Prota 1: Cereals and pulses/Céréales et légumes secs
Sp. pl. 2: 971 (1753).
2n = 20
Maize, corn, Indian corn (En). Maïs (Fr). Milho (Po). Mhindi, muhindi (Sw).
Origin and geographic distribution
Maize was domesticated in southern Mexico around 4000 BC. Early civilizations of the Americas depended on maize cultivation. When the Europeans arrived in the Americas, maize had already spread from Chile to Canada. Maize was reported for the first time in West Africa in 1498, six years after Columbus discovered the West Indies. The Portuguese brought floury grain types from Central and South America to São Tomé, from where they spread to the West African coast. Portuguese and Arab traders introduced Caribbean flint maize types into East Africa in the mid 1500s, from where they spread to southern Africa. Through the trans-Saharan trade, the Arabs introduced the flinty types that had been brought to northern Africa into sub-Saharan Africa. The flinty types still predominate in northern parts of West Africa while the floury types prevail in the southern parts, with some variation from this pattern. Maize had become a staple food in East and southern Africa by the 1930s.
Maize has an extremely wide distribution. It is grown from latitude 58°N in Canada and Russia, throughout the tropics, to latitude 42°S in New Zealand and South America, and in areas below sea level in the Caspian Plain up to areas as high as 3800 m in Bolivia and Peru. It is grown in all countries of Africa, from the coast through savanna regions to the semi-arid regions of West Africa, and from sea-level to the mid- and high-altitudes of East and Central Africa.
Maize grain is used for three main purposes: as a staple food, as feed for livestock and poultry, and as a raw material for many industrial products. In tropical Africa nearly all maize grain is used for human food, prepared and consumed in many ways. It may be eaten fresh on the cob and simply roasted, but the grain is usually ground and the meal is boiled into porridge or fermented into beer. In tropical Africa maize is mainly consumed as thick porridge (‘ugali’ in East Africa, ‘sadza’ in Zimbabwe). It is commonly eaten with cooked vegetables and, when available, meat. A thin porridge (‘uji’ in East Africa, ‘ogi’ in Nigeria, ‘koko’ in Ghana) is also commonly eaten especially as weaning food. In Ethiopia local beer (‘tella’) and spiritual liquor (‘arakie’) are prepared from maize grain malt. Popcorn is a popular snack.
The main industrial products obtained from maize are breakfast products such as cornflakes, starch, sugar and oil. The main product is starch that is used for human consumption or made into syrup, alcohol, but also among others as laundry starch and as a source material for many chemical products. Most industrial products are obtained by the wet-milling process, in which the grain is first steeped in water, after which the germ and bran are separated from the endosperm. The various products are subsequently obtained by physical or chemical processes, and e.g. sugars from maize now account for half of the sugars used in human nutrition. Dry milling produces grits, consisting of coarsely ground endosperm from which most of the bran and the germ have been separated. The germ yields an oil that can be refined for human consumption, widely used as cooking or salad oil and in margarines. It is the second most widely consumed vegetable oil in the United States and is also made into soap or glycerine. The residues from the production of starch or oil, together with the bran, are used in animal feeds (corn gluten meal and corn gluten feed).
Unripe cobs are consumed as vegetable or green maize, boiled or roasted. Very young female inflorescences (‘baby cobs’) are a fancy vegetable in Western countries and in Asia. Mature maize plants are used for animal feed. Silage maize is one of the leading crops in industrialized Western countries, where special cultivars and production technologies have been developed. The stalks are used for fuel, fodder and thatching and as compost. The fibre in the stems and the inner leaves surrounding the cob are made into paper. These cob leaves are often used to wrap foods, and may also be made into cloth or mats, and be used for mattress filling. Ash of the burnt stem is sometimes a substitute for salt. The cob is made into pipe-bowls. In southern Africa the incinerated cob is included in a snuff.
Maize has a range of uses in traditional African medicine. Urino-genital problems are treated with prescriptions based on the whole or parts of the maize plant, especially a decoction of the styles, which is also used to treat jaundice. A leaf maceration is drunk to treat fever. Charcoal made from the culms is included in medicines to treat gonorrhoea; an infusion from the burnt cob is used to wash wounds.
Production and international trade
According to FAO estimates, the average world production of maize in 1999–2003 amounted to 611 million t/year from 139 million ha. The main producing countries are the United States (243 million t/year in 1999–2003, from 28 million ha), China (117 million t/year from 24 million ha), Brazil (38 million t/year from 12 million ha), Mexico (19 million t/year from 7 million ha), France (15 million t/year from 2 million ha), Argentina (15 million t/year from 3 million ha) and India (12 million t/year from 7 million ha). South Africa produced 9.4 million t/year from 3.6 million ha. Maize production in tropical Africa in 1999–2003 was 26.6 million t/year from 21.2 million ha. The main producing countries in tropical Africa are Nigeria (4.7 million t/year from 4.2 million ha), Ethiopia (2.9 million t/year from 1.6 million ha), Tanzania (2.6 million t/year from 1.6 million ha), Kenya (2.5 million t/year from 1.6 million ha) and Malawi (2.0 million t/year from 1.5 million ha). From 1961–1965 to 1999–2003 the annual maize production in tropical Africa increased from 9.1 to 26.6 million t/year, and the harvested area from 10.2 to 21.2 million ha.
Average world export of maize amounted to 80.1 million t/year in 1998–2002, with the United States (47.5 million t/year), Argentina (10.3 million t/year), France (7.9 million t/year) and China (7.4 million t/year) as main exporters. Export of maize from tropical Africa was only 307,000 t/year, with Zimbabwe (143,000 t/year), Tanzania (42,000 t/year) and Uganda (25,000 t/year) as main exporters. The main importers were Japan (16.3 million t/year) and South Korea (8.3 million t/year). Maize imports into tropical Africa were 1.8 million t/year.
The composition of mature white maize grain per 100 g edible portion is: water 10.4 g, energy 1527 kJ (365 kcal), protein 9.4 g, fat 4.7 g, carbohydrate 74.3 g, dietary fibre 7.3 g, Ca 7 mg, Mg 127 mg, P 210 mg, Fe 2.7 mg, Zn 2.2 mg, thiamin 0.39 mg, riboflavin 0.20 mg, niacin 3.6 mg, vitamin B6 0.62 mg, folate 19 μg and ascorbic acid 0 mg. The essential amino-acid composition per 100 g edible portion is: tryptophan 67 mg, lysine 265 mg, methionine 197 mg, phenylalanine 463 mg, threonine 354 mg, valine 477 mg, leucine 1155 mg and isoleucine 337 mg. The principal fatty acids per 100 g edible portion are: linoleic acid 2097 mg, oleic acid 1247 mg and palmitic acid 569 mg (USDA, 2004). Maize is deficient in tryptophan and lysine, but cultivars with higher content of these amino acids have been bred using the recessive gene Opaque-2 with modifiers. These cultivars are referred to as Quality Protein Maize (QPM). In general 100 kg of whole maize, with 16% moisture content, yields about 64 kg starch and 3 kg oil. The endosperm, which accounts for 80% of the weight of the grain, is poor in phosphorus and calcium and contains most of the starch and two-thirds of the protein. More than 80% of the fat and most minerals are in the embryo or germ, which constitutes about 12% of the grain. The starch of the endosperm usually consists of a mixture of about 75% amylopectin and 25% amylose, but waxy maize contains only amylopectin. The most common grain colours are yellow and white. Yellow maize predominates in the United States, China and Brazil, whereas white maize predominates in tropical Africa, Central America and the northern part of South America. White maize has harder grain and gives sweeter, more flavourful products; it is primarily grown for food, whereas yellow maize is mainly used as animal feed. Yellow maize contains the provitamin A cryptoxanthin. Most vitamins are found in the outer layers of the endosperm and in the aleurone layer. Maize is deficient in gluten and therefore unsuitable for making leavened bread; it is tolerated by patients with coeliac disease. Maize oil is considered a premium oil for human consumption, due to its flavour, colour and stability and the presence of linoleic acid and vitamin E.
Maize grain in tropical Africa often contains mycotoxins such as aflatoxins and fumosinins, which are harmful to humans and livestock. Aflatoxins are produced by Aspergillus spp., especially Aspergillus flavus; they are powerful carcinogens, especially affecting the liver, and have immunosuppressive properties. Fumosinins are produced by Fusarium spp., especially Fusarium verticillioides; they have been implicated in various animal diseases. Human health risks due to fumosinins are possible, but so far there is no conclusive evidence, although correlation studies have suggested a link between consumption of maize with fumosinins and high incidence of human oesophageal carcinoma.
Robust annual grass up to 4(–6) m tall; root system consisting of adventitious roots, developing from the lower nodes of the stem near the soil surface, usually limited to the upper 75 cm of the soil, but single roots sometimes penetrating to a depth of over 2 m; stem (culm) usually single and simple, solid. Leaves alternate, simple; leaf sheaths overlapping, auricled at the top; ligule c. 5 mm long, colourless; blade linear-lanceolate, 30–150 cm × 5–15 cm, acuminate, margins smooth, midrib pronounced. Male and female inflorescences separate on the same plant; male inflorescence (‘tassel’) a terminal panicle up to 40 cm long, lateral branches with paired spikelets 8–13 mm long, one sessile, the other on a short pedicel, each spikelet with 2 glumes and 2 florets, each floret with an ovate lemma, a thin palea, 2 fleshy lodicules and 3 stamens; female inflorescence a modified spike, usually 1–3 per plant in leaf axils about half way up the stem, composed of a thick spongy axis with paired sessile spikelets in 8–20 longitudinal rows and enclosed by 8–13 modified leaves (spathes), spikelet with 2 glumes and 2 florets, lower floret sterile, consisting solely of a short lemma and palea, upper floret with a short, broad lemma and palea, a single superior ovary and a long threadlike style and stigma (‘silk’) up to 45 cm in length and emerging from the top of the inflorescence, receptive throughout most of its length. Fruit a caryopsis (grain), usually obovate and wedge-shaped, variously coloured from white, through yellow, red and purple to almost black, up to 1000 together in an infructescence (‘cob’) enclosed by modified leaves up to 45 cm × 8 cm.
Other botanical information
Zea comprises 5 species, including cultivated Zea mays and 4 wild relatives, all from tropical America and called teosintes. Zea mays is a heterogeneous species and cultivars can be divided into 8 types (or cultivar groups) according to the structure and shape of the grain:
– Dent maize: the sides of the grain have corneous endosperm, but the inside has soft white starch, extending to the apex, shrinking on drying to produce the characteristic dent, the wedge-shaped grains are usually yellow or white; it is the principal maize in the United States and northern Mexico;
– Flint maize: the grain can be coloured variously and consists mainly of hard endosperm with a little soft starch in the centre, it has rounded ends and is generally smaller than the grain of dent maize, it matures earlier, is harder, and when dry it is more resistant to insect attack; it is the predominant type grown in Europe, Asia, Central and South America and parts of tropical Africa;
– Flint-dent maize: this group resulted from hybridization between flint and dent maize, and has intermediate characteristics; it appeared first in the United States at the end of the 18th century, and spread to Europe in the 20th century, where it is widely cultivated;
– Pod maize: this is the most primitive type of maize in which the grain is enclosed in bracts; it is not grown commercially;
– Pop maize or popcorn: it has small grains with a high proportion of very hard corneous endosperm and a little soft starch in the centre; on heating the steam generated inside the grain causes it to pop and explode, the endosperm becoming everted about the embryo and hull to produce a palatable white fluffy mass (‘popcorn’); in ‘rice popcorn’ cultivars the grains are pointed and in ‘pearl popcorn’ cultivars rounded; popcorn is most important in the United States and Mexico, but has also become a popular snack in tropical Africa;
– Flour maize or soft maize: the grain can have all types of colours, it usually has no dent and the endosperm consists of soft starch, when parched it can be chewed more easily than flint maize and it is also easier to grind, but it is susceptible to mould and breakage during handling; it is one of the oldest maize types and was widely grown in the drier parts of the United States, western South America and South Africa, and it is still widely grown in the Andes and small amounts are grown in the United States; people in the southern parts of West Africa relish flour maize;
– Sweet maize: the grain contains glossy endosperm with little starch, giving a wrinkled appearance after drying, and it is usually eaten in the immature state as a fresh vegetable; it is mostly grown in the United States, but has become popular among the elites in African countries;
– Waxy maize: the starch is composed entirely of amylopectin and is used for the manufacture of adhesives; it is mainly grown in eastern Asia for human consumption, but also in the West for industrial applications..
Within the various grain types, there are many cultivars grown in different parts of the world.
Growth and development
The first leaf of maize emerges from the soil usually 4–6 days after planting. The minimum temperature for germination is 10°C; the optimum around 20°C. The plant sometimes has a few tillers that are of value in low density stands. At a later stage some whorls of aerial roots (‘brace roots’) may develop from the lower nodes above the ground which partly help to anchor the plant while also contributing to the uptake of water and nutrients. Flower initiation is generally 20–30 days after germination. Maize is protandrous: in cultivars that mature in 4 months the male inflorescence emerges 50–60 days after planting and the styles of the female inflorescence appear about a week later. Maize is mature 7–8 weeks after flowering. The period from planting to harvesting varies considerably. It may be as short as 70 days in some extra early cultivars and as long as 200 days in some very late cultivars. Climatic conditions, latitude and altitude influence growth duration. In tropical highland areas it may take 9–10 months to maturity. Maize is predominantly cross-pollinating (90–95%), but is self-fertile. Maize follows the C4-cycle photosynthetic pathway.
Maize is adapted to a wide range of environments, but it is essentially a crop of warm regions where moisture is adequate. The bulk of the crop is grown in tropical and subtropical regions. In West and Central Africa the Guinea savanna zone offers the best ecological conditions for maize. The mid-altitude regions of East and southern Africa are also suitable for maize production. In Ethiopia, for instance, maize is mainly grown at 1000–2400 m altitude. Maize is generally less suited to semi-arid or equatorial climates, although drought-tolerant cultivars adapted to semi-arid conditions are now available. The crop requires an average daily temperature of at least 20°C for adequate growth and development; the optimum temperature for growth and development is 25–30°C; temperatures above 35° reduce yields. Frost is not tolerated. Maize requires abundant sunlight for optimum yields. The time of flowering is influenced by photoperiod and temperature; maize is considered a quantitative short-day plant. Maize is less drought-resistant than sorghum, pearl millet and finger millet. In the tropics it does best with 600–900 mm well-distributed rainfall during the growing season. It is especially sensitive to drought and high temperatures around the time of flowering.
Maize can be grown on a wide range of soils, but performs best on well-drained, well-aerated, deep soils containing adequate organic matter and well supplied with nutrients. The high yield of maize is a heavy drain on soil nutrients and maize is therefore often grown as a first crop in the rotation. It can be grown on soils with a pH of 5–8, but 5.5–7 is optimal. It does not tolerate waterlogging and is sensitive to salinity. Since a young crop leaves much of the ground uncovered, soil erosion and water losses can be severe and attention should be paid to adequate soil and water conservation measures.
Propagation and planting
Maize is propagated by seed and direct sowing is common. The 1000-grain weight is 150–300 g. Sowing should preferably be done early in the season, as soon as soil conditions and temperature are favourable and the rainfall is well established. Smallholders plant maize by hand while mechanical planting is practised on large commercial farms. Planting by hand requires 5–10 man-days/ha. Seed is dropped in the plough furrow or in holes made with a planting stick or hoe. Planting may be done on hills or in rows, on flat land or on ridges. Ridging or heaping is usually done on heavy soils, to improve drainage. The seed rate is up to 25 kg/ha in sole cropping, and 10–15 kg/ha in intercropping. When maize is sown in rows, the spacing is usually 75–90 cm between rows and 25–50 cm within the row, with 1–3 seeds per pocket, resulting in a plant density of 40,000–80,000 plants/ha. Wide spacing results in more weed growth and increases erosion. To obtain a high yield, a uniform crop stand is very important, as the tillering capacity of maize is limited. The sowing depth is commonly 3–8 cm, depending on soil conditions and temperature. Deep sowing is recommended on light, dry soils. On smallholdings the land is usually cultivated by hand or by animal traction. The usual depth of ploughing is 8–10 cm and ploughing is done just before or at planting time. Sometimes animal manure or fertilizers are applied at the time of planting.
Maize may be grown as a sole crop or in intercropping with other food crops such as common bean, cowpea, pigeon pea, groundnut, yam, cassava, sweet potato, pumpkin, melon or watermelon. In some parts of tropical Africa two crops of maize are planted per year. In areas where the rainy season is shorter, the crop is planted only once, although a second planting is possible under irrigation, on residual moisture on heavy soils or on hydromorphic soils.
Maize is very sensitive to weed competition during the first 4–6 weeks after emergence, and weed control is very important. The crop should be planted as soon as possible after the preparation of the seed-bed. Interrow cultivation to control weeds and to break up a crusted soil surface may be done until the plants reach a height of about 1 m. Weeding is mostly done by hand, requiring at least 25 man-days/ha. Chemical weed control is gaining importance in tropical Africa, because hand weeding is time-consuming and expensive as a result of the increasing scarcity of labour. Ridging or earthing-up is sometimes practised. Most maize production in tropical Africa is rainfed. Occasionally it is grown on bunds in irrigation schemes. Maize usually responds well to fertilizers. A maize crop yielding 2 t grain and 5 t stover per ha removes about 60 kg N, 10 kg P and 70 kg K per ha from the soil. Nitrogen uptake is slow during the first month after planting, but increases to a maximum during formation of the inflorescences. Maize has a high demand for nitrogen, which is often the limiting nutrient. High nitrogen levels should be applied in 2 doses; the first dose at planting or 2–3 weeks after emergence and the second one about 2 weeks before flowering. Phosphate is not taken up easily by maize and, moreover, many tropical soils are deficient in available phosphate. It is advisable to apply organic manures before ploughing to improve soil structure and supply nutrients. Smallholder farmers in tropical Africa apply little or no fertilizer to the maize crop. When they do, it is usually only once, about 4 weeks after planting when the crop is knee high.
Maize is grown in rotation with groundnut, common bean, cowpea, cotton and tobacco. Rotation with soya bean is gaining popularity in northern Nigeria; it increases maize yields by providing nitrogen and by reducing parasitism. In the United States maize is often grown in rotation with soya bean.
Diseases and pests
The most important fungal diseases of maize in tropical Africa are rots affecting female inflorescences (Fusarium spp. and other fungi), the stalk-rot complex (Diplodia maydis, Fusarium moniliforme, Macrophomina phaseoli and Pythium aphanidermatum) and leaf blights (Exserohilum turcicum and Bipolaris maydis). Of more local importance are downy mildew (Peronosclerospora sorghi), smut (Ustilago maydis) and rusts (Puccinia sorghi and Puccinia polysora). Grey leaf spot (Cercospora zeae-maydis) is important in East and southern Africa, but in West and Central Africa it occurs only in mid-altitude regions. Host-plant resistance is the most effective disease control measure. Cultivars resistant to Exserohilum turcicum leaf blight and downy mildew are available. Maize cultivars that possess resistance to multiple diseases are now available in tropical Africa. Measures to reduce mycotoxin contamination of the cob include early harvesting, rapid drying, sorting out of damaged and infected grains, sanitation (removal of crop residues, cleaning of stores, removal of heavily damaged cobs), improved storage, and the use of fungicides. The most important virus disease of maize is maize streak virus (MSV), which is restricted to Africa and may cause 100% yield loss. It is transmitted by leafhoppers (Cicadulina spp.) and is most serious in late-planted crops. Cultivars resistant to maize streak virus are available. Of lesser importance in tropical Africa are maize dwarf mosaic virus (MDMV), sugar cane mosaic virus (SCMV) and maize chlorotic mottle virus (MCMV). Maize is relatively tolerant to nematodes occurring in tropical soils.
The most serious insect pests of maize in tropical Africa are cutworms (Agrotis spp.), stem borers (especially Busseola fusca, Eldana saccharina, Sesamia calamistis and Chilo partellus), cob borer (Mussidia nigrivenella), cotton bollworm (Helicoverpa armigera), armyworm (Spodoptera exempta), leafhoppers (Cicadulina spp.) and less commonly variegated grasshopper (Zonocerus variegatus). Occasionally termites and locusts also infest maize fields. Application of insecticides may be necessary to control these pests. Cultural methods for insect control include early planting and burying or burning of crop residues. Although biological control of stem borers using natural enemies has not been very successful, it is still considered a potentially viable control option. Maize is not prone to bird damage.
Common storage pests of maize are grain moths (Sitotroga cerealella and Ephestia cautella), grain weevils (Sitophilus spp.) and the larger grain borer (Prostephanus truncatus). Grains may be mixed with small amounts of an insecticide (e.g. malathion) to control these pests. Rodents are also major storage pests in tropical Africa.
The parasitic witchweed (Striga spp.) is a serious constraint to maize production in many parts of tropical Africa, especially Striga hermonthica (Delile) Benth. in West and Central Africa and Striga asiatica (L.) Kuntze in southern Africa. No single control measure is effective for this weed, and an integrated approach is recommended, involving planting maize seed that is free from Striga seeds, planting resistant cultivars, adequate fertilizer application (especially N), crop rotation (e.g. with cotton, soya bean or cowpea), and removal of Striga plants before they flower.
Maize is usually harvested by hand. Mechanical harvesting is practised on large farms. Indicators of maturity are yellowing of the leaves, yellow dry papery leaves around the cobs, and hard grains with a glossy surface. In the dry season, maize is often left in the field until the moisture content of the grain has fallen to 15–20%. In case of harvesting by hand, the cobs should be broken off with as little attached stalk as possible. Cobs may be harvested with the surrounding leaves still attached. These may be turned backward and used to tie several cobs together and hung up to dry. Alternatively, the leaves are completely removed from the cobs, which are then stored in cribs to dry.
Maize has the highest yield potential among the cereal crops. The current average world yield of maize is 4.4 t/ha, but grain yields over 20 t/ha are possible. Average grain yields of maize in tropical Africa are about 1.25 t/ha, varying greatly from less than 1 t/ha for smallholders to about 6 t/ha in commercial farms. Yields higher than 10 t/ha have been recorded, but these are exceptional. In 2001 the average yields of maize in the different sub-regions of tropical Africa were: West Africa 1.3 t/ha, Central Africa 1.0 t/ha, East Africa 1.6 t/ha and southern Africa 1.4 t/ha.
Handling after harvest
The major post-harvest problems of maize in most production areas are reducing the moisture content of the grain to 12–15%, protection from insects and rodents, and proper storage. A high grain moisture content combined with high ambient temperatures can cause considerable damage, making the product unsuitable for consumption by humans and livestock. Maize grain for home consumption is either sun-dried for several days by hanging up whole cobs tied together by their leaves, or these are put in a well-ventilated store or crib. Shelling (the removal of grains from the cob) is usually carried out by hand, although mechanical shellers are available. The average shelling percentage is about 75%. The shelled grain is dried for a few days and then stored in bags, tins or baskets. The optimum moisture content for storage is 12–13%, but often it is not below 18%. Smallholder farmers generally select seed for the next crop from the last harvest. The selected cobs are stored at home in the surrounding leaves above the fireplace to prevent insect damage.
The largest germplasm collections of maize are held in India (Indian Agricultural Research Institute, New Delhi, 25,000 accessions), Mexico (International Maize and Wheat Improvement Center (CIMMYT), Mexico City, 22,140 accessions), the United States (USDA-ARS North Central Regional Plant Introduction Station, Iowa State University, Ames, Iowa, 17,910 accessions) and China (Institute of Crop Germplasm Resources (CAAS), Beijing, 15,840 accessions). In tropical Africa substantial germplasm collections are held in Kenya (Kenya Agricultural Research Institute (KARI), National Agricultural Research Centre, Kitale, 1780 accessions), Malawi (Malawi Plant Genetic Resources Centre, Chitedze Agricultural Research Station, Lilongwe, 970 accessions), Rwanda (Institut des Sciences Agronomiques du Rwanda (ISAR), Butare, 580 accessions).
Maize breeding in tropical Africa started with introduction of improved materials from Central and South America. Some of the cultivars were multiplied and distributed to farmers directly, while others were subjected to genetic improvement. Breeding for resistance to various diseases, such as rust, blight, smut and leaf spots, was a major objective. Many cultivars resistant to the prevalent diseases were released. At the initial stages of maize improvement in the region, composites (mixtures of genotypes from various sources that are maintained by normal pollination) and synthetic cultivars (cultivars produced by crossing a number of genotypes in all possible combinations, with subsequent maintenance by open pollination) were developed for the farmers. F1 hybrids (first generation progeny of crosses between genetically distinct parents) with greatly increased grain yield were produced in the United States and in some tropical African countries during the early part of the 20th century. Zimbabwe, for example, adopted hybrids at that time; most other tropical African countries could not produce hybrids because there were no seed companies that could produce and distribute hybrid seed in commercial quantities. Two international research institutes, IITA (International Institute of Tropical Agriculture) and CIMMYT, established breeding programmes in the region and greatly boosted the development of improved cultivars. With time, many African countries also established their own breeding programmes and developed maize cultivars for their special needs. Germplasm from CIMMYT and IITA has been widely used in these programmes. The demand for maize continued to increase, thus necessitating attempts to improve the yield of the cultivars grown by the farmers. Hybrid seed is commonly used in high-input farming with high fertilizer use and adequate facilities for seed production. In tropical Africa breeding methods such as recurrent selection, inbreeding and hybridization have been used in maize breeding. Seed companies are now being established in many of the countries, thereby making it possible to produce hybrid seed in commercial quantities. In low-input farming, composite or synthetic cultivars may be preferable, as they permit farmers to save seed from one crop to the next, while their wider genetic base provides a better adaptation to variable growth conditions. Emphasis in maize breeding is on incorporation of resistance to biotic and abiotic stresses. Several open-pollinated cultivars and F1 hybrids that are resistant to one or more stress factors, including Striga, diseases, insect pests, drought and low soil N, are now available for farmers in tropical Africa. ‘Obatanpa’, a Quality Protein Maize (QPM) with higher contents of tryptophan and lysine, developed by maize breeders in Ghana, is widely grown in West and Central Africa, and also in some East and southern African countries. Compared to other crops, the adoption level of improved maize cultivars is relatively high in tropical Africa. It has been estimated that 35–50% of the maize area in tropical Africa is planted with improved open-pollinated cultivars and F1 hybrids, but large differences exist between countries.
A range of techniques is available for in-vitro regeneration of maize, using callus tissue, cell suspensions, excised plant parts and immature embryos. Genetic transformation of maize is possible using Agrobacterium -mediated and biolistic methods, but the efficiency of the latter is relatively low. Genetic transformation of maize is now routinely and commercially employed, although genotype-independent techniques are not yet available. In 2001 the world area under transgenic maize was estimated at 9.8 million ha, and maize was only second to soya bean in area planted to transgenic crops. The main transgenic maize types planted are Bt maize (maize that possesses genes from Bacillus thuringiensis conferring resistance to the European maize borer, Ostrinia nubilalis), herbicide-tolerant maize or types with both traits. Bt maize has been commercially released in South Africa. CIMMYT is working on Bt maize for tropical Africa, especially to control stem borers. Industrial and academic research is testing transgenes capable of improving grain quality, e.g. by increasing the lysine content. Maize was one of the earliest crops to be subjected to molecular mapping; the first molecular map was reported in 1986. Many genetic linkage maps have been constructed since then, using mainly RFLP, SSR and SNP markers; maps have been integrated into a high-density linkage map. Quantitative trait loci (QTLs) for a wide range of traits have been localized, including grain yield, resistance to diseases and pests, drought tolerance, and oil and protein contents of the grain. Genome sequencing of maize is difficult because of its large size (2500 Mbp), complexity and highly repetitive character.
Maize will continue to play a large and important role in Africa’s food production. It is the principal staple food in large parts of East and southern Africa. Although less important in West and Central Africa, it is a major source of energy in these regions, especially in parts of Côte d’Ivoire, Ghana, Benin and Nigeria. Of the cereals, maize gives the highest yield per man-hour invested; it is usually the first crop to be harvested for food during the hunger period of the year; it is easy to grow as sole crop or intercropped with other crops; it is easy to harvest, it does not shatter and is not liable to bird damage. Many maize technologies have been developed in national and international research stations in Africa but most of these are yet to be adopted by the farmers. This has led to a large yield gap between the researchers’ and the farmers’ fields. High-quality seed is in short supply because most countries, especially in West and Central Africa, do not have adequately organized seed sectors. Farmers also need improved access to fertilizers, crop protection chemicals and other inputs. Cultivars and cropping techniques that fit well into the prevailing cropping systems are now being developed in collaboration with farmers in what is called participatory plant breeding.
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Sources of illustration
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Correct citation of this article:
Badu-Apraku, B. & Fakorede, M.A.B., 2006. Zea mays L. In: Brink, M. & Belay, G. (Editors). PROTA 1: Cereals and pulses/Céréales et légumes secs. [CD-Rom]. PROTA, Wageningen, Netherlands.
1, basal plant part; 2, central plant part with female inflorescences; 3, upper plant part with male inflorescence; 4, infructescence.
detail of male inflorescence
plant with female inflorescences
drying maize cobs