As with any bioenergy crop, biomass composition is important to breeders, producers and end users. However, the definition of quality varies greatly depending on who is defining it and the specific method being used for processing. In addition, the specific type of sorghum, the stage of growth, and the production environment strongly affect composition [31,32]. For example, in forage sorghum, high protein content is important for forage quality, but lower protein content is more desirable for biomass sorghum, as nitrogen is of little value in the biomass; it is more valuable when it is returned and retained in the field [33]. Since bioenergy sorghum is grown to produce carbohydrates, the composition of both non-structural (sugar and starch) and structural carbohydrates (cellulose, hemicelluloses and lignin) is important. In biomass sorghum, structural composition of the biomass is the critical factor while in sweet sorghum, non-structural carbohydrates are of primary importance. In either case, sorghum has a significant range of variation in composition for both non-structural and structural composition [34,35].

For structural carbohydrates, Stefaniak et al. [36] reported a twofold range in variation among sorghum types for lignin, hemicelluloses and cellulose. Dahlberg et al. [37] eval­uated commercially available forage sorghum hybrids for composition and reported that existing hybrids could be used as a biomass source for ethanol production. While some of this variation is dependent on the environment and maturity [35, 38], there is a genetic component as well [35]. In addition to selection for optimum composition, all approaches will select against ash content, as excessive ash is an undesirable trait for biofuel processing. It must be noted that harvest approaches will also influence this trait, and therefore any process that reduces ash uptake is good.

Research examining variation in forage composition and its effects on digestion in animal systems is of relevance to the bioenergy breeder because several fermentation approaches mimic a ruminant digestive system. There is considerable debate as to the net gain of energy using current and proposed lignocellulosic ethanol conversions techniques [39]. However, the consensus is that this strategy of ethanol production from starch has a positive net energy gain utilizing current technologies [40]. There is also evidence that the overall energy balance for lignocellulosic ethanol will be more favorable than starch-based ethanol, if logistical and technical processing hurdles can be solved [41]. Additional improvements are needed to make lignocellulosic energy production more cost effective as compared to fossil fuels or other renewable energy sources [42,43].

The biomass composition of energy sorghum varies depending upon genotype and envi­ronment in which it is grown, but the relative importance of these sources of variation is not well known. In a study containing six sorghum genotypes, ranging from grain to forage and sweet types, biomass yield varied by 82%, indicating the existence of genetic variability for biomass production among sorghum types [44]. In sweet sorghum, variations in composition of traits such as glucans (cellulose) ranging from 24.7 to 38.5%, xylans (hemicellulose) from 8.5 to 13.9% and lignin from 9.3 to 13.0% were reported [45]. Hoff­mann [35] reported variation in biomass composition across 15 genotypes of photoperiod sorghum in five environments, cellulose ranged from 26.9 to 31.8%, xylan from 14.9 to 18.4% and lignin from 8.3 to 18.9%. The environment had a greater effect on composition than genotype per se in the six bioenergy sorghum cultivars [35].