Residual biomass potential of commercial and pre-commercial sugarcane cultivars

Sugarcane (Saccharum spp.) is an efficient and sustainable alternative for energy generation compared to non-renewable sources. Currently, during the mechanized harvest process, the straw left in the field can be used in part for the second generation ethanol and increasing the electric energy production. Thus, this study aimed to provide information on the potential for residual biomass cultivars of sugarcane cropping system. This study provides the following information: yield of straw, depending on the calculated leaf area index and the number of tillers per linear meter; primary energy production of several sugarcane genotypes; contribution of dry tops and leaves; biomass yield; and evaluation of fiber, cellulose, hemicellulose and lignin. Preliminary results obtained by researchers of the State of São Paulo, Brazil, and reCviews related studies are presented. The results suggest that the production of sugarcane straw content varies according to the cultivars; the greater mass of sugarcane straw is in the top leaves and that the potential for the crude energy production of sugarcane per area unit can be increased using fiber-rich species or species that produce more straw. The straw indexes was shown to be a good indicator and allow the estimation of straw volumes generated in a sugarcane crop. The cellulose, hemicellulose and lignin composition in sugarcane is distinct among varieties. Therefore, it is possible to develop distinct biomass materials for energy production and for the development of sugarcane mills using biochemical processes and thermal routes.


Introduction
The mechanized harvesting process for sugarcane (Saccharum spp.) generates significant dry mass deposition on the soil during cropping.Among the several uses of this residue are energy co-generation and raw material for ethanol and biochemical production.It is still unknown the amount of residual straw that should be removed from the soil surface to be used in energy generation and how much should remain for soil conservation, given the benefits it brings to tropical soils and sugarcane cropping.
The straw mass generated is directly related to the sugarcane crop yield, which varies according to the cultivar, environment and management system adopted.The straw mass generated is an intrinsic characteristic of a cultivar and ranges from 8 to 23 % of the stem weight (Franco et al., 2013) and 14 % of the dry weight (Paes and Oliveira, 2005).It is crucial to determine the amount of residual dry mass that should remain on the soil and the amount that can be removed after harvest.
This study aimed to provide information about the residual biomass potential of specific sugarcane cultivars under a mechanized harvesting unburned system.Part of this information was generated through a series of studies developed within the project: Sustainable Bioenergy Sugarcane Breeding and Cultivar Development (Bioen project -FAPESP).

Straw
Sugarcane genetic improvement should no longer be based solely on characteristics to increase sugar yield, Sugar Tons per Hectare (STH), but it should be based on biomass production indicated by the fiber mass, Fiber Tons per Hectare; (FTH) that is available for energy and second-generation ethanol production.The STH and FTH composition provides the total biomass or Dry Matter Tons per Hectare (DMTH), where the saccharose-rich syrup constitutes the feedstock for sugar production and the residual fiber (bagasse plus soil surface straw) is the raw material for energy, biochemical and biofuel production.Currently, these new technologies are hindered by the economic costs of biomass use, in other words, cost of sugarcane per ton should not exceed US$20.00.
In 2031, all sugarcane straw from sugarcane harvest in Brazil will be available for use (Gomes et al., 2010).Harvesting methods for sugarcane straw will be implemented based on the principles and concepts that are compatible with the needs and economic processes, which will, therefore, place sugarcane straw in a prominent position, along with sugarcane bagasse, as a primary biomass sources for second-generation ethanol production.
When identifying which plant characteristics are determinant for the straw content variation of a sug-Sci.Agric.v.70, n.5, p.299-304, September/October 2013 arcane cultivar in accordance with studies performed at the Centro de Cana/IAC -Instituto Agronômico de Campinas (Sugarcane Center/IAC), we found that leaf area (leaf length) and leaf width are the most important.These characteristics are described in sugarcane botanical studies by the National Service for Cultivar Protection of the Ministry for Agriculture, Livestock and Supply (MAPA) in Brazil regarding leaf width and foliar canopy volume.Leaf length (L) and leaf width (W) comprise the leaf area (LA) and can be expressed as follows: LA = L W 0.75 (Francis et al., 1969).Considering 1.4 m of length and 0.045 m of width (standard leaf) (Table 1), the LA per leaf would be 0.047 m².Considering 13 tillers per linear meter and 16 internodes representing 16 potential leaves, approximately 1.4 million leaves in one hectare would be obtained, which means that 66 m² LA and 6.6 leaf layers would cover the entire hectare.The sheaths and top leaves not included in these figures should also be considered.Effectively, these results represent a highly productive commercial crop area with numbers greater than 30 t ha -1 .Miocque (1999) investigated sugarcane growth and biomass for ten crops in São Paulo State (Brazil) and concluded that the period of highest biomass accumulation is between the months of December and February, when sugarcane shows increased growth.Miocque (1999) also reported that sugarcane top leaves account for 26 % of the stem weight at harvest.Thus, after dehydration, the top leaf dry mass in a crop of 100 t ha -1 , for example, would be approximately 8 t ha -1 .

Cultivars
The mechanized sugarcane harvesting system, which prevails in Brazilian crops over crop burning, at first left producers concerned because the high volume of remaining straw on the soil hindered plant sprouting primarily in low-temperature regions, such as the southwestern states of São Paulo, Paraná and Mato Grosso do Sul.This hindrance occurred because the cultivars used at that time had not been developed for mechanized harvesting.In recent years, new cultivars have been developed, studied and used in this new environmental context.Therefore, these new cultivars are better adapted with increased sprouting capacity under the residual straw.
The sprouting difficulty of certain cultivars is less significant in high-temperature regions, such as centralwestern Brazil (Salton et al., 2008).The reduction of light incidence together with temperature reduction and the temperature range in the soil can cause disease proliferation and increased presence of pests, which may decrease crop yield (Ripoli et al., 2005).On the other hand, straw plays an essential role in increasing organic matter and provides benefits regarding water retention and water loss to the atmosphere (Bayer et al., 2002).This aspect shows that distinct criteria are used to define the amount of straw that should be removed for energy production when considering regions with lower or higher temperatures, such as central-western Brazil, where there is a higher hydric deficit (Salton et al., 2008).
After 2007, the Sugarcane Program/IAC ("Programa Cana/IAC") started to investigate new sugarcane cultivars, not only for their capacity to produce sugar, but also for their potential for primary energy production.This parameter accounts for all of the energy that can be produced from sugars and fibers in sugarcane stems and straw, and it was used to characterize the new cultivars at the IAC (Table 2).The primary energy was estimated as follows: PE (MJ t -1 ) = (18 x kg Fiber) + (16 x kg Saccharose) + [15.6 x RS (kg)] where: PE is the primary energy expressed in MJ t -1 ; Fi-  ber is the water insoluble matter in sugarcane expressed as a percentage (Fernandes, 2003).Saccharose is the main technological sugarcane quality parameter.That sugar is directly crystallized in the production process and is directly related to Pol (apparent saccharose expressed in percentage) in the cane syrup; and RS is the Reduction Sugars, which is a term used to designate the sugars (glucose and fructose) that are capable of reducing copper from a cupric to a cuprous state (Fernandes, 2003).
The primary energy production estimated for IAC91-1099 and IACSP95-5000 was 15% and 10% higher than that for RB72454 (Table 2) indicating that the energy yields can be increased by changing sugarcane varieties.Therefore, the initial concerns with the lack of adaptation of some varieties to mechanized harvesting opened the opportunity to investigate the potential use of sugarcane biomass in several sugarcane genotypes.
In a previous study (Manechini, 1997), the type of cultivar was identified as a determinant factor for the volume of straw mass produced, determined by the distinct proportions of leaves (Table 3).Studies performed at the Sugarcane Center/IAC have reported the straw content in several IAC cultivars, with straw percentage values ranging between 9 and 19 % (Table 4).However, Paes and Oliveira (2005) noticed that the straw percentage values in studies by many authors ranged from 2 to 35 % in different productive regions around the world (Table 5).Romero et al. (2007) also noticed a similar trend (Table 6).In other words, that yield levels affect straw content with lower productivity, generally resulting in higher levels of straw content in global biomass production and, therefore, higher straw/TSH ratios (%).

Relationship among straw, stem straw and top leaves in sugarcane
In previous studies, we reported values of straw dry mass produced per hectare was for the IACSP93-3046 (15 t), IAC91-1099 (17 t), IACSP95-5000 (20 t) and IACSP94-2094 (21 t) cultivars.According to Ripoli et al. (2005), the crude energy obtained from straw is approximately 18 MJ kg -1 .Therefore, it is estimated that the IACSP94-2094 variety can produce approximately 379 MJ ha -1 .Ripoli et al. (2005) quantified straw production and concluded that straw productivity varies as follows: soil surface straw from 1.1 to 2.4 t ha -1 ; stem straw from 6.5 to 8.4 t ha -1 ; and top leaves from 6.4 to 11.1 t ha -1 (Figure 1).
We investigated the same materials (Table 7) and observed that there was a ratio that tended to repeat for biomass volumes in top leaves, stem straw, surface soil straw and stem mass within a sugarcane variety.However, these ratios varied according to the variety.The indexes showing the ratio of the Top Leaves Dry Mass (TLDM) to Stem Mass (SM) are between 0.074 and 0.121; therefore, the TLDM accounts for approximately 10 % of the stem mass, which was similar to the observations of Miocque (1999) and Franco et al. (2013).The Stem Straw Dry Mass (SSDM) ratio to SM ranged from 0.071 to 0.094.Thus, on average, the Stem Straw Dry Mass (SSDM) accounted for approximately 9 % of the SM.Given that the Soil Straw Dry Mass (SoSDM) represented approximately 2 % of the sugarcane SM, the ratios for the sugarcane Total Mass of Sugarcane Dry Straw (TMSDS/SM) ranged from 0.174 to 0.239.
Technically, it is important to leave part of the straw on the soil.Based on the ratios presented, sugarcane stem straw, which can be collected during mechanized harvesting, accounted for approximately 42 % of the crop total straw.Some sugarcane varieties show spontaneous defoliation, which should be considered when adopting a strategy for crop management.Spontaneous defoliation was observed (Table 7), mainly for the IACSP94-2094 cultivar, which presented a SoSDM/ SM ratio of only 1 %.
When soil surface straw is collected, large amounts of minerals are also removed.Therefore, it is recommended to leave surface straw on the soil.Despite being more abundant and accounting for approximately 49 % of the total straw mass, top leaf straw  results in high humidity at harvesting (approximately 79 %).Thus, straw requires dehydration in the field to increase its capacity to generate thermal energy.As observed previously, in addition to the different volumes of biomass generated, it is important to consider that straw fiber content varies according to the sugarcane variety.
est (53 % ± 1).These results demonstrated that there is great genetic variation in the cellulose contents (approximately 10 %) of the materials, thus indicating a great genetic potential among these materials to obtain more productive commercial cultivars for 6-carbon sugar production, such as lignocellulose compounds, which may be used to produce second-generation ethanol and in the development of other products by sugarcane mills.
According to Calmonovici (ETH BIOENERGY Director; personal communication) and Sheldon (2011), "green economy" or "bioeconomy" allows using these sugars as building blocks in an array of biochemical scopes to obtain products, such as "green" plastic, hydroxymethyl furfural, succinic acid, glutamic acid, glycerol, sorbitol, and farnesene, which can replace petrochemical use and generate biomass demand for this application.
Regarding 5-carbon sugars, hemicellulose contents ranged from 21 % ± 1 for the IAC 122 genotype to 25 % ± 1 for the IAC 94 genotype (Table 8).Although the content variation in this sugar type was only 4 % among the materials, it is possible to develop pentose-richer cultivars to produce 5-carbon materials for xylitol and furfural production or for special ethanol fermentations from these sugars.Reddy and Yang (2005) also found similar hemicellulose contents (19 to 24 %).
Lignin contents ranged from 21 % ± 0.4 to 35 ± 0.6 % in the IAC 94 and IAC 122 genotypes (Table 8), respectively.The data had a variation of almost 15 % in lignin contents among the different genotypes.Reddy and Yang (2005) also found similar lignin results, which ranged from 23 to 32 % in sugarcane.The relation of fiber contents in each genotype allows inferring that cellulose-rich genotypes are also lignin-poor.This information is very interesting for plant breeding because it allows developing cellulose-rich and ligninpoor materials, which is in line with their intended use in second-generation ethanol production.In obtaining cellulose-rich materials (6-carbon sugar ethanol source for ethanol production), it is possible to simultaneously obtain lignin-poor materials, which is a structural component of the lignocellulosic material that hinders the cellulose hydrolysis process, thereby increasing the costs of cellulose processing.

Conclusions
The production of sugarcane straw content varies according to cultivars.The greatest mass of sugarcane straw is in the top leaves, but stem straw accounts for more than 40 % of the total straw in a sugarcane crop.The straw index could be used as a good tool to estimate the straw production in sugar crop, but these indexes should be obtained separately for each sugarcane variety to obtain more accurate forecasts of crop production.The potential for crude energy production of sugarcane can be increased using fiber-rich species or species that produce more straw, but the products of interest should be taken into account before choosing the crop variety.Finally, cellulose, hemicellulose and lignin composition in sugarcane is distinct among varieties, so it is possible to develop distinct materials for energy production and for the development of sugarcane mills using biochemical processes and thermal routes.--------------------------------------------% --------------------------------------------

Figure 1 -
Figure 1 -Straw yield estimates in four sugarcane IAC varieties.

Table 1 -
Leaf area index (LA), leaf (+3) calculated as a function of leaf length (L) and leaf width (W), of sugarcane.

Table 2 -
Production of primary energy of sugarcane cultivars from fiber and total sugars stratification.

Table 6 -
Total Stems per Hectare (TSH) and straw percentage prior to harvesting, in four sugarcane cultivars with distinct yield levels.

Table 4 -
Straw content of different cultivars.

Table 5 -
Straw content in different producing regions around the world.