The cultivated sorghums are all varieties or hybrids from S. bicolor L. Moench. The sorghum species is quite diverse but very consistent both genetically and botanically. All S. bicolor is a genetic diploid with base chromosome number of n = 10 and 2n = 2x = 20. While there is evidence that the crop is an ancient tetraploid, it is a functional diploid [2]. The genome size is larger than rice but substantially smaller than corn (Zea mays) or other grasses species and the complete genome has been sequenced [3]. In addition to S. bicolor, there are several additional Sorghum species, but none are systematically cultivated and many are perennial. Those species have variable growth habits and chromosome numbers but they do represent an additional source of genetic variability if needed. Of these, Sorghum halepense is likely the most infamous because it is the persistent and aggressive weed known as johnsongrass.

Botanically, sorghum is a member of the Panicodeae family. It is a typical grass with a deep and fibrous root system, primary culm and the capacity for both basal and axillary tillering. Reproductively, sorghum has a complete flower that is exerted through the top leaf sheath prior to anthesis. Sorghum species are predominantly self-pollinated but outcrossing does occur at rates between 2 and 30%; the precise amount of cross-pollination is a function of both genotype and environment [4]. While it is self-pollinated, sorghum is highly amenable to commercial hybrid seed production through the use of cytoplasmic male sterility systems. The crop is typically grown and managed as an annual but it is technically a perennial; once harvested, it will regrow unless environmental conditions (e. g., freezing temperatures) preclude further growth. Among commonly grown cereal crops, sorghum has a high water use efficiency and very good heat and drought tolerance. Plant maturity and height are highly variable and influenced by both the genetics of the species and the environment.

As an energy crop, sorghum is unique in that there are several types of sorghum that are and can be used for biofuel or bioproduct production, with the defining factor in these crops being the primary source of fermentable carbohydrates. Grain sorghum, long used as feed grain, is now a significant contributor to biofuel production in the United States [5]. Sweet sorghum, long used as a sweetener, is now being deployed in conjunction with sugarcane in sugar to ethanol conversion facilities and biomass sorghums produce significant quantities of structural carbohydrates. Different types of forage sorghums, commonly grown for animal grazing, hay or silage, are integrated into the biomass sorghums and occasionally, and sometimes incorrectly, into the sweet sorghum group.