Pennisetum glaucum (PROSEA)
Pennisetum glaucum (L.) R. Br.
- Protologue: Prodr. Fl. Nov. Holl. 1: 95 (1810).
- Family: Gramineae
- Chromosome number: 2n= 14
- Pennisetum americanum (L.) Leeke (1907),
- P. typhoides (Burm.f.) Stapf & Hubbard (1933).
- Pearl millet, bulrush millet, cattail millet (En)
- Mil à chandelle, mil pénicillaire (Fr)
- Bajra (Hindi)
- Vietnam: cỏduôi voi, kê voi, lúa miêu.
Origin and geographic distribution
After Africa's most recent humid era from 5000-3000 BC, pearl millet originated in the period 3000-2000 BC in the African Sahel zone by domestication from its wild ancestor P. violaceum (Lamk) Richard. From there it spread to East Africa, India, Spain and the United States. It is commonly grown as a grain crop in the semiarid regions of West Africa and the driest parts of East Africa and the Indian subcontinent. In Australia, South Africa and the United States, it is grown as a fodder crop.
Pearl millet is the staple food for about 90 million people in parts of tropical Africa and India, which are too hot, dry and sandy for sorghum production. Decorticated and pounded into flour it is mostly consumed as a stiff porridge (tô) or gruel in Africa, or as flat unleavened bread (chupatty) in India. There are various other preparations such as couscous, rice-like products, snacks of blends with pulses, and fermented and non-fermented beverages in Africa. In several Indian preparations parched seeds are used. The stalks are used for thatching and building, as fuel and as a poor quality fodder. Outside India and Africa it is mostly grown as a green fodder crop.
Production and international trade
Production statistics on millet often combine data on all millet species. Estimates based on total millet production and relative importance of pearl millet indicate an annual production of 13 million t from a planted area of 25 million ha. The largest acreages occur in India and the dry regions of Africa. Production figures over the past 25 years show considerable fluctuations; in Africa, area and yield have remained constant; in India, the cultivated area has declined steadily, but yields have increased proportionately. Negligible quantities are traded internationally.
Average composition of the seed per 100 g edible portion is: water 12 g, protein 10-12 g, fat 3-5 g, carbohydrates 60-70 g, fibre 1.5-3 g, ash 1.5-2 g. The energy value is about 1525 kJ/100 g. The protein is rich in tryptophane and cystine, poor in lysine and methionine. Its nutritional value is somewhat superior to maize, rice, sorghum and wheat. 1000-seed weight is 3.5-16 g.
- A robust, strongly tillering, annual grass, usually 1-4 m tall, with basal and nodal tillering.
- Root system extremely profuse; sometimes the nodes near ground level produce thick, strong prop roots.
- Stem slender, 1-3 cm in diameter, solid, often densely villous below the panicle, nodes prominent.
- Leaf sheath open and often hairy; ligule short, membranous, with a fringe of hairs; leaf blade linear to linear-lanceolate, up to 1.5 m × 5-8 cm, margins with small teeth, scaberulous and often pubescent.
- Inflorescence a cylindrical or ellipsoidal, contracted, stiff and compact panicle, suggesting a spike, 15-200 cm long; rachis cylindrical, bearing densely packed clusters of spikelets; spikelets usually borne in pairs, sometimes 1-4 or more per cluster, subtended by a tuft (involucre) of 25-90 bristles, that are about as long as the spikelets, but in some cultivars a terminal bristle is elongated.
- Spikelet 3-7 mm long, consisting of 2 glumes and usually 2 florets, the lower one staminate or sterile, the upper one bisexual; lemma and palea not clasping the grain; lodicules absent; ovary obovoid, smooth, with 2 styles, connate at the base; stamens 3; anthers 2-5 mm long, tipped with brush-like bristles.
- Caryopsis globose to subcylindrical or conical, 2.5-6.5 mm long, colour variable, from white, pearl-coloured or yellow to grey-blueish and brown, occasionally purple; hilum marked by a distinct black dot at maturity.
Growth and development
Cultivars vary in time to maturity from 55-280 days, but mostly from 75-180 days. Time to flower initiation is the main factor determining the life cycle of a cultivar. Photo-sensitive cultivars require shorthening days of less than 12 hours. In shortduration cultivars the developmental stages (from germination to flower initiation, to flowering and to maturity) are of approximately equal duration.
Field establishment of pearl millet is affected by its relatively small seed size, especially in crusting soils. During early development the roots grow more than the aboveground parts. Pearl millet produces an extensive and dense root system, which may reach a depth of 1.2-1.6 m, sometimes even of 3.5 m. Basal tillering occurs between 2-6 weeks after sowing, and up to 40 tillers may be produced. Tillering from the upper nodes of the stems is common and occurs in response to drought, or damage to the main stem. These secondary tillers produce 2-3 leaves and an inflorescence within 10-20 days; they may contribute 15% and occasionally up to 50% of grain yield. Its asynchronous tillering habit compensates for the effects of drought before anthesis by an increased yield from tillers.
It takes 15-20 days from inflorescence differentiation to flowering. Pearl millet is protogynous and normally crossfertilized, but about 10% or more selfing may occur, depending on overlap in flowering of tillers. Heavy rainfall, low temperature and moisture stress reduce seedset. The length of the grain-filling period varies greatly with genotype and is temperature dependent, but normally takes 22-25 days. The harvest index is very low (0.15-0.20), attaining 0.35 in improved cultivars.
Other botanical information
P. glaucum belongs to a complex of 3 taxa which hybridize freely and which are sometimes considered as belonging to one species. However, as long as the complicated taxonomy of pearl millet has not been fully cleared up, it is preferable to keep those taxa separate:
- P. glaucum (synonym: P. americanum (L.) Leeke subsp. americanum). The crop plant, with persistent, stipitate involucral bristles usually subtending 2 spikelets.
- P. sieberanum (Schlecht.) Stapf & Hubbard (synonyms: P. stenostachyum (A. Br. & Bouché) Stapf & Hubbard, P. dalzielii Stapf & Hubbard, P. americanum (L.) Leeke subsp. stenostachyum (A. Br. & Bouché) Brunken). These plants (also collectively called "shibras") are weeds in pearl millet fields in West Africa and northern Namibia (not in Asia). They commonly mimic the companion pearl millet in inflorescence size and shape, vegetative morphology and time of flowering. They differ from pearl millet in having deciduous, stipitate involucral bristles. They are obligate weeds of cultivation and do not persist more than one generation in abandoned fields. They are derived from introgression between wild P. violaceum and cultivated P. glaucum.
- P. violaceum (Lamk) Richard (synonyms: P. fallax (Fig. & De Not.) Stapf & Hubbard, P. americanum (L.) Leeke subsp. monodii (Maire) Brunken). This is a polymorphic weed of disturbed places, commonly occurring around villages from Senegal to the Sudan, and is harvested as a wild cereal in times of scarcity. It differs from P. glaucum in having deciduous, sessile involucral bristles which always contain a single spikelet. Some authors restrict P. violaceum to only the true wild pearl millet, limited in its distribution to very dry areas in the northern Sahel and consider the weedy forms of disturbed places as belonging to P. sieberanum, together with the shibras.
Mainly based on differences in grain shape, four cultivar groups have been distinguished in P. glaucum:
- cv. group Globosum (synonym: race Globosum). Caryopsis globose, more than 2.4 mm in diameter. The inflorescence is cylindrical, often longer than 1 m. It is most common in the Sahel west of Nigeria.
- cv. group Leonis (synonym: race Leonis). Caryopsis oblanceolate in outline, 3.8-6.3 mm × 1.9-2.5 mm × 1.9-2.5 mm, apex acute. The inflorescence is cylindrical. At maturity about one-third of the grain protrudes beyond the floral bracts. This is the smallest cv. group and is grown in Sierra Leone, Senegal and Mauritania.
- cv. group Nigritarum (synonym: race Nigritarum). Caryopsis obovoid but extremely angular in cross-section, 3-5 mm × 1.7-2.5 mm × 1.5-2.2 mm. The inflorescence is cylindrical. The mature grain generally protrudes beyond the floral bracts. It is most common in semi-arid regions from Nigeria to Sudan.
- cv. group Typhoides (synonym: race Typhoides). Caryopsis obovoid but terete in cross-section, 2.5-5.5 mm × 1.5-3 mm × 1.2-2.4 mm. The inflorescence is cylindrical or ellipsoidal, usually less than 0.5 m long. Occasionally the grains remain enclosed by the floral bracts. It is the most primitive, the most variable and most widely distributed cv. group, occurring all over the pearl millet range in Africa and also in India. Probably the other cv. groups were derived from this group.
Based on growth duration, agronomically two main groups of cultivars are recognized in West Africa: short duration Gero or Souna cultivars and long duration Maiwa or Sanio cultivars. Gero cultivars are less photo-sensitive and, being early maturing, are adapted to regions with a short rainy season. They are more widely grown than Maiwa cultivars. Flowering in Maiwa cultivars is strongly controlled by daylength and they are grown in regions where the rainy season is longer and where sorghum is the major cereal, but on poorer, more drought-prone soils than sorghum. Certain Maiwa millets are transplanted from seed-bed into the field and are known as Dauro millet.
In India, both improved cultivars and single-cross hybrids are grown on about 40% of the area sown to pearl millet. Most are early (80 days) to very early (65 days) maturing and less photo-sensitive than African cultivars.
Pearl millet is not closely related to other millet species, although foxtail millet (Setaria italica (L.) P. Beauvois) was once classified as a Pennisetum.
Pearl millet is characterized by the C4-cycle photosynthetic pathway and has a very high rate of biomass production, despite being a crop adapted to the drier parts of the semi-arid tropics. Its northern limit in West Africa is the zone with about 250 mm annual rainfall, where cultivars requiring only 55-65 days to mature are grown. In the 250-400 mm rainfall zone, where very high temperatures are common especially at planting time, it is the dominant cereal. Further south it is found with sorghum. Optimum temperature for germination is 33-35 °C; no germination occurs below 12 °C. Optimum temperature for tiller production and development is 21-24 °C, and for spikelet initiation and development about 25 °C. Extreme high temperatures before anthesis reduce panicle size and spikelet density, thus reducing yield. Pearl millet is tolerant of various soil conditions, especially of light and acid soils. Its large and dense root system allows it to grow on soils with a low nutrient status. On light soils it is less affected by nematodes than sorghum. Soil crusting is a major cause of poor seedling establishment. It does not tolerate waterlogging. Once established, the crop is tolerant of salinity.
Propagation and planting
Propagation is by seed, usually sown directly in the field. Transplanting is carried out on a very limited scale in India and West Africa (Dauro millet).
In Africa, shortduration cultivars are sown as early as possible after the onset of the rains and land preparation is limited to a light hoeing. Land preparation for long-duration cultivars, which are sown later, is done more thoroughly. Pearl millet is mostly sown in pockets on hills or ridges; in drier areas and on light soils it is also sown in furrows. Plant density depends on rainfall and amounts to 20 000-50 000 plants/ha in pure stands.
In India the usual method of land preparation is to make 2-3 passes with a traditional plough. The seed is broadcast with the first rains and is then covered with a brush harrow. Occasionally, traditional seed drills are used. Row spacing varies between 45-60 cm. Seed rates vary with desired stand and soil type from 3-11 kg/ha.
In Africa pearl millet is often intercropped with sorghum (Sorghum bicolor (L.) Moench), cowpea (Vigna unguiculata (L.) Walp.) or groundnut (Arachis hypogaea L.), its place and importance in the system being dependent on rainfall. In India pearl millet is often intercropped with a great variety of pulses, e.g. hyacinth bean (Lablab purpureus (L.) Sweet), mung bean (Vigna radiata (L.) Wilczek), horse gram (Macrotyloma uniflorum (Lamk) Verdc.), and less commonly with castor (Ricinus communis L.) or cotton (Gossypium spp.).
The weeding of shortduration cultivars in Africa coincides with land preparation and planting of later crops and therefore shortage of labour often leads to weeding being neglected.
In India pearl millet is weeded using a bullock-drawn peg-tooth harrow, followed by hand weeding. Usually, 1-2 harrowings and 1-2 hand weedings are needed. Broadcast crops can only be hand weeded. Pearl millet is grown in rotation with sorghum, cotton, groundnut, other millets and occasionally rice. In northern India, if the rainfall pattern allows, it is often doublecropped with wheat and sometimes finger millet.
Under traditional, rainfed conditions the application of manure and chemical fertilizers is limited. Vigorous early growth, promoted by nitrogen, may consume water required for later crop development and grain growth. Response to phosphorus is not uncommon, but the requirement under rainfed conditions does not appear to be high. The requirement for potassium is relatively high. A new cultivar yielding about 3.1 t/ha in the West African savanna is reported to have removed N 132 kg, P 28 kg, K 65 kg and Ca 31 kg per ha.
Diseases and pests
Green ear caused by downy mildew (Sclerospora graminicola), smut (Tolyposporium penicillariae), rust (Puccinia penniseti) and ergot (Claviceps microcephala) are important diseases in both Asia and Africa. Sources of resistance against all four have been identified and are being incorporated into new cultivars (except for resistance to ergot which is polygenic and recessive).
Birds are the major pest in pearl millet: Quelea spp. in Africa and sparrows, parakeets, crows and migrating rosy pastors in India. Bird scaring for several weeks before the harvest is essential. Farmers in West Africa often do not expect to harvest a larger area than they can protect from birds. Cultivars with long, hard bristles are attacked less severely than awnless ones.
Insect pests are of more importance in West Africa than in India. Stem borer (Coniesta ignefusalis) and earworm (Heliocheilus albipunctella) are locally important. Other pests are millet midge (Geromiya penniseti), white grubs, grasshoppers, locusts, and various Lepidoptera.
Pearl millet is attacked by Striga hermontica (Del.) Benth. in West Africa, sometimes seriously, and by white-flowered S. asiatica (L.) Kuntze in parts of northern India. In southern Africa, red-flowered S. asiatica that parasitizes sorghum does not affect pearl millet, not even cultivars that are attacked in West Africa. Apparently, S. asiatica has not yet evolved a pearl millet biotype in southern Africa.
Pearl millet is harvested by hand, either by picking the panicles or by harvesting whole plants. In strongly tillering cultivars, where spikes (heads) ripen unevenly, several pickings are required.
Yields vary considerably with amount and distribution of rainfall and range from 250 kg/ha in the driest areas, to 500-1500 kg/ha in the main production areas. Average yields in West Africa and India are about 600 kg/ha. Under optimal growth conditions maximum yields from improved cultivars are 3-3.5 t/ha. Hybrids can reach 5 t/ha in 85 days and yields of 8 t/ha have been reported.
Handling after harvest
The harvested crop is dried in the sun for a few days. In Africa it is commonly stored on the panicle in elevated granaries, built of mud or plant materials and covered with thatch. Sometimes it is stored in pits. Threshing is normally done manually when grain is needed. In India the grain is threshed soon after harvesting and drying. Threshing is carried out by beating with sticks or trampling with cattle. Threshing recovery is about 55%. The grain may then be stored in containers which are kept in granaries or store rooms, or sometimes in pits in the ground. Seed may be covered with sand or mixed with leaves of the neem tree (Azadirachta indica Juss.) to reduce insect attack. Pearl millet grain can be stored more easily and longer than maize or sorghum. Grain kept for seed can be stored adequately at room temperature for several years.
Landraces of pearl millet have been selected by farmers for yield, adaptability to unfertilized soil, resistance to drought and diseases, and for grain type. Frequent cross pollination with wild relatives in West Africa further contributes to the crop's genetic diversity. Genetic variation is conserved and evaluated at the Coastal Plains Experiment Station, Tifton (Georgia, United States) and the International Crops Research Institute for the SemiArid Tropics (ICRISAT) in Hyderabad (India), where the world collection of 21 000 entries is housed. The International Plant Genetic Resources Institute (IPGRI) supports a programme, started in Burkina Faso, to improve the description and evaluation of material at the time and location of collection.
One particular germplasm source, the Iniadi cultivar from northern Togo and Ghana has had a profound effect on pearl millet breeding. Selections from it have been successful as cultivars in northern India, Namibia and Botswana, and it has been extensively used in breeding programmes.
Although genetic manipulation is facilitated by its profuse tillering, protogynous flowering habit and high fertility, breeding of pearl millet only started in the 1950s and traditional cultivars are still commonly used. Breeding work by the Indian Council of Agricultural Research and ICRISAT has been most successful in developing cultivars rapidly adopted by farmers. The discovery of cytoplasmic male sterility in 1958 in the forage breeding programme at Tifton (Georgia, United States), led to the production of grain hybrids in India, which covered 3 million ha by 1970. Despite disease epidemics, now overcome, some 40% of the Indian millet crop on peasant farms now consists of hybrids and improved cultivars, and yields have increased by 30% since 1965. Early breeding work in West Africa by the "Institut de Recherches Agronomiques Tropicales et de Cultures Vivrières" and the East African Agriculture and Forestry Organization, produced improved cultivars, but adoption has been negligible. The International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), working in Niger and Zimbabwe since the early 1980s, have produced improved cultivars which, prompted by increasing food shortages, are being grown in 8 countries in West Africa and 5 countries in southern Africa. Work at ICRISAT is focused on the identification of stable stress tolerance, wide adaptability and high yield potential. Sources of tolerance of the major diseases have been identified and are being incorporated into new cultivars.
Breeding work on fodder millets is done mainly in the United States, Australia and South Africa and has produced pearl millet hybrids and interspecific hybrids between pearl millet and elephant grass (Pennisetum purpureum Schumach.). The latter hybrid is a sterile perennial, usually propagated by cuttings. It is well adapted to use by small farmers in Asia, East Africa and parts of South America. Pearl millet has been bred as a new feed grain crop in the United States and commercial production for poultry feed has commenced in Georgia.
Compared to other major cereals, little research effort has been invested in pearl millet. However, genetic yield gains have been good, 1-2% per year as measured in India over the last 30 years. It has one of the highest rates of dry matter gain among the C4-cycle cereals. Only a small proportion of the available genetic variability has been used from the primary pearl millet gene pool. Thus the potential for further improvement is high, both as a staple cereal for low to medium productivity agriculture in drought-stress and nutrient-deficient conditions in developing countries, and as a high yield nutritious food or feed grain crop in tropical or warm temperate intensive agriculture.
Some testing has been carried out in all SouthEast Asian countries with variable results, but its potential under limited rainfall and poor light soils merits further study.
- Andrews, D.J. & Kumar, K.A., 1992. Pearl millet for food, feed and forage. Advances in Agronomy 48: 89-139.
- Andrews, D.J. & Bramel-Cox, P., 1993. Breeding cultivars for sustainable crop production in low-input dryland agriculture in the tropics. In: Buxton, D.R., Shibles, R., Forsberg, R.A., Blad, B.L., Asay, K.H., Paulsen, G.M. & Wilson, R.F. (Editors): International crop science I. Crop Science Society of America. Madison, Wisconsin, United States. pp. 211-222.
- Andrews, D.J. & Kumar, K.A., 1996. Use of West African pearl millet cultivar Iniadi in cultivar development. Plant Genetic Resources Newsletter 105: 15-22.
- Brunken, J., de Wet, J.M.J. & Harlan, J.R., 1977. The morphology and domestication of pearl millet. Economic Botany 31: 163-174.
- Clayton, W.D. & Renvoize, S.A., 1982. Pennisetum. In: Polhill, R.M. (Editor): Flora of tropical East Africa. Gramineae (Part 3). A.A. Balkema, Rotterdam, the Netherlands. pp. 672-673.
- Hanna, W.W., 1989. Characteristics and stability of a new cytoplasmatic nuclear male-sterile source in pearl millet. Crop Science 29: 1457-1459.
- Kumar, K.A. & Andrews, D.J., 1993. Genetics of qualitative traits in pearl millet: a review. Crop Science 33: 1-20.
- Pearson, C.J. (Editor), 1985. Pearl millet, special issue. Field Crops Research 11 (2, 3): 111-290.
- Rachie, K.O. & Majmudar, J.V., 1980. Pearl millet. Pennsylvania State University Press, University Park, United States. 305 pp.
- Stoop, W.A., 1986. Agronomic management of cereal/cowpea cropping systems for major toposequence land types in the West African savanna. Field Crops Research 14: 301-320.
- L.P.A. Oyen & D.J. Andrews