Grain sorghum is grown primarily for starch, a primary component of the grain and like corn, a substantial portion of the U. S. grain sorghum crop (approximately 33%) is used for ethanol production [5]. The primary goal of most grain sorghum breeding programs is to enhance and protect productivity of the grain crop. While variation for seed composition (i. e., starch) exists in sorghum [6,7], simply increasing grain yield increases ethanol yield, because total starch per unit area is increased. In addition to maximizing productivity, protection of yield potential is equally important, especially drought tolerance, since production of grain sorghum occurs mostly drier areas of the world.

Sweet sorghum genotypes produce high quantities of simple sugars by having a very juicy stalk and high concentrations of soluble sugars. Traditionally, sweet sorghums have been used as a sweetener; extracted juice is cooked and sugars are concentrated in syrup. Sweet sorghums are known in various parts of Africa and they became popular in Asia and North America in the seventeenth and eighteenth centuries [8]. In the early twentieth century, the United States was producing 20 million gallons of sorghum syrup annually. Production dropped after World War II as crystal sugar became more available; today, sorghum syrup production is essentially artisanal and it is concentrated within the southeastern United States. Sweet sorghums are usually tall, with thick stalks and low grain yields compared to grain sorghum varieties. For energy production, sweet sorghum is processed in a similar manner to sugarcane; in fact, sugarcane processes and equipment provide a logical starting point for utilizing sweet sorghum [9]. Sweet sorghums have relatively high biomass yield potential; Hunter and Anderson [10] estimated that sorghum has the potential to produce up to 8000 liters of ethanol per hectare, or about twice as much as that of maize and 30% more than sugarcane ethanol in Brazil. Much of the carbohydrate content of the stalk juice of sweet sorghum is sucrose and/or glucose and it is fermentable without starch hydrolysis. This has advantages and disadvantages because the fermentation process can proceed without pretreatment (advantage) but fermentation must be initiated quickly because of the instability of the sugars in the stalks and/or juice (disadvantage). Preliminary results indicate that there can be a reduction of 16.8% sugar yield if juice extraction is delayed by 48 hours [11]. The residue (bagasse) is also a rich source of structural carbohydrates that can be used for energy production or as an animal feed [12].

For these reasons, the duration of the harvest season is important for sweet sorghum production and crop complementation production systems or storage processes must be developed [13]. Subtropical and tropical regions, which have longer harvest seasons, have an inherent production advantage for sweet sorghum. In addition, there is a logical and economically beneficial complementation between sweet sorghum and sugarcane and this can extend the harvest and milling season [14]. Sweet sorghum complements sugarcane in this scheme because it can be harvested twice in one year in tropical environments and because of its enhanced drought tolerance and water use efficiency.

Energy sorghum is a specific type of photoperiod sensitive sorghum that, when grown in long day environments, accumulates large quantities of biomass [1, 15]. Because they are photoperiod sensitive, they will not flower until day length drops below a specific length of time under temperate climates and the plants will never reach anthesis due to cold temperatures [16]. This characteristic causes an extended vegetative growing season, allowing the plant to capture and convert solar energy into biomass, provided adequate moisture is available for growth. In addition, a plant that is in a vegetative growth stage is inherently more drought tolerant than when in reproductive growth stages. Biomass sorghum cultivars will reduce growth during periods of drought but then resume growth when moisture is available. Therefore, this reduces the crop’s sensitivity to drought stress and its timing. Biomass sorghum is produced primarily for structural carbohydrates but it does produce some non-structural carbohydrates, albeit at lower levels than either sweet sorghum or grain sorghum.