Sweet sorghum is an “opportunity crop” for resource-poor farmers. The plant looks much like grain sorghum, except that it is often taller (sometimes much taller, reaching up to 4 meters) and it accumulates a great deal of sugary juice in the stalk.
Long used as livestock fodder and silage (fermented fodder), sweet sorghum stalks can also be crushed to extract juice for ethanol production. The leftover crushed stalks (called “bagasse”) can be used as livestock feed.
Sweet sorghum grows far more quickly than grain sorghum, maize, or sugarcane but produces more biomass. Compared with other cereals, it is considerably more tolerant of environmental stresses such as drought, water logging and temperature extremes. The crop requires less moisture, which makes it a good fit for arid and semi-arid tropical production environments.
Grown primarily for its juicy stalks, farmers also value sweet sorghum grain, both as food and as a nutritious livestock feed. The grain contains high levels of iron and zinc (more than 70 ppm and 50 ppm, respectively), offering the potential for reducing micronutrient malnutrition.
Origins of the crop
Sweet sorghum, like grain sorghum, emerged in Northeast Africa, and was probably first domesticated in Ethiopia. Historical evidence that might pinpoint the crop’s geographic roots is unavailable. However, Northeast Africa is known to be a primary center of genetic diversity.
Where sweet sorghum is grown today
Sweet sorghum can grow almost everywhere grain sorghum is grown, and is now found in a number of developing and industrialized countries. In developing countries, the decision by farmers to produce sweet sorghum rests mainly on the farmers’ ability to market it for ethanol production and sweet syrups, or to use it as livestock feed in mixed crop/livestock farming systems.
Main threats to production
As with grain sorghum, various pests and diseases, as well as abiotic constraints, limit production. These include:
- Shoot fly, stem borer, head bug and aphid insect pests;
- Grain mold, anthracnose diseases, and leaf blight;
- Weed competition and (in Africa) the parasitic plant Striga spp.; and
- Abiotic stresses such as drought (especially terminal drought), high temperatures, acid soils (which are often associated with toxic levels of aluminum saturation) and low soil fertility (in terms of both macronutrients such as nitrogen and phosphorus, as well as micronutrients such as iron and zinc).
Current and future research
- Crop improvement research at ICRISAT has demonstrated that sweet sorghum has good genetic variability for stalk sugar content and juice volume, thus providing ample scope for increasing the crop’s sugar content, and therefore its ethanol yield.
- Research done by the Center and its partners has further demonstrated the sensitivity of sweet sorghum varieties to the environmental conditions in which they are grown. Scientists refer to this as Genotype x Environment (G x E) interaction, meaning that new cultivars need to be customized for different agro-ecological zones and seasons.
- Researchers have developed new sweet sorghum hybrids that produce more juice and sugar. The more productive of these can produce about 50 liters of ethanol per ton of stalk.
- ICRISAT scientists have shown that there are no food to fuel tradeoffs with sweet sorghum. In on-farm tests, improved sweet sorghum varieties yielded 1.5 to 2 t/ha of sugar while still producing 2 to 2.5 t/ha of grain. Such trials have also demonstrated that farmers who use cultivars and crop production technologies suited to their environments achieve yields that are 50-160% higher than they were getting with traditional, unimproved varieties.
- Post-harvest research has shown that sweet sorghum residue after crushing is valuable for feeding livestock. Feed blocks made from such residues are highly palatable, cost effective to produce, and improve yields of milk from cows and meat from sheep.
- As global climate change progresses, new areas for sweet sorghum are likely to open up. To capitalize on these opportunities, ICRISAT researchers are working on breeding new varieties and hybrids that are less sensitive to temperatures and day length, and that have genetic resistance to multiple pests and diseases. This will allow sweet sorghum to be grown in a wider range of environments.
- Research aimed at broadening the “harvest window” of sweet sorghum is also underway. The idea is to breed varieties that mature at different rates – some early, some medium, and some late maturing – that will help farmers provide ethanol producers with sugar-rich raw material over a longer period, and in so doing increase farm-level incomes.
- If cultivars with different maturities are grown in the same areas, however, insect pest populations are likely to build up and have a proportionately larger impact on the later-maturing varieties. ICRISAT researchers are therefore giving additional attention to insect resistance in late-maturing cultivars.