Regardless of the type of sorghum being grown, the basic growth and development of the crop is similar. Differences between grain sorghum, forage sorghum and energy sorghum are due to specific changes in a few very important traits within the species. Those traits include tillering and regrowth, plant height, stem type, maturity and grain yield potential.

All commercial sorghums have a fibrous non-rhizomatous root system. For a mature sorghum plant, roots can extend at least 1.5 m in all directions from the base of the plant. In addition, tillering is especially important in sorghum, but tiller characteristics and their desirability depend on which type of sorghum is being produced. Sorghum has the potential for both basal and nodal tillering. In general, profuse tillering is desirable in forage sorghum while reduced tillering is more desirable for both grain and energy sorghum. If ratoon crops are desired, then basal tillering is essential to facilitate regrowth after cutting or grazing. Increased tillering is also associated with reduced stem size [17]. Temperature and plant density also influence tillering potential [18].

Stem thickness and composition are additional defining characteristics of different sorghum types. In general, thicker stems are desirable in energy and grain sorghum while thinner stems are appropriate for forage sorghum. Stem thickness in sweet sorghum is highly variable, depending on regional preferences and the primary use of the cultivar (i. e., syrup, feed or ethanol production). While there is significant genetic variation for this trait, stem diameter is strongly influenced by management and environmental conditions. For example, planting density is the largest factor affecting stem diameter. Numerous studies show that plant density and stem thickness are negatively correlated: the greater plant density, the finer the stem [19].

In addition to stem diameter, stem morphology and sugar concentration in the stems also define sorghum type. Sweet sorghum cultivars have a very juicy stem while biomass sorghum typically has a pithier stalk. Juicy and pithy stemmed characteristics are both found in grain and forage sorghum lines. A single gene locus D was reported to control juiciness of the stem [20] but more recent analysis indicates that, while this gene is important, it is not the sole genetic factor influencing juiciness of the stalk [21]. Sugar concentration is critical for sweet sorghum but less so for other sorghum types. Sugar concentrations are also strongly influenced by both genotype and environment. For example, sugar concentrations typically peak in sweet sorghum approximately 20-30 days post anthesis and different genotypes with unique profiles of fermentable sugars have been identified [22].

Plant height is a primary factor in overall biomass yield potential; higher yields are correlated with taller plants [23,24]. However, there are limits to height as taller plants are more prone to both stalk and root lodging, so the benefits of height must be effectively balanced with the risks [25]. Sorghum height is easily manipulated; four major height genes were identified [26] and many modifiers of these genes have been subsequently described [27,28].

Of all sorghum traits, maturity is likely the most important because it influences so many agronomically important characteristics. Sorghum maturity is controlled by genes that are influenced by both day length (photoperiod sensitivity) and temperature [16].

Photoperiod sensitive sorghum cultivars require a defined length of darkness to induce panicle differentiation and development [15]. As sorghum was moved to more temperate climates and away from lower latitudes, photoperiod insensitive sorghums were necessary for the crop to produce grain before the growing season ended. Much of the early work to develop photoperiod insensitive sorghum in the United States was completed by producers who would identify and save seed of individual, early maturing mutants or segregants [26]. In these photoperiod insensitive sorghums, maturity is primarily influenced by temperature.

Six maturity genes have been described in photoperiod sensitive and insensitive sorghum and there are likely many more [29,30]. These loci interact to produce an array of matu­rities in all types of sorghum cultivars. Maturity is important in forage sorghum because forage quality tends to decrease once the crop flowers and starts to produce grain. This is particularly critical in sorghum-Sudan grass hybrids that are grown for hay production. Because of the relationship between maturity and quality, sorghum breeders have devel­oped photoperiod sensitive sorghum-Sudan grass hybrids and forage sorghum hybrids that maintain forage quality with yields similar to photoperiod insensitive hybrids [30].