Throughout the world, sorghum is known and grown for its ability to use water efficiently and to maintain productivity during drought [55]. This is important because biomass pro­duction from energy crops is expected to be rainfed. The water use efficiency and/or drought tolerance of sorghum is due to a number of morphological and physiological traits [56]. As would be expected, there is significant variation in the Sorghum genus for drought tolerance among the types of sorghum (i. e., energy, grain, or sweet) and the genotypes within each group [57]. This variation has been used by breeders to enhance drought tolerance of grain sorghums [58] and has created interest in deploying it in energy sorghum cultivars as well.

There are differences in drought tolerance between sweet sorghum and energy sorghum that are related to maturity of the two groups [1]. In essentially all crops, reproductive growth, from the initiation to completion, is more sensitive to water stress than vegetative crop growth [59]. Since energy sorghum does not initiate reproductive growth until very late in the season (and in some cases, not at all), it has an inherently greater level of tolerance to water stress during the growing season than either grain or sweet sorghum cultivars. When drought occurs, energy sorghum can essentially become dormant and resume growth when moisture is available. Because sweet and grain sorghum genotypes flower, post-anthesis drought tolerance (i. e., “stay-green”) is critical in these types [60]. Over the past 30 years, the stay-green trait has been integrated into many grain sorghum hybrids and cultivars. It is much less common in sweet sorghum because there has been a limited breeding effort in sweet sorghum until very recently. Given that stay-green is associated with increased non-structural carbohydrate accumulation in the stalk [61] and greater tolerance to charcoal rot lodging [62], it is likely that there will be little or no detrimental effect to introgression of this trait into sweet sorghum.