Sorghum bicolor (PROTA)

Plant Resources of Tropical Africa Introduction

List of species

General importance
Geographic coverage Africa
Geographic coverage World
Cereal / pulse
Stimulant
Carbohydrate / starch
Dye / tannin
Medicinal
Fuel
Ornamental
Forage / feed
Fibre
Food security
Climate change

distribution in Africa (planted)

panicles and spikelets of the 5 basic races: 1, bicolor; 2, caudatum; 3, durra; 4, guinea; 5, kafir. Source: PROSEA

plant habit

flowering plants

panicles

flowering panicle

field with fruiting plants

harvested panicles

girl carrying harvested panicles

stalks for dye extraction

dye extraction by rubbing the stalks

Sorghum bicolor (L.) Moench

Protologue: Methodus: 207 (1794).

Family: Poaceae (Gramineae)

Chromosome number: 2n = 20

Vernacular names

• Sorghum, sorgo, guinea corn, great millet, durra (En).
• Sorgho, gros mil, sorgho rouge (dye cultivars), sorgho des teinturiers (dye cultivars) (Fr).
• Sorgo, milho miúdo, massambala (Po).
• Mtama (Sw).

Origin and geographic distribution

The greatest diversity in both cultivated and wild types of Sorghum is found in north-eastern tropical Africa. The crop may have been domesticated in that region, possibly Ethiopia. Various hypotheses have been put forward as to when the crop was domesticated, from as early as 5000–3000 BC to around 1000 BC, but the latter period is more widely accepted now. From north-eastern Africa sorghum was distributed all over Africa and along shipping and trade routes through the Middle East to India. From India it is believed to have been carried to China along the silk route and through coastal shipping to South-East Asia. From West Africa sorghum was taken to the Americas through the slave trade. It was introduced into the United States for commercial cultivation from North Africa, South Africa and India at the end of the 19^th century. It was subsequently introduced into South America and Australia. It is now widely cultivated in drier areas of Africa, Asia, the Americas, Europe and Australia between latitudes of up to 50°N in North America and Russia and 40°S in Argentina. Sorghum types exclusively cultivated for the dye in the leaf sheaths can be found from Senegal to Sudan.

Uses

Sorghum is an important staple food, particularly in semi-arid tropical regions of Africa and Asia, and an important feed grain and fodder crop in the Americas and Australia. In the simplest food preparations, the whole grain is boiled (to produce a food resembling rice), roasted (usually at the dough stage), or popped (like maize). More often the grain is ground or pounded into flour, often after hulling. Sorghum flour is used to make thick or thin porridge, pancake, dumplings or couscous, opaque and cloudy beers and non-alcoholic fermented beverages. In Africa sorghum grain is germinated, dried and ground to form malt, which is used as a substratum for fermentation in local beer production. White grain is generally preferred for cooking while red and brown grains are normally used for beer making. Where bird pressure is high, e.g. around Lake Victoria, red and brown types rich in tannin may be grown for food instead of white types. In China sorghum is extensively distilled to make a popular spirit and vinegar. Sorghum grain is a significant component of cattle, pig and chicken feeds in the United States, Central and South America, Australia and China, and is becoming important in chicken feed in India. It requires grinding, rolling, flaking or steaming to maximize its nutritional value.

Several non-edible sorghum cultivars are exclusively grown for the red dye present in the leaf sheaths and sometimes also in adjacent stem parts. In Africa this dye is used particularly for goat-skin leather (e.g. in Nigeria), but also for mats, textiles, strips of palm leaves and grasses used in basketry and weaving, ornamental calabashes, wool (e.g. in Sudan), as a body paint and to colour cheese and lickstones for cattle (e.g. in Benin). A similar dye can be extracted from the grain refuse (glumes and grain wall) of several red sorghum cultivars grown for food or for beer-making. In Nigeria the red sorghum dyes were traditionally used by the Bunu, Aworo, Igbira and Okpella people for a fabric called ‘abata’, used as a funeral hanging, decorated with patterns made by thick threads added to the weft of the fabric. The fabrics in which the dominant colours were derived from sorghum were known as ‘ifala’. Sorghum is also used to provide the violet colours decorating the masks worn during certain dances by Yoruba people in southern Benin and in south-western Nigeria. In Côte d’Ivoire sorghum and other tannin-rich dyes are used in combination with mud to create the patterns of the painted cloths produced in the Korhogo region. The dye was formerly exported to Morocco where it was used in the leather industry. In China sorghum types with red panicles and leaf sheaths were also used for dyeing. In the 19^th century red sorghums were exported to Europe where the dye was known as ‘carmin de sorgho’. It was extracted by squeezing out the juice, which was then fermented. Used with wool or silk mordanted with tin or chrome, the result was a colourfast red-brown that was once known as ‘rouge badois’. ‘Durra red’, a similar product, was imported from India into the United Kingdom where the dye was known as ‘Hansen brown’ or ‘Meyer brown’. Recently the use of sorghum dye in hair dying products has been patented.

The stems of sweet sorghum types are chewed like sugar cane and, mainly in the United States, a sweet syrup is pressed from them. In North America and eastern Europe special types with very long, fibrous and few-seeded inflorescences, known as ‘broomcorn’, are grown to make brooms. Sorghum plant residues are used extensively as material for roofing, fencing, weaving and as fuel. The stems can be used for the production of fibre board. Danish scientists have made good panelling using stem chips of sorghum. The stover remaining after harvesting the grain is cut and fed to cattle, sheep and goats, or may be grazed. Some farmers grind harvested stover and mix it with sorghum bran or salt to feed livestock. Sorghum is also grown for forage, either for direct feeding to ruminants or for preservation as hay or silage. Sorghum flour is used to produce an adhesive in the manufacture of plywood. Sweet sorghum is suitable for the production of alcohol, while the bagasse is a suitable source of paper pulp for the production of kraft paper, newsprint and fibre board. Sorghum has various applications in African traditional medicine: seed extracts are drunk to treat hepatitis, and decoctions of twigs with lemon against jaundice; leaves and panicles are included in plant mixtures for decoctions against anaemia. The Salka people in northern Nigeria use sorghum in arrow-poisons. The red pigment is said to have antimicrobial and antifungal properties and is also used as a cure for anaemia in traditional medicine.

Production and international trade

Sorghum grain is the fifth most important cereal in the world after wheat, rice, maize and barley. In Africa it comes second after maize in terms of production. According to FAO estimates, the average world production of sorghum grain in 1999–2003 amounted to 57.7 million t/year from 42.6 million ha. The production in sub-Saharan Africa was 19.0 million t/year from 22.8 million ha. The main producing countries are the United States (12.0 million t/year in 1999–2003 from 3.2 million ha), India (7.6 million t/year from 9.8 million ha), Nigeria (7.6 million t/year from 6.9 million ha), Mexico (6.0 million t/year from 1.9 million ha), Sudan (3.4 million t/year from 5.3 million ha), Argentina (3.0 million t/year from 630,000 ha), China (3.0 million t/year from 840,000 ha), Australia (1.9 million t/year from 690,000 ha), Ethiopia (1.4 million t/year from 1.2 million ha) and Burkina Faso (1.3 million t/year from 1.4 million ha). In sub-Saharan Africa annual production increased from around 10 million t from 13 million ha in the early 1960s to about 20 million t from 25 million ha in the early 2000s.

Almost all sorghum traded on international markets is for use as livestock feed. Average world exports of sorghum in 1998–2002 amounted to 6.3 million t/year, almost all from the United States (5.6 million t/year). The main importers are Mexico and Japan. In tropical Africa most sorghum is grown for home consumption (except for beer production). In southern and eastern Africa malting sorghum for beer brewing has developed into a large-scale commercial industry, using about 150,000 t of sorghum grain annually. In Uganda commercial production of lager beer using sorghum instead of barley is becoming a great success (annual requirement of sorghum is 3000 t) and is very promising for other African countries. In Nigeria sorghum malting has become a major industry for lager and stout beer brewing and for malt beverages, using about 15,000 t of sorghum annually. In South Africa an instant breakfast cereal is made from sorghum that is similar in quality but much cheaper than wheat or maize products. Annual production is 12,000 t and is increasing steadily.

In West Africa small tied bundles of 4–6 leaf sheaths of sorghum dye cultivars are offered for sale on local markets (in the 1990s the price was about 150 CFA). In 1993 in Burkina Faso, the red pigment was successfully extracted chemically from sorghum leaf sheaths and offered for sale as dry powder on the world market.

Properties

The composition of sorghum grain per 100 g edible portion is: water 9.2 g, energy 1418 kJ (339 kcal), protein 11.3 g, fat 3.3 g, carbohydrate 74.6 g, Ca 28 mg, P 287 mg, Fe 4.4 mg, vitamin A 0 IU, thiamin 0.24 mg, riboflavin 0.14 mg, niacin 2.9 mg and ascorbic acid 0 mg. The essential amino acid composition per 100 g edible portion is: tryptophan 124 mg, lysine 229 mg, methionine 169 mg, phenylalanine 546 mg, threonine 346 mg, valine 561 mg, leucine 1491 mg and isoleucine 433 mg. The principal fatty acids are per 100 g edible portion: linoleic acid 1305 mg, oleic acid 964 mg and palmitic acid 407 mg (USDA, 2004). Sorghum grain is first limiting in lysine, then in methionine and threonine. Much of the protein in sorghum is prolamine ( 39–73%), which is poorly digestible. As a result, maximum available protein in sorghum grain is usually 8–9%. The tannin content of sorghum also affects its nutritional value. High- and low-tannin sorghum types are distinguished. High-tannin sorghum types (sometimes called ‘brown sorghums’, although the grain may also be white, yellow or red) have less nutritional value but have agronomic advantages, including resistance to birds, insects, fungi and decreased sprouting in the panicle. Sorghum types without a pigmented grain wall (‘white sorghums’) do not contain condensed tannins and have a nutritional value similar to that of maize. Decortication, parboiling, malting or steeping in alkali solutions significantly reduce the tannin content of sorghum grain. In general the endosperm accounts for 82–84% of the grain weight, the germ for 9–10% and the grain wall for 6–8%. The starch granules in the endosperm have a diameter of (4–)15(–25) μm. The starch normally contains 70–80% amylopectin and 20–30% amylose, although some types contain 100% amylopectin and others up to 62% amylose. The gelatinization temperature ranges from 68–75°C. Sorghum grain does not contain gluten and cannot be used for leavened products unless mixed with wheat.

The composition of the green plant varies according to age and cultivar but it normally contains 78–86 g of water per 100 g of fresh material. On a dry basis it contains per 100 g: protein 12 g, carbohydrate 40–50 g and fibre 20–30 g. The glycoside dhurrin occurs in the aerial parts of most sorghum. Dhurrin is hydrolyzed to hydrocyanic acid (HCN), which is highly toxic and can kill grazing animals. It is particularly concentrated in the young leaves and tillers and in plants that are suffering from drought. HCN content usually declines with age, reaching non-toxic levels 45–50 days after planting, and HCN is destroyed when the fodder is made into hay or silage.

The red pigment in sorghum dye cultivars is composed of anthocyanic compounds, particularly rich (95%) in the stable apigeninidin chloride (3-deoxyanthocyanidin) and tannins of the condensed proanthocyanidins group (producing phlobaphen reds). The red pigment in the sorghum leaf sheath makes up to over 20% of the dry weight. The role of the non-pathogenic fungus Bipolaris maydis in the production of apigeninidin in these cultivars deserves further research. Used without a mordant, the dye obtained from sorghum gives a dark red that is fairly colourfast and still much used in eastern Africa, particularly Sudan and Ethiopia, for dyeing leather, cotton and the grasses and reeds used for woven matting. Black colours are obtained with natron salt and iron mordants. From red sorghum grain the pigments apigenin, quercimeritrin, kaempferol glucosides, apigenidin glucosides, apigeninidin, luteolinidin and 7-O-methyl-luteolin-glucoside have been isolated. From the stem of red sorghum cultivars the constituents of the red dye were the anthocyanidin apigeninidin (17%) and the flavonoids luteolin (9%) and apigenin (4%). The anaemia curing property of the red pigment has been confirmed in tests with rats.

Description

• Annual grass up to 5 m tall, with one to many tillers, originating from the base or stem nodes; roots concentrated in the top 90 cm of the soil but sometimes extending to twice that depth, spreading laterally up to 1.5 m; stem (culm) solid, usually erect.
• Leaves alternate, simple; leaf sheath 15–35 cm long, often with a waxy bloom, with band of short white hairs at base near attachment, reddish in dye cultivars, auricled; ligule short, c. 2 mm long, ciliate on upper free edge; blade lanceolate to linear-lanceolate, 30–135 cm × 1.5–13 cm, initially erect, later curving, margins flat or wavy.
• Inflorescence a terminal panicle up to 60 cm long; rachis short or long, with primary, secondary and sometimes tertiary branches, with spikelets in pairs and in groups of three at the ends of branches.
• Spikelet sessile and bisexual or pedicelled and male or sterile, with 2 florets; sessile spikelet 3–10 mm long, with glumes approximately equal in length, lower glume 6–18-veined, usually with a coarse keel-like vein on each side, upper glume usually narrower and more pointed, with central keel for part of its length, lower floret consisting of a lemma only, upper floret bisexual, with lemma cleft at apex, with or without kneed and twisted awn, palea, when present, small and thin, lodicules 2, stamens 3; ovary superior, 1-celled with 2 long styles ending in feathery stigmas; pedicelled spikelet persistent or deciduous, smaller and narrower than sessile spikelet, often consisting of only two glumes, sometimes with lower floret consisting of lemma only and upper floret with lemma, 2 lodicules and 3 stamens.
• Fruit a caryopsis (grain), usually partially covered by glumes, 4–8 mm in diameter, rounded and bluntly pointed.

Other botanical information

Sorghum comprises 20–30 species. Sorghum bicolor belongs to section Sorghum, together with the 2 perennial species Sorghum halepense (L.) Pers. and Sorghum propinquum (Kunth) Hitchc. At present, Sorghum bicolor is mostly considered as an extremely variable crop-weed complex, comprising wild, weedy and cultivated annual types (classified as subspecies) which are fully interfertile. The cultivated types are classified as subsp. bicolor (synonyms: Sorghum ankolib Stapf, Sorghum caudatum Stapf, Sorghum cernuum Host, Sorghum dochna (Forssk.) Snowden, Sorghum durra (Forssk.) Stapf, Sorghum membranaceum Chiov., Sorghum nigricans (Ruiz & Pav.) Snowden, Sorghum subglabrescens (Steud.) Schweinf. & Asch., Sorghum vulgare Pers.) and they are subclassified into different races on the basis of grain shape, glume shape and panicle type. Five basic races and 10–15 hybrid combinations of 2 or more of these races are recognized and grouped into subsp. bicolor. A classification into cultivar groups would, however, be more appropriate. The 5 basic races are:

• Bicolor: the most primitive cultivated sorghum, characterized by open inflorescences and long clasping glumes that enclose the usually small grain at maturity. Cultivars are grown in Africa and Asia, some for their sweet stems to make syrup or molasses, others for their bitter grains used to flavour sorghum beer, but they are rarely important. They are frequently found in wet conditions.

• Caudatum: characterized by turtle-backed grains that are flat on one side and curved on the other; the panicle shape is variable and the glumes are usually much shorter than the grain. Cultivars are widely grown in north-eastern Nigeria, Chad, Sudan and Uganda. The types used for dyeing also belong here and are known as ‘karan dafi’ by the Hausa people in Nigeria.

• Durra: characterized by compact inflorescences, characteristically flattened sessile spikelets, and creased lower glumes; the grain is often spherical. Cultivars are widely grown along the fringes of the southern Sahara, western Asia and parts of India. The durra type is predominant in Ethiopia and in the Nile valley in Sudan and Egypt. It is the most specialized and highly evolved of all races and many useful genes are found in this type. Durra cultivars range in maturity from long to short-season. Most of them are drought resistant.

• Guinea: characterized by usually large, open inflorescences with branches often pendulous at maturity; the grain is typically flattened and twisted obliquely between long gaping glumes at maturity. Guinea sorghum occurs primarily in West Africa, but it is also grown along the East African rift from Malawi to Swaziland and it has also spread to India and the coastal areas of South-East Asia. Many subgroups can be distinguished, e.g. with cultivars especially adapted to high or low rainfall regimes. In the past the grain was often used as ship’s provisions because it stored well.

• Kafir: characterized by relatively compact panicles that are often cylindrical in shape, elliptical sessile spikelets and tightly clasping glumes that are usually much shorter than the grain. Kafir sorghum is an important staple across the eastern and southern savanna from Tanzania to South Africa. Kafir landraces tend to be insensitive to photoperiod and most commercially important male-sterile lines are derived from kafir type sorghum.

Hybrid races exhibit various combinations and intermediate forms of the characteristics of the 5 basic races. Durra-bicolor is found mainly in Ethiopia, Yemen and India, guinea-caudatum is a major sorghum grown in Nigeria and Sudan, and guinea-kafir is grown in East Africa and India. Kafir-caudatum is widely grown in the United States and almost all of the modern North American hybrid grain cultivars are of this type. Guinea-caudatum with yellow endosperm and large seed size is used in breeding programmes in the United States.

The wild representatives are classified as subsp. verticilliflorum (Steud.) Piper (synonyms: Sorghum arundinaceum (Desv.) Stapf, Sorghum bicolor (L.) Moench subsp. arundinaceum (Desv.) de Wet & J.R.Harlan): tufted annual or short-lived perennial, with slender to stout culms up to 4 m tall; leaf blade linear-lanceolate, up to 75 cm × 7 cm; panicles usually large, somewhat contracted to loose, up to 60 cm × 25 cm, branches obliquely ascending, spreading or pendulous. Wild types extend across the African savanna and have been introduced into tropical Australia, parts of India and the New World.

The weedy plants are usually considered as hybrids between subsp. bicolor and subsp. verticilliflorum, and named subsp. drummondii (Steud.) de Wet (synonyms: Sorghum × drummondii (Steud.) Millsp. & Chase, Sorghum aterrimum Stapf, Sorghum sudanense (Piper) Stapf); they occur in Africa wherever cultivated sorghum and its wild relatives are sympatric because they cross freely. These weedy plants occur in recently abandoned fields and field margins as a very persistent weed; stem up to 4 m tall; leaf blade lanceolate, up to 50 cm × 6 cm; panicle usually rather contracted, up to 30 cm × 15 cm, often with pendulous branches. A well-known forage grass, ‘Sudan grass’, belongs to this complex.

Growth and development

The optimum temperature for sorghum seed germination is 27–35°C. Seedling emergence takes 3–10 days. Panicle initiation takes place after approximately one third of the growth cycle. By this stage the total number of leaves (7–24) has been determined and about one-third of total leaf area has developed. Rapid leaf development, stem elongation and internode expansion follow panicle initiation. Rapid growth of the panicle also occurs. By the time the flag leaf is visible, all but the final 3 to 4 leaves are fully expanded and light interception is approaching its maximum; lower leaves have begun to senesce. During the boot stage, the developing panicle has almost reached its full size and is clearly visible in the leaf sheath; leaf expansion is complete. The peduncle grows rapidly and the panicle emerges from the leaf sheath. Flowering follows soon after panicle emergence, with the interval largely determined by temperature. Individual panicles start flowering from the tip downwards and flowering may extend over 4–9 days. Sorghum is predominantly self-pollinating; cross-pollination may range from 0–50%, but is on average about 5–6%. Grain filling occurs rapidly between flowering and the soft dough stage, with about half the total dry weight accumulating in this period. Lower leaves continue to senesce and die. By the hard dough stage, grain dry weight has reached about three-quarters of its final level. At physiological maturity, determined by the appearance of a dark layer at the hilum (where the grain is attached to the panicle), maximum dry weight has been achieved. Moisture content of the grain is usually between 25–35% at this stage. The time taken between flowering and maturity depends on environmental conditions but normally represents about one-third of the duration of the crop cycle. Further drying of the grain takes place between physiological maturity and harvest, which usually occurs when grain moisture content has fallen below 20%. Leaves may senesce rapidly or stay green with further growth if conditions are favourable. Early maturing sorghum cultivars take only 100 days or less, whereas long-duration sorghum requires 5–7 months. Sorghum follows the C₄-cycle photosynthetic pathway.

Ecology

Sorghum is primarily a plant of hot, semi-arid tropical environments that are too dry for maize. It is particularly adapted to drought due to a number of morphological and physiological characteristics, including an extensive root system, waxy bloom on leaves that reduces water loss, and the ability to stop growth in periods of drought and resume it when the stress is relieved. A rainfall of 500–800 mm evenly distributed over the cropping season is normally adequate for cultivars maturing in 3–4 months. Sorghum tolerates waterlogging and can also be grown in areas of high rainfall. It tolerates a wide range of temperatures and is also grown widely in temperate regions and at altitudes up to 2300 m in the tropics. The optimum temperature is 25–31ºC, but temperatures as low as 21ºC will not dramatically affect growth and yield. Sterility can occur when night temperatures fall below 12–15°C during the flowering period. Sorghum is susceptible to frost, but to a lesser extent than maize and light night-frosts during ripening cause little damage. Sorghum is a short-day plant with a wide range of reactions to photoperiod. Some tropical cultivars fail to flower or to set seed at high latitudes. In the United States, Australia and India the existence of mild photoperiod-sensitive to virtually insensitive cultivars has been recorded.

Sorghum is well suited to grow on heavy Vertisols commonly found in the tropics, where its tolerance of waterlogging is often required, but is equally suited to light sandy soils. The best growth is achieved on loams and sandy loams. Sorghum tolerates a range of soil pH from 5.0–8.5 and is more tolerant of salinity than maize. It is adapted to poor soils and can produce grain on soils where many other crops would fail.

In the floodplains of the Senegal and Niger rivers and in parts of Chad and Cameroon sorghum is sown in the early dry season when the water recedes, and the crop survives on residual moisture (‘culture de décrue’).

Propagation and planting

Sorghum is normally grown from seed. The 1000-grain weight is 13–40 g. Seed dormancy is not common in cultivated sorghum. A fine seedbed is preferable but is often not achieved. The seed is usually sown directly into a furrow following a plough, but can also be broadcast and harrowed into the soil. Optimum plant spacing depends on soil type and availability of moisture. In low-rainfall areas a population of 20,000 plants/ha is normal, in high-rainfall areas 60,000 plants/ha. For favourable conditions, spacings of 45–75 cm between rows and 15–25 cm within the row, resulting in 80,000–180,000 pockets per ha, are normal; for drier or less fertile conditions rows 1 m apart, or broadcasting at 6 kg seed per ha. A planting depth of 2.5–5 cm is common, and up to 25 seeds may be sown per pocket. Occasionally, seedlings are grown in a nursery and transplanted into the field early in the dry season, e.g. on the floodplains round Lake Chad in Africa (‘sorgho repiqué’). Sweet sorghum in the United States is also sometimes transplanted. Sorghum can also be propagated vegetatively by splitting tillers from established plants and transplanting them, a practice that is often used by small farmers to fill gaps. Sorghum may be harvested more than once as a ratoon crop, e.g. in locations with a bimodal rainfall pattern. Sorghum is often grown in intercropping systems with maize, pearl millet, cowpea, common bean, groundnut and bambara groundnut; in India also with pigeonpea.

Dye cultivars are never grown in large quantities. Farmers usually grow a few plants in or around their normal sorghum field or near the house.

Management

Sorghum does not compete well with weeds during the early stages of growth, and it is recommended that weeding be done early during the seedling stage. In tropical Africa weeding is commonly done once or twice with a hoe but sometimes animal-drawn or tractor-drawn cultivators are used. Where couch grass (Cynodon dactylon (L.) Pers.) is a problem more frequent weeding is necessary. Sorghum may be weeded by a combination of inter-row cultivation with animal-drawn implements and hand weeding within rows. Chemical weed control is almost non-existent among small farmers. Thinning can be carried out at the same time as hand weeding, or at intervals during the crop cycle, particularly where thinnings are used to feed livestock. Subsistence farmers rarely apply fertilizer, but application of farmyard manure or ash is common. In South Africa and the United States high doses of fertilizers are used in the production of sorghum. In tropical Africa sorghum is grown mainly as a rainfed crop, but it is grown under irrigation in Sudan. It is grown in rotations with maize, pearl millet, finger millet, cotton and other crops. It is often planted late in the rotation, as it tolerates low soil fertility. Under certain conditions decomposing roots of sorghum have an allelopathic effect on the subsequent crop, including sorghum.

Diseases and pests

Common seed and seedling rot diseases in sorghum are caused by soil- and seed-borne Aspergillus, Fusarium, Pythium, Rhizoctonia and Rhizopus spp. They are controlled by treatment of the seed with fungicides. Anthracnose (Colletotrichum graminicola) is common in hot and humid parts of Africa. Control measures include the use of resistant cultivars and crop rotation. Downy mildew (Peronosclerospora sorghi) may cause serious yield losses; the use of resistant cultivars and seed treatment are recommended. Smuts (Sporisorium spp.) are important panicle diseases. Loose and covered kernel smut are controlled by seed treatment with fungicides; head smut and long smut by using resistant cultivars and cultural practices such as crop rotation and removal of infected panicles. Grain mould is caused by a complex of fungal pathogens (predominantly Cochliobolus lunatus (synonym: Curvularia lunata), Fusarium spp. and Phoma sorghina) that infect the grain during development and can lead to severe discoloration and loss of quality. It is most severe in seasons when rains continue through the grain maturity stage and delay the harvest. Control measures include adjustment of the sowing date to avoid maturation during wet weather, and the use of resistant cultivars.

Important pests of sorghum in tropical Africa are shoot fly (Atherigona soccata) and stem borers (particularly Busseola fusca, Chilo partellus and Sesamia calamistis). Shoot fly larvae attack shoots of seedlings and tillers, and cause ‘dead hearts’. Stem borers cause damage in all crop stages. Damage by both shoot fly and stem borers can be reduced by early, non-staggered planting and seed or soil treatment with insecticides. Resistance to shoot fly is associated with low yield. Foliage pests include army worms (Spodoptera and Mythimna spp.); they are controlled by contact insecticides. Larvae of the sorghum midge (Stenodiplosis sorghicola, synonym: Contarinia sorghicola) feed on the young grains in the panicle. Damage can be limited by sowing early-maturing cultivars and avoiding staggered planting. Head bugs (Eurystylus and Calocoris spp.) suck on developing grains, resulting in yield loss, grain deformation and discoloration and infection by moulds. Guinea type sorghum is generally less affected.

In practice, control methods of diseases and pests are mainly preventative or cultural, including selection of optimum planting dates, seed treatment and crop rotation. Early sowing is particularly important as a mechanism to avoid large insect populations at times when plants are most susceptible to damage. High levels of host plant resistance are available for sorghum midge, but only low levels of resistance for the other pests. Chemical control of diseases and insect pests is rarely practised in tropical Africa.

Birds, especially Quelea quelea, cause important yield losses. Control measures include the choice of suitable planting dates, timely harvesting, bird scaring and the destruction of roosting and nesting sites. Brown sorghum is less preferred by birds than the tannin-free white sorghum.

Sorghum is very susceptible to damage by storage pests, the main ones being rice weevil (Sitophilus oryzae), flour beetle (Tribolium castaneum) and the grain moth (Sitotroga cerealella). Damage can be minimized by drying grain adequately before storage. Cultivars with hard grain also suffer less damage.

The parasitic weed Striga (especially Striga hermonthica (Del.) Benth., but also Striga asiatica (L.) Kuntze, Striga densiflora Benth. and Striga forbesii Benth.) has become a major constraint to sorghum cultivation, particularly in Africa, where severe infestations can lead to grain losses of 100% and land being abandoned. Striga can be controlled by cultural methods such as rotation with trap crops or with crops that are not susceptible (e.g. groundnut, cotton or sunflower), rigorous removal of the weeds before flowering and application of nitrogen fertilizer and herbicides. A few sorghum cultivars that are resistant or tolerant to Striga have been identified.

Harvesting

Sorghum is usually harvested when the grain moisture content has fallen below 20%, and the grain has become hard. Harvesting is done by hand using a knife to cut the panicles, which are temporarily stored in sacks before being taken to the threshing floor for further drying to a moisture content of 12–13%. Alternatively, the whole plant is cut or pulled up and the panicle removed later. Combine harvesting is possible, but many small farmers cannot afford to buy the machinery. In South Africa combine harvesting is more common.

For dye production, leaf sheaths are harvested when the plant comes to maturity, about 4–6 months after sowing. They can be used immediately or dried and stored.

Rainfed forage sorghum is usually cut only once, soon after flowering. Forage sorghum crops grown under more favourable conditions, often with irrigation and high levels of fertilizer, can be harvested and then left to regrow (ratoon).

Broomcorn is harvested by hand as mechanical harvesters are not available. Sweet sorghum is harvested when the seed is in the soft dough stage when the sugar content of the stalk is highest.

Yield

Average sorghum grain yields on farmers’ fields in Africa are as low as 0.5–0.9 t/ha because sorghum is often grown in marginal areas under traditional farming practices (low inputs, traditional landraces). Under favourable conditions sorghum can produce grain yields up to 13 t/ha. In South Africa, with intensive agricultural practices and improved cultivars, average commercial yield was 2.3 t/ha in 2001. In China, where sorghum is grown with high levels of inputs, yield averages 3.6 t/ha and in the United States 3.8 t/ha.

Forage yields from single-cut cultivars and hybrids can reach 20 t/ha of dry matter. Multi-cut cultivars and hybrids usually give only slightly higher total yields but produce better quality forage. Sweet sorghum yields about 1000 l syrup per ha in the United States. Average broomcorn yields are 300–600 kg/ha, enough to make 350–800 brooms.

Handling after harvest

The harvested grain of sorghum is usually sun-dried, often in the panicle. Panicles, particularly those to be retained for seed, may be stored hanging from the ceiling of kitchens over cooking fires where the smoke helps to deter insect attack. Alternatively, the heads may be threshed after drying and the grain stored in granaries, above or below ground, designed to prevent insect attack.

Traditional food preparation of sorghum is quite varied. The whole grain may be ground into flour or decorticated before grinding to either a fine particle product or flour which is then used in various food products. To prepare porridge, water is boiled and sorghum flour is gradually added until the desired consistency of the paste is reached. Regular stirring is needed to mix the contents thoroughly. Another simple form of sorghum food preparation is to boil the grain before or after decorticating. To make beer, sorghum grain is germinated, dried, pounded into flour and mixed with water and left to ferment in a warm place for some days. To make the non-fermented drink ‘mageu’ in Botswana and South Africa, milled sorghum malt is mixed with water and kept at room temperature for 2–3 days. Occasional stirring may be necessary.

In a traditional method of dyeing hides with sorghum dye in West Africa, a watery extract of wood ashes, preferably from the wood of Anogeissus leiocarpa (DC.) Guill. & Perr., is prepared and allowed to stand for 3–4 hours. The major active compound of the lye is potassium- or sodium carbonate. The red leaf sheaths are pulverized and placed in a large vessel in which the dyeing is carried out. From time to time a little lye is added and diluted with plain water as desired, obtaining a crimson liquid. The tanned hide that has been dressed with oil is folded with the tanned side outwards, the hide is immersed for about two minutes in the dye bath, wrung out and shaken. Alternatively, the dye liquid is painted on the tanned surface with the fingers or a brush. The hide is then rinsed in cold water acidulated with lime juice or tamarind pulp. After the hide has been dried, the process is completed by rubbing the hide with a smooth stone on a wooden block. It is estimated that 1.25 l of dye bath is sufficient for about 6 skins of medium size. Another recipe uses about 30 leaf-sheaths of sorghum, about half a spoonful of soda, a handful of ‘sant’ pods (Acacia nilotica (L.) Willd. ex Delile) or 2 handfuls of chips of mangrove bark, 2 spoonfuls of palm oil and 1.5 l of water. These are all mixed together and boiled, the juice of 5 or 6 limes added, and the liquid is left to simmer for 2 hours. It is then ready for application on the skin by brushing or rubbing.

To obtain a dye of constant high quality, a laboratory extraction technique has been designed in Burkina Faso. Sorghum leaf sheaths are crushed into fine particles, a solvent is added in an acid or basic medium (both give similar results) and a red liquid is produced. By addition of an acid the dyestuff is precipitated and is centrifuged off. The end product is a fine, burgundy-red powder with an apigeninidin concentration of 50–60%, ready for use as a dye. Pure apigeninidin can be obtained by further processing of the powder.

Forage sorghum can be fed to livestock while still green or can be stored in various ways for later use. The forage is often dried and stacked or can be made into silage. Stover left after harvest of grain is often grazed by animals.

Genetic resources

A major collection of sorghum germplasm is maintained and distributed to interested researchers by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, India. The collection extends to over 36,000 accessions from all the major sorghum-growing regions of the world (90 countries). Large germplasm collections of sorghum are also held in the United States (Southern Regional Plant Introduction Station, Griffin, Georgia, 30,100 accessions; National Seed Storage Laboratory, Fort Collins, Colorado, 10,500 accessions) and China (Institute of Crop Germplasm Resources (CAAS), Beijing, 15,300 accessions). In tropical Africa large germplasm collections of sorghum are held in Zimbabwe (SADC/ICRISAT Sorghum and Millet Improvement Program, Matopos, 12,340 accessions), Ethiopia (Institute of Biodiversity Conservation (IBC), Addis Ababa, 7260 accessions), Kenya (National Genebank of Kenya, Crop Plant Genetic Resources Centre, KARI, Kikuyu, 3410 accessions) and Uganda (Serere Agricultural and Animal Production Research Institute, Serere, 2635 accessions).

Breeding

The main objectives in sorghum breeding include high grain yield, white grain for human consumption with improved nutritional value and processing quality, and red or brown grain for feed purposes and brewing. In many countries the emphasis is on producing cultivars which combine high grain yield with high stover yields because of the importance of the residues as animal feed. Incorporation of resistance to major yield-limiting diseases and pests, and tolerance of abiotic stresses are also of high priority. Resistance to grain moulds and other diseases as well as to insect pests such as head bugs and sorghum midge has been identified. High-yielding improved cultivars of sorghum are available in most of the main producing countries. These include cultivars and hybrids produced using cytoplasmic male sterility. Compared to traditional landraces they have a weak photoperiodic response and they are less hardy, less tall, with a lower grain quality but a higher yield potential. Striga -resistant cultivars have been released in Africa and India, e.g. ‘Framida’ in Ghana and Burkina Faso. Cultivars resistant to grain mould have also been released. Special cultivars with high biomass production and good forage quality are bred for animal feed. Modern sorghum cultivars predominate in the Americas, China and Australia, but in Africa they occupy probably less than 10% of the area under sorghum. In India about 50% of the sorghum area is sown to modern cultivars and 50% to traditional landraces.

The sorghum genome is relatively small (about 760 Mbp) compared to that of maize (about 2500 Mbp), and construction of a physical genome map is in progress. Several genetic linkage maps have been developed, mainly based on RFLP markers. Various genes have been tagged, e.g. genes associated with head smut resistance, leaf blight resistance and shattering. Many QTLs have been mapped, including those associated with plant height, tillering, seed size, drought resistance and rust resistance. In-vitro plant regeneration has been achieved from calli derived from young leaf bases, shoot apices, immature inflorescences and immature embryos. Protocols have been developed for the production of stably transformed sorghum plants using microprojectile bombardment or Agrobacterium-mediated transformation, but the efficiency is generally low, especially with the former technique.

Prospects

Sorghum is a hardy, drought-tolerant crop with a high potential yield, which plays an important role in tropical Africa and elsewhere, especially as source of food and fodder, but also for a range of other uses, including as a source of dye. Sorghum has lost part of its traditional area in tropical Africa to maize, which yields better in more favourable environments, is less liable to bird damage and easier to process. It is to be expected, however, that sorghum will remain an important food security crop in less favourable environments in tropical Africa. Important problems in sorghum cultivation to be addressed by research and breeding activities are the large yield losses caused by parasitic weeds (especially Striga hermonthica), anthracnose, downy mildew, grain moulds, sorghum midge and stem borers. Improved sorghum cultivars are not widely grown in tropical Africa, and the improvement of seed supply systems should accompany sorghum improvement programmes in this region. Demand for sorghum for non-traditional uses is likely to increase. In particular, the use of sorghum as a feed grain, already well established in many industrialized countries, is likely to become more common in developing countries. However, sorghum faces strong competition from maize in the international feed grain market. Similarly, as increased affluence results in increased demand for meat and dairy products, the use of sorghum as a forage crop in intensive production systems in many tropical regions is likely to expand. The use of sorghum as a raw material for industrial processes will also increase. Research should focus on innovations that are likely to reduce the costs of production of sorghum. This should include research to increase yield levels of available cultivars, and to improve agronomic practices. Emphasis should be placed on enhancing resistance to the main biotic and abiotic stresses and on production of cultivars richer in high quality proteins.

Sorghum dye may profit from the trend of increasing use of natural colourants in foods and cosmetics. Rising harvesting costs of broomcorn in North America and Europe may offer possibilities for expanding this commodity in Africa.

Major references

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Sources of illustration

• Stenhouse, J.W. & Tippayaruk, J.L., 1996. Sorghum bicolor (L.) Moench. In: Grubben, G.J.H. & Partohardjono, S. (Editors). Plant Resources of South-East Asia No 10. Cereals. Backhuys Publishers, Leiden, Netherlands. pp. 130–136.

Author(s)

• T.V. Balole, Botswana College of Agriculture, Private Bag 0027, Gaborone, Botswana

• G.M. Legwaila, Botswana College of Agriculture, Private Bag 0027, Gaborone, Botswana

Correct citation of this article

Balole, T.V. & Legwaila, G.M., 2006. Sorghum bicolor (L.) Moench. In: Brink, M. & Belay, G. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. Accessed 18 December 2024.

• See the Prota4U database.