Sorghum - Bioenergy Crop Plants - page 492

under marginal environmental conditions, especially under hot and dry conditions. Furthermore,
sweet sorghum is frequently grown in environments that are normally too harsh for other C 4 plants.
Also important is the amount of energy used to produce ethanol. Historically, for each unit of
energy it took to plant and harvest a crop and process it into ethanol, the fuel returned 0.92 units of
energy. Ethanol had a negative “energy balance” of 1 unit in for 0.92 units out (1:0.92). However,
steady improvements have been made in corn yield, harvesting, and ethanol processing efficiency.
The latest studies show corn ethanol boasts a positive energy balance of 1:1.25—a 25% net increase
in net energy (Farrell et al. 2006). Today, corn ethanol is made by converting the starch in corn to
sugars and then into alcohol by a fermentation process. Sugarbeets ( Beta vulgaris L.) are a better
ethanol source, producing nearly 2 units of energy for every unit used in production. However,
sugarcane is by far the most efficient of the current feedstocks, yielding more than 3 units as much
energy as is needed to produce the ethanol derived from it (Hopkinson and Day 1980). Sweet
sorghum's positive energy balance, with a ratio of 1:3, is comparable to that of sugarcane (Worley
et al. 1992). Given their positive energy balances and higher yields, it makes more sense to produce
ethanol from sugar crops than from starchy grains.
Switchgrass ( Panicum virgatum ) has gained a great deal of attention as a biomass crop in
North America. However, establishment issues and relatively low annual dry matter yields
suggest other crops may be better suited for cellulosic ethanol production in Iowa, Nebraska,
and the surrounding states. There have been multiple reports of switchgrass biomass yields from
the north-central U.S. states. Second-year switchgrass dry matter at four Nebraska locations was
1.6-7.3 Mg/ha (Schmer et al. 2006). Annual yields of fully established switchgrass swards were
13-21 Mg/ha at Mead, NE and 6.5-11.0 Mg/ha at Arlington, WI (Casler et al. 2004). Switchgrass
dry matter yields at Ames, IA were 4.5-14.3 Mg/ha depending on the genotype (Hopkins et al.
1995). Biomass production on Conservation Reserve Program land in South Dakota totaled
3-4 Mg/ha with 56 kg N/ha (Mulkey et al. 2006). Biomass yields of switchgrass fertilized with
120 kg N/ha and harvested at maturity stages R3-R5 averaged 10.5-11.2 Mg/ha at Mead, NE and
11.6-12.6 Mg/ha at Ames, IA (Vogel et al. 2002). The average dry matter yield of 20 switchgrass
populations grown in southern Iowa was 9.0 Mg/ha (Lemus et al. 2002). In comparison, final dry
matter yields of grain-type winter triticales in Iowa were 8-16 Mg/ha with no more than 33 kg N/ha
(Schwarte et al. 2005).

19.7 varIetIes oF sWeet sorGhum
Variety selection is an important decision in sweet sorghum production. Improved varieties have
been developed in recent years at the U.S. Sugar Crops Field Station near Meridian, MS. Seed of
older varieties originating at other places may still be available in some areas. Important varieties
are described (Table 19.2).

19.8 BIoenerGy-related traIts

19.8.1 f lowEring and m aturity
Sorghum is a short-day species and requires short days (10-11 h) and long nights to stimulate the
reproduction phase. Quinby (1967) reported that four genes influenced inheritance of duration of
growth in sorghum. Manipulation of delayed flowering is an important trait to obtain a higher total
biomass. Photoperiods longer than 11 h promote the vegetative growth. Most U.S. collection is
sensitive to the photoperiod. Efforts were made in the early 1960s in Texas to convert more than
500 unique sorghum lines to photoperiod-insensitive lines to enhance the genetic variability Grain
sorghum requires less water than corn, under low to modest yield conditions, and it is an alternative
to corn in production environments with frequent severe water deficits (Carter et al. 1989; Bennett
et al. 1990; Maman et al. 2004; AFRIS-FAO 2006; Wikipedia 2006).