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Potential agronomic options for energy-efficient sugar beet-based bioethanol production in northern Japan

Authors

  • NOBUHISA KOGA,

    1. Climate and Land-Use Change Research Team, National Agricultural Research Center for Hokkaido Region, Shinsei, Memuro, Kasai, 082-0081 Hokkaido, Japan
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  • HIROYUKI TAKAHASHI,

    1. Hokkaido Region Biomass Research Team, National Agricultural Research Center for Hokkaido Region, Shinsei, Memuro, Kasai, 082-0081 Hokkaido, Japan
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  • KAZUYUKI OKAZAKI,

    1. Hokkaido Region Biomass Research Team, National Agricultural Research Center for Hokkaido Region, Shinsei, Memuro, Kasai, 082-0081 Hokkaido, Japan
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  • TSUTOMU KAJIYAMA,

    1. Hokkaido Prefectural Tokachi Agricultural Experiment Station, Shinsei, Memuro, Kasai, Hokkaido 082-0081, Japan
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  • SOHEI KOBAYASHI

    1. Rhizosphere Environment Research Team, National Agricultural Research Center for Hokkaido Region, Hitsujigaoka 1, Toyohira, Sapporo, Hokkaido 062-8555, Japan
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Nobuhisa Koga, tel. +81 155 62 9274, fax +81 155 61 2127, e-mail: nkoga@affrc.go.jp

Abstract

Sugar beet (Beta vulgaris L. subsp. vulgaris) is deemed to be one of the most promising bioethanol feedstock crops in northern Japan. To establish viable sugar beet-based bioethanol production systems, energy-efficient protocols in sugar beet cultivation are being intensively sought. On this basis, the effects of alternative agronomic practices for sugar beet production on total energy inputs (from fuels and agricultural materials during cultivation and transportation) and ethanol yields (estimated from sugar yields) were assessed in terms of (i) direct drilling, (ii) reduced tillage (no moldboard plowing), (iii) no-fungicide application, (iv) using a high-yielding beet genotype, (v) delayed harvesting and (vi) root+crown harvesting. Compared with the conventional sugar beet production system used in the Tokachi region of Hokkaido, northern Japan, which makes use of transplants, direct drilling and no-fungicide application contributed to reduced energy inputs from raising seedlings and fungicides, respectively, but sugar (or ethanol) yields were also reduced by these practices, to a greater equivalent extent than the reductions in energy inputs. Consequently, direct drilling (6.84 MJ L−1) and no-fungicide application (7.78 MJ L−1) worsened the energy efficiency (total energy inputs to produce 1 L of ethanol), compared with conventional sugar beet production practices (5.82 MJ L−1). Sugar yields under conventional plow-based tillage and reduced tillage practices were similar, but total energy inputs were reduced as a result of reduced fuel consumption from not plowing. Hence, reduced tillage showed improved energy efficiency (5.36 MJ L−1). The energy efficiency was also improved by using a high-yielding genotype (5.23 MJ L−1) and root+crown harvesting (5.21 MJ L−1). For these practices, no major changes in total energy inputs were noted, but sugar yields were consistently increased. Neither total energy inputs nor ethanol yields were affected by extending the vegetative growing period by delaying harvesting.

Introduction

The first generation of bioethanol production from sugary and starchy components of agricultural biomass, e.g. sugar cane (Saccharum officinarum L.) in Brazil and maize (Zea mays L.) in the United States, has led to this fuel being used as practicable and sustainable energy source (Goldemberg, 2007). In cropping systems fully dedicated to bioethanol production, active attempts have been made to improve such systems' energy balances through alternative cultivation, transportation and transformation (conversion of feedstock to ethanol) practices (von Blottnitz & Curran, 2007). Such studies have been undertaken for sugarcane (Macedo, 1998; Renouf et al., 2008), maize (Renouf et al., 2008; Liska et al., 2009), sugar beet (Beta vulgaris L. subsp. vulgaris) (Malça & Freire, 2006; Halleux et al., 2008; Renouf et al., 2008) and some cereal crops (Rosenberger et al., 2001). The cultivation step is the first step in generating primary biomass energy through photosynthesis, but is also the step that expends large amounts of energy through the burning of fossil fuels to operate agricultural machinery, as well as other agricultural inputs: seeds, chemical fertilizers, biocides (fungicides, insecticides and herbicides), etc. To heighten the contribution of bioethanol utilization toward mitigating net global warming and conserving fossil energy resources, agronomic practices suitable for energy-efficient bioethanol production are needed at the cultivation step. For example, Rosenberger et al. (2001) showed that, although it reduced ethanol yields significantly, lowering production intensity (nitrogen fertilization and plant protection) contributed to an improved energy balance in a winter wheat-based ethanol production system.

As elsewhere in the world, in the context of the globally flourishing bioethanol production industry, research and development on first generation bioethanol production has opened up the search for viable domestic biofuel production in Japan. Hokkaido prefecture, located in the northern part of Japan, plays a central role in Japan's food supply, and the Tokachi region of Hokkaido, in particular, is a typical large-scale farming area, producing winter wheat (Triticum aestivum L.), beans [adzuki bean –Vigna angularis (Willd.) Ohwi & Ohashi; kidney bean –Phaseolus vulgaris L.; soybean –Glycine max Merr.], potato (Solanum tuberosum L.) and sugar beet in rotation (Tokachi Subprefectural Office, Hokkaido Government, 2007). An energy balance of the major crops grown in the Tokachi region of Hokkaido implied that sugar beet, given its high-energy efficiency in cultivation and potential ethanol yields, was the most promising crop to serve as a bioethanol feedstock, despite the fact that this crop required more energy inputs from fuels and other agricultural inputs than other potential crops (Koga, 2008). Currently, sugar beet grown in the Tokachi region of Hokkaido is solely directed toward sugar production. Under the conventional sugar beet production system, large amounts of energy are consumed in raising seedlings. Instead of direct drilling, a paper pot transplanting system is presently implemented in Hokkaido to ensure sugar yields. A wide range of tractor operations and agricultural inputs from chemical fertilizers, biocides and agricultural machinery also contribute to the energy consumption (Department of Agriculture, Hokkaido Government, 2005). Thus, it is important to test alternative agronomic technologies for energy-efficient sugar beet-based bioethanol production in northern Japan. This can be achieved by employing high-yielding varieties and improving agronomic practices such as soil (e.g. tillage method) and crop management (e.g. planting method and pest management). To establish viable sugar beet-based bioethanol production systems in northern Japan, the efficiency of agronomic options was assessed by calculating both ethanol yields and total energy inputs during cultivation and transportation of feedstock.

Materials and methods

Conventional sugar beet production in the Tokachi region of Hokkaido

Under the conventional sugar beet production system in use in the Tokachi region of Hokkaido, a paper pot transplanting system is employed, where sugar beets are sown in cylindrical paper pots (13 cm tall × 2 cm in diameter) in the middle of March, and the 5–6-week-old seedlings raised in a heated vinyl house are transplanted to the field in late April (Department of Agriculture, Hokkaido Government, 2005). In addition, compound fertilizers containing N, P, K and Mg are typically used for fertilization, and pest and weed management are performed mainly by biocide spraying. A number of tractor-based field operations including tillage, transplanting, fertilization and harvesting are used. Typically, sugar beet is harvested in the middle of October, and transported to a sugar refinery by dump trucks.

Agronomic options assessed

For each agronomic option tested, harvestable root biomass yields, sugar contents and sugar yields, along with alternative practice vs. conventional practice ratios of these parameters were determined in field trials (Table 1). The field trials were performed in 2007 and 2008 in Memuro township in the Hokkaido prefecture, northern Japan (Fig. 1). For reduced tillage, field trials were performed in Memuro and Sapporo city. The soil type at both sites was an Andisol, a volcanic ash-derived soil typical of the Tokachi region of Hokkaido. In a separate set of experiments, the effects of using a high-yielding genotype on harvestable root biomass yields, sugar contents and sugar yields were assessed at five locations (differing in soil types) in Hokkaido. The effects of alternative agronomic options on root biomass yields, sugar contents and sugar yields were tested by a paired t-test, and all statistical analyses were performed using r version 2.7.1 (R Development Core Team, 2008).

Table 1.   Effects of alternative agronomic options on harvestable biomass yields, sugar contents and sugar yields in field trials in 2007 and 2008
Agronomic options*Harvestable biomass yield
(Mg ha−1)
Sugar content (%)Sugar yield (Mg ha−1)
CAA/CEffect†,‡CAEffect†,‡CAA/CEffect†,‡
  • *

    Detailed cultural conditions for each option are described in ‘Materials and methods’.

  • Effects of alternative agronomic options on harvestable biomass yields, sugar contents and sugar yields were tested by a paired t-test.

  • ***, **, *, # and ns represent the significance levels of 0.001, 0.01, 0.05, 0.1 and no significance, respectively.

  • §

    §Before transplanting, soils were treated with pathogens of leaf spot disease and root rot disease in the field trials.

  • The high-yielding genotype ‘Hokkai No. 99,’ a promising high sugar-yielding line in development at NARCH was assessed in the field trials.

  • C, conventional practice; A, alternative agronomic option practice; n, the number of pairs.

Direct drilling (n=4)74.361.90.833*18.618.8ns13.811.60.841#
Reduced tillage (n=6)62.365.11.04ns17.817.8ns11.111.61.05ns
No fungicides§ (n=10)48.837.50.768***18.116.2***8.836.080.689***
High-yielding genotype (n=10)69.375.91.10***16.817.1*11.613.01.12***
Delayed harvesting (n=4)74.371.60.964ns18.619.0ns13.813.60.986ns
Root+crown harvesting (n=4)74.386.31.16**18.618.1*13.815.61.13**
Figure 1.

 Location of the study site.

Direct drilling

Under the conventional transplanting system in the Hokkaido region, electricity for soil sieving and sowing, and kerosene for heating a vinyl house were consumed in raising seedlings (Zhang et al., 2008). Comparatively, under the direct-drilling system, the use of electricity and kerosene was eliminated. In tractor-based field operations, a general seeder (also used for bean sowing) was used for sowing sugar beet seeds, instead of using the beet transplanter used in conventional transplanting. This switch from a beet transplanter to a general seeder led to changes in diesel fuel consumption in tractor operations (Table 2) and also material consumption-linked energy consumption in agricultural machinery (Table 3) (Hokkaido Prefectural Tokachi Agricultural Experiment Station, 2003). Under the conventional transplanting system, one herbicide spraying occurred after transplanting while one more herbicide spraying was necessary in the direct-drilling system during the growing seasons, as recommended by the Department of Agriculture, Hokkaido Government (2005). Therefore, diesel consumption for herbicide spraying doubled, and the expenditure for biocides increased by 42.9 × 103 yen ha−1. Compared with conventional transplanting, effects of direct drilling on root biomass yields, sugar contents and sugar yields were assessed in four field trials using two sugar beet lines (‘Hokkai No. 87’ and ‘Hokkai No. 98’).

Table 2.   Energy inputs in fuel and electricity consumption for raising seedlings, tractor operations and truck transport of harvests (GJ ha−1)*
OperationsTractor
implements
Agronomic options
Conventional
practice
Direct
drilling
Reduced
tillage
No
fungicides
High-yielding
genotype
Delayed
harvesting
Root+
crown
harvesting
  • *

    Energy equivalents: 3.6 MJ kWh−1, 36.7 MJ L−1 and 37.8 MJ L−1 for electricity, kerosene and diesel, respectively (Greenhouse Gas Inventory Office of Japan, 2008).

  • 49.5 kWh ha−1 of electricity for soil sieving and sowing and 37 L ha−1 of kerosene for heating a vinyl house (Zhang et al., 2008).

  • Data in conventional tractor operations for sugar beet were cited from Department of Agriculture, Hokkaido Government (2005).

  • §

    §Diesel fuel consumption rates in harvesting and transporting of harvests depended upon the levels of harvestable root biomass yields.

  • Assuming 10 km of one-way distance, diesel fuel consumption in round-trip truck transportation (total 20 km) for harvested materials between the field and the bioethanol plant was calculated.

Raising seedlings (electricity and kerosene) 1.541.541.541.541.541.54
Tractor operations (diesel)
 LimingLime sower0.2270.2270.2270.2270.2270.2270.227
 Soil preparationRotary harrow1.561.560.7821.561.561.561.56
 Basal fertilizationFertilizer applicator0.3590.3590.3590.3590.3590.3590.359
 TransplantingBeet transplanter0.7140.7140.7140.7140.7140.714
 SowingGeneral seeder0.359
 Herbicide sprayingBoom sprayer0.07180.1440.1440.07180.07180.07180.0718
 Inter-row cultivationCultivator0.2150.2150.2150.2150.2150.2150.215
 Pesticide sprayingBoom sprayer0.3590.3590.3590.1440.3590.3590.359
 Harvesting§Beet harvester1.231.021.280.9431.351.181.43
 PlowingMoldboard plow0.8620.8620.8620.8620.8620.862
Transport of harvests (diesel)§,¶Truck1.341.121.401.031.481.301.56
Total 8.486.237.027.678.748.398.90
Table 3.   Mean expenditure and energy inputs from seeds, chemical fertilizers, biocides and agricultural machinery
 Agronomic options
Conventional
practice
Direct
drilling
Reduced
tillage
No
fungicides
High-yielding
genotype
Delayed
harvesting
Root+crown
harvesting
Expenditure (103 yen ha−1)*
 Seeds26.026.026.026.026.026.026.0
 Chemical fertilizers160160160160160160160
 Biocides93.813798.764.193.893.893.8
 Agricultural machinery67.640.662.767.667.667.667.6
Energy input (GJ ha−1)
 Seeds0.5190.5190.5190.5190.5190.5190.519
 Chemical fertilizers18.818.818.818.818.818.818.8
 Biocides6.8910.17.254.716.896.896.89
 Agricultural machinery3.412.053.163.413.413.413.41
 Total29.631.529.727.429.629.629.6

Reduced tillage

In the reduced tillage system, moldboard plowing (roughly 25 cm plowing depth) after harvesting of a previous crop was omitted, and only one spring harrowing for soil preparation was performed (0.782 GJ ha−1), compared with the conventional tillage system that included two harrowings for soil preparation. These changes in tillage operations led to a significant reduction in diesel fuel consumption (Table 2). In this study, it was assumed that extra nonselective herbicide (glyphosate) spraying was necessary for weed control after sugar beet harvesting under reduced tillage. Each spraying operation consumed an energy equivalent of 0.0718 GJ ha−1. The price of the glyphosate herbicide was assumed to be 4.9 × 103 yen ha−1 (personal communication). Assuming 1.4 × 106 yen of the retail price of a typical moldboard plow (Japan Agricultural Mechanization Association, 2006), 8 years of mean life expectancy of the implement and 35.8 ha of mean cropland area per farm (2001–2005) in the Tokachi region (Tokachi Subprefectural Office, Hokkaido Government, 2007), the expenditure for agricultural machinery could be reduced by 4.9 × 103 yen ha−1 from not doing any moldboard plowing (Table 3). Field trials for reduced tillage were performed for 2 years with two conventional varieties (‘Lycka’ in Memuro and ‘Freuden R’ in Sapporo). In 2008, the field trials were conducted under two soil conditions in terms of different preceding crops for each variety.

No fungicides

In the conventional pest management system, pesticides (fungicides and insecticides) were sprayed five times in total over the season, using a boom sprayer (Department of Agriculture, Hokkaido Government, 2005). In the no-fungicide treatment, the number of pesticide sprayings was assumed to be two (insecticide spraying only), given that no fungicides were applied. Consequently, the mean expenditure for fungicides (29.7 × 103 yen ha−1) could be reduced (Ministry of Agriculture, Forestry and Fisheries of Japan, 2004–2008). The effects of no fungicides on root biomass production and sugar contents were investigated for five conventional varieties in the field trials. In the fields, pathogens (Cercospora beticola Saccardo, leaf spot disease, and Rhizoctonia solani Kühn, root rot disease) were incorporated into the soil before transplanting sugar beet seedlings.

Using a high-yielding genotype

The National Agricultural Research Center for Hokkaido Region (NARCH) has been developing high sugar-yield sugar beet lines for several decades. ‘Hokkai No. 99’ is one of the promising high sugar-yield lines in development at NARCH. The sugar yield of ‘Hokkai No. 99’ was compared with that of the conventional variety, ‘Monohomare’ in field tests, held at five locations in the Hokkaido prefecture in 2007 and 2008. Other than the genotypes used, cultural conditions were the same as those under conventional sugar beet production.

Delayed harvesting

In the Tokachi region of Hokkaido, sugar beet is typically harvested in the middle of October. Harvest was delayed for 2 weeks, compared with the conventional sugar beet harvest to investigate the effects of extending the vegetative growing period on sugar yields. Excluding harvesting timing, all cultural conditions were those followed in conventional sugar beet production. Field trials to assess the effects of delayed harvesting on sugar yields were performed for two sugar beet genotypes (‘Hokkai No. 87’ and ‘Hokkai No. 98’).

Root and crown harvesting

In conventional sugar beet production, only fresh root biomass was harvested because the crown component contains impurities detrimental to the sugar refining process. Under root+crown harvesting, both root and crown biomass components were harvested to increase sugar yields for bioethanol production. Two sugar beet genotypes (‘Hokkai No. 87’ and ‘Hokkai No. 98’) were used in assessing the effects of root+crown harvesting vs. root only harvesting on sugar yields in the field trials.

Energy inputs from fuel-consuming operations and agricultural materials

For each agronomic practice, energy inputs from fuels (cultivation and transportation of the feedstock) and other input materials such as seeds, chemical fertilizers, biocides and agricultural machinery were taken into consideration. In raising seedlings, 49.5 kWh ha−1 of electricity for soil sieving and sowing, and 37 L ha−1 of kerosene for heating a vinyl house (Zhang et al., 2008), represented an equivalent energy consumption of 1.54 GJ ha−1. The total energy inputs in diesel-consuming tractor operations were calculated from total diesel fuel consumption (Table 2), depending on the agronomic practices examined. Assuming that diesel fuel consumption in harvesting depends on fresh root biomass yields, diesel consumption in the harvesting operation was calculated as follows:

image(1)

where Fh is the diesel fuel consumption for harvesting (L ha−1), RF is root biomass yield factor (the ratio of alternative practice to conventional practice in terms of harvestable root biomass yield) for each agronomic option (Table 1) and 32.5 is the rate of diesel fuel (L ha−1) consumed in harvesting under conventional sugar beet production practices (Department of Agriculture, Hokkaido Government, 2005).

Assuming a 10 km one-way distance, diesel fuel consumption in round-trip truck transportation (total 20 km) for harvested materials between the field and the bioethanol plant was calculated as follows:

image(2)

where Ft is diesel fuel consumption for transport (L ha−1), FE is the fuel efficiency (3.5 km L−1) of a 10 Mg (loading capacity) truck (Center for Environmental Information Service, 1998), 62.2 is the mean fresh root biomass yield for sugar beet in the Tokachi region of Hokkaido in 2002–2006 (Sugar Beet Association, 2007), 20 is the total transport distance (km) and 10 is the loading capacity of a truck (Mg).

The consumptions of 1 L of kerosene and diesel fuel were considered equivalent to 36.7 and 37.8 MJ L−1 (means of 2004–2006), respectively (Greenhouse Gas Inventory Office of Japan, 2008). Each kWh of electricity consumption was equivalent to 3.6 MJ of energy consumption.

Energy inputs from the consumption of agricultural materials such as seeds, chemical fertilizers, biocides and agricultural machinery (mainly tractors and tractor implements) were calculated using the mean expenditure (2002–2006) (Ministry of Agriculture, Forestry and Fisheries of Japan, 2004–2008) and the energy consumption rate cited from a Japanese Input–Output Table for 2000 (Center for Global Environmental Research, 2007) for each commodity (Table 3). For agricultural machinery, machine repair costs were excluded from the calculation of mean expenditures. The energy equivalents used for seeds, chemical fertilizers, biocides and agricultural machinery were 19.98, 117.24, 73.41 and 50.41 kJ yen−1, respectively (Center for Global Environmental Research, 2007).

Ethanol yields

The sugar yield under each agronomic practice normalized to the mean sugar yield in the Tokachi region (Table 4) was calculated by multiplying 10.9 Mg ha−1 (62.2 Mg ha−1 of mean root biomass yield × 17.5% of mean sugar content), the mean sugar yield (2002–2006) in the Tokachi region of Hokkaido (Sugar Beet Association, 2007), and the sugar yield factor determined in field trials for each practice (Table 1). Assuming a 51% theoretical conversion of sugars to ethanol (Mg ha−1) and that 95% of theoretically yielded ethanol was substantially produced (personal communication), ethanol yields were estimated from sugar yields. For the density of ethanol, a value of 0.806 kg L−1 was used.

Table 4.   Ethanol yields and total energy inputs from fuels, electricity and agricultural materials for cultivation and transportation
Agronomic practiceSugar yield*
(Mg ha−1)
Ethanol yield†,‡
(kL ha−1)
Energy input (GJ ha−1)
Fuel and electricityMaterialsTotal
  • *

    Sugar yield=10.9 (mean sugar yield in the Tokachi region) × sugar yield factor (A/C) for each agronomic option (Table 1).

  • Ethanol yield=sugar yield × 0.51 × 0.95.

  • 0.806 kg L−1 for the density of ethanol.

Conventional practice10.96.558.4829.638.1
Direct drilling9.175.516.2331.537.7
Reduced tillage11.46.857.0229.736.7
No fungicides7.514.517.6727.435.1
High-yielding genotype12.27.338.7429.638.3
Delayed harvesting10.76.438.3929.638.0
Root+crown harvesting12.37.398.9029.638.5

Results

Root biomass and sugar yields under different agronomic practices

In field trials in 2007 and 2008 (Table 1), harvestable biomass yields were significantly increased by root+crown harvesting (16% increase over conventional root harvesting, P<0.01) and using a high-yielding genotype (10% increase over the conventional cultivar, P<0.001). Meanwhile, root biomass yields were significantly reduced by direct drilling (17% reduction, P<0.05) and no fungicides (23% reduction, P<0.001) over conventional sugar beet production. Reduced tillage and delayed harvesting had no significant impact on harvestable yield biomass production. Sugar contents were significantly reduced by the absence of fungicides (P<0.001) and root+crown harvesting (P<0.05), while it was increased by using the high-yielding genotype, ‘Hokkai No. 99’ (P<0.05).

The yield increase in sugar as a substrate for bioethanol was most pronounced by root+crown harvesting (13% increase, P<0.01). The sugar yield was also enhanced by using a high-yielding genotype (12% increase, P<0.001). Reduced tillage and delayed harvesting had no significant impact on sugar contents. Sugar yields were markedly reduced by direct drilling (16% reduction, P<0.10) and the absence of fungicides (31% reduction, P<0.001), compared with conventional sugar beet production in northern Japan.

Energy inputs from fuels and other agricultural inputs

Under conventional sugar beet production, 8.48 GJ ha−1 was consumed in raising seedlings, tractor-based field operations and transport of harvested sugar beets (Table 2). Of the tractor operations, soil preparation (1.56 GJ ha−1), moldboard plowing (0.862 GJ ha−1) and harvesting (1.23 GJ ha−1) were the major fuel-consuming tractor operations, accounting for 18%, 10% and 15% of the total. Other than tractor operations, energy inputs from fuel and electricity consumption were attributed to raising seedlings (1.54 GJ ha−1) and transport of harvested sugar beets (1.34 GJ ha−1, 20 km of round-trip transport distance).

When raising seedlings before transplanting were omitted, 1.54 GJ ha−1 could be eliminated in the direct-drilling system. Energy consumption was also reduced in harvesting and transport of harvests due to significant reductions in harvestable biomass yields under direct drilling (Tables 1 and 2). Moreover, under the direct-drilling system, a general seeder was used for sowing, and extra herbicide spraying was necessary as a tractor operation. Therefore, energy inputs under direct drilling totaled 6.23 GJ ha−1 (27% reduction compared with the conventional practice). Under the reduced tillage system, total energy input amounted to 7.02 GJ ha−1 (17% lower than the conventional practice). This reduction arose mainly from the fact that moldboard plowing in previous autumn was omitted and soil preparation in spring was simplified under reduced tillage. The reduction in energy input was also observed when no-fungicide application was made (7.67 GJ ha−1, 10% reduction) as the frequency of pesticide spraying was reduced. Moreover, the root biomass yield was significantly reduced when no fungicides were applied. Unlike the abovementioned agronomic options, using a high-yielding genotype and root+crown harvesting marginally increased fuel consumption-linked energy inputs (3% and 5% increase, respectively) due to increased diesel consumption during harvesting and transport of harvests resulting from increased harvestable biomass yields. There was no clear difference in the energy input between conventional harvesting and delayed harvesting.

Under the conventional sugar beet production practices in the Tokachi region of Hokkaido, energy inputs from seeds, chemical fertilizers, biocides and agricultural machinery were equivalent to 29.6 GJ ha−1 (Table 3), which was 3.5-fold greater than fuel consumption-derived energy inputs. Chemical fertilizers and biocides represented 64% and 23% of the material consumption-linked energy inputs, respectively. As the use of seeds and chemical fertilizer was identical between all agronomic options, there was no difference in energy inputs from these commodities. In direct drilling, a general seeder was used, instead of a beet transplanter, and total expenditure and resulting energy inputs from agricultural machinery were significantly reduced (2.05 vs. 3.41 GJ ha−1 for conventional practice). However, extra herbicide spraying was required for the direct-drilling system, and energy inputs from biocides were increased (10.1 over 6.89 GJ ha−1 for conventional practice). Similarly, under reduced tillage, energy inputs from agricultural machinery were reduced as a result of no moldboard plowing while energy inputs from biocides were increased given that nonselective herbicide spraying after harvest was required for weed control. No-fungicide application resulted in a significant reduction in energy inputs from biocide use (4.71 over 6.89 GJ ha−1 for conventional biocide application).

Total energy inputs from fuels and agricultural materials are summarized in Table 4. Total energy inputs were reduced by direct drilling, reduced tillage and no fungicides, accounting for 1%, 4% and 8% reductions over the conventional practice, respectively. Conversely, total energy inputs were slightly increased using a high-yielding genotype and root+crown harvesting.

Agronomic practices for sugar beet-based ethanol production

Under conventional sugar beet production, one would expect 6.55 kL ha−1 of ethanol to be generated from 10.9 Mg ha−1 of harvested beet sugars (Table 4). On the other hand, ethanol yields under different agronomic practice options varied considerably, from 4.51 kL ha−1 for no fungicides to 7.39 kL ha−1 for root+crown harvesting.

Total energy inputs during cultivation and transport of harvested products to produce 1 L of ethanol are presented in Fig. 2. Under the conventional sugar beet production system in northern Japan, 5.82 MJ of energy equivalents was inputted to produce 1 L of ethanol. In direct-drilling and no-fungicide systems, total energy inputs from fuels and agricultural materials could be reduced considerably, but reductions in ethanol yields were more pronounced. Consequently, total energy inputs under direct drilling and no-fungicide application were 18% and 34% higher, respectively, than those under conventional practice. Reduced tillage, using a high-yielding cultivar and root+crown harvesting reduced energy inputs per unit ethanol produced, accounting for 8%, 10% and 10% reductions, respectively, compared with the conventional practice. In terms of reduced tillage, significant reductions in fuel consumption contributed to the improved energy efficiency. Regarding the high-yielding cultivar and root+crown harvesting, the improved energy efficiency was mainly attributed to increased ethanol yields as a result of increased sugar yields. Delayed harvesting had no advantages for improving the energy efficiency.

Figure 2.

 Total energy input (cultivation and transport of feedstock) to produce 1 L of ethanol from sugar beet under different agronomic options.

Discussion

Sugar beet is grown in a wide range of temperate and cool climate regions in the world. Although it requires large energy inputs from fuels and agricultural materials such as fertilizers and biocides, sugar beet has attracted growing attentions in its high potentials of biomass production (Kuesters & Lammel, 1999; Hülsbergen et al., 2002; Koga, 2008). Recently, the high biomass production potentials in sugar beet is linked with sugar beet-based bioethanol production in the global context of renewable energy generation and utilization. In a wide range of conventional sugar beet production systems, over a long period, efforts have been made to improve sugar productivity through the employment of excellent sugar beet varieties and the implementation of effective agronomic practices (e.g. fertilization, pest control and soil management). Hence, shifts of cultivation practices for conventional sugar production are needed for bioethanol feedstock production to establish more energy-efficient sugar beet-based bioethanol production systems. However, such changes in agronomic practices may cause significant changes in sugar or ethanol yields. Therefore, it is essential to use selected agronomic options that contribute to both saving energy and ensuring bioethanol yields.

Our results showed that six agronomic options for sugar beet production in northern Japan had positive or negative impacts on the energy efficiency (total energy inputs to produce 1 L of ethanol) in comparison with conventional sugar beet production practices (Fig. 2). Extending vegetative periods by delaying harvesting for 2 weeks had little effects on both total energy inputs and sugar yields, resulting in no advantages in improving the energy efficiency. Using high-yielding sugar beet lines and root+crown harvesting were thought to be reliable options to improve the energy efficiency for sugar beet-based ethanol production. These operations increased sugar yields consistently with marginal increases in energy inputs (Table 4). The improved energy efficiency was also noted in reduced tillage. Under reduced tillage in the Tokachi region of Hokkaido, sugar yields were ensured even with significant reductions in energy inputs as a result of no plowing and simple harrowing. Equal or better sugar yields in sugar beet production under reduced tillage were also reported in other studies for a Japanese Andisol (Imura et al., 1985), a Canadian clay loam soil (Hao et al., 2001) and German loess soils (Koch et al., 2009). Thus, such positive effects of reduced tillage on the energy efficiency can be expected in a wide range of soil types.

The implementation of no-fungicide application had negative impacts on the energy efficiency for ethanol production from sugar beet (Fig. 2). This result arose mainly from the fact that a remarkable decrease in sugar yields (31% reduction, Table 1) exceeded a reduction in energy inputs (8% reduction, Table 4). However, it should be noted that the soils were pretreated with pathogens of leaf spot disease and root rot disease in field trials. This must be the severest case in which sugar beets were tremendously damaged. Anyhow, such pest management makes ethanol yields lower and more unstable. Considering the importance of stable feedstock supply for ethanol production, proper pest management through biocide spraying is necessary to stabilize ethanol production from sugar beet. Likewise, direct drilling lowered the energy efficiency (Fig. 2), as a result of significant reductions in sugar yields. In the Tokachi region of Hokkaido, acreage of direct drilling for sugar beet (instead of conventional paper pot transplanting) is now expanding as a means of labor saving (Sugar Beet Association, 2007), particularly for large-scale producers. Although direct drilling may have adverse effects on sugar yields and the energy efficiency, it is a possible option for efficient and labor-saving bioethanol-oriented sugar beet production.

As presented in Tables 3 and 4, chemical fertilizer contributed to 49% of total energy inputs under conventional sugar beet production. This illustrates the importance of reducing chemical fertilizer use without sacrificing sugar yields for the improved energy efficiency. The Tokachi region of Hokkaido is also an area of livestock farming, producing milk and beef (Tokachi Subprefectural Office, Hokkaido Government, 2007). The reduction in chemical fertilizer use through proper use of animal manures may contribute to improved energy efficiency of the Tokachi region's sugar beet-based bioethanol production system.

In this study, we proposed some effective options that contribute to the improved energy efficiency. To develop further energy-efficient sugar beet production protocols suitable for bioethanol in northern Japan, it is also a key to integrate effective individual agronomic technologies. In this context, a simple estimation was made for the combination of reduced tillage, using a high-yielding line and root+crown harvesting. In this combination, 14.5 Mg ha−1 of the sugar yield (or 8.72 kL ha−1 of ethanol yield) would be expected, using 1.33 of the combined sugar yield factor for these practices (calculated from Table 1). Meanwhile, considering increased energy inputs in the harvesting operation and feedstock transport resulting from increased harvestable biomass yield (1.33 of the combined root biomass yield factor), the energy input from fuel consumption was equivalent to 7.76 GJ ha−1, and that from material consumptions was identical to 29.7 GJ ha−1 for reduced tillage. Consequently, the energy use for the combination totaled 37.5 GJ ha−1, and the total energy input to produce 1 L of ethanol was estimated at 4.30 MJ L−1 (26% reduction compared with the conventional sugar beet production). For practicable bioethanol feedstock production from sugar beet, such a combination of the selected effective measures can largely contribute to both improved energy efficiency and a substantial increase in ethanol yields.

Conclusion

To establish sugar beet production systems suitable for bioethanol production in northern Japan, we assessed the relationship between total energy inputs (fuels, fertilizers, biocides, agricultural machinery and so on) and ethanol yields for each included agronomic option. Reduced tillage, using a high-yielding genotype and root+crown harvesting improved the energy efficiency (total energy inputs for cultivation and transportation to produce 1 L of ethanol) by 8%, 10% and 10%, respectively, over the conventional sugar beet production in use in northern Japan. In contrast, direct drilling and no-fungicide application worsened the energy efficiency as a result of significant reductions in sugar yields. In Japan, challenges for domestic bioethanol production from various types of crops such as potatoes, sugarcane, rice (Oryza sativa L.), sorghum [Sorghum bicolor (L.) Moench] and sweet potato [Ipomoea batatas (L.) Lam.] have been launched, and the optimization of agronomic practices for each crop is intensively sought. The approach used in this study can be used as a guide to help agricultural managers and researchers to optimize efficient bioethanol feedstock crop production protocols.

Acknowledgement

This study was financially supported by the Ministry of Agriculture, Forestry and Fisheries of Japan (Rural Biomass Research Project, BUM-Cm1320, BCD-A1111, BCD-A1113 and BCD-A1121).

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