than L (+1.4%). 4. Discussion Understanding a proper harvest management is necessary to maximize long-term biomass yields of switchgrass, while maintaining moisture and ashes low in biofuels. Perennial crops like switchgrass are conventionally harvested once per year in autumn or after killing-frost. Alternatively, a diversified and more flexible harvest management such as a two-cut system could provide a longer round-year supply of biomass to conversion plants, while allowing farmers to easy dry the summer-harvested biomass in the field, that is without additional drying costs. Of course, whether or not a two-cut system can be feasible will depend on biomass quality (e.g. ash content), seasonal yield, long-term regrowth capacity, economic life-span of the plant and harvesting costs. To the best of our knowledge the effects of harvest frequency on switchgrass productivity and quality is still not

clearly understood. Literature does not report European studies on this issue, while US researches have emphasized a single harvest of mature senescing crops (Sanderson et al., 1999), a double harvest when the re-growth was harvested in fall (George and Obermann, 1989), or either cutting systems depending on summer precipitation (Reynolds et al., 2000). The results of this research show that under southern European climatic conditions, with an average summer rainfall of about 250 L m2 (June–September), cutting switchgrass once a year produced the best results both in term of total dry biomass and quality. The two-cut system lead to increase seasonal biomass yield only in the two first productive years (+15% and +38% of TDM, respectively), while by the third year biomass yields were clearly lower (on average 28% TDM compared to one-cut system). Very similar conclusions were reported by Mulkey et al. (2007) comparing switchgrass harvested once annually at anthesis and after killing-frost, and by Cuomo et al. (1996) comparing once, twice and thrice annually harvested switchgrass for three consecutive years. Rainfall course did not generally reflect plant response to cut management, thus supporting previous findings showing minor effects of rainfall in limiting biomass yield under a two-cut management (Fike et al., 2006b). Rather, yield depression under two-cut system was reasonably due to the reduction of plant vigour consequent to the below ground reserve depletion because of the incomplete mineral translocation to rhizomes which naturally occurs during the crop senescence (Radiotis et al., 1996; Sanderson et al., 1996). Vigour depression may be reasonably quantified either by plant height or number of tillers. Anderson et al. (1989) found that frequent harvests decreased by about 50% the tiller density of switchgrass during a 2-year period. Unfortunately, in this research the effect of cut management on tillers density could not be clearly distinguished as the number of tillers was only measured in the two first years and after the summer harvest 2006. Nonetheless, it could be somewhat remarkable that tiller density did not appreciably change between cut systems during the two first years (1091 tillers m2, average of all cytotypes), while it appreciably decreased under twocut system by about 50% (548 tillers m2) in the last year (2006) with respect to one-cut system. Plant height was also clearly affected by cut management with both cytotypes considerably reducing the height in the last 2 years, espe-

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cially L cytotypes. Possibly, the physiological stage at the occurrence of summer harvest was the main reason of this different behavior in plant height and tiller density of U and L cytotypes. Vogel et al. (2002) reported that in a two-cut system the best harvest time to maximize switchgrass yield is at about a maturity index score of 3.3, which corresponds to all spikelets visible and panicle fully emerged from the boot (Moore et al., 1991). In our research, when summer harvest occurred, U were generally at anthesis, i.e. proximate to the best maturity index score of 3.3, while L having longer growth cycle were still elongating the stems. Nonetheless the more responsiveness of U to two-cut system is somehow spurious as biomass yield of U deriving from the first cut in summertime often exceeded the seasonal biomass obtained from a single annual harvest after killing-frost. Therefore, the contribution of the second cut to the total biomass yield of U was trivial, accounting for about only 10% of the total seasonal biomass. As a result, the re-growth biomass of U was clearly too low to economically offset the additional harvest costs. In a previous study addressed to assess the economic analysis of switchgrass, Monti et al. (2007) calculated an average harvesting/baling cost under a conventional agricultural practice equal to € 280 ha1. This means that, with a reasonable market price of about € 55 Mg1 of dry biomass, not less than 5 Mg ha1 of regrowth biomass would be needed to offset additional harvest costs. Since the re-growth biomass yield of upland cytotypes never exceeded 2 Mg ha1 (2004), the second harvest can be considered clearly uneconomic for U. Nonetheless, from time-management point of view, the attitude of U to accumulate nearly all the biomass much early in the season may be attractive for farmers. Summer harvest when other field work has been already completed would in fact allow farmers to keep biomass in the field for few days thus to reduce moisture content below 15% without additional costs. Also, autumn or post-frost harvests could be not always practicable because of weather conditions and soil characteristics. Nonetheless, it should be underlined that especially in U, ash content of early cut biomass was almost twofold than in biomass harvested once per year, thus resulting in a clearly lower quality of biofuel. Alike U, L showed an higher productivity under two-cut system in the two first years after the establishment, though it was significant only in 2004 when the re-growth biomass reached 5.5 Mg ha1, an amount very close to break-even yield as calculated by Monti et al. (2007). Therefore, it could be concluded that two harvests per year seem generally not worthwhile with L too. 5. Conclusions Harvesting switchgrass twice per year would have several advantages from time-management point of view. However, though it first appeared worthwhile, especially with U upland cytotypes, it does not offset additional harvest costs due to the too low re-growth biomass yield, both

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of upland and lowland cytotypes. Furthermore, starting from the third productive year, the two-cut system sensibly decreased the plant vigour in both cytotypes. Also, summer and re-growth harvested biomass showed a considerable lower quality being much richer in ashes with respect to biomass harvested after killing-frost. Acknowledgement This research was carried out in the framework of the EU Project ‘‘Bioenergy Chains from perennial crops in southern Europe”. References Anderson, B.E., Matches, A.G., Nelson, C.J., 1989. Carbohydrate reserves and tillering of switchgrass following clipping. Agron. J. 81, 13–16. Burvall, J., 1997. Influence of harvest time and soil type on fuel quality in reed canary grass (Phalaris arundinacea L.). Biomass Bioenerg. 3, 149– 154. Casler, M.D., Vogel, K.P., Taliaferro, C.M., Wynia, R.L., 2004. Latitudinal adaptation of switchgrass populations. Crop Sci. 44, 293–303. Ciais, P., Reichstein, M., Viovy, N., Granier, A., Oge´e, J., et al., 2005. Europe-wide reduction in primary productivity caused by the heat and drought in 2003. Nature 437, 529–533. Condon, A.G., Richards, R.A., Rebetzke, G.J., Farquhar, G.D., 2004. Breeding for high water-use efficiency. J. Exp. Bot. 55, 2447–2460. Cuomo, G.J., Anderson, B.E., Young, L.J., Wilhelm, W.W., 1996. Harvest frequency and burning effects on monocultures of 3 warmseason grasses. J. Range Manage. 49, 157–162. Elbersen, H.W., Ocumpaugh, W.R., Hussey, M.A., Sanderson, M.A., Tischler, C.R., 1999. Field evaluation of switchgrass seedlings divergently selected for crown node placement. Crop Sci. 39, 475–479. Fazio, S., Monti, A., Venturi, G., 2007. Life cycle assessment of switchgrass under variable scenarios from ‘‘Cradle to Farm Gate”. In: Proc. 15th European Conference and Exhibition, Berlin, Germany, 7–11 May, pp. 653–657. Fike, J.H., Parrish, D.J., Wolf, D.D., Balasko, J.A., Green Jr., J.T., Rasnake, M., Reynolds, J.H., 2006a. Switchgrass production for the upper southeastern USA: influence of cultivar and cutting frequency on biomass yields. Biomass Bioenerg. 30, 207–213. Fike, J.H., Parrish, D.J., Wolf, D.D., Balasko, J.A., Green Jr., J.T., Rasnake, M., Reynolds, J.H., 2006b. Long-term yield potential of switchgrass-for-biofuel systems. Biomass Bioenerg. 30, 198–206. George, J.R., Obermann, D., 1989. Spring defoliation to improve summer supply and quality of switchgrass. Agron. J. 81, 47–53. Hultquist, S.J., Vogel, K.P., Lee, D.J., Arumuganathan, K., Kaeppler, S., 1996. Chloroplast DNA and nuclear DNA content variations among cultivars of switchgrass, Panicum virgatum L. Crop Sci. 35, 565– 571. Jenkins, B.M., Baxter, L.L., Miles Jr., T.R., Miles, T.R., 1998. Combustion properties of biomass. Fuel Process. Technol. 54, 17–46. Keeney, D.R., De Luca, T.H., 1992. Biomass as an energy source for the Midwestern US. Am. J. Altern. Agric. 7, 137–144. Lewandowski, I., Scurlock, J.M.O., Lindvall, E., Christou, M., 2003. The development and current status of perennial rhizomatous grasses as energy crops in the US and Europe. Biomass Bioenerg. 25, 335–361. Madakadze, I.C., Stewart, K.A., Peterson, P.R., Coulman, B.E., Smith, D.L., 1999. Cutting frequency and nitrogen fertilization effects on yield and nitrogen concentration of switchgrass in a short season area. Crop Sci. 39, 552–557. McLauglin, S.B., Kszos, L.A., 2005. Development of switchgrass (Panicum virgatum) as a bioenergy feedstock in the United States. Biomass Bioenerg. 28, 515–535.

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