Geography and distribution
Sorghum bicolor is widely cultivated in the drier areas of Africa, Asia, Australia, North and South America and Europe. It is also found in tropic, subtropic and warm-temperate regions.
The greatest diversity of cultivated and wild types of sorghum is found in north-eastern Africa where the crop was first cultivated in 5,000 to 3,000 BC.
Although the largest bulk producer today is the USA, about 90% of the area planted to sorghum lies in developing countries, mainly in Africa, and Asia where it is grown by subsistence farmers.
Overview: Sorghum bicolor is an annual with erect, robust stems which can grow up to 600 cm long.
Leaves: The leaf-blade base is broadly rounded and the leaf blades themselves can be up to one metre long.
Flowers: The inflorescence is a panicle (a highly branched inflorescence) 4–50 cm long and 2–20 cm wide. The spikelets (clustered units of flowers and bracts) are in pairs, the fertile spikelets being fixed directly onto the axis and the sterile spikelets on stalks. The sterile spikelets are well developed and as long as the fertile ones. The upper and lower glumes (empty bracts that enclose the florets) are of different sizes, the lower wider than the upper. Both can be of a leathery or papery texture. The flower contains two lodicules (small structures at the base of the stamens) the surface of which is covered in hair-like projections.
Fruits: The fruit is a caryopsis (a fruit in which the seed is fused to an outer wall) which is exposed at maturity.
Sorghum bicolor is cultivated in many parts of the world today. It is an important food crop in the semi-arid tropics and it is commonly cultivated for feeding livestock in the Americas and Australia.
The grain itself is quite small and is easy to prepare. It can be boiled, roasted or popped (like maize) and when ground into flour it can be used to make porridge, pancakes, dumplings or couscous. Sorghum flour is also used in the production of beers and other non-alcoholic beverages and can be used to make an adhesive in the manufacture of plywood.
Birds also enjoy the taste of sorghum so when bird pressure is high some farmers prefer to grow a variety of sorghum in which the grains are bitter on account of their high tannin content. Both birds and humans find this unprocessed sorghum variety unpalatable although, when cooked or fermented, a delicious dish can be made. In general, white grain is preferred for cooking and the reddish, bird-resistant grain is used for beer making.
In the southern United States, a sweet syrup is made from pressing the stems of sorghum to extract their juice. The sweet sorghum variety is also grown as an energy crop, producing ethanol for use as a biofuel. The roots of sorghum can be used as fuel for cooking.
Beyond the use of the grain as a food source, the entire plant can be used for forage, hay or silage. The stems are used for building, fencing, weaving, broom making and firewood. Industrially, it can be used for vegetable oil, waxes and dyes. The rich violet tones of sorghum dye are used to decorate the masks worn during certain dances by Yoruba people in Southern Benin and in south-western Nigeria.
Sorghum has a number of medicinal applications. In African traditional medicine seed extracts are drunk as a remedy for hepatitis, and decoctions of twigs with lemon against jaundice. Leaves and panicles are included in plant mixtures for decoctions against anaemia. The red pigment of some sorghum varieties is thought to have antimicrobial and antifungal properties and is also used as a cure for anaemia in traditional medicine.
The Salka people in Nigeria use sorghum in arrow poisons.
Crop wild relatives of sorghum
The most serious pest of sorghum is a parasitic plant, Striga asiatica. Nicknamed 'the witch weed', this pest can destroy 70-100% of the potential yield of staple crops in the semi-arid tropics. The good news is that varying degrees of resistance to the witch weed have been found in the wild relatives of sorghum and these resistance genes can be passed onto cultivated sorghum. Striga-resistant varieties of sorghum have thickened root walls which make it impossible for Striga to penetrate – without a host, the witch weed withers and dies.
The centre of origin of Sorghum bicolor is Africa, and Ethiopia is reported to contain the highest genetic diversity for this species. Asia is also a hotspot for sorghum diversity due to the early introduction of the crop there. The wild relatives of cultivated sorghum that grow in these centres of diversity are valuable sources of resistance to insect pests, diseases and other stresses such as drought and high temperatures. They can also be exploited to source traits that improve yield and food and fodder quality.
Many of the wild relatives of sorghum are under-represented in genebank collections, because they are rare or located in extremely isolated areas. In addition, many species are under threat due to habitat destruction, commercial agricultural practices and industrial activities.
The Millennium Seed Bank and the Global Crop Diversity Trust are engaged in a ten-year project, called 'Adapting Agriculture to Climate Change'. The project aims to protect, collect and prepare the wild relatives of 29 key food crops, including sorghum, so that they are available to pre-breeders for the development of new varieties that are more resilient to the effects of climate change.
Millennium Seed Bank: Seed storage
The Millennium Seed Bank Partnership aims to save plants worldwide, focusing on those plants which are under threat and those which are of most use in the future. Once seeds have been collected they are dried, packaged and stored at -20°C in Kew’s Millennium Seed Bank vault.
Ugandan tray made from the stems of sorghum.
Description of seeds: Average weight of 1,000 seeds = 14.3 g
Number of seed collections stored in the Millennium Seed Bank: Two
Seed storage behaviour: Orthodox (the seeds of this plant can be dried to a low moisture content without significantly reducing their viability which means they are suitable for long-term frozen storage)
Germination testing: Successful
This species at Kew
The Economic Botany Collection houses a number of sorghum artefacts, among them a Ugandan bowl made from stems of sorghum.