Radiation use efficiency, chemical composition, and methane yield of biogas crops under rainfed and irrigated conditions

Europ. J. Agronomy 87 (2017) 8–18

Contents lists available at ScienceDirect

European Journal of Agronomy journal homepage: www.elsevier.com/locate/eja

Radiation use efficiency, chemical composition, and methane yield of biogas crops under rainfed and irrigated conditions Burkhard Schoo a,∗ , Henning Kage b , Siegfried Schittenhelm a a b

Julius Kühn-Institute (JKI), Institute for Crop and Soil Science, Bundesallee 50, D-38116 Braunschweig, Germany Kiel University (CAU), Institute of Crop Science and Plant Breeding, Hermann-Rodewald-Straße 9, D-24118 Kiel, Germany

a r t i c l e

i n f o

Article history: Received 30 August 2016 Received in revised form 3 March 2017 Accepted 8 March 2017 Keywords: Bioenergy Biogas Drought stress PAR interception

a b s t r a c t For biomethane production, the cup plant (Silphium perfoliatum L.) is considered a promising alternative substrate to silage maize (Zea mays L.) due to its high biomass potential and associated ecological and environmental benefits. It has also been suggested to grow cup plant on less productive soils because of its presumed drought tolerance, but robust information on the impact of water shortage on biomass growth and substrate quality of cup plant is rare. Therefore, this study assesses the effects of soil water availability on the chemical composition and specific methane yield (SMY) of cup plant. Furthermore above-ground dry matter yield (DMY) was analysed as a function of intercepted photosynthetic active radiation (PAR) and radiation use efficiency (RUE). Data were collected in a two-year field experiment under rainfed and irrigated conditions with cup plant, maize, and lucerne-grass (Medicago sativa L., Festuca pratensis Huds., Phleum pratense L.). The cup plant revealed a slight decrease of −6% in the SMY in response to water shortage (less than 50% of plant available water capacity). The average SMY of cup plant [306 l (kg volatile solids (VS))−1 ] was lower than that of maize [362 l (kg VS)−1 ] and lucerne-grass [334 l (kg VS)−1 ]. The mean drought-related reduction of the methane hectare yield (MHY) was significantly greater for cup plant (−40%) than for maize (−17%) and lucerne-grass (−13%). The DMY reduction in rainfed cup plant was mainly attributed to a more severe decrease in RUE (−29%) than for maize (−16%) and lucernegrass (−12%). Under water stress, the mean cup plant RUE (1.3 g MJ−1 ) was significantly lower than that of maize (2.9 g MJ−1 ) and lucerne-grass (1.4 g MJ−1 ). Compared to RUE, the reduced PAR interception was less meaningful for DMY in rainfed crops. Hence, the cup plant is not suitable for growing on drought prone lands due to its high water demand required to produce reasonably high MHYs. © 2017 Elsevier B.V. All rights reserved.

1. Introduction

Abbreviations: ADL, acid detergent lignin; AWC, available water content; DMY, above-ground dry matter yield; DMYirrigated , above-ground dry matter yield in irrigated plots; DMYrainfed , above-ground dry matter yield in rainfed plots; GAI, green area index; IPAR, cumulative intercepted photosynthetically active radiation; IPARirrigated , cumulative intercepted photosynthetically active radiation in irrigated plots; IPARrainfed , cumulative intercepted photosynthetically active radiation in rainfed plots; KPAR , extinction coefficient for photosynthetically active radiation; LIPAR , above-ground dry matter yield-loss due to reduced cumulative intercepted photosynthetically active radiation; LRUE , above-ground dry matter yield-loss due to reduced cumulative radiation use efficiency; MHY, methane hectare yield; NFE, nitrogen-free extract; PAR, photosynthetically active radiation; Qi , transmitted radiation; Q0 , incident radiation; RUE, radiation use efficiency; RUEirrigated , radiation use efficiency in irrigated plots; RUErainfed „ radiation use efficiency in rainfed plots; SMY, specific methane yield; VS, volatile solids; WUE, water use efficiency. ∗ Corresponding author. E-mail addresses: [email protected] (B. Schoo), kage@pflanzenbau.uni-kiel.de (H. Kage), [email protected] (S. Schittenhelm). http://dx.doi.org/10.1016/j.eja.2017.03.001 1161-0301/© 2017 Elsevier B.V. All rights reserved.

Among a variety of crops used for biomethane production in Germany, silage maize (Zea mays L.) attains the highest MHY (Weiland, 2010). Besides the favourable chemical composition, this superiority mainly results from a high biomass yield (Amon et al., 2007a; Brauer-Siebrecht et al., 2016). Consequently, maize became the key substrate for biomethane production in Germany (DMK, 2013) and is increasingly cultivated in short crop rotations or monoculture, with the resulting negative effects on biodiversity (Gardiner et al., 2010) and plant health (Meissle et al., 2010). In the search for alternative biogas substrates, the perennial cup plant (Silphium perfoliatum L.) is gaining increasing attention due to its high biomass potential (Gansberger et al., 2015) and ecological benefits in comparison to continuously grown maize (Sanderson and Adler, 2008). Furthermore, some authors expect this C3 crop to be drought tolerant (Sontheimer, 2007; Bauböck et al., 2014; Franzaring et al., 2014, 2015).

B. Schoo et al. / Europ. J. Agronomy 87 (2017) 8–18

The ecologically and economically sustainable biogas production depends on choosing the most suitable crops for the available sites. In this context, knowledge of the crop specific water use patterns is essential, because drought stress is one of the most important yield-limiting factors of crop production. Drought stress has been shown to alter the chemical composition, i.e. increased fibre and decreased nitrogen-free extract (NFE) content in maize (Meibaum et al., 2012), whereas no significant effect of drought stress on chemical composition in lucerne (Medicago sativa L.) was found (Testa et al., 2011). Several studies have been focussing on the cup plant’s SMY (Dandikas et al., 2014; Mast et al., 2014; Haag et al., 2015), DMY (Gansberger et al., 2015) and MHY (Stolzenburg and Monkos, 2012; Mast et al., 2014; Haag et al., 2015) under a wide range of growing conditions. However, none of these studies provided detailed information on the impact of drought stress on the respective target traits of the cup plant. The SMY of cup plant is considerably lower than that of maize (Dandikas et al., 2014; Mast et al., 2014; Haag et al., 2015). Therefore, the economic methane production with cup plant is mainly achieved via a high DMY. Model calculations for various bioenergy cropping systems have shown that a high DMY essentially depends on an efficient use of the crop growth resources water and incident solar radiation (Wienforth, 2011). With respect to water use efficiency (WUE), the cup plant is regarded to be far less efficient than maize. However, its higher soil water extraction ability compensated partly for this deficiency (Schoo et al., 2016). The interception of PAR by the cup plant and the photosynthetic transformation of intercepted radiation into biomass, expressed by the RUE (Monteith and Moss, 1977), have not yet been studied. In principle a higher crop DMY can be achieved with increased and extended radiation interception and high RUE (Loomis and Amthor, 1999; Wienforth, 2011). The amount of PAR interception is determined by the crop’s leaf area development (Bonhomme, 2000). Under drought stress, leaf area reduction, leaf senescence as well as wilting and rolling of leaves will reduce the amount of PAR interception, whereas decreased leaf photosyn-

9

thetic rates reduce RUE (Sinclair and Muchow, 1999). Contrary to annual crops, perennial cup plant exhibits an earlier development of PAR absorbing leaf area right from the beginning of the vegetation cycle. On the other hand, the RUE of C3 plants is lower than that of C4 plants (Sinclair and Muchow, 1999). If the cup plant is actually drought tolerant, it should be able to maintain a relatively high PAR interception and RUE even under water deficit conditions. The present study addresses the determination of the energy potential of the cup plant and its suitability as an alternative substrate for biogas production with respect to the use of water and radiation. The analyses covered the impact of drought stress on the substrate quality as well as the radiation interception and radiation use efficiency. Maize and lucerne-grass were used as reference crops.

2. Materials and methods 2.1. Field experiment The field experiment was conducted in 2013 and 2014 at the experimental field (52.296◦ N, 10.438◦ E, altitude 76 m) of the Julius Kühn Institute for Crop and Soil Science in Braunschweig, Germany. The soil was classified as a Haplic Luvisol (FAO, 1997) with locally occurring clay rich bandings typical of a Lamellic Luvisol (FAO, 1997). The plant available soil water content at field capacity amounted to 185 mm in the upper 150 cm. The experimental plots (6 m × 40 m) were established in 2012 in a two-factorial split-plot design with four replications. In the subplots, perennial cup plant, semi-perennial lucerne-grass (Medicago sativa L., Festuca pratensis Huds., Phleum pratense L.) and a maize monoculture were grown under rainfed and irrigated conditions (main plots). Supplemental overhead or drip irrigation was provided for maintaining 50–80% of the plant available water content (AWC). Details on crop management are provided in Table 1.

Table 1 Details of cultural practices in the years 2013 and 2014. Cup plant

Maize

Lucerne-grass

Cultivars1

Population of Russian origin

Atletas (ripening group: late)

Sowing date Plant spacing or seeding rate Harvest dates

Crop established May 9, 2012 already 12 rows; 0.5 m between and within rows (4 plants m−2 ) August 20 (rainfed) and August 29 (irrigated) in 2013; August 6 (rainfed) and August 14 (irrigated) in 2014

April 26, 2013 and April 16, 2014 8 rows; 0.75 m between rows (9 plants m−2 ) October 1, 2013 and September 29, 2014

Harvested area Irrigation2

160 m2 (8 central rows) 2013: 215 mm (11); 2014: 230 mm (11) 41, 237, 33, 39 of P, K, Mg, S in 2013 and 30, 199, 61, and 76 of P, K, Mg, and S in 2014 170

120 m2 (4 central rows) 2013: 235 mm (12); 2014: 120 mm (6)

COUNTRY 2056 (80% Medicago sativa L., 15% Festuca pratensis Huds., and 5% Phleum pratense L.) Crop established March 21, 2012 already Four tracks with a 1.5 m seed drill at a seeding rate of 20 kg ha−1 Four cuttings on May 14, June 24, August 1, and September 17 in 2013; 5 cuttings on April 28, June 14, July 15, August 18, and October 6 in 2014 120 m2 (two central seed drill tracks) 2013: 235 mm (12); 2014: 120 mm (6)

41, 237, 33, 39 of P, K, Mg, S in 2013 and 40, 252, 61, and 74 of P, K, Mg, and S in 2014 180

41, 270, 36, and 43 of P, K, Mg, and S in 2013 and 30, 274, 30, and 33 of P, K, Mg, and S in 2014 30

May 3, 2013 and April 30, 2014

March 27, 2013 and March 10, 2014

Basic fertilisation (kg nutrient ha−1 yr−1 ) Nitrogen fertiliser (kg N ha−1 yr−1 ) Application date Form Weed control3

Pest and disease control3

March 3, 2013 and March 10, 2014 Uniformly calcium ammonium nitrate None

Tank mix with 650 g ha−1 boscalid and 100 g ha−1 pyraclostrobin against grey mould (Botrytis cinerea) at June 4 and 6, 2013 and May 5 and 15, 2014

−1

Tank mix with 100 g ha mesotrione, 43.2 g ha−1 nicosulfuron, and 15 g ha−1 prosulfuron 100 g ha−1 pirimicarb at July 7, 2013 against aphids

Side effect of multiple cutting

None

1 Seedlings of the cup plant were purchased from N.L. Chrestensen, Erfurt. Seeds of maize and lucerne-grass were kindly provided by the KWS Saat AG and the Deutsche Saatveredelung AG (DSV), respectively. 2 Number of irrigation events in brackets. Overhead and drip irrigation were applied. 3 Amount of active ingredients.

10

B. Schoo et al. / Europ. J. Agronomy 87 (2017) 8–18

2.2. Monitoring of soil moisture

utilised:

Soil moisture was measured twice a week during crop growth in 10 cm intervals up to a soil depth of 160 cm by means of the Diviner 2000 capacitance soil moisture probe (Sentek Technologies, Stepney, Australia). In each cup plant and maize plot, two PVC access tubes were installed, one within and one midway between rows of the respective crops. Only one access tube per plot was used in the densely cropped lucerne-grass. The dimensionless ‘scale frequency readings’, representing the ratio of frequency response in soil, air and water, were converted to soil water volume percent (vol.-%) through a calibration equation. A site-specific calibration was gained from a depth-specific regression analyses between measured and gravimetrically determined soil moisture contents (R2 = 0.64; n = 1714). Higher coefficients of determination were not reached due to the large size of the experimental area causing increased risks for variable soil conditions, i.e. by the afore mentioned bandings with increased clay contents and bulk densities. 2.3. Green area index (GAI) The green area index (GAI) of all plots was assessed weekly throughout the two growing seasons. For early determinations with low leaf area, photographs of marked subplots (1 × 1 m) were taken with a digital camera. The soil coverage ratio was estimated from these images by means of a self-written image analysis software (Dr. Ulf Böttcher, Kiel University, Germany). In conformity with Wienforth (2011), the soil coverage values were used to calculate the GAI by a transformation of the Lambert-Beer law which describes the relationship of green area and radiation transmittance:

PAR interception(%) =

(incident global radiaiton − transmitted global radiation) incident global radiation

×100

(2)

The weekly estimated proportion of intercepted PAR was interpolated on a daily basis by means of a fitted least squares quadratic function with the PROC REG procedure of SAS (SAS Institute, Cary, NC) following Dohleman and Long (2009). Thereafter, the daily sum of intercepted PAR (MJ m−2 ) was calculated as the product of the proportion of intercepted PAR (%) and the daily sum of global radiation (MJ m−2 ), considering a ratio of PAR to global radiation of 0.5 following Sinclair and Muchow (1999). The radiation use efficiency (RUE; g MJ−1 ) was calculated for the duration of the growing seasons as the ratio of DMY to cumulative intercepted PAR (IPAR; MJ m−2 ). Furthermore, drought-related DMY losses were associated to reductions of IPAR and RUE by a simple mechanistic yield model adopted from Earl and Davies (2003): LIPAR = DMY irrigated × (1–IPAR rainfed /IPAR irrigated )

(3)

where LIPAR is the yield loss contributed to the reduced IPAR and DMYirrigated is DMY of the irrigated plots. Yield losses associated with reduced RUE (LRUE ) were calculated based on the equation: LRUE = (DMY irrigated −LIPAR ) × (1–RUErainfed /RUEirrigated )

(4)

2.5. Biomass sampling and chemical analyses ln (Q i /Q 0 ) = −kPAR × GAI

(1)

with transmitted (Qi ) and incident (Q0 ) radiation and the crop specific radiation extinction coefficient (kPAR ). The parameter kPAR was calculated from subsequent measurements of incident and transmitted solar radiation and amounted to 0.8, 0.7 and 0.7 for cup plant, maize and lucerne-grass, respectively. Along with the formation of consecutive leaf layers, measurements were conducted with a SunScan Canopy Analysis System (Delta-T Devices, Cambridge). The SunScan probe was held below the canopy to measure the transmitted solar radiation while an external beam fraction sensor recorded the incident solar radiation above the canopy. On each measurement date, 20 readings were taken per plot. Measurements were conducted between 10:00 a.m. and 2:00 p.m. above the actual senescent leaf layer. The ellipsoidal leaf angle distribution parameter was set to 4 for cup plant and to 1 for maize and lucerne-grass. This parameter characterises the average leaf angle (horizontal to vertical leaf orientation) in a canopy and is assessed by dividing the number of horizontal leaves by the number of vertical leaves, multiplied by  and divided by 2 (Webb et al., 2008). By this means the canopy structure can be taken into account in the calculation of the extinction coefficient according to the equation of Campbell (1986). The reliability of the measured GAI values was confirmed by means of repeated destructive measurements and control readings with a LAI-2200 Plant Canopy Analyzer (Li-Cor, Inc., Nebraska, USA). 2.4. Radiation interception and use For the estimation of the PAR interception (in%) and the determination of the critical GAI (GAI required for 95% PAR interception), values of the incident and transmitted global radiation, gained from GAI measurements with the SunScan Canopy Analysis System were

Cup plant and maize plots were harvested with a tractorpowered single-row field chopper (Pöttinger Ltd.), while lucernegrass plots were harvested with a green fodder plot harvester with integrated chopper (HALDRUP Ltd.). Twenty kg samples of the chopped fresh mass were taken from each plot and homogenised before sub-sampling. Triplicate subsamples were oven dried to determine dry matter concentration (105 ◦ C for 48 h) and chemical composition (60 ◦ C for 48 h). Subsamples for chemical analysis were ground with a rotary mill (Brabender, Duisburg, Germany). The nutrient analysis for volatile solids (VS), crude ash, crude fat, crude protein, crude fibre and nitrogen-free extract (NFE), was conducted according to VDLUFA (1976). The procedure was essentially the same as that described by Schittenhelm (2010). Crude fat concentration was analysed in 2013 only. Further, three subsamples of 1000 g of chopped fresh mass were stored in polyethylene bags at −20 ◦ C for subsequent measurements on anaerobic digestion. 2.6. Batch anaerobic digestion The batch tests were conducted following the VDI Guideline 4630 (VDI-Richtlinie 2006) as described in detail by Pfitzner et al. (2010). In brief, the deep frozen fresh mass was thawed, filled into 20 l polyethylene vessels and inoculated with communal sewage sludge. The fermentation process was carried out for 35 days at a temperature of 37 ◦ C under mesophilic conditions. The volume of gas produced was analysed with a drum-type gas meter TG3 (Ritter GmbH, Bochum, Germany) and a Xam-7000 gas detection device (Drägerwerk AG, Lübeck, Germany). Methane production is given in litre per kg of volatile solids. The SMY (l (kg VS)−1 ) was determined according to the VDI Guideline 4630 (VDI-Richtlinie 2006). The MHY (m3 VS ha−1 ) was calculated by multiplying DMY with SMY.

B. Schoo et al. / Europ. J. Agronomy 87 (2017) 8–18

2.7. Statistics

11

3. Results

was around 4 (Fig. 4). Contrary to the perennial crops, maize attained this level of PAR interception only in mid-July (beginning of stem elongation), at a time when the daily global radiation intensity was already declining. The drought stress in 2013 caused a significantly lower (F = 89.55; P = 0.018) PAR interception of cup plant and lucerne-grass in the rainfed compared to the irrigated treatment, whereas the PAR interception of maize was not significantly reduced. In 2014, no differences in PAR interception were observed between water regimes for maize and lucerne-grass. The cup plant on the contrary exhibited a significant (F = 19.43; P = 0.0217) drought-related reduction of PAR interception by the beginning of flowering. Even in the irrigated cup plant treatment, the crops’ typical and strong reduction in GAI towards harvest caused a notable decrease of PAR interception by the end of stem elongation. Averaged across water regimes and years, IPAR was highest in cup plant (2056 MJ m−2 ), followed by lucerne-grass (1880 MJ m−2 ) and maize (1341 MJ m−2 ).

3.1. Growth conditions

3.4. Radiation use

In 2013, by the middle of June (stem elongation), the high initial soil moisture level in rainfed and irrigated plots of the cup plant fell below the critical level of 50% AWC (Fig. 1). In rainfed maize (stem elongation) and lucerne-grass (third regrowth), the soil moisture level dropped below 50% AWC in mid-July. The water deficits in rainfed plots caused severe drought stress in all crops. Despite high rainfall in May and high irrigation intensity from June to August (end of stem elongation/flowering), the target soil moisture of the irrigated cup plant plots could not be met. Due to the even distribution of precipitation in 2014, the soil water content in rainfed maize and lucerne-grass plots was almost always above 50% AWC considering the growth phase from June to August. However, the soil water content of rainfed cup plant again dropped to the low level of 2013, causing severe drought stress right from the beginning of stem elongation. Contrary to 2013, the minimum soil moisture target of 50% AWC could be maintained in irrigated cup plant through additional drip irrigation.

Averaged across water regimes and years, the RUE of the cup plant was significantly lower (−57%) than that of maize (Table 2). Under irrigated conditions, cup plant and lucerne-grass had similar RUE. The cup plant attained its maximum RUE of 1.51 g MJ−1 in the irrigated treatment of 2014. Under drought stress, IPAR and RUE of the crops were significantly reduced. Crops interacted significantly with water regimes and years to affect IPAR and RUE. In all crops, the reduced RUE under drought stress caused a significantly greater (F = 186.44; P = 0.0008) yield loss than the reduced IPAR (Table 3). There were significant interactions between crops and yield loss components. Averaged across years, the yield losses through reduced RUE amounted to 27, 14 and 10% for the cup plant, maize and lucerne-grass, respectively.

The linear mixed models procedure (GLIMMIX) of SAS (Version 9.3, SAS Inst., Cary, NC) was used for the analysis of variance, with water regime and crop as fixed effects. The residuals of the data sets were tested for normal distribution (Shapiro-Wilk test) and variance homogeneity (Levene test). Analyses were conducted for single years (crop + water regime + crop × water regime) and across years (crop + water regime + year + crop × water regime + crop × year + water regime × year + crop × water regime × year). Insignificant interactions were not excluded from analyses. Comparisons of means (t-test: Least Significant Difference (LSD)) were performed when the respective effects were significant (P < 0.05). Figures were created with SigmaPlot 12 (Systat Software Inc., San Jose, California).

3.2. Green area development In both experimental years, the perennial cup plant and the semi-perennial lucerne-grass exhibited a rapid leaf area development starting right from the beginning of the vegetation cycle (Fig. 2). In both water regimes and both years, the cup plant reached its maximum GAI of ≈10 around mid-June, when the elongating stems had 8 to 9 nodes. Thereafter, the low penetration of radiation into the dense cup plant crops resulted in a severe die back of leaves causing a strong decline of green leaf area. Due to soil water deficiency in 2013, the GAI of rainfed cup plant plots fell more rapidly and remained on a significantly (F = 73.70; P < 0.0033) lower level than that of the irrigated plots. Just before harvest in 2013, the GAI had dropped to values of about 2 in the rainfed and 4 in the irrigated plots. The differentiation between the two water regimes was less pronounced in 2014, because of the higher precipitation. In each growing season, maize attained the maximum GAI (≈5) at the beginning of August (end of stem elongation), whereas lucerne-grass reached its maximum GAI at the time of the first cut. Contrary to cup plant, rainfed maize and lucerne-grass only exhibited a significant GAI reduction in 2013. 3.3. Radiation interception and critical GAI In both water regimes and years, cup plant (leaf development) and lucerne-grass exceeded the 95% PAR interception threshold already at the beginning of May due to their rapid leaf area development (Fig. 3). For all crops, the critical GAI for 95% PAR interception

3.5. Dry matter and chemical composition The dry matter content of the cup plant was more severely affected by drought than that of the reference crops (Table 4). Averaged across years, the dry matter content of cup plant was 5% higher under rainfed compared to irrigated conditions. The highest dry matter content of the cup plant was measured under the severe drought stress conditions of 2013 with the dieback of whole shoots. Those dead shoots contributed with 29% to the dry matter yield. The crude fibre content of the cup plant was significantly higher than that of the two reference crops. All crops showed significantly lower crude fibre contents in the rainfed than in the irrigated plots. The cup plant had a much lower crude fat concentration than maize and lucerne-grass. In both water regimes and years, the crude protein content of the cup plant ranged on a similar level as that for maize. The NFE content of the cup plant was 21% lower than that of maize but 8% higher than that of lucerne-grass. Compared to irrigated conditions, crude protein contents of rainfed cup plant and maize increased significantly. Crops interacted significantly with water regimes and years for dry matter, crude protein, crude fibre and NFE. 3.6. Methane yields Averaged across water regimes and years the SMY of the cup plant was 15 and 8% lower than that of maize and lucernegrass, respectively (Table 4). Under drought, the cup plant’s SMY decreased significantly by 6% in both years, whereas no effects of drought stress on SMY were observed for the reference crops. For SMY, there was a significant interaction between crops and years. Averaged across years, drought related reduction of DMY was higher for the cup plant (33%) than for maize (18%) and lucerne-

12

B. Schoo et al. / Europ. J. Agronomy 87 (2017) 8–18

Fig. 1. Growth conditions for rainfed and irrigated cup plant, maize and lucerne-grass: air temperature, precipitation and dynamics of soil water content (n = 8 or 16) in the entire rooted soil layer (0–150 cm) in the years 2013 and 2014. The dashed line represents 50% available water capacity (AWC) which was the targeted lower soil moisture limit for the irrigated plots.

grass (14%). The highest MHY was observed for irrigated maize in 2013, whereas the rainfed cup plant marked the lowest level in the same year. Averaged across years, MHY in rainfed plots of cup plant, maize and lucerne-grass was reduced by 40, 17 and 13%, respectively, compared to irrigated plots. Crops, water regimes and years interacted significantly for DMY and MHY. 4. Discussion This study evaluates the cup plant as a potential biogas crop with special focus on its productivity under well-watered and drought conditions. Therefore, the parameters determining the quality and

quantity of the biomass were analysed and related to those of the well-known agricultural raw materials maize and lucernegrass. 4.1. Effect of water supply on substrate quality The SMY of cup plant measured in the present study was higher than those reported by Dandikas et al. (2014, Mast et al. (2014, and Haag et al. (2015. The values determined for maize and lucerne-grass were in line with findings of Amon et al. (2007b), Schittenhelm (2008), Weiland (2010), Hakl et al. (2012), Dandikas et al. (2014), Mast et al. (2014) and Herrmann et al. (2016). The

B. Schoo et al. / Europ. J. Agronomy 87 (2017) 8–18

13

Fig. 2. Green area index (GAI) of cup plant, maize and lucerne-grass depending on the water regime in the growing seasons of 2013 and 2014. Vertical bars represent ±1 SD of the mean (n = 4) where these exceed the size of the symbol.

low SMY of the cup plant seems mainly attributable to the relatively high crude fibre content, because Dandikas et al. (2014) and Herrmann et al. (2016) could show that acid detergent lignin (ADL) is the most limiting chemical constituent for SMY. Drought stress caused lower fibre contents in all crops (Table 4). This was somewhat surprising because all rainfed crops exhibited a substantially higher degree of leaf senescence than their irrigated counterparts (Fig. 2). For example, Meibaum et al. (2012) observed a considerably increased crude fibre content of maize under restricted water supply. In lucerne-grass on the other hand, Testa et al. (2011) found only an insignificant alteration of the crude fibre content as a result of restricted water supply. Furthermore, the decrease in crude fibre content would be expected to allow the SMY to increase. However, for the cup plant the opposite was true.

The contradicting results may be explained by an unexamined shift in the crude fibre composition towards a higher ADL and cellulose content with simultaneously decrease of total crude fibre concentration. This assumption could explain the decrease of SMY in rainfed cup plant despite decreasing crude fibre content. However, considering the reduction of SMY in rainfed compared to irrigated cup plant, the decrease of SMY appears very moderate with respect to the comprehensive dieback of whole shoots. Nevertheless, the relatively low SMY of the cup plant emphasizes the role of DMY as the main driver of high MHY. There were a number of interactions between crops and water regimes as well as between crops, water regimes and years for chemical composition, DMY and methane yields. These interactions were mainly attributable to the distinctly different water use patterns of the

14

B. Schoo et al. / Europ. J. Agronomy 87 (2017) 8–18

Fig. 3. Interception of photosynthetic active radiation (PAR) of cup plant, maize and lucerne-grass depending on the water regime in the growing seasons of 2013 and 2014. Vertical bars represent ±1 SD of the mean (n = 4) where these exceed the size of the symbol.

crops. Here, the cup plant could not attain potential evapotranspiration even under intensive water supply. By this means, the cup plant reacted somewhat contrarily in comparison to the reference crops. Furthermore the different rainfall distribution and the application of different irrigation practices between years (uniform in 2013 and crop specific in 2014) may have caused statistical interactions. 4.2. GAI development and radiation interception A high crop DMY depends on a favourable combination of IPAR and RUE (Loomis and Amthor, 1999; Wienforth, 2011). As expected, cup plant and lucerne-grass exceeded the critical GAI of 95% PAR interception earlier than maize due to their perennial growth and the lower temperature requirements of the C3 metabolism, which

allowed them to intercept higher amounts of IPAR than maize (Table 2 and Fig. 4). Despite a shorter growth period, cup plant had a markedly higher IPAR than the repeatedly cut lucerne-grass. The severe drought stress in 2013 caused a strong leaf area reduction in rainfed cup plant and lucerne-grass (Fig. 2). Only maize was able to maintain the critical GAI (Fig. 3) due to its high WUE (Schoo et al., 2016). The critical GAI determined for maize and lucerne-grass in the present study was in accordance with findings of Madonni and Otegui (1996) and Teixeira et al. (2007). As cup plant attained its critical GAI already in early May, the further unrestricted leaf area increase until mid-June was redundant with respect to PAR interception. In fact, the high GAI was responsible for the subsequent leaf area reduction as lower leaves became increasingly shaded. While a portion of senesced leaves remained on the plants, it can be assumed that yield losses occurred through crushing of the brittle

B. Schoo et al. / Europ. J. Agronomy 87 (2017) 8–18

15

Fig. 4. Mean critical GAI of cup plant, maize and lucerne-grass. The lines indicate 95% intercepted PAR.

leaves during the mechanical harvest. This yield reduction also contributed to lower RUE and is strongly counter-productive in terms of the main goal of cultivating biomass crops.

The high GAI of the cup plant is attributable to the low storage capacity of its reproductive sinks (root stocks and seeds). Thus, mainly the stems and to a lesser extent also the leaves represent

Table 2 Cumulative radiation interception (IPAR) and radiation-use efficiency (RUE) for cup plant, maize and lucerne-grass depending on the water regime in the growing seasons of 2013 and 2014. Year (Y) Water regime (W) 2013 Rainfed

Irrigated

2014 Rainfed

Irrigated

2013 and 2014 Rainfed

Irrigated

Mean

Crop (C)

IPAR1 [MJ m−2 ]

RUE1 [g MJ−1 ]

Cup plant Maize Lucerne-grass Cup plant Maize Lucerne-grass Source of Variation W C WxC

1.1e 2.3b 1.2d 1.4c 3.1a 1.6c

DF2 1; 3 2; 12 2; 12

1,887b 1,215f 1,542d 2,017a 1,347e 1,729c Significance of F-values 114** 765*** 1.81

1.1e 3.1b 1.4d 1.5c 3.2a 1.4d

DF 1; 3 2; 12 2; 12

2,097c 1,407d 2,153b 2,222a 1,395d 2,096c Significance of F-values 4.38 2,971*** 36.4*** 1,992b 1,311f 1,848d 2,120a 1,371e 1,913c 2,056a 1,341c 1,881b

1.1e 2.7b 1.4d 1.5c 3.2a 1.5c 1.3c 3.0a 1.5b

Significance of F-values 101** 2,655*** 1,064*** 6.79** 61.2*** 178*** 17.3***

375*** 2,916*** 57.5*** 23.2*** 108*** 66.9*** 22.1***

Cup plant Maize Lucerne-grass Cup plant Maize Lucerne-grass Source of variation W C WxC Cup plant Maize Lucerne-grass Cup plant Maize Lucerne-grass Cup plant Maize Lucerne-grass Source of variation W C Y WxC WxY CxY WxCxY

DF 1; 3 2; 30 1; 30 2; 30 1; 30 2; 30 2; 30

**P < 0.01, and ***P < 0.001. 1 Means followed by different letters for a given character are significantly different (P < 0.05; t-test). 2 Degrees of freedom for numerator (first number) and denominator (second number).

329*** 787*** 19.2***

70.3** 3,341*** 33.8***

16

B. Schoo et al. / Europ. J. Agronomy 87 (2017) 8–18

Table 3 Drought-related yield losses (%) attributable to reduced IPAR and RUE of cup plant, maize and lucerne-grass grown under rainfed conditions in 2013 and 2014. Year (Y)

Crop (C)

IPAR

2013

Cup plant Maize Lucerne-grass DF2 2; 6 1; 3 2; 6

6.4c1 9.8c 10.8c Significance of F-valueschar 0.40 120*** 5.82*

29.2a 24.5b 22.0b

Cup plant Maize Lucerne-grass DF 2; 6 1; 3 2; 6

5.6b −0.9c −2.7c Significance of F-valueschar 82.2*** 53.6** 21.5*

24.8a 3.8b −1.5c

Cup plant Maize Lucerne-grass DF 2; 6 1; 3 1; 3 2; 21 2; 21 1; 21 2; 21

−0.9c −2.7c 4.1d Significance of F-valueschar 36.0*** 175*** 166*** 22.3*** 25.8*** 16.2*** 1.00

27.0a 14.2b 10.3c

Source of variation C L CxL 2014

Source of variation C L CxL 2013 and 2014

Source of variation C L Y CxL CxY LxY CxLxY

RUE Contribution to yield loss (L; %)

1 Means followed by different letters for a given character are significantly different (P < 0.05; t-test). **P < 0.01, and ***P < 0.001. 2 Degrees of freedom for numerator (first number) and denominator (second number).

the most important sinks for assimilates. In its natural habitat, the temperate latitudes of North America (Gansberger et al., 2015), the cup plant occurs solitarily or in loose stands (Penskar and Crispin, 2010) allowing radiation to reach even the lowest leaves. The evolutionary reason for this source-sink allocation strategy may lie in an increased ability to attract pollinators. The cross-pollinated cup plant forms little ‘cups’ through the fused leaf pairs, which collect drinking water for pollinators. Therefore, many nodes and the high interception of precipitation by the large leaf area (Schoo et al., 2016) may be advantageous. However, in the context of yield formation, the leaf area of the reference crops was by far more efficient with respect to the use of radiation for yield formation (Figs. 2 and 3). Maize stopped the leaf area growth after having reached the maximum PAR interception (tassel emergence). The large green area of lucerne-grass through the multiple cuttings contributed itself to dry matter yield. 4.3. Radiation use efficiency affects the methane hectare yield Although the cup plant attained the highest IPAR among the three crops studied, its DMY due to the low RUE was significantly lower than that of maize (Table 2). The values of RUE calculated for maize and lucerne-grass matched with those reported in other studies (Sinclair and Muchow, 1999; Earl and Davies, 2003; Brown et al., 2006; Dohleman and Long, 2009; Wienforth, 2011 Wienforth, 2011). Besides the less efficient C3 physiology (Sinclair and Muchow, 1999) and the crop growth through colder seasons (Wienforth 2011), low RUE of cup plant and lucerne-grass may be attributable to a larger partitioning of assimilates to rootstock and roots towards the end of the growth period. In this context Khaiti and Lemaire (1992) observed that RUE of lucerne-grass decreased from 0.9 g MJ−1 in summer to 0.55 g MJ−1 in autumn. Lucerne-grass had a higher RUE than the cup plant despite the higher energy content of its biomass (Table 4), a deeper and more intensive root

growth (Schoo et al., 2016) and an energy demanding symbiotic nitrogen fixation (Cuttle et al., 2003) which are all at the expense of the legume´ıs DMY. Earl and Davies (2003) also found RUE more important than IPAR for DMY under drought conditions in maize. However, the extent of RUE reduction under drought and thus the effect on DMY was by far highest in cup plant. The reduction of RUE of rainfed compared to irrigated crops averaged 29% for cup plant, 16% for maize, and 12% for lucerne-grass. This is in accordance with model calculations based on data from the same field experiment (see Schoo et al., 2016), which revealed a markedly greater reduction of transpiration under drought stress for cup plant than for maize. Contrary to the cup plant, lucerne-grass maintained relatively high RUE because of its better water acquisition ability through deep and intense rooting. Therefore, the cup plant showed the most severe reduction of MHY under water shortage. The MHYs found in the present study were in line with literature reports for cup plant (Mast et al., 2014), maize (review by Herrmann and Rath, 2012) and lucerne-grass (Hakl et al., 2012). In view of the inefficient transformation of intercepted PAR into biomass and the high drought susceptibility of this process, it seems highly unlikely that the cup plant attains DMYs comparable to maize, especially at drought prone sites. However, cultivation of the less warmth requiring cup plant may be advantageous with increasing elevation, because the RUE of maize is strongly affected by low air temperatures (Andrade et al., 1993). This was impressively demonstrated in practice in the southwest of Germany on elevated sites (beyond 500 m above sea level) which also provide favourable water supply by high precipitation. At those locations, the cup plant was competitive with or even higher yielding than maize (Janzing, 2015). However, these climate conditions are rather an exception considering total acreage cultivated with biogas crops in Germany.

B. Schoo et al. / Europ. J. Agronomy 87 (2017) 8–18

17

Table 4 Contents of dry matter (DM), crude ash, crude fat, crude protein, crude fibre and nitrogen free extracts (NFE) as well as specific methane yield (SMY), above ground dry matter yield (DMY) and methane hectare yield (MHY) of cup plant, maize and lucerne-grass depending on the water regime in the growing seasons of 2013 and 2014. Crude fat content was only determined in 2013 using three representative samples of cup plant, maize and lucerne-grass. Year (Y) Water regime (W) 2013 Rainfed

Irrigated

Source of variation W C WxC 2014 Rainfed

Irrigated

Source of variation W C W×C 2013 and 2014 Rainfed

Irrigated

Mean

Source of variation W C Y W×C W×Y C×Y W×C×Y 1 2

Crop (C)

DM

Crude ash

Crude fat

Crude protein

Crude fibre

NFE

SMY

DMY

MHY

[%]

[%]

[%]

[%]

[%]

[%]

[l (kg VS)−1 ]

[t ha−1 ]

[m3 VS ha−1 ]

Cup plant Maize Lucerne-grass Cup plant Maize Lucerne-grass DF2 1; 3 2; 12 2; 12

29.9b 1 11.7a 35.7a 4.0b 18.8d 11.4a 26.0c 11.4a 34.3a 3.6b 15.7e 11.6a Significance of F-values 50.6** 0.760 156*** 724*** 1.50 1.31

1.7 3.6 3.4 1.7 3.6 3.4

9.2b 7.1c 23.4a 7.4c 5.8d 22.9a

34.1a 21.2d 24.6c 34.8a 23.3c 28.2b

43.2b 65.1a 37.3c 44.7b 64.7a 33.9d

290d 351a 334ab 310c 333ab 329b

10.0d 13.7c 9.5d 15.5b 20.9a 14.0c

2,889c 4,807b 3,166c 4,789b 6,955a 4,618b

– – –

28.5* 3021*** 3.95*

17.3* 515*** 7.39**

2.17 2318*** 15.7*

0.040 26.3*** 5.02*

766*** 273*** 13.7***

310*** 322*** 7.38**

Cup plant Maize Lucerne-grass Cup plant Maize Lucerne-grass

26.1b 36.7a 17.3d 19.8c 35.7a 17.1d

– – – – – –

8.0a 7.0c 24.7a 6.7c 7.0c 24.8a

33.9b 21.4d 25.3c 38.3a 21.3d 25.4c

45.5b 65.1a 35.3d 42.1c 65.5a 35.6d

303c 376a 338b 321bc 389a 336b

11.7d 21.6a 15.0c 16.8b 22.4a 14.5c

3,543e 8,134b 5,080cd 5,399c 8,714a 4,864d

DF 1; 3 2; 12 2; 12

Significance of F-values 46.4** 0.82 – 1271*** 1086*** – 36.0*** 2.18 –

6.83 5589*** 7.57**

11.9* 688*** 19.5***

7.70 2943*** 14.9***

3.52 63.5*** 1.40

54.8** 433*** 50.3***

48.3** 545*** 32.1***

Cup plant Maize Lucerne-grass Cup plant Maize Lucerne-grass Cup plant Maize Lucerne-grass

28.0c 36.2a 18.1e 22.9d 35.0b 16.4f 25.4b 35.6a 17.2c

– – – – – – – – –

8.6b 7.0c 24.1a 7.1c 6.4d 23.8a 7.8b 6.7c 24.0a

34.0b 21.3e 24.9d 36.6a 22.3e 26.8c 35.3a 21.8c 25.9b

44.4b 65.1a 36.3c 43.4b 65.1a 34.7d 43.9b 65.1a 35.5c

296d 363a 336b 315c 361a 333b 306c 362a 334b

10.8f 17.7b 12.2e 16.1c 21.7a 14.2d 13.5b 19.7a 13.2b

3,216f 6,471b 4,123e 5,094c 7,835a 4,741d 4,155c 7,153a 4,432b

DF 1; 3 2; 30 1; 30 2; 30 1; 30 2; 30 2; 30

Significance of F-values 75.8** 0.920 1789*** 1984*** 22.4* 7.54 24.0*** 0.620 0.290 0.020 56.0*** 1.83 8.92** 3.15

– – – – – – –

24.2* 8063*** 9.47 10.1*** 9.81* 36.9*** 0.71

25.2* 960*** 0.13 3.09 1.93 11.2*** 19.0***

8.86 4317*** 0.010 2.81 0.030 0.390 22.6***

1.48 82.6*** 30.7* 4.21* 2.28 9.22*** 2.01

559*** 696*** 371*** 36.5*** 155*** 32.8*** 32.7***

349*** 772*** 362*** 28.2*** 63.1*** 70.3*** 14.6***

11.0a 4.0b 11.3a 11.2a 3.7b 10.8a

11.3a 4.0b 11.3a 11.3a 3.7b 11.2a 11.3a 3.8b 11.3a

Character means followed by different letters for a given character are significantly different (P < 0.05; t-test). *P < 0.05, **P < 0.01, and ***P < 0.001. Degrees of freedom for numerator (first number) and denominator (second number).

5. Conclusion Results of the present study indicate a lower energy production potential of the cup plant in comparison with maize, especially on drought prone sites. The DMY has been shown to be of decisive importance for MHY of the cup plant under drought stress as chemical composition and SMY were less affected. The cup plant exhibited higher drought susceptibility than maize and lucerne-grass, mainly because of the strong decrease of RUE. If high cup plant DMY is to be achieved, high water availability is essential. Nevertheless the less efficient use of agricultural land must be set against possible ecological benefits when evaluating sustainability of cultivating cup plant as co-substrate. Further research is required to specify ecological benefits as well as site characteristics most suitable for the cultivation of the cup plant. Acknowledgements The technical assistance of Martin Dohlenburg, Frank Höppner, Bernd Kahlstorf, Sabine Peickert, Martina Schabanoski, and

Jan-Martin Voigt is greatly appreciated. The authors thank Henrike Mielenz for her critical review of an earlier version of the manuscript<. Funding for this research was provided by the German Federal Ministry of Food and Agriculture (BMEL) via the Agency for Renewable Resources (FNR; project number 22037311) based on a decision of the Parliament of the Federal Republic of Germany.

References Amon, T., Amon, B., Kryvoruchko, V., Machmüller, A., Hopfner-Sixt, K., Bodiroza, V., Hrbek, R., Friedel, J., Pasch, E., Wagentristl, H., Schreiner, M., Zollitsch, W., 2007a. Methane production through anaerobic digestion of various energy crops grown in sustainable crop rotations. Bioresour. Technol. 98, 3204–3212. Amon, T., Amon, B., Kryvoruchko, V., Zollitsch, W., Mayer, K., Gruber, L., 2007b. Biogas production from maize and dairy cattle manure-Influence of biomass composition on the methane yield. Agric. Ecosyst. Environ. 118, 173–182. Andrade, F.H., Uhart, S.A., Cirilo, A., 1993. Temperature affects radiation use efficiency in maize. Field Crop Res. 32, 17–25. Bauböck, R., Karpenstein-Machan, M., Kappas, M., 2014. Computing the biomass potentials for maize and two alternative energy crops, triticale and cup plant (Silphium perfoliatum L.), with the crop model BioSTAR in the region of Hannover (Germany). Environ. Sci. Eur. 26, 19.

18

B. Schoo et al. / Europ. J. Agronomy 87 (2017) 8–18

Bonhomme, R., 2000. Beware of comparing RUE values calculated from PAR vs. solar radiation or absorbed vs. intercepted radiation. Field Crop Sci. 68, 247–252. Brauer-Siebrecht, W., Jacobs, A., Christen, O., Götze, P., Koch, H.-J., Rücknagel, J., Märländer, B., 2016. Silage maize and sugar beet for biogas production in rotations and continuous cultivation: dry matter and estimated methane yield. Agronomy 6, 2, http://dx.doi.org/10.3390/agronomy6010002. Brown, H.E., Moot, D.J., Teixeira, E.I., 2006. Radiation use efficiency and biomass partitioning of lucerne (Medicago sativa) in a temperate climate. Eur. J. Agron. 25, 319–327. Campbell, G.S., 1986. Extinction coefficients for radiation in plant canopies using ellipsoidal inclination angle distribution. Agric. Meteor. 36, 317–321. Cuttle, S., Shepherd, M., Goodlass, G., 2003. A review of leguminous fertility-building crops, with particular reference to nitrogen fixation and utilization. Available at: http://www.swarmhub.co.uk/index.php?dlrid=4130 (Accessed 10 January 2017). DMK, 2013. Ein Drittel der Maisanbaufläche für Biogas. Deutsches Maiskomitee e.V. News 11/2013. Dandikas, V., Heuwinkel, H., Lichti, F., Drewes, J.E., Koch, K., 2014. Correlation between biogas yield and chemical composition of energy crops. Bioresour. Technol. 174, 316–320. Dohleman, F.G., Long, S.P., 2009. More productive than maize in the midwest: how does miscanthus do it? Plant Physiol. 150, 2104–2115. Earl, H.J., Davies, R.F., 2003. Effect of drought stress on leaf and whole leaf canopy radiation use efficiency and yield of maize. Agron. J. 95, 688–696. FAO, 1997. FAO/UNESCO Soil Map of the World. Revised Legend, with Corrections and Updates. World Soil Resources Report 60. FAO, Rome (Reprinted with updates as Technical Paper 20, ISRIC, Wageningen, Netherlands). Franzaring, J., Schmid, I., Bäuerle, L., Gensheimer, G., Fangmeier, A., 2014. Investigations on plant functional traits, epidermal structures and the ecophysiology of the novel bioenergy species Sida hermaphrodita Rusby and Silphium perfoliatum L. J. App. Bot. Food Qual. 87, 36–45. Franzaring, J., Holz, I., Kauf, Z., Fangmeier, A., 2015. Responses of the novel bioenergy plant species Sida hermaphrodita (L.) Rusby and Silphium perfoliatum L. to CO 2 fertilization at different temperatures and water supply. Biomass Bioenergy 81, 574–583. Gansberger, M., Montgomery, L.F.R., Liebhard, P., 2015. Botanical characteristics, crop management and potential of Silphium perfoliatum L. as a renewable resource for biogas production. A review. Ind. Crops Prod. 63, 362–372. Gardiner, M.A., Tuell, J.K., Isaacs, R., Gibbs, J., Ascher, J.S., Landis, D.A., 2010. Implications of three biofuel crops for beneficial arthropods in agricultural landscapes. Bioenergy Res. 3, 6–19. Haag, N.L., Nägele, H.-J., Reiss, K., Biertümpfel, A., Oechsner, H., 2015. Methane formation potential of cup plant (Silphium perfoliatum). Biomass Bioenergy 75, 126–133. ˇ ˚ cek, J., 2012. The biogas production from lucerne Hakl, J., Fuksa, P., Habart, J., Santr uˇ biomass in relation to term of harvest. Plant Soil Environ. 58, 289–294. Herrmann, A., Rath, J., 2012. Biogas production from maize: current state, challenges, and prospects: 1. Methane yield potential. Bioenergy Res. 5, 1027–1042. Herrmann, C., Idler, C., Heiermann, M., 2016. Biogas crops grown in energy crop rotations: linking chemical composition and methane production characteristics. Bioresour. Technol. 206, 23–35. Janzing, B., 2015. Praktiker ebnen der Silphie den Weg. Biogas J. 6, 42–45. Khaiti, M., Lemaire, G., 1992. Dynamics of shoot and root growth of lucerne after seeding and after cutting. Eur. J. Agron. 1, 241–247. Loomis, R.S., Amthor, J.S., 1999. Yield potential, plant assimilatory capacity, and metabolic efficiencies. Crop Sci. 39, 1584–1596. Madonni, G., Otegui, M., 1996. Leaf area light interception, and crop development in maize. Field Crop Res. 48, 81–87.

Mast, B., Lemmer, A., Oechsner, H., Reinhardt-Hanisch, A., Claupein, W., GraeffHönninger, S., 2014. Methane yield potential of novel perennial biogas crops influenced by harvest date. Ind. Crops Prod. 58, 194–203. Meibaum, B., Riede, S., Schröder, B., Manderscheid, R., Weigel, H.-J., Breves, G., 2012. Elevated CO2 and drought stress effects on the chemical composition of maize plants: their ruminal fermentation and microbial diversity in vitro. Arch. Anim. Nutr. 66, 473–489. Meissle, M., Mouron, P., Musa, T., Bigler, F., Pons, X., Vasileiadis, V.P., Otto, S., Antichi, D., Kiss, J., Pálinkás, Z., Dorner, Z., van der Weide, R., Groten, J., Czembor, E., Adamczyk, J., Thibord, J.B., Melander, B., Cordsen Nielsen, G., Poulsen, R.T., Zimmermann, O., Verschwele, A., Oldenburg, E., 2010. Pests, pesticide use and alternative options in European maize production: current status and future prospects. J. Appl. Entomol. 134, 357–375. Monteith, J.L., Moss, C.J., 1977. Climate and the efficiency of crop production in Britain [and discussion]. Philos. Trans. R. Soc. Lond. B 281, 277–294. Penskar, M.R., Crispin, S.R., 2010. Special Plant Abstract for Silphium perfoliatum (cup plant) Michigan Natural Features Inventory. Michigan State University, Lansing, MI, USA. Pfitzner, C., Höppner, F., Greef, J.-M., 2010. Assessment of the biogas production potential of renewable resources with near infrared spectroscopy. J. Cultivated Plants 62, 451–460. Sanderson, M.A., Adler, P.R., 2008. Perennial forages as second generation bioenergy crops. Int. J. Mol. Sci. 9, 768–788. Schittenhelm, S., 2008. Chemical composition and methane yield of maize hybrids with contrasting maturity. Eur. J. Agron. 29, 72–79. Schittenhelm, S., 2010. Effect of drought stress on yield and quality of maize/sunflower and maize/sorghum intercrops for biogas production. J. Agron. Crop Sci. 196, 253–261. Schoo, B., Schroetter, S., Böttcher, U., Kage, H., Schittenhelm, S., 2016. Drought tolerance and water use efficiency of biogas crops: a comparison of cup plant, maize and lucerne-grass. J. Agron. Crop Sci., http://dx.doi.org/10.1111/jac. 12173. Sinclair, T.R., Muchow, R.C., 1999. Radiation use efficiency. Adv. Agron. 65, 215–265. Sontheimer, A., 2007. Alternativen lassen hoffen. Biogas J. 3, 42–45. Stolzenburg, K., Monkos, A., 2012. Erste Versuchsergebnisse mit der Durchwachsenen Silphie (Silphium perfoliatum L.) in Baden-Württemberg. Landwirtschaftliches Technologiezentrum Augustenberg, Karlsruhe. Teixeira, E., Moot, D., Brown, H., Pollock, K., 2007. How does defoliation management impact on yield, canopy forming processes and light interception of lucerne (Medicago sativa L.) crops? Eur. J. Agron. 27, 154–164. Testa, G., Gresta, F., Cosentino, S.L., 2011. Dry matter and qualitative characteristics of alfalfa as affected by harvest times and soil water content. Eur. J. Agron. 34, 144–152. VDI-Richtlinie 4630, 2006. Fermentation of organic materials—Characterization, of substrate, sampling, collection of material data, fermentation tests. VDI-Gesellschaft Energietechnik, Düsseldorf, Germany. VDLUFA, 1976. Die chemische Untersuchung von Futtermitteln. Methodenhandbuch. Band III, 4. Ergänzung 1997. VDLUFA-Verlag, Darmstadt, Germany. Webb, N., Nichol, C., Wood, J., Potter, E., 2008. User Manual for the SunScan Canopy Analysis System Type SS1 User Manual Version: 2.0. Delta-T Devices Ltd, Cambridge, United Kingdom. Weiland, P., 2010. Biogas production: current state and perspectives. Appl. Microbio. Biotechnol. 85, 849–860. Wienforth, B., 2011. Cropping systems for biomethane production: a simulation based analysis of yield, yield potential and resource use efficiency. In: Ph.D. Thesis. Kiel University (CAU), Institute of Crop Science and Plant Breeding.