Sorghum bicolor (PROSEA)
Sorghum bicolor (L.) Moench
- Protologue: Methodus pl.: 207 (1794).
- Family: Gramineae
- Chromosome number: 2n= 20
- Holcus bicolor L. (1771),
- Andropogon sorghum (L.) Brot. (1804),
- Sorghum vulgare Pers. (1805).
- Sorghum, sorgo, guinea corn (En)
- Sorgho (Fr)
- Indonesia: cantel, jagung cantel
- Malaysia: jagung, jagong
- Philippines: batad, batag (Tagalog), bukakau (Ilokano)
- Laos: khauz fa:ngz
- Thailand: khao-panghangchang (northern), samutkhodom (central), mutkhodom (southern)
- Vietnam: miền, cỏmia hai mầu.
Origin and geographic distribution
The greatest variability in cultivated and wild sorghum is found in north-eastern Africa. It is thought that the crop was domesticated in Ethiopia by selection from wild sorghum types (S. bicolor (L.) Moench subsp. verticilliflorum (Steud.) Piper, synonym: S. arundinaceum (Desv.) Stapf), between 5000 and 7000 years ago. It was probably distributed from this centre of origin along shipping and trade routes through the Middle East to India at least 3000 years ago. From there, it is thought to have been carried to China along the silk route and through coastal shipping to Burma (Myanmar) and other parts of South-East Asia. One distinctive group of sorghums, amber cane sorgos, is found in India, Burma (Myanmar) and coastal areas of China and Korea. These are quite different from the sorghums of mainland China. Glutinous-starch sorghums are found in Burma (Myanmar), Thailand and other parts of South-East Asia where glutinous starch is preferred, and these types were presumably selected locally by farmers. Sorghum was first taken to the Americas through the slave trade from West Africa. It was introduced into the United States for commercial cultivation from North Africa, South Africa and India during the latter years of the 19th Century. It was subsequently introduced into South America and Australia, where it has become an established grain and fodder crop. It is now widely distributed in drier areas of Africa, Asia, the Americas and Australia. It is cultivated between sea-level and altitudes of 2200 m and at latitudes of up to 50N in the United States and Russia and 40S in Argentina.
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. However, the grain is normally ground or pounded to form a flour, often after hulling to remove pigmented pericarp. The flour is used to make thin or thick porridge, leavened or unleavened bread, or beer. The processes, which vary considerably from region to region, often involve the use of alkali and fermentation to improve digestibility, particularly in the case of high-tannin sorghums. Beer production is important in Africa, where sorghum grain is germinated, dried and ground to form malt, and sorghum flour is used as a substratum for fermentation. 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 processing by grinding, rolling, flaking, or steaming, to maximize its nutritional value.
Sorghum plant residues are also used extensively as building material, fuel and animal feed. The dried stems can be used as roofing or fencing materials. The stover remaining after harvesting the grain is cut and fed to cattle, sheep and goats, or may be grazed. In many dryland areas where animal feed is in short supply, this use is as important as grain production. Sorghum is also grown for forage, either for direct feeding to ruminants or for preservation as hay or silage. Sweet-stemmed sorghums are used for animal feed and for extraction of sugar and syrup.
Production and international trade
Sorghum grain is the fifth most important cereal in the world after wheat, rice, maize, and barley. World production is between 55 and 70 million t of grain per annum from between 40 and 45 million ha. Because the crop is a dryland one and is generally grown in marginal environments, productivity and areas sown vary substantially from season to season due to variation in rainfall. Average figures must therefore be treated with some caution. The most important producers are the United States, with an annual production of 15 million t from 4 million ha, India (12 million t from 13 million ha), China (5.5 million t from 1.5 million ha), Mexico (5 million t from 1.5 million ha), Nigeria (4.5 million t from 4.5 million ha) and Sudan (3 million t from 5 million ha). Sorghum is a minor crop in South-East Asia. The main producers are Thailand (250 000 t from 175 000 ha) and Burma (Myanmar) (180 000 t from 150 000 ha). Small areas are also grown in Indonesia, the Philippines and Vietnam. World trade in sorghum is limited. Most sorghum is consumed by its producers and little enters the cash economy for sale.
100 g of air-dried sorghum grain contains: water 8-16 g, protein 8-15 g, fat 2-6 g, carbohydrates 70-80 g, fibre 1-3 g, and ash 1-2 g. The energy value ranges from 1300-1600 kJ/100 g edible portion. The tannin content of sorghum is important in considering nutritional value, as tannins bind proteins and reduce their digestibility. Much of the protein in sorghum is prolamine, which is nutritionally valueless. Maximum available protein is usually 8-9%. Sorghum grain is deficient in lysine, methionine and threonine.
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, carbohydrates 40-50 g, and fibre 20-30 g. The cyanogenic glycoside dhurrin occurs in the aerial parts of most sorghums. The quantity depends on the cultivar, the plant parts, their age and environmental conditions. Dhurrin is hydrolysed 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 and is destroyed when the fodder is made into hay or silage. The 1000-grain weight is 13-40 g.
- Vigorous annual grass, 0.5-5.0 m tall, with one to many tillers, originating from the base or later from stem nodes.
- Seedling radicle replaced by fibrous adventitious roots emerging from lowest nodes below and immediately above ground level; roots concentrated in the top 90 cm but may extend to twice that depth, spreading laterally up to 1.5 m.
- Stem solid, usually erect, dry or juicy, insipid or sweet; the centre may become spongy with spaces in the pith; intercalary meristems above the root band capable of differential growth, allowing fallen stems to regain upright position; at each node an axillary bud may develop into a tiller from the base, or into a branch from the stem.
- Leaves 7-24, according to cultivar, initially erect, later curving, borne alternately; sheath 15-35 cm long, encircling the stem with overlapping margins, often with a waxy bloom, with band of short white hairs at base near attachment; ligule usually present, short, about 2 mm long, ciliate on upper free edge; auricles triangular or lanceolate; blade lanceolate to linear-lanceolate, 30-135 cm × 1.5-13 cm, margins flat or wavy, midrib white or yellow in dry pithy cultivars or green in juicy cultivars, stomata on both leaf surfaces.
- Inflorescence a panicle; peduncle erect, sometimes recurved forming a goose-neck; rachis short or long, with primary, secondary and sometimes tertiary branches, bearing racemes of spikelets; length of rachis and length and closeness of panicle branches determine panicle shape, which may be densely packed, conical or ovoid, to spreading and lax.
- Spikelets borne in pairs, one sessile and hermaphrodite, the other pedicelled and male or sterile, occasionally hermaphrodite; terminal spikelets of a raceme borne in groups of three with one sessile and two pedicelled spikelets; sessile spikelet 3-10 mm long; glumes 2, approximately equal in length, coriaceous (leathery) or chartaceous (papery), ovate, elliptical or obovate; lower glume partially enveloping the upper, 6-18-nerved, usually with a coarse keel-like nerve on each side; upper glume usually narrower and more pointed, with central keel for part of its length; glumes enclosing 2 florets; lower floret infertile, consisting of a lemma only, forming a broad ciliate membranous bract which partially enfolds the upper floret; upper floret hermaphrodite, with a membranous lemma with a two-toothed cleft at apex, teeth free or adnate to an awn, when present, from the sinus; awn kneed and twisted; palea, when present, small and thin; 2 lodicules adjacent to lemma, short, broad, truncate, fleshy with ciliate margins; stamens 3; ovary single-celled with 2 long styles ending in feathery stigmas; pedicelled spikelet variable, with long or short pedicel, persistent or deciduous, smaller and narrower than sessile spikelet; often consisting of only two glumes, sometimes upper floret with lemma, no palea, 2 lodicules and 3 stamens with functional pollen, while lower floret consisting of lemma only.
- Fruit a caryopsis, usually partially covered by glumes, rounded and bluntly pointed, 4-8 mm in diameter and varying in size, shape and colour.
Growth and development
The coleoptile emerges from the soil 3-10 days after sowing and leaf emergence follows soon after, with the rate depending largely on temperature. Panicle initiation, when the apical meristem switches from producing fresh leaf primordia to producing the floral primordium, takes place after approximately one third of the growth cycle of the cultivar. By this stage the total number of leaves has been determined and about one third of total leaf area has developed. Rapid leaf development, stem elongation and internode expansion follow growing-point differentiation. 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 heads start flowering from the tip downwards and flowering may extend over 4-9 days. 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. Additional leaves have been lost. 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.
The time to maturity varies greatly among cultivars, some early types taking only 100 days or less, whereas long-duration sorghums require 5-7 months.
Other botanical information
At present, S. bicolor is considered as an extremely variable crop-weed complex, comprising wild, weedy and cultivated annual forms (classified as subspecies) which are fully interfertile. In the most widely accepted system, the cultivated forms are classified into different races on the basis of grain shape, glumes and panicle type. Five basic races and ten hybrid combinations of these races are recognized and grouped into subsp. bicolor. Instead of races grouped into a subspecies, a direct classification into cultivar groups is preferred. The 5 basic groups are:
- cv. group Bicolor (based on race "bicolor" in the sense of Harlan & de Wet): 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.
- cv. group Caudatum (based on race "caudatum" in the sense of Harlan & de Wet): 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. Caudatum sorghums are widely grown in Chad, Sudan, northeastern Nigeria and Uganda.
- cv. group Durra (based on race "durra" in the sense of Harlan & de Wet): characterized by compact inflorescences, characteristically flattened sessile spikelets, and creased lower glumes; the grain is often subspherical. Durra sorghums are widely grown along the fringes of the southern Sahara, across arid West Africa, the Near East and parts of India.
- cv. group Guinea (based on race "guinea" in the sense of Harlan & de Wet): characterized by usually large, open inflorescences whose branches are 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, Guinea sorghums were often used as ship's provisions because their hard grains stored well.
- cv. group Kafir (based on race "kafir" in the sense of Harlan & de Wet): 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 sorghums are important staples across the eastern and southern savanna from Tanzania to South Africa.
The other 10 cultivar groups (based on the hybrid races of the same names in the sense of Harlan & de Wet) exhibit various combinations and intermediate forms of the characteristics of the 5 basic cultivar groups, e.g. cv. group Guinea-Bicolor, cv. group Caudatum-Bicolor, cv. group Kafir-Bicolor, cv. group Durra-Bicolor, etc.
The wild representatives are classified as S. bicolor (L.) Moench subsp. verticilliflorum (Steud.) Piper (synonyms: S. arundinaceum (Desv.) Stapf, S. bicolor (L.) Moench subsp. arundinaceum (Desv.) de Wet & Harlan): tufted annuals or weak biannuals, 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; racemes 1-5-noded, fragile; sessile spikelet lanceolate to elliptical, 5-8 mm long, usually ciliate, glumes coriaceous, lemmas ciliate and upper one usually awned, grain pointed obovoid; pedicelled spikelet male or neuter and often longer than the sessile spikelet. The subspecies is further divided into 4 overlapping types ("races"), of which verticilliflorum is most widely distributed, extending across the African savanna and introduced into tropical Australia, parts of India and the New World; it has large and open inflorescences with spreading but not pendulous branches.
The weedy representatives are classified into the complex of hybrids, collectively named Sorghum ×drummondii (Steud.) Millsp. & Chase (synonyms: S. sudanense (Piper) Stapf, S. bicolor (L.) Moench subsp. drummondii (Steud.) de Wet); it occurs as a weed in Africa wherever cultivated grain sorghums and their closest wild relatives are sympatric because they cross freely; it represents the intermediate population occurring in the intermediate habitat of recently abandoned fields and of field margins as a very persistent weed; stem up to 4 m tall; leaf blade lanceolate, up to 50 cm × 6 cm; panicle variable, usually rather contracted, up to 30 cm × 15 cm, often with pendulous branches; racemes more or less crowded, mostly 3-5-noded, tardily disarticulating at maturity; sessile spikelet pointed, elliptical, 5-6 mm long, lemmas hyaline ciliate, grain resembling the grain from the cultivar from which the weed was derived; pedicelled spikelet male or neuter. A well-known forage grass, Sudan grass, belongs to this complex.
Sorghum is adapted to a wide range of environmental conditions and will produce significant yields under conditions that are unfavourable for most other cereals. Sorghum is particularly adapted to drought and has a number of morphological and physiological characteristics that contribute to this, 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. Sorghum is characterized by the C4-cycle photosynthetic pathway. It is a short-day plant with a wide range of different reactions to photoperiod. It is also highly influenced by temperature, with the result that many cultivars can show markedly different appearance and productivity in different environments. Some tropical cultivars fail to flower or to set seed at high latitudes.
Sorghum also tolerates waterlogging and can be grown in areas of high rainfall. It is, however, primarily a plant of hot, semi-arid tropical environments with rainfall from 400-600 mm that are too dry for maize. It is also grown widely in temperate regions and at altitudes of up to 2300 m in the tropics. Sorghum tolerates a wide range of temperatures. Sterility can occur when night temperatures fall below 12-15°C during the flowering period. Sorghum is killed by frost.
Sorghum can be grown successfully on a wide range of soil types. It is well suited to heavy Vertisols found commonly in the tropics, where its tolerance of waterlogging is often required, but is equally suited to light sandy soils. It 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.
Propagation and planting
Sorghum is normally grown from seed. A fine seed-bed 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. For favourable conditions, row spacings of 45-60 cm and plant-to-plant spacings of 12-20 cm, giving populations of about 120 000 plants per ha, are normal. For drier or less fertile conditions, wider spacing and lower plant populations are usually optimal. The seed rate varies from 3 kg/ha in very dry areas to 10-15 kg/ha under irrigation. 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. 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. Ratooned crops give low grain yields.
Sorghum is usually grown as a rainfed crop, sown after the onset of the monsoon season. Seeding rates are often higher than optimum to compensate for poor seed-bed or to allow for unfavourable moisture conditions. Subsistence farmers rarely apply fertilizer, as responses depend on moisture availability which is usually very uncertain. Under more favourable conditions, chemical fertilizers and farmyard manure are used with advantage, but even so the quantities used are usually below optimum. The application of 25-50 kg/ha of nitrogen often proves to be appropriate. In large-scale cultivation in the United States, high fertilizer doses are applied, comparable to those for maize.
The crop is usually weeded by a combination of inter-row cultivation with animal-drawn implements and hand weeding within rows. Thinning is carried out at the same time as hand weeding, or at intervals during the crop cycle, particularly where thinnings are used to feed livestock. Little chemical weed control is practised as it is usually uneconomical, although effective herbicides are available.
The parasitic weed Striga is a major pest of sorghum, particularly in Africa, where severe infestations can lead to land being abandoned. Striga is also an important problem in Burma (Myanmar) and India. Striga can be controlled by cultural methods such as the use of rotations of crops that are not susceptible and rigorous removal of the weeds before flowering. Application of fertilizer also helps to control Striga.
Diseases and pests
The most severe disease problem of sorghum in South-East Asia is grain moulds, caused by a complex of fungal pathogens (predominantly Curvularia lunata, Fusarium spp., and Phoma sorghina) that infect the grain during development and can lead to severe discolouration and loss of quality. This disease is most severe in seasons where rains continue through the grain maturity stage and delay the harvest. In Thailand, sorghum is sown late in the rainy season in order to escape grain mould damage. Important foliar diseases in South-East Asia include anthracnose (Colletotrichum graminicola), leaf blight (Exerohilum turcicum), zonate leaf spot (Gloeocercospora sorghi), and tar spot (Phyllacora sorghi). Charcoal rot (Macrophomina phaseolina) is an important root and stem rot of sorghum in Thailand and the Philippines, particularly when terminal drought stress is severe. Chemical control of these diseases is rarely if ever practised. Other diseases of sorghum that are important in other areas of the world include downy mildew (Peronosclerospora sorghi), rust (Puccinia purpurea), and ergot (Claviceps sorghi).
The main insect pests of sorghum are shoot fly (Atherigona soccata), stem borers (Busseola fusca and Chilo partellus), sorghum midge (Contarinia sorghicola), and head bugs (Calocoris angustatus). The main control methods for these pests are cultural. 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 also available for sorghum midge, but only low levels of resistance for the other pests. As in the case of diseases, chemical control of insect pests is rarely practised. Sorghum is very susceptible to damage by storage pests, the main ones being rice weevil (Sitopholus 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.
Sorghum is usually harvested by hand. The heads can be cut or the whole plant cut and the heads removed later. Sorghum is harvested by combine in Thailand, where short-statured cultivars are preferred for grain production.
Sorghum is often grown in marginal areas that are not suitable for many other crops. Its average grain yields on farmers' fields are therefore often low. The average grain yield in Thailand is 1.5 t/ha and that in Burma (Myanmar) is 1.2 t/ha. However, sorghum has high yield potential and under favourable conditions can produce much higher yields. In China, where it is grown with high levels of inputs, sorghum yield averages 3.6 t/ha and in the United States it averages 3.8 t/ha. Yields up to 6 t/ha have been reported from experimental fields.
Rainfed forage sorghum is usually cut only once, soon after flowering. Other forage sorghum crops are grown under more favourable conditions, often with irrigation and high levels of fertilizer applied, and can be harvested and then left to regrow (ratoon). 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 over all cuts but produce better quality forage.
Handling after harvest
Sorghum grain is highly susceptible to storage pests, and proper handling after harvest is very important. The harvested grain is usually sun-dried, often on the head. Heads, particularly those to be retained for seed, may be stored hanging from the ceilings 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 various traditional granaries, above or below ground, designed to prevent insect attack. 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.
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) in India. The collection extends to over 35 000 accessions of landraces from all the major sorghum-growing regions of the world. Other smaller collections are maintained in regional genebanks in Africa and by many national research programmes in Africa, Asia and the Americas.
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 systems. The main breeding objectives include high grain yield, white grain for human consumption, and red or brown grain for feed purposes and brewing. Incorporation of resistance to major yield-limiting diseases and pests, and tolerance of abiotic stresses is of high priority for most sorghum improvement programmes. In many countries the emphasis is on producing cultivars which combine high grain yield with high stover yields because of the importance of sorghum crop residues as animal feed. Special cultivars with high biomass production and good forage quality are bred for animal feed.
Sorghum has traditionally been used as human food in most sorghum-growing areas of the world. But, as the population becomes more affluent, eating habits change and the consumption of other preferred cereals increases and the consumption of sorghum declines. This trend is likely to continue, although the demand for sorghum as food is still likely to remain substantial in many production areas. 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. 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.
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