18 February 2008

7 minute read

Introduction

The term sweet sorghum is used to describe varieties of sorghum (Sorghum bicolor (L.) Moench), a summer annual, that has a high concentration of soluble sugars in the plant sap or juice. This crop is attractive because of the easy accessibility of readily fermentable sugars combined with very high yields of green biomass.

Figure 1. Occurrence of Sorghum bicolor in the U.S. (CREDITS: http://plants.usda.gov/java/profile?symbol=SOB12)

In all varieties, the primary carbohydrate is sucrose, with variable amounts of reducing reducible sugars and starch. Similar to sugarcane, the sap of sweet sorghum is extracted by milling. Once extracted, the sugars from sweet sorghum can be easily fermented to produce ethanol. Other products from sweet sorghum include syrup, molasses, and crystal sugar.

Current Potential for Use as Biofuel

Sweet sorghums have generated interest as a feedstock for ethanol production since the 1970s. Juice from sweet sorghum can be converted to ethanol using currently available, conventional fermentation technology (similar to ethanol produced from sugarcane juice in Brazil). The bagasse (crushed stalks) that remains after removal of the juice can be burnt to generate electricity or steam as part of a co-generation scheme. Additionally, the bagasse could be utilized as a feedstock if the technology for cellulosic ethanol production becomes viable on a commercial scale. Typically, sweet sorghum varieties have low grain yield, but recently varieties with more balanced grain/sugar production have been developed in China and India. These varieties can be used as a dual-purpose crop, whereby the grain is harvested for human or animal consumption, and the sugars are fermented to ethanol. Alternatively, these varieties can be used as a dedicated bioenergy crop, where both the sugars and the grain are used for ethanol production.

Figure 2. Sweet Sorghum planted in Florida

Biology of Sweet Sorghum

Sorghum is an annual crop with considerable variability in growth characteristics. Heights range from 1.5 ft for grain sorghum types to over 16 ft tall for sweet and forage types. The thickness of stalks also varies, ranging between 0.5 to 1.5 inches. Brace or prop roots often grow from the lower nodes. Juice content of stalks at maturity is typically lower in grain and forage types compared to sweet types. Seeds are produced by self-pollination from a panicle that emerges at the top of the plant and contains both the male and female inflorescences. Sorghum seeds are small, round and may be white, yellow, brown, or red in color. Although sorghum is predominately self-pollinating, hybrids and crosses can be produced using male-sterile plants as the maternal parent. After harvesting the stalks, most varieties will regrow or ratoon. The ability to form a ratoon enables multiple harvests per season in certain environments although yields typically decrease in ratoon crops Sugar content in the juice increases with maturity, and is low prior to seed development.

Production

Sweet sorghum is a warm-season crop that tolerates drought and high-temperatures better than many crops, but it does not grow well under low temperatures. Optimal planting times in Florida will vary between locations, but soil temperatures at planting should be above 65 °F. Late planted crops will mature more rapidly, but should be planted early enough to ensure that the crop matures before the first expected killing frost.

Proper variety selection will play a large role in the success of sweet sorghum production for ethanol. The ideal variety for a particular location should produce high yields with minimal inputs, have a high percentage of high quality and easily extractable juice, be disease and insect tolerant, and tolerate both drought and wet conditions.

Sweet sorghum can be produced in a wide variety of soil types, but yields are typically highest in deep, well-drained soils with good fertility. Sorghum grown in shallow soils or soils very low in organic matter may be more prone to drought stress. Although sorghum is more tolerant of drought stress than many other crops, ample moisture during the growing season is important for good yields of stalks and juice.

Soil tests should be taken to determine soil fertility requirements for sweet sorghum. Nitrogen typically has the greatest impact on yields and will likely be needed on most soils. Lime should be applied to soils with a pH below 6.0 to correct soil acidity. In Florida, minimum of 80 lbs nitrogen (N) per acre would be needed for moderate yields and soil test recommendations call for 150 lbs of N and 50 lbs of both P2O5 and K2O2 for soils with medium fertility levels (http://edis.ifas.ufl.edu/SS163). Sweet sorghum is typically seeded in widely spaced rows (30-40 inches) using a corn planter. The ideal seeding rate for most sweet sorghum varieties is 3-4 seeds per linear ft of row with a final stand of 2-3 plants per linear ft of row. If plant populations are too high, the canes will be spindly and contain less juice than an equal tonnage of larger diameter canes.

Currently, the only commercially viable harvest method for sweet sorghum is removing the entire crop with a forage harvester and transporting it to a mill/ethanol facility. Using this method, transportation costs and proximity to the mill/ethanol facility will play a large role in determining where sweet sorghum production is profitable. Several research groups have developed prototype harvesters that extract the juice and leave the bagasse in the field, but it is unclear if this technology will become commercially viable.

Potential Yields

Sweet sorghum yields vary considerably depending on the varieties/hybrids that are used, the location (soil, water, climate, pests and diseases), inputs, and production practices. When considering sweet sorghum for ethanol production via conventional fermentation, biomass yield, juice yield, and sugar production per acre are the most important yield components. The concentration of soluble sugars in sorghum ranges widely depending upon variety. For example, some forage varieties have some sweet sorghum parental background to increase the palatability and energy value of the feed. While these forage types produce high amounts of biomass and some sucrose, they typically contain much less sucrose than dedicated sweet sorghum types (up to 15%). At present, there are limited data available on sweet sorghum yields and production practices in Florida. In preliminary small plot trials conducted on organic soils at the Everglades REC in Belle Glade, FL, yields ranged from 17 to 36 tons of green biomass per acre. The best performing varieties were Keller and Theis, which produced 1.4 and 1.3 tons sugar per acre, respectively. The following information is based on data collected from multiple sources, and -- because of the wide range of genetic variability, production practices, and growing environments in which sweet sorghum is produced -- it should not be assumed that these yields are typical of what will occur when producing sweet sorghum in Florida. Biomass yields of sweet sorghums can range from 8 to 48 tons per acre and juice content ranges from 65 to 80%. The combined sugar content of the juice varies between 9 and 15%. Sugar yields vary from 1.6 to 6.9 tons per acre. The bagasse and leaves make up the remainder of the wet biomass. The bagasse represents approximately two-thirds of the dry matter. Fermentation of the sugar in the juice yields between 400 to 600 gallons of ethanol per acre.

Production Challenges

The costs associated with transportation of the crop to the mill will be the major limiting factor for where sweet sorghum can be profitably grown. Varieties that have higher sugar contents per ton of biomass will be more efficient to process and haul to the mill.

Currently, there is a limited number of varieties for which seed is commercially available. If sweet sorghum is widely and rapidly adopted as an energy crop, seed may become difficult to obtain. Disease and insect problems may also limit yield potentials because there are no sweet sorghum varieties that have been specifically bred for Florida growing conditions.

Estimated Production Costs

Currently, sweet sorghum is not produced in Florida on a commercial basis, so there is limited information on production costs. However, grain and silage/forage sorghum are produced in north Florida and their production costs are likely similar. Information can be found at: (http://nfrec.ifas.ufl.edu/Hewitt/budgets.htm).

Environmental Concerns

Compared to many other crops, sweet sorghum has high water and nutrient use efficiencies and is considered environmentally sustainable. Unlike some proposed high biomass energy crops, sweet sorghum is not a threat to become an invasive weed in Florida.

Summary

It should be noted that there are no commercial facilities converting sweet sorghum to ethanol currently operating in Florida. Consequently, growing sweet sorghum for purposes other than research or a green manure crop is not recommended unless you have a legally binding contract with an ethanol facility.

January 2008