Sorghum - Bioenergy Crop Plants - page 489

factors, including ethanol yield, efficiency of conversion, and quality of the “distiller's grain” (grain
residue and yeast mass remaining after the fermentation process).
Sorghum has the potential for being used in the production of bioindustrial products, including
bioethanol. Sorghum is a starch-rich grain with similar composition to maize, and, as with all
cereals, its composition varies significantly because of genetics and environment (Rooney and
Serna-Saldivar 2000). Starch ranges of 60-77% and 64-78% have been reported for sorghum and
maize, respectively (Shelton and Lee 2000). As such, sorghum grain would be appropriate for use
in fermentation similar to the use of maize for the production of bioethanol. Its use may be of
particular benefit in countries where rainfall is limiting and maize does not grow well. With regard
to the United States, approximately 95% of the bioethanol is currently produced from maize starch,
primarily in the maize-growing regions. Sorghum production in the United States in 2004 was
11.6 million t (http://faostat.fao.org), equivalent to approximately 457 million bushels, and 10-20%
of those were used for ethanol production (http://www.sorghumgrowers.com). In the same year
in the United States, 3.4 billion gal of ethanol were produced from 1.22 billion bushels of grain
(http://www.ksgrains.com/ethanol/useth.html). From this, it may be calculated that 1.2-2.3 million
metric t sorghum was used for ethanol production, 3.7-7.5% of the grain for ethanol production
was sorghum, and 0.13-0.25 billion gal of ethanol originated from sorghum. Although significant
research into the production of ethanol from maize grain has been conducted, comparatively little
research has been done on the conversion of sorghum grain into bioethanol.
19.4 ForaGe sorGhums
Forage sorghum differs from grain sorghum primarily in utilization. Forage sorghum is an important
annual forage source in the Midwestern and Plains regions of the United States and can be planted
later than maize ( Z. mays L.) to provide fodder for stock between late spring and autumn. The crop
may be cut only once (single cut) or several times (multicut) during the growing season, whereas
maize can be cut only once. It uses water more efficiently, yields greater biomass, and provides
an acceptable yield when exposed to drought. These include sudan grass and tunis grass and are
used for pasture and forage. Forage sorghums ranged from 2- to 5-m tall, and whole-plant yields
ranged between 3.1 and 10.1 t of dry matter per acre. They are annuals and grow quickly. They are
generally used for summer pasture. Johnson grass, perennial grass sorghum, is considered a pest
when out of control, but it makes an excellent hay and cattle feed. It is important to remember that
forage-sorghum varieties vary widely with respect to agronomic characteristics.
Sweet sorghum is one of the many types of cultivated sorghum, characterized by the high sugar
content in its stem juice. Some lines attain juice yields of 78% of total plant biomass and contain
15-23% soluble fermentable sugar (comparable to sugarcane). The sugar is composed mainly of
sucrose (70-80%), fructose, and glucose. Most of the sugars are uniformly distributed in the stalk,
whereas approximately 2% are in the leaves and inflorescences (Vietor and Miller 1990). Even in
dry climates, sweet sorghum can yield high levels of fermentable sugars, together with grain and
lignocellulosics (Gnansounou et al. 2005). Stalks of sweet sorghum contain fermentable sugars
capable of producing 400-800 gallons of ethanol per acre (Reddy et al. 2008), which is comparable
to ethanol yield from corn grain (assuming an average irrigated corn yield of 170 bushels/acre
results in 470 gal of ethanol); however, the conversion of sweet sorghum into ethanol does not
require the energy-intensive steam cooking step necessary to produce ethanol from corn grain.
Sweet sorghum reproduces by seed and produces tillers, but it has no rhizomes. It is a perennial
grass under tropical conditions, but it is winter-killed in areas where frost occurs. Some sweet
sorghum cultivars are grown for syrup production, whereas others are grown for forage (silage).
Sweet sorghum is adapted to widely differing climatic and soil conditions, rendering it ideally suited
as a biofuel crop for marginal land production. Utilization for bioenergy conversion processes is
higher for sweet sorghum compared with other crops because it produces high biomass, fermentable
carbohydrates yields, and a small amount of grain.