Until recently, systematic breeding of sweet sorghum was sporadic with the USDA-ARS Meridian, Mississippi sweet sorghum breeding program being the only project devoted exclusively to this crop. Though it closed in 1988, many of the existing sweet sorghum varieties in the United States were derived from that program [86-88]. These varieties represent an array of different maturities, heights and agronomic packages adapted to regions as best identified by producers. They have become standards throughout the world and form the basis for sweet sorghum breeding programs that are being established in many regions of the world. With increasing interest in bioenergy, newer sweet sorghum varieties have been released in India and China [11].

Prior breeding efforts in energy and sweet sorghum focused on pure-line cultivar develop­ment. Sweet sorghum cultivars were selected from intentional crosses and advanced through several generations via self-pollination and selection to uniformity. Traits of importance included maturity, height, sugar yield, sugar concentration, sugar quality and agronomic adaptation to stalk rot, drought, and stem borers. Initial breeding efforts did not emphasize bagasse quality because it was of little importance to small-scale sweet sorghum producers. In an industrial setting, however, bagasse is substantially more important, as it represents an energy source to produce electricity or to be converted to ethanol itself once lignocellulosic conversion processes become profitable [43].

For reasons previously mentioned, hybrid sweet sorghum cultivars are crucial for indus­trial production systems. Because sugar concentration is a primarily additive trait, a true sweet sorghum hybrid is often difficult to produce, since most parental seed lines have low sugar concentrations in the juice and the resulting hybrids have intermediate sugar concentrations [89]. Consequently, even though juice volumes were heterotic, overall sugar yields were reduced relative to sweet sorghum varieties because of lower sugar concentra­tions in the parental seed lines. Therefore, it was necessary to develop sweet sorghum seed parents to produce true sweet sorghum hybrids. Development of pollinator parents was also important, but most sweet sorghum varieties can serve as pollinator parents as they typically restore fertility to their hybrids made using either A1 or A2 cytoplasm.

Sweet sorghum parental seed lines are being developed in several programs around the world [11]. Studies of these hybrids are superior to seed parents and numerically equal if not superior to pollen parents (i. e., sweet sorghum varieties) [45,46]. While first generation hybrids did not always outperform their respective pollinator parents, hybrid seed production capacity was four to six times greater than for a pure-line sweet variety and it was much easier to harvest. Furthermore, it is logical to expect that yield and quality of subsequent hybrids will be better than those currently available. In India, excellent progress has been made in developing hybrid sweet sorghum lines from sweet and grain parents. From the ICRISAT sorghum improvement program, Reddy et al. [90] described six hybrids significantly lower in brix than the control genotype that were nonetheless significantly higher in sugar yield than the control. Additionally, and not surprisingly, these six hybrids also produced significantly greater grain yields than the sweet sorghum control.

Hybrid sweet sorghum breeding methodologies follow traditional sorghum breeding approaches with modifications in traits and selection protocol. For example, because of the additive effect of sugar concentration, it is critical to select for sugar concentration in both seed and pollinator parents. However, juice volume is more of a dominant trait; consequently, it may be selected in either parent and will be expressed in the hybrid. Most breeding programs will use a pedigree approach followed by sterilization of the seed parent and test-crossing of both types on standard testers [65]. In addition, and just like grain crops, a range of hybrid maturities will be necessary to ensure an optimal distribution of hybrids for a continual harvest season [14].

For improvement purposes it is best to define total sugar yield using the individual components that contribute to it, which are juice yield and soluble sugar concentration. Juice yield is related to biomass yield, and thus total biomass yield is critical in sweet sorghum breeding [34]. Often, reports in the literature estimate sugar yield using total dry biomass yield and a coefficient for juice content and soluble sugar concentration reported in the brix units. However, juice extractability and the proportion of sugar to total soluble materials must be considered in selection, which if based on brix and/or moisture content alone could be misleading [45]. Lodging and stress tolerance are also traits of interest for sweet sorghum breeders insofar as they affect harvestability, stability and fermentable sugar yield. Finally, an important trait unique to sweet sorghum (as compared to grain sorghum) is the duration of optimal sugar yield in the hybrids. It is generally agreed that sugar yields peak in sorghum prior to physiological maturity and, therefore, if that yield can be maintained for a longer period it could extend the economic harvest season, which is crucial for development of a sustainable biofuel industry.

Willow biomass crops can be grown on marginal land using a coppice management system so that multiple harvests are generated from a single planting of genetically improved shrub willow varieties [21]. The system typically includes three-year rotations with one year of site preparation prior to planting. After the first growing season, the willow is coppiced and material is typically left in the field since first year production is very low, typically between 0.5 and 1.0 tons ha-1. The willow resprouts the following spring and produces multiple stems on each plant. The willow is left to grow for three years and then harvested during the dormant season. After harvest the willows resprout and grow for another three-year cycle. Up to seven three-year rotations are currently projected before the willow stools spread out and limit access for harvesting and chip collection equipment. Following the final harvest, the willow can be killed with herbicide and stools ground down and incorporated into the soil [21].

Effective establishment of perennial energy crops like willow is essential to their biologi­cal and economic success, so conducting proper site preparation is essential. Site preparation should begin with control of existing weeds using a combination of chemical and mechan­ical techniques. These activities should begin in the fall before planting if the field contains perennial weeds, which is often the case with marginal land, or after crops are harvested if the land is currently being used for the production of an annual crop. It is essential to control competing vegetation and prepare the soil before willows are planted in the spring. Improper or incomplete site preparation often results in strong weed competition during the establishment phase and has frequently been noted as one of the main limitations to successful establishment of willow biomass crops [6, 9, 22].

Willows are planted as unrooted, dormant hardwood cuttings at about 15 000 plants per ha-1 as early in the spring as the site is accessible with mechanized planters attached to farm tractors. Over the years several versions of planters have been developed and two of them, the Step planter and the Egedal (Figure 12.1), are currently being used in the United States. With both machines, one-year-old stems are fed into the planter, cut at an appropriate length (15-20 cm) and either actively inserted into the ground (Step planter) or placed into a slit opened by the planter (Egedal). Both machines are capable of planting around 0.8 ha h-1 when site preparation has been done properly and ground conditions are appropriate.

The use of dormant cuttings allows planting of selected varieties of genetically improved willow. In North America, varieties are generally planted in individual blocks with several different genetic varieties planted across a field to maintain diversity. In parts of the United Kingdom, where pressure from willow leaf rust (Melampsora spp.) is stronger, studies have shown that planting random mixtures of different willow varieties can maintain or even increase yields [23, 24]. Differences in growth rates, stem form, canopy width and other characteristics among varieties may influence the effectiveness of this approach if some external pressure, such as a disease or pest, is not a major influence on the system. Research is underway in North America to characterize willow varieties and to explore the potential benefits associated with mixed random planting designs.

Current recommendations for planting designs and densities for willow in North America are based on the double row system developed in Sweden and research from Europe largely based on the growth of S. viminalis [25] and trials in North America [26]. Research in

North America was based on a single variety, ‘SV1’ (Salix dasyclados), over multiple rotations with densities ranging from about 15 000 to 111 000 plants ha-1. The current recommended spacing for a double row system allows 1.8 m between each set of double rows, 0.75 m between individual rows, and 0.55 m between plants along each row. This results in a planting density of about 14 600 plants ha-1. However, recent studies with new willow varieties developed in New York suggest that their growth rate is rapid enough that there is no significant yield difference among planting densities ranging from 8800 to 17 500 plants ha-1 [27].

Following the first year of growth, the willows are cut back close to the soil surface during the dormant season to force coppice regrowth. This increases the average number of stems per stool from 1-4 to 8-13 depending on the variety [18] (Figure 12.2). After an additional three to four years of growth the stems are mechanically harvested during the dormant season after the willows have dropped their leaves so those nutrients are maintained in the system (Figure 12.3). In addition, most end users do not want foliage in

the biomass delivered to their facility because of its higher ash and nutrient content. The chipped material is then delivered to end users for conversion to bioenergy, biofuels and/or bioproducts. The plants will sprout again the following spring and are allowed to grow for another three or four years before being harvested. Projections indicate that the crop can be maintained for 7-10 rotations before the rows of willow stools begin to expand to the point that they are no longer accessible with harvesting equipment. At this point the crop can be replanted by removing the existing stools with herbicides after harvesting, followed by chopping using a heavy disk and/or grinding machine, and subsequently planting new cuttings that year or the following year.

Nutrient removal from willow biomass crops is limited because only the aboveground woody portion of the crop is harvested during the dormant season after the leaves have dropped and most nutrients have been translocated to the root system. Nutrients not translo­cated from the foliage are returned to the system in litterfall. For most soils in the region where willow is being deployed, the only nutrient addition that is recommended is nitro­gen, which is typically added at the rate of about 100 kgN ha-1 once every 3-4 years in the spring after the crop is harvested. However, recent research has indicated that for a number of sites in the northeast there was no yield response when nutrients either in the form of commercial fertilizers or organic amendments were applied to willow crops [28, 29]. Marginal agricultural soils in the northeast United States are typically limited by poor drainage and wet conditions rather than nutrient supply. Additionally, tight nutrient cycling in these systems [30] and relatively low nutrient removal rates in the woody biomass [31] are other factors that may further reduce the need for fertilization on a wide range of favorable sites for woody crops in the region. If regular nutrient additions are not required, willow systems will reduce production costs and greenhouse gas emissions, improve the net energy balance, and preserve or improve water quality in natural streams and waterways when compared to annual cropping systems.

A rapid growth rate is one of the attributes that makes shrub willows an appealing biomass crop. Yields in research plots of fertilized [32] or fertilized and irrigated [31] unimproved varieties of willow grown for three years have exceeded 27 oven-dried metric tonnes (odt) ha-1 yr-1. Due to costs associated with irrigation and the relatively low value for biomass, irrigation will not be used for most large-scale production operations. The exception with regard to irrigation is where willow biomass crops could be irrigated with wastewater as part of an overall nutrient management plan. Nonetheless, these studies set a benchmark for the potential production of willow biomass, with even higher yields being possible with improved genetic material from current breeding and selection programs.

First-rotation, non-irrigated research-scale trials across a range of sites planted between 1993 and 2007 with a range of willow varieties have produced yields of 6.9 odt ha-1 yr-1 [33]. Trials planted after 2005 that included new willow varieties developed in North America produced 9.2 odt ha-1 yr-1, an increase of 33%. Many of these trials involved the testing of a wide range of varieties, some of which were later determined to be unproductive and were eliminated as potential commercial varieties. If just the top five varieties in each trial are included, then the reported yield is 9.2 odt ha-1 yr-1 in the trials with older varieties and 11.2 odt ha-1 yr-1 when new varieties were included, a 22% increase. Second rotation yields of willow are typically higher than first rotation yields because the plant’s root system is already established and more of the carbon that is fixed by the plant in the second rotation can be allocated to aboveground growth. In one trial, second rotation yields increased by about 20% while third and fourth rotation yields were maintained and largely dependent on weather conditions [33].

Among the fertile germplasm, accessions should be selected as parents to be used in crosses for breeding purposes or to create mapping populations for genetic studies. As a Miscanthus crop takes several years to mature, genetic markers and phenotypic methods need to be developed to speed up the breeding process.

4.3.2.1 Tools for Genetic Studies and Breeding

The polyploid nature and the relative large size of the Miscanthus genome complicate genetic analyses. Using flow cytometry and stomatal cell analyses, Rayburn et al. [131] found M. x giganteus had a genome size of 7.0 pg (6.8 Gb, the number of DNA base pairs per nucleus being assumed 0.965 x 109 bp per pg by the author) while Miscanthus sinensis and Miscanthus sacchariflorus had genome sizes of 5.5 pg (5.3 Gb) and 4.5 pg (4.3 Gb) respectively (Table 4.5). It is clear that there are many gaps that require further

investigation. However, further studies will be facilitated by the use of plants such as sugarcane, sorghum and maize, which are likely to be good models for genomics and breeding issues in Miscanthus.

Detailed DNA mapping and sequencing studies in plants related to Miscanthus will pro­vide relevant genetic tools and information. For example, Sorghum Bicolor is diploid and, with a relatively small genome of about 730 Mb, has been completely sequenced [132]. Although sugarcane is related to Miscanthus [11], its genome is much more complicated due to its very high degree of polyploidy (about 12x for modern cultivars, Le Cunff et al. [133]). The monoploid genome size for S. officinarum (x = 10) is about 926 Mb while that of S. spontaneum (x = 8) is about 760 Mb (Butterfield et al. [132]). Maize is less related to Miscanthus than sugarcane and sorghum but its genome has been fully sequenced [134]. Syntenic regions or candidate gene sequences can be expected and exploited for compar­ative genetic studies. From the conserved syntenic regions, markers can be developed in Miscanthus and related to traits of interest for marker-assisted selection.

A wide diversity of molecular markers are available from plants related to Miscanthus but their transferability for use in Miscanthus needs to be determined. First comparisons are promising, however, with Hernandez et al. [136] showing that 75% of the maize microsatel­lites tested gave highly reproducible amplification with Miscanthus DNA. More recently, Swaminathan et al. [137] showed that sorghum could be used as a reference genome sequence for Andropogoneae grasses. In a survey of the complex Miscanthus x giganteus genome using 454 pyrosequencing of genomic DNA and Illumina sequencing-by-synthesis of small RNA, Swaminathan et al. [137] found that the coding fraction of the Miscant — hus x giganteus genome had a high level of sequence identity to that of other grasses (sorghum, maize and rice). In addition, Kim et al. [138] designed SSRs from sugarcane expressed sequence tags (ESTs) and in applying these to a Miscanthus mapping population succeeded in generating EST-SSR-based genetic maps of Miscanthus.

Genetic linkage maps offer an efficient tool in the study of the inheritance of quanti­tative traits. Most Miscanthus species are self-incompatible, resulting in a high level of heterozygosity from outcrossing. Grattapaglia and Sederoff [139] proposed a two-way pseudo-testcross model for the genetic mapping of highly heterozygous organisms.

Several maps are available for marker-assisted studies in Miscanthus. The first genetic map of Miscanthus was constructed with this pseudo-testcross strategy using intraspecific hybrids from a cross between two Miscanthus sinensis clones [10]. 383 RAPD markers were developed for this map but a higher density of molecular markers was needed due to the high number of linkage groups (28) relative to the basic chromosome number (x = 19). This map had a total length of 1074.5 cM. A decade later, Kim et al. [138] developed a genetic map with highly heterozygous individuals being interspecific hybrids from a controlled cross between heterozygous single plants of M. sacchariflorus Robustus (2n = 2x = 38) and M. sinensis (2n = 2x = 38). Their map used cDNA-derived SSR loci and comprised 23 linkage groups with 303 markers and was 2238.3 cM in total length. Ma et al. [140] created a high-resolution genetic map of Miscanthus sinensis using genome sequencing and comprising 3745 SNP markers spanning cM on 19 linkage groups with a 0.64 cM average resolution.

Miscanthus linkage groups of the map developed by Kim et al. [138] were aligned suc­cessfully to the Sorghum chromosomes. A duplication of the whole genome was produced and corresponds to the Miscanthus lineage after the divergence of subtribes Sorghinae and Saccharinae [138]. Comparative genomics analyses of their map to the genomes of sorghum, maize, rice and Brachypodium distachyon [140] indicated that sorghum had the closest syntenic relationship to Miscanthus. This validates the use of sorghum as a model for the genomics of Miscanthus.

Breeding programs will be directly guided in the future by the genome sequencing of Miscanthus x giganteus and its close relatives, to capture, for example, the genes of interest present in these species. It is noticeable that sequencing efforts of four Miscanthus species (M. x giganteus, M. sinensis, M. sacchariflorus, and M. floridulus) are ongoing along with the creation of genomic resources by the Energy Biosciences Institute (http://www. energybiosciencesinstitute. org/) and by the Joint Genome Institute (http://www. jgi. doe. gov/).

Pines evolved in the Northern Hemisphere, with most species naturally occurring between 20 and 70° latitude. The largest genus of conifers contains over 100 species, which may be divided into two or three subgenera. Some say the “hard pines” (subgenus Pinus) contain 64 species while “soft pines” (subgenus Strobus) contain 37 species. Members of the subgenus Pinus have two flbrovascular bundles per needle while the subgenus Strobus only has one.

Pine wood is composed of cellulose, hemicellulose, lignin, oleoresin, uronic anhydride, acetyl and ash. Cellulose, lignin and hemicellulose typically comprise more than 90% of the wood while resins may make up 9% of the oven-dry weight. On an equal mass basis, the economic value of the resins is generally higher than that for wood. The chemical structures and amounts can vary by species because of genetics involved in producing the cell wall and its components.

William L. Rooney

Department of Soil & Crop Sciences, Texas A&M University, U. S.A.

7.1 Introduction

Sorghum (Sorghum bicolor L. Moench) is an important crop species in the United States and around the world. Because of its substantial heat and drought tolerance, sorghum pro­duction is traditional in semi-arid, subtropical and tropical regions. In addition to abiotic stress tolerance, sorghum is very responsive (in terms of productivity) to more favorable conditions. While primarily known as a cereal grain, sorghum is grown throughout the world as a forage, syrup and more recently, energy crop. In 2008, U. S. farmers harvested about 2.9 million hectares of grain sorghum (USDA NASS, http://www. nass. usda. gov/). Worldwide, the top 20 grain sorghum producing countries harvested 49 472 518 met­ric tonnes of grain in 2007 (FAOSTAT, http://faostat. fao. org/). Unfortunately, production statistics for the other types of sorghum (i. e., forage and sweet) are not reported.

With renewed national and international emphasis on sustainable bioenergy, interest in sorghum as a bioenergy crop has increased for several reasons [1]. Firstly, sorghum has an established production history as a crop in the United States and around the world. This history eliminates the time required for crop domestication, production and market develop­ment and reduces concerns regarding producer acceptance and adoption. Secondly, there is a well established seed industry that is knowledgeable regarding genetic improvement and proficient in seed production. Thirdly, the annual nature of the crop, while a detraction to some, increases the speed and efficiency at which sorghum can be genetically improved and deployed in a production environment. Finally, sorghum has evolved as a standard genetic model for improvement of bioenergy crops. Combined, these factors confirm that sorghum will play an important role in the development and evolution of dedicated energy crops.

Cellulosic Energy Cropping Systems, First Edition. Edited by Douglas L. Karlen. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

Within this chapter, a brief summary of sorghum growth and development, composition, relevant production issues and genetic improvement approaches are discussed.

In Sweden, timber is harvested and sent to the saw mill while branches and tops are placed in piles along the road. Tarps are placed over the pile to help keep the pile dry. After several months of drying, the piles are transported to a central heat plant. The dried pine residue is burned to produce steam and hot water. The hot water is then used in homes, reducing the need for residential hot water heaters.

In the United States, branches and tops are often left in the forest to decay. Once pine logs have been harvested for sawtimber and pulpwood, there are often 20-80 tonnes/ha of biomass remaining on the site. In some harvest operations, the branches and tops and non­merchantable stems are ground or chipped for use as energy. Crews with specialized biomass harvesting equipment process the pine tops, branches and non-merchantable hardwoods. The chips are then transported to a power company to produce electricity and heat. Some of the excess heat is used to dry chips prior to burning. In many cases, removing a residue of 60 tonnes/ha will reduce the cost of land clearing needed prior to establishing the next pine plantation. This savings could exceed $60/ha [14]. Currently, much of the fuel wood chips harvested in this manner in the United States are used by pulp mills to supplement mill residues and natural gas for cogeneration of steam and electricity.

Since biomass sorghum hybrids are photoperiod sensitive, seed production relies on either genetic control of photoperiod sensitivity or strategic planting in seed production environ­ments. In the latter, day length during winter months in tropical environments is sufficiently short to allow seed production of hybrids. In this situation, the greatest challenge is planting seed and pollinator seed stock in time to ensure both reach anthesis at the same time. In temperate environments, such production is not possible because of cool temperatures dur­ing the winter season. Therefore, seed production must rely on genetic systems that allow production of a photoperiod sensitive hybrid using two photoperiod sensitive parental lines. Such a system was identified and characterized in forage sorghums [47, 65] and can be readily deployed within a bioenergy breeding program.

Breeding for biomass production uses approaches similar to those currently used to produce hybrid forage sorghum. Vegetative biomass yield will be the most important trait, as is the case with current forage types. In most cases, biomass sorghum cultivars are being bred for a single harvest management scheme. Although multiple cut types could be used, they most likely will be forage types, as those perform very well in multiple-cut production systems [91]. Inbred line development will follow the same approaches used for grain and forage sorghum. For energy sorghums, most of the breeding effort will focus on the pollinator parent because existing seed parent lines are suitable for use to produce biomass sorghum hybrids. Potential pollinator parents range from existing elite sorghum germplasm with good general combining ability to unique genotypes that maximize photoperiod sensitivity and are derived from exotic sorghum accessions. Initial screening for maturity, yield, composition and agronomic desirability will be used to identify pollinator parents, which can be improved through further breeding to complement existing seed parents for both maturity and dwarfing loci. This will allow for the production of hybrids using lines that are moderately short and photoperiod insensitive but that produce a hybrid that is tall and photoperiod sensitive [26, 47].

Despite the wide array of benefits associated with willow biomass crops, expansion and rapid deployment of this system has been restricted by high production costs and, in some situations, a lack of market acceptance. The economics of willow biomass crops has been analyzed using a cash flow model (EcoWillow) that is publically available from SUNY — ESF (State University of New York-College of Environmental Science and Forestry) [34]. The model incorporates all the stages of willow crop production from site preparation and planting through to harvesting over multiple rotations, and transportation of harvested chips to an end user. The removal of the stools once the crop has expired at the end of seven rotations is also included in the model. The cash flow model is based on experience establishing and maintaining willow biomass crops in New York State. The model is flexible enough that it can be applied across the range of sites where shrub willow might be grown. Users can vary input variables and calculate cash flow and profits throughout the entire production chain from site preparation and crop establishment to the delivery of wood chips to an end user.

For the base case scenario in EcoWillow, a productivity of 12 odt ha-1 and a biomass price of $60 odt-1 showed an internal rate of return (IRR) over seven, three-year harvest cycles (i. e., 22 years) of 5.5% [35] with profits of $101 ha-1 yr-1 or $10 odt-1. The model shows that payback is reached in the thirteenth year with revenues from the third harvest neutralizing the project’s expenses. Harvesting, establishment, and land rent are the main expenses associated with willow biomass crops over their entire lifespan making up 32%, 23%, and 16% of the total undiscounted costs. The remaining costs including crop removal, administrative costs and fertilizer applications account for about 29% of the total costs.

For willow biomass crops, harvesting is the largest single cost factor, accounting for just under one-third of the final delivered cost. Harvesting, handling, and transportation account for 45-60% of the delivered cost [34]. Harvesting operations have a significant impact on the final cost of production because it is the operation that occurs most frequently during the life span of the willow crop. If seven, three-year rotations are run for this system, harvesting operations need to be conducted seven times. Each of those operations requires a harvester and system of collection wagons or trucks. Since this part of the production system makes up such a large portion of the final delivered cost and the harvesting systems being used are relatively new, opportunities for cost savings are significant. Improving harvesting efficiency by 25% could reduce the delivered cost of willow by approximately $0.50/MMBtu ($7.50/ton). Research and development work is underway to reach these targets using a cut-and-chip harvesting system that is based on a New Holland forage harvester and cutting head (Figure 12.4) specifically designed for short rotation woody crops like willow and poplar. Recent field trials of this harvesting system have generated throughput rates exceeding 50 green tons h-1 [35].

Harvest costs are also significantly influenced by the field design. Missing or inadequate headlands can create costly delays when handling harvesting equipment. Furthermore, maximizing row length and, thus, reducing unproductive turn-around time is crucial. For instance, increasing row length from 200 to 400 m, ceteris paribus, increases the IRR by 11% [34].

Another approach for decreasing harvest cost is to reduce the number of harvests over the crop’s lifespan. Increasing the production cycle to four years would result in five, four-year harvests instead of seven, three-year harvests, thus decreasing harvest costs per ton by about 14% and increasing the IRR for the entire system by about 11% [34]. This improvement in return is primarily associated with an increase in biomass at the time of harvest. However, this also assumes that mean annual willow growth does not decrease as rotation length increases and that the harvester can efficiently and effectively handle larger diameter stems.

Another alternative to reduce harvest costs is to use a smaller harvesting system that has lower capital and operating costs. One example is the NyVarra harvester, which could be used to harvest willow on a two-year rotation. This system is particularly appealing when biosolids are being land applied to willow fields and the producers are also generating some revenue from this operation. This has become more common in Europe in recent years. The limitations of a shorter harvest cycle and smaller harvesting system are that fewer tons are harvested in each rotation, so the fixed costs associated with both the harvester and chip collection system are spread over fewer tons and the rate of production of these systems is typically lower. Whether or not a smaller harvesting system and a more frequent harvest cycle is more economically attractive than a larger harvesting system is being explored further.

Willow biomass crop establishment is the second largest cost category in the production system, accounting for almost one quarter of the final delivered biomass cost [34]. The high upfront establishment costs are a barrier to the deployment of willow, especially since any return on these investments is not realized until the first harvest, which typically occurs four or five years after planting. Costs for planting stock typically account for over three quarters of the establishment expense for willow biomass crops. Current planting stock costs are in the range of $0.12 to $0.15 per cutting and with the current recommended planting density of 14 600 plants ha-1, the cost of planting material alone is $1752 to $2190 ha-1. There are two approaches to reduce costs associated with planting stock. One is to reduce the planting density and the second is to reduce planting material cost. As noted above, recent studies have suggested that the planting density could be reduced to 8800 plants ha-1 [27]. Furthermore, as willow biomass crops are expanded and demand for planting material increases, improvements in production of planting material in the nursery are anticipated to lower costs about $0.10 per cutting. If both a lower planting density and lower stock cost were implemented, costs of planting stock could be reduced by 50-60%. This would have a significant impact on both establishment costs and overall returns from the system.

Improving yields will increase revenues from willow biomass crops and will improve returns. Yield improvement is a key focus of research efforts to overcome the economic barrier to commercialization of this system [16]. Increasing yields by 50% from 11.3 odt ha-1 yr-1 would improve the IRR from 5.5 to 14.6% [34]. As noted above, significant improvements in yield have already been made with the production of new willow varieties and additional improvements will occur with new genotypes and improvements in the management of willow biomass crops.

Willow biomass cropping systems are in their infancy in North America and there is potential for large gains in yield by optimizing production practices and through breeding. By addressing system components that have the greatest influence on costs, the overall economics of these systems can be improved so that they can be deployed across the landscape. As the knowledge base about how willow grows and the roles it plays expands, it will be deployed more effectively so that in addition to biomass, other landscape benefits derived from this system can be optimized.

Miscanthus can be vegetatively propagated by rhizome division but this process is time consuming. Developing an efficient tissue culture system would provide an alternative to rhizome division and be useful for breeding purposes. Tissue culture enables a large number of plants to be generated and stored regardless of the season. In addition, the risk of transferring diseases between fields is lower than propagation by manual rhizome separation [141].

Somatic embryogenesis and clonal propagation are two methods used for in vitro prop­agation of Miscanthus. With somatic embryogenesis, considerable differences exist in the capacity of explants types of the same genotype to produce an embryogenic callus and regenerate plants [142, 143]. The growth stage of inflorescences used for the somatic embryogenesis is very important, with younger inflorescences showing a significantly higher callus induction rate than more developed inflorescences [144,145]. Immature inflo­rescences are abundant and can easily be obtained from field-grown M. x giganteus during the summer or from greenhouse grown plants throughout the year. In vitro propagated plants are more cold tolerant in their first season than plants obtained in vivo [146]. Plants propa­gated from rhizome division are larger and have a higher yield than plants propagated via somatic embryogenesis [66]. In clonal propagation involving organogenesis, new plants are produced from shoots obtained from a culture of axillary buds [64, 147]. Plantlets from vegetative regeneration are genetically identical (Rambaud, personal communica­tion). In addition, clonal propagation can also be applied to seedlings. Figure 4.7 presents the different stages of the clonal propagation of Miscanthus from seeds.

This last efficient plant regeneration system would be helpful for genetic improve­ment through future biotechnology research. It is interesting, for example, to handle tissue culture in order to produce transgenes. Recently, particle bombardment-mediated transformation [148] and Agrobacterium transformations [149] were used to insert genes of interest in Miscanthus genome for agronomical genetic traits and introduce genetic variations.

The heat of combustion for wood is expressed as the high heating value (HHV) or low heating value (LHV). The HHV can be thought of as the gross amount of energy trapped in an odMg of wood while the LHV is the net energy after accounting for the moisture content of the wood. The HHV can be considered a theoretical value while the LHV is closer to the utilizable energy.

The quantity of usable heat produced by the complete combustion of pine logs (LHV) will vary depending upon the moisture content [4]. Although a cubic meter of wood contains the same mass of cellulose either green, air dried or oven dried, the amount of usable heat produced is greater for oven-dried wood (Table 10.1). This is because energy is required to turn water into steam. The more water the wood contains, the more energy will be used to produce steam.

Two methods are used when calculating the moisture content. In some reports, the denominator includes both wood and water and in other reports it does not include water (i. e. oven-dry basis). When the method is not specified, this difference can cause confusion. For example, if someone says wood has a moisture content of 50%, it would not be clear if this means half of the mass is water (i. e. green basis) or if 33% of the mass is water (i. e. 1/3 water and 2/3 wood). Therefore, in this chapter, we will include the designation “od” to indicate the denominator does not include water.

The energy contained in pine logs will also vary depending upon the resin content. The heat of combustion for pine increases by about 16.8 kWh/dry tonne for each percentage point increase in extractive content. Pine logs with high resin might have 20% more energy than logs with no resin [5]. In fact, the heat of combustion of pine resin can be higher than coal (Table 10.1). Certain “hard pines” produce more resin than other species. Pines known for their ability to produce lots of resin include Pinus elliottii and Pinus palustris. Wood from pines from the southern United States may contain about 5% extractives, but heartwood segments from old growth may contain 30-35% extractive content. This will cause the wood to be dense enough to sink when placed in water. This type of wood is commonly referred to as “fatwood” or “lighter wood” and is sold as kindling on the Internet for about $2.50 per kg (or $2500 per tonne). Unfortunately, the demand is generally low and, therefore, it may take some time to sell one Mg of “fatwood.”