Zea mays (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Zea mays L.

Protologue: Sp. pl.: 971 (1753).

Family: Gramineae

Chromosome number: 2n= 20

• Maize, Indian corn (En)
• Corn (Am)
• Maïs (Fr)
• Indonesia: jagung
• Malaysia: jagong
• Philippines: mais
• Cambodia: pôôt
• Laos: khauz ph'ô:d, khauz sa:li:
• Thailand: khao-phot
• Vietnam: ngô, bắp.

Maize was first cultivated by Indian tribes some 7000 years ago and is thought to have originated in Mexico and Central America. Early civilizations of the Americas depended on maize cultivation. In the 16th Century it was introduced into SouthEast Asia by the Portuguese. It is still one of the most important grain crops and is geographically one of the most widely planted cereals. It is grown from latitudes up to 58 °N in Canada and Russia, throughout the tropics, to latitudes of 42 °S in New Zealand and the South American continent, and in areas below sea-level in the Caspian Plain and up to areas as high as 3600 m in Peru. Maize is not known from the wild.

Maize kernels are used for three main purposes:

• as a staple food, particularly in the tropics;
• as feed for livestock and poultry, particularly in the industrialized countries of the temperate zones, providing over twothirds of the total trade in feed grains;
• as a raw material for many industrial products.

Maize grain is prepared and consumed in a multitude of ways. For human consumption it is usually ground or pounded and the meal may be boiled, roasted or fermented. The main industrial products are starch, oil, syrup, organic liquids and alcoholic beverages. Most industrial products are usually obtained by the wetmilling process, in which the grain is steeped, after which the germ and bran are separated from the endosperm. The main product is starch. Oil obtained from the germ is made into soap or glycerine, but can be refined to produce a cooking or salad oil. The residues from the production of starch or oil, together with the bran, are used in animal feeds. 100 kg of whole maize, with a 16% moisture content, yields about 64 kg pearl starch and 3 kg oil; the remainder is used as feed. The starch may be used as human food or made into sizing, laundry starch and other products. Dry milling produces grits, consisting of coarsely ground endosperm from which most of the bran and the germ have been separated.

Maize also has a great number of subsidiary uses. Mature plants are used for animal feed. Silage maize is one of the leading crops in industrialized western countries; special cultivars and production technology have been developed. Crop residues such as the stalks are used for fuel or compost. The inner husks of the ear and the fibre in the stems have been used for making paper. Unripe ears can be consumed as a vegetable (baby corn, boiled or roasted corn).

Present world production (1993) is about 470 million t grain from about 127 million ha. The major producing countries are the United States with an annual production of 161 million t, China, the second largest producer (103 million t), Brazil (30 million t), Mexico (19 million t), and France (15 million t). The main producing countries in SouthEast Asia are Indonesia with an annual production of 6.6 million t, the Philippines (5.4 million t) and Thailand (3.8 million t). Only a small proportion of the total production enters world trade. The United States is the principal exporter, followed by France, China, Argentina, South Africa and Thailand. Other countries exporting small amounts include Indonesia. The largest importers of maize are Japan and the former Soviet Union. In Indonesia and the Philippines maize is mainly grown as a subsistence crop; between 60-75% of the production is consumed directly by the farm households, the remainder is processed by animalfeed mills and maizeoil factories. Maize is less profitable per ha than some other food crops and has a negative income elasticity (decreasing demand with rising income). Cultivation of maize remains popular in areas where limited water availability or relatively low temperatures do not permit rice production, as in parts of China, western Pakistan, northern India, the Philippines, Laos, Cambodia, Vietnam and Indonesia. In South-East Asia, it is mainly grown by smallholders.

The average composition per 100 g edible portion is approximately: water 10 g, protein 10 g, fat 4.5 g, carbohydrates 70 g, fibre 2 g, ash 2 g. The energy value averages 1525 kJ/100 g. The protein content varies from 6-15%, of which zein predominates. Maize is deficient in tryptophane and lysine, but cultivars with higher lysine and tryptophane content have been bred, using the recessive gene Opaque-2. The starch of the endosperm usually consists of a mixture of about three-quarters amylopectin and onequarter amylose. The endosperm, which accounts for 80% of the weight of the kernel, is poor in phosphorus and calcium and contains most of the starch and twothirds of the protein. More than 80% of the fat and most minerals are in the embryo or germ, which constitutes about 12% of the kernel. Yellow maize contains some provitamin A due to cryptoxanthin. Most vitamins are found in the outer layers of the endosperm and in the aleurone layer. Maize is unsuitable for making leavened bread as it lacks gluten. The 1000kernel weight is usually 250-300 g.

• A robust, monoecious, annual grass, 1-4 m tall.
• Root system consisting of adventitious roots, developing from the lower nodes of the stem below and often also just above the soil surface, usually limited to the upper 75 cm of the soil, but single roots sometimes penetrating to a depth of 200 cm and more.
• Stem usually simple and 1.7-3 m tall, solid, with clearly defined (8-)14(-21) nodes and internodes.
• Leaves 8-21, borne alternately on either side of the stem at the nodes, with overlapping sheaths, auricled above, and linearlanceolate blades, 30-150 cm × 5-15 cm, acuminate, with pronounced midrib; ligule about 5 mm long, colourless.
• Male and female inflorescences separate on the same plant.
• Male inflorescence ("tassel") a terminal panicle, up to 40 cm long, axis bearing a variable number of lateral branches, with paired spikelets, one sessile and the other with short pedicels, 8-13 mm long, each with 2 glumes and 2 florets, consisting of an ovate lemma, a thin palea, 2 fleshy lodicules, and 3 stamens.
• Female inflorescence ("ear") a modified spike, usually 1-3 per plant, developed in the axil of one of the largest leaves, about half way up the stem, enclosed by 8-13 modified leaves ("husks"), with paired, sessile spikelets, each with 2 glumes enclosing 2 florets, the lower of which is sterile, consisting solely of a short lemma and palea, and the upper pistillate, with a short, broad lemma and palea, no lodicules, and a single basal ovary and a long threadlike style and stigma ("silk"), which grows up to 45 cm in length and emerges from the top of the inflorescence, and is receptive throughout most of its length.
• Mature infructescence ("ear" or "cob") enclosed by husks, 8-42 cm × 3-7.5 cm. Grains or "kernels" (caryopsis) 30-1000 per ear, usually obovate and wedgeshaped, variously coloured from white, through yellow, red and purple to almost black.

The coleoptile emerges from the soil usually 4-6 days after planting. The plant may sometimes have a few basal branches ("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 firmly, while also contributing to the uptake of water and nutrients. Flower initiation is generally 20-30 days after germination. With a fourmonth cultivar the tassel emerges 50-60 days after planting and the silk appears about a week later. Compared with rice and other cereals, maize has a long postfloral period of 7-8 weeks. The period from planting to harvesting varies considerably. It may be as short as 70 days in some very early cultivars and as long as 200 days in some very late cultivars. Climatic conditions, latitude and altitude influence growth duration.

Z. mays is a highly variable, cross-pollinating, markedly heterogeneous, complex species, in which all forms hybridize freely. The cultivars can be divided into groups according to the structure and shape of the grain. The following cultivar groups are distinguished:

• cv. group Dent Corn or Dent Maize (synonyms: Z. mays L. convar. dentiformis Koern., Z. indentata Sturt., Z. mays L. var. indentata (Sturt.) Bailey). The sides of the grain have corneous endosperm, but soft white starch, extending to the apex, shrinks on drying to produce the characteristic dent. The wedge-shaped grains are usually yellow or white. It is the principal maize of the Corn Belt of the United States and northern Mexico.

• cv. group Flint Corn or Flint Maize (synonyms: Z. mays L. convar. mays or group vulgaris Koern., Z. indurata Sturt., Z. mays L. var. indurata (Sturt.) Bailey). 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 smaller than dent maize; it matures earlier, is hardier, and when dried 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.

• cv. group Pod Corn or Pod Maize (synonym: Z. tunicata Sturt.). This is the most primitive form of maize in which the grain is enclosed in floral bracts. It is not grown commercially but is preserved by some Indians who believe it to have magical properties. The earliest domesticated maizes were pod corns.

• cv. group Pop Corn or Pop Maize (synonym: Z. mays L. convar. microsperma Koern., Z. everta Sturt., Z. mays L. var. everta (Sturt.) Bailey). Popcorn 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. In rice popcorn cultivars the grains are pointed and tend to be imbricated; in pearl popcorn cultivars the grains are rounded and very compact. Popcorn is most important in the United States and Mexico.

• cv. group Soft Corn or Soft Maize, Flour Corn or Flour Maize (synonyms: Z. amylacea Sturt., Z. mays L. var. amylacea (Sturt.) Bailey, Z. mays L. convar. amylacea (Sturt.) Grebensc.). The grain can have all colours, it usually has no dent and the endosperm consists of soft starch. When parched it can be chewed more easily than flint corn and it is also easier to grind. It is one of the oldest maize forms and was widely grown in the drier parts of the United States, western South America and South Africa.

• cv. group Sweet Corn or Sweet Maize (synonyms: Z. mays L. convar. saccharata Koern., Z. mays L. var. rugosa Bonaf., Z. saccharata Sturt.). The grain contains a glossy endosperm with little starch, giving a wrinkled appearance after drying. The grain is usually eaten as a fresh vegetable; it is mostly grown in the United States, but becoming popular in South-East Asia.

• cv. group Waxy Corn or Waxy Maize (synonyms: Z. mays L. convar. ceratina Kuleshov, Z. mays L. subsp. ceratina (Kuleshov) Zhuk.). The starch is composed entirely of amylopectin and is used for the manufacture of adhesives. It is mainly grown in eastern Asia.

A compact filling gives the endosperm a horny, transparent appearance and improves the storability of the grain; a less compact filling gives the kernel a chalky, meallike appearance.

A great many cultivars belonging to the various grain types are grown in different parts of the world. The important aspects influencing tropical smallholders in choosing cultivars are growth duration, taste and cooking quality, yield stability and production, storability and suitability for intercropping.

With its large number of cultivars differing in period to maturity, maize has a wide range of tolerance of temperature conditions. It is characterized by the C4cycle photosynthetic pathway. It is essentially a crop of warm regions where moisture is adequate. The bulk of the crop is grown in tropical and subtropical regions. It is less suited to semiarid or equatorial climates. It is predominantly grown in areas with isotherms of 21-30°C at tasseling. The minimum temperature for germination is 10 °C. The crop requires an average daily temperature of at least 20 °C for adequate growth and development. The time of flowering is influenced by photoperiod and temperature. Maize is considered to be a quantitative shortday plant. It is grown mainly from 50 °N to 40 °S and from sea-level up to about 3000 m altitude at the equator. At higher latitudes, up to 58 °N, it can be grown for silage. Maize is specially sensitive to moisture stress around the time of tasseling and fertilization. It also needs optimum moisture conditions at the time of planting. In the tropics it does best with 600-900 mm of rain during the growing season. The shoot/root ratio is fairly high, rendering maize sensitive to drought.

Maize can be grown on a wide variety of soils, but performs best on welldrained, wellaerated, deep soils containing adequate organic matter and well supplied with available nutrients. The high yield of maize is a heavy drain on soil nutrients. Maize is often used as a pioneer crop, because of the high physical and chemical demands it makes of the soil. Maize can be grown on soils with a pH from 5-8, but 5.5-7 is optimal. It belongs to the group of crops that is considered to be 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.

Maize is nearly always planted through direct seeding. In China and Vietnam, however, a transplanting technique has been adopted. Maize should preferably be sown early in the season, as soon as soil conditions and temperature are favourable. Seed may be planted mechanically, but in smallholder cultivation it is usually sown by hand. This requires 5-10 man-days/ha. Seed is dropped in the plough furrow or in holes made with a planting stick. Planting may be done on hills or in rows, on flat land or on ridges. On heavy soils ridging is advisable, to improve drainage. Distance between the rows varies from 60-100 cm; crop density depends on soil conditions, rainfall, method of irrigation, cultivar and cropping system. Wide spacing results in more weed growth and increases the occurrence of erosion. A uniform crop stand is very important, as the tillering capacity of maize is limited. Average plant density varies from 20 000-80 000 plants/ha. An average seed rate of 10-25 kg/ha is fairly common; in Indonesia higher rates are not unusual to ensure reasonable plant stands at harvest time. The depth of planting is commonly 3-6 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 about 8-10 cm. In Indonesia ploughing is done just before or at planting time. Sometimes animal manure or fertilizers are applied at the time of planting.

Maize cropping in SouthEast Asia is mainly carried out within the following three cropping systems: (a) permanent upland cultivation, (b) wetrice system, and (c) shifting cultivation. Rotations or intercropping with other rainfed crops include soya bean, groundnut, other pulses, cassava, sweet potato, vegetables, tobacco and cotton. Maize is suitable for offseason cropping in rice fields, provided moisture and drainage are adequate.

Adequate weed control is very important. Maize is very sensitive to weed competition during the first 4-6 weeks after emergence. It 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 by hand requires a minimum of 25 man-days/ha. Chemical weed control is gradually gaining importance, because hand weeding is timeconsuming and is usually carried out rather late in the growing season. The herbicide most widely used for postemergence spraying is 2,4D. Ridging or earthingup is sometimes practised. Irrigation is used in areas of low rainfall and is particularly valuable at the time of tasseling and fertilization.

Maize usually responds well to fertilizers, provided other growth factors are adequate. The quantity of manure applied by smallholders is usually very limited. Improved cultivars can only reach their high yield potential when supplied with sufficient nutrients. A maize crop of 2 t/ha grain and 5 t/ha stover removes about 60 kg N, 10 kg P and 70 kg K from the soil. Nitrogen uptake is slow during the first month after planting, but increases to a maximum during ear formation and tasseling. Maize has a high demand for nitrogen, which is often the limiting nutrient. High nitrogen levels should be applied in 3 doses, the first at planting, the second when the crop is about 50 cm tall, and the third at silking. 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 to improve soil structure and supply nutrients, all before ploughing.

Downy mildew (Sclerospora spp.) is the most serious disease of maize in SouthEast Asia. Severe losses are recorded annually in India, Indonesia, the Philippines and Thailand. Maize is most susceptible during the first 2 weeks after planting. Several cultural practices reduce the severity of downy mildew either by eliminating the pathogens from a particular area, reducing primary inoculum, or by stimulating early plant growth. Ridomil is an effective fungicide now widely used against downy mildew. Commercial cultivars (open-pollinated or hybrid) released in South-East Asia usually have some resistance. Other diseases are leaf blight (Helminthosporium turcicum and H. maydis), rust (Puccinia spp.), stalk and ear rots caused by various pathogens, and maize smut (Ustilago maydis). Stem borers, cornear worms and army worms are among the most serious pests. The principal pests of stored maize are Angoumois grain moth (Sitotroga cerealella), grain weevils (Oryzaephilus surinamensis, Sitophilus oryzae) and rodents.

The semiparasitic weed striga (Striga spp.), a serious problem in maize in Africa, is not of great importance in SouthEast Asia.

Maize is usually harvested by hand. Mechanical harvesting is practised on large farms in Thailand and parts of the Philippines. The stage of maturity can be recognized by yellowing of the leaves, yellow dry papery husks, 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 hand harvesting the ears should be broken off with as little attached stalk as possible. They may be harvested with the husks still attached. These may be turned back and the ears tied together and hung up to dry.

Maize yields vary greatly, from about 1 t/ha for smallholders up to about 9 t/ha. Average yields of maize in t/ha are as follows (1993): United States 6.3, Europe 4.7, Africa 1.7, South America 2.7, Indonesia 2.2, the Philippines 1.7 and Thailand 3.2.

The major problems in most maizeproducing areas are reducing the moisture content of the grain to 12-15%, protection from insects and rodents, and proper storage after harvest. A high moisture content with high temperatures can cause considerable damage, making the product unsuitable for human consumption. Maize for home consumption is either sundried on the cob for several days by hanging up tied husks, or put in a wellventilated store or crib. Shelling (the removal of grains from the cob) is usually carried out by hand, though several hand and pedalpowered mechanical shellers are now available. The average recovery is about 75%. The shelled grain is dried again 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%. In Indonesia seed for the next crop is generally selected from the last harvest. The selected ears are stored at home in the husk above the fireplace to prevent losses by insects. Crop residues are removed from the field and then used as fodder, fuel, etc.

International institutes such as CIMMYT (International Maize and Wheat Improvement Center, Mexico) and IITA (International Institute of Tropical Agriculture, Nigeria) play a major role as conservators and suppliers of germplasm. Both institutes frequently cooperate with national breeding programmes. Many tropical countries maintain their own germplasm collections.

Maize is a crosspollinated crop with many cultivars, most of which are adapted to or bred for particular geographical areas. Many tropical countries have their own breeding programmes producing cultivars for their special needs, e.g. human nutrition, silage, or industrial processing. The extinct wild maize and the first domesticates in Mexico and Central America were pod corns and popcorns with very small ears. Maize evolved into a highly productive crop in a comparatively short time. Greatly increased yields became attainable through the discovery and development of hybrid maize in the early part of the 20th Century, created by crossing two or more inbred lines. Attempts to improve yield of openpollinated maize resulted in the production of synthetic (mixture of inbred lines) and composite (mixture of improved selections) cultivars. These are usually superior to local, openpollinated cultivars, but not as productive as the best single and doublecross cultivars adapted to a particular environment. Hybrid seed is commonly used in highinput farming with high fertilizer use and adequate facilities for seed production. In lowinput farming composite or synthetic cultivars may be preferable, as they permit seed to be kept from one crop to the next. The wider genetic base of these cultivars provides a better adjustment to variable growing conditions. The use of hybrid seed under such conditions would be hampered by problems such as the production and distribution of high-quality seed by government agencies or commercial seed firms, and the need for higher inputs. In the meantime the use of composites or synthetics, which are better adapted to the smallholder's needs, could be an improvement over the existing maize cultivars. It is estimated that 45% of the maize area in South-East Asia is still planted with local unimproved cultivars, 45% with improved open-pollinated cultivars and only 10% with hybrids. In maize breeding attention is paid to grain yield, duration of growth, resistance to diseases and pests, response to nitrogen, tolerance of heat and drought, tolerance of acid soils, resistance to lodging, ear characteristics and protein content.

The potential yield of maize is higher than that of rice or wheat. It may be expected that maize will assume a proportionally larger and more important role in world food production. Maize will remain an important cereal in SouthEast Asia because:

• it gives the highest yield per man-hour of invested labour;
• the husks provide protection against birds and rain;
• it is easy to harvest and to store and it does not shatter;
• it can be harvested over a long period (first the immature ears, a few weeks later the mature ones);
• it can tolerate a wide range of temperatures; and
• the demand for maize as food and feed is increasing.

Yields can be improved considerably. Low yields are due to a combination of the following factors:

• maize is mainly cultivated as a rainfed crop;
• farmers have been slow to adopt available improved cultivars and advanced cropping techniques;
• there is a shortage of high quality seed;
• maize cropping is much less profitable than certain other food crops.

These aspects are closely related to marketing, prices, transport facilities, drying, storage, processing and usage. Often there is no efficient agency for the distribution of seed. Farmers need to have access to improved seed, fertilizers, crop protection chemicals and other inputs. Cultivars and cropping techniques that fit well into the prevailing cropping systems should be developed. The improved cultivars available today are not always suitable for local mixed intercropping systems. Therefore research should be more closely geared to farmers' needs.

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• A. Koopmans, H. ten Have & Subandi