Manihot esculenta (PROSEA)
Manihot esculenta Crantz
- Protologue: Inst. rei herb. 1: 177 (1766).
- Family: Euphorbiaceae
- Chromosome number: 2n= 36
- Cassava, tapioca, Brazilian arrowroot (En).
- Manioc (Fr)
- Indonesia: ubi kayu, singkong, ketela pohon
- Malaysia: ubi kayu, ubi benggala, ubi belanda
- Papua New Guinea: mandioca, cassava
- Philippines: kamotingkahoy (Tagalog, Bisaya), balangay (Bisaya), kasaba (Ilokano).
- Burma (Myanmar): palawpinanupin
- Cambodia: dâmlô:ng chhë:
- Laos: man tônz
- Thailand: man-sampalang (general), man-samrong (central), man-mai (southern)
- Vietnam: sắn, khoai mì.
Origin and geographic distribution
Cassava is indigenous to tropical America, as are all species (98-200) of the genus Manihot Miller. It is not known in a wild state and the origin of cassava as a crop is unknown. It is thought to have first been cultivated in northeastern Brazil (related wild species are very abundant there) but Mexico and Central America are also mentioned as centres of domestication. Cassava was certainly already cultivated in many parts of tropical America in the first millennium BC, and its initial cultivation is thought to be considerably more ancient. In the postColumbian Era, the cassava plant spread from the American continent. It is known to have been introduced early on in several Asian countries by the Portuguese. It was probably introduced around 1810 in Indonesia. Spanish explorers and traders from Mexico are thought to have introduced it in the Philippines. Initially, its adoption was slow in all countries of SouthEast Asia. In Java, cassava cultivation had spread little by 1880. Good export prospects for cassava starch and dried storage roots, failures of rice and maize crops causing famine during several years, and promotional campaigns in favour of cassava organized by the colonial Dutch government were the main reasons for the rapid expansion of cassava cultivation in Indonesia at the beginning of the 20th Century.
It is now a prominent crop in SouthEast Asia, particularly in Indonesia and Thailand.
Of the world production of cassava, 65% is used directly for human consumption, 20% for animal feed and the remaining 15% for starch and industrial uses. For human consumption, the storage roots are peeled and chopped and then boiled, steamed, fried or roasted, directly or after drying or fermenting.
In SouthEast Asia, the use differs considerably between different countries. In Thailand, local use for human consumption is unimportant. Ninety-five per cent is exported, mainly for animal feed in European countries. The remainder is for local human consumption and industrial uses. In Indonesia, 60% of production is used for direct human consumption, 25% fresh and 35% after drying; 25% is used for starch production, most of which is for human food as kerupuk, cookies and other snacks. The remaining 15% is exported. In Malaysia, the starch industry uses about 90% of all cassava produced. The remaining 10% is used for animal feed. Only a very small proportion of local production is used directly for human consumption in Malaysia. The cassava produced in the Philippines is split equally between human consumption, animal consumption and starch production. In China, cassava is presently used mainly for industrial processing. Starch and monosodium glutamate are the traditional industrial products. With the improvement of cassava industrial technology, other products such as alcohol and starch-based sweeteners, e.g. glucose and fructose, have become more important. In Vietnam, the use of cassava for human consumption (actually 80% of the production) is decreasing because of the improvement of the rice supply. Presently about 10% of cassava production is processed into starch, the remaining 10% is for animal consumption.
Throughout the region cassava leaves are used for human or animal consumption.
Production and international trade
Production of fresh storage roots from cassava in Asia is 50 million t per year, which represents about one third of world production. The main producers are Thailand with 21 million t and Indonesia with 16 million t. Indonesia has been the leading producer for many years. The very rapid increase in Thailand is striking: from about 2 million t in 1964 to 20 million t in 1984. The export of cassava to Europe started at the beginning of the 1960s, serving as a component of animal feed, initially as meal, later as chips and since about 1969 as pellets. In the last decade, with a brief depression in 1986 and a peak in 1989, the yearly production in Thailand has been about 20 million t.
Other Asian countries with a significant cassava production are India (5.3 million t), China (3.4 million t), Vietnam (2.6 million t) and the Philippines (1.8 million t). Production in Asian countries was more or less stable between 1983-1993, which means a per capita decline.
The edible portion of fresh storage roots represents 80-90% of the total storage root weight. Its average composition per 100 g is: water 62 g, protein 1.0 g, fat 0.3 g, carbohydrates (mainly starch) 35 g, and minerals 1.0 g. The energy value amounts to 600 kJ/100 g. The contents of protein, fat, most vitamins and minerals are low. Only the vitamin C content is of importance. The protein of cassava storage roots is especially deficient in sulphurbearing essential amino acids. The dry matter content of the roots varies from 30-40%. The protein content of fresh cassava leaves is up to 7%. The leaves also contain reasonable amounts of carotene and vitamin C.
Cassava roots can contain dangerous amounts of cyanogenic glucosides. The glucoside content (as HCN) in the central part of fresh storage roots varies from 10-490 mg/kg. Small amounts are tolerable, but a person should not consume more than 1 mg HCN per kg body weight per day. Glucoside content is also influenced by ecological conditions and mineral supply. High content of nitrogen and low content of potassium in the soil increase the glucoside content. The first rains after a dry season may also cause a large increase in glucoside. If the cells of storage roots are crushed, glucosides and the enzyme linamarase make contact and the HCN is produced. This is the key to methods of getting rid of HCN. The volatile HCN should be allowed to escape. Boiling is not always a guarantee that the product is safe, as the HCN can be trapped in the starchy paste. Grating and slowly drying the resulting product is effective. Though consumers in villages usually know how to prepare a safe product, accidents still occur, especially with children. Cassava leaves also contain considerable amounts of cyanogenic glucosides, but no accidents have been reported in relation to their consumption. It is advisable, however, to cut the leaves into pieces before cooking and to throw away the cooking water.
- A perennial, monoecious, cultivated shrub, up to 4 m tall, all parts containing white latex and varying concentrations of a cyanogenic glucoside.
- Seedling forming a taproot with generally slender secondary roots.
- Adventitious roots arising from stem cuttings, very variable in shape, size, position and number, usually 5-10(-20) per plant, usually tapering but also long and slender, cylindrical to globose, up to 100 cm × 15 cm, serving as storage organ of starch in the parenchymatous cells of the white, yellowish or reddish pith; storage roots white, brownish or reddish, becoming lignified with age.
- Stem woody, unbranched to variously branched, predominantly brownish or greyish, usually with prominent leaf scars.
- Leaves arranged spirally with phyllotaxis 2/5, petiolate, simple; petiole 5-30 cm long, attached basally to the blade or slightly peltate; blade entire to 3-10partite to near the base; lobes oblong, obovate, linear or lanceolate, 4-20 cm × 1-6 cm, entire, acuminate.
- Inflorescence a lax terminal raceme, 3-10 cm long.
- Flowers unisexual with 5 united sepals and no petals, the pistillate flowers are basal and open first, the staminate flowers are apical; male flower with pedicel 4-6 mm long, calyx about 1 cm long, divided to or beyond the middle, yellowish, stamens 10 in two whorls; female flower with pedicel 0.5-2.5 cm long, calyx up to 1.3 cm long, stigmas 3, thick, wartylobed, ovary 3carpeled.
- Fruit a 6-winged subglobose capsule, 1-1.5 cm in diameter, with up to 3 ellipsoid seeds, 12 mm long, carunculate, variously marked or plain.
Growth and development
All commercial crops are grown from stem cuttings. Planted cuttings start to root from the soilcovered nodes, at the base of the axillary buds and the stipule scars, some 5 days after planting. Sprouting starts about 10 days after planting. At that time, callus can also be observed at the base of the cutting, from which a large number of adventitious roots emerge. Two to four months after planting, storage roots start to develop by secondary thickening of a number of the adventitious roots. In tropical regions, an almost constant proportion of dry matter production is stored in the roots once secondary thickening has started. This proportion depends on cultivar and ecological conditions.
The number of shoots per planted cutting depends on the length and orientation of the cuttings: the longer the cutting, and the more horizontally it is planted, the greater the number of shoots. New leaves are formed continuously, but the rate decreases with time. Older leaves die and fall after 40-200 days. After a certain number of nodes (leaves) has been formed (depending on cultivar and ecological conditions), the growing point becomes reproductive. This entails a number of axillary buds just below the growing point sprouting and developing into similarly sized branches (generally 2-4). Later in the growth period this process may be repeated once or more. However, some cultivars do not branch at all. There is strong evidence that long days stimulate flower initiation in some cultivars. Premature abortion of inflorescences may prevent flowering. Flower initiation, however, results in the stem forking.
In cassava, both crosspollination and selfpollination occur naturally. Male and female flowers hardly ever open simultaneously in the same raceme; however, female and male flowers on different branches of the same plant commonly open simultaneously. Male sterility is frequent. Crossincompatibility has not been found. The fruit matures three to five months after fertilization.
Other botanical information
There is no satisfactory general botanical classification of cassava below species level. Cassava's pantropical distribution by man and its cultivation since ancient times have resulted in an enormous number of cultivars which, when compared, show continuous variation in every characteristic studied. Many attempts to classify the cultivars formally have failed and usually only have historical or local practical value.
All cassava cultivars contain cyanogenic glucosides (mainly linamarin), which liberate toxic HCN by enzymic breakdown. Glucosides are present in all plant parts. Cultivars used to be divided into two groups on the basis of the glucoside content in the central part of the storage roots: "sweet" and "bitter". This distinction is not justified as all kinds of intermediates occur, and correlation between the glucoside content and the taste is far from straightforward.
Distribution of cassava is worldwide in regions between 30°N and 30°S. In equatorial areas, cassava can be grown up to 1500 m altitude. At the more extreme latitudes the growth period is limited because of the occurrence of periods of frost, which result in an immediate die-off of the plant. The optimum temperature range is 20-30°C. Specific cultivars are necessary for successful cultivation at an average temperature of 20°C.
Cassava is grown in regions with 500-6000 mm of rainfall per year. Optimum annual rainfall is 1000-1500 mm, without distinct dry periods. Once established, cassava can resist severe drought. With prolonged periods of drought, cassava plants shed their leaves but resume growth after the rains start, making it a suitable crop in areas with uncertain rainfall distribution. Because of its drought resistance, in many regions cassava is planted as a reserve crop against famine in dry years. This practice has often been the reason for its introduction in a certain area. Good drainage is essential for cassava; the crop does not tolerate waterlogging.
A linear relationship has been observed between the amount of absorbed incoming radiation and growth, thus high irradiance is preferred.
Cassava is grown on soils with very different physical and chemical characteristics. Best growth and yield are obtained on very fertile sandy loams. It is able to produce reasonable yields on severely depleted or even eroded soils where other crops fail. Gravelly or stony soils causing problems with root penetration are unsuitable, as are heavy clay or other poorly drained soils. Cassava growth and yield are reduced drastically on saline soils with an electrical conductivity of more than 50 mS/m and on alkaline soils with a pH above 8.0. The optimum pH is between 5.5 and 7.5, but cultivars are available that tolerate a pH as low as 4.6 or as high as 8.0. Reasonably salttolerant cultivars have also been selected. Cassava is tolerant of high levels of exchangeable aluminium and available manganese.
Propagation and planting
In commercial production, cassava is propagated exclusively from stem cuttings. Propagation from storage roots is impossible as the roots have no buds. Propagation by seed is possible but is only practised in breeding. Cuttings, well lignified, 20-30 cm long and 20-25 mm in diameter, preferably from the middle of the stems of plants 8-14 months old, are most suitable. Healthy material should be taken and should be dipped in fungicide and insecticide before planting.
The interval between cutting stems and planting should be as short as possible (not more than a couple of days). Whole stems can be stored in shady places for 3 months.
In Asia, cassava is usually planted vertically. The drier the soil the bigger the part of the stem placed in the soil. Under very dry conditions, cuttings should be planted at an angle and covered for the larger part with soil. There is no clear relation between planting angle and yield. Horizontal planting leads to a large number of thin stems, which may cause lodging. Cuttings should not be planted upside down, as this drastically reduces yield.
Soil preparation varies from practically zero under shifting cultivation to ploughing, harrowing and possibly ridging in more intensive cropping systems. Planting on ridges is recommended, especially for areas with rainfall of more than 1200 mm per year. Ridging may not give higher yield, but harvesting is easier and soil erosion may be reduced, especially by contoured ridges. Plant density is 10 000-15 000 plants per ha in sole cropping. In intercropping, densities are usually lower. Cassava for home consumption is often planted in mixtures with crops such as maize, groundnuts, other grain legumes, coconuts or bananas. Sole cropping is the common method for largescale production.
Planting is usually at the beginning of the rainy season. It is mostly done by hand, though largescale planting may be mechanized. Cuttings are then planted horizontally.
In Java, a special grafting technique has been developed by Mukibat, a farmer. Manihot glaziovii Muell. Arg. ("tree cassava") is used as a scion and ordinary cassava as the rootstock. This method is used by many small farmers in Java, especially for home garden production. Very high yields can be obtained with this method, especially under dry conditions. The reason may be that Mukibat plants have a more extensive root system, allowing greater uptake of water and nutrients. It is a very labourintensive method and probably not suitable for largescale production.
Weeding is necessary every 3-4 weeks until 2-3 months after planting. Afterwards the canopy may cover the soil and weeding is less necessary. Herbicide use is still limited, though preemergence and postemergence herbicides are increasingly being used where labour is scarce, e.g. in largescale production.
As yet, fertilizers are little used, though yield declines in the long term if fertilizers are not applied. In fact, use of fertilizers is often not economic because of low and uncertain prices. Moreover, the crop can still produce reasonably well on soils of poor fertility where other crops such as maize do badly. In general, cassava responds well to farmyard manure. Nutrient removal by 1 t of fresh storage roots is N 2.3 kg, P 0.5 kg, K 4.1 kg, and Ca 0.6 kg. Nutrient removal by 1 t of total plant (storage roots, stems and leaves) is N 4.9 kg, P 1.1 kg, K 5.8 kg and Ca 1.8 kg. Stems are often taken from the field and used as firewood. Fertilizer recommendations for cassava are not easy to make. In East Java, good yield responses are obtained with nitrogen fertilizers and more and more farmers are applying small amounts of nitrogen. Critical contents of major elements to prevent deficiencies in the youngest fully expanded leaf blades of cassava plants 2-5 months old are: N 5.0%, P 0.4%, K 1.2%, Ca 0.7% and Mg 0.3%.
Under certain conditions symbiotic fungi of cassava roots (mycorrhiza) can significantly increase phosphate availability.
There is a wide variety of cropping patterns and rotations with cassava. Though rotation with other crops is preferable, cassava is sometimes grown continuously on the same land, especially in dry areas not suitable for other crops (but in Thailand also because of the economic importance of the crop). When grown in bush-fallow systems, cassava is usually planted at the end of the rotation cycle, as it still produces relatively well at lower fertility levels and also allows a smooth transition to the fallow.
Diseases and pests
Damage by diseases and pests is relatively moderate in SouthEast Asia. Cassava bacterial blight ( Xanthomonas spp.) is present in Asia but no severe damage has been reported. Leaf spot ( Cercospora spp.) is quite common but there are no clear data about the yield reduction it causes. The major pest in SouthEast Asia is probably red spider mite ( Tetranychus spp.). Locally, scale insects can seriously reduce yield.
Cassava has no distinct period of harvesting, because the crop is perennial. At harvest the top part of the stem is cut off and then the plant is pulled up or levered out of the ground with a tool. For human consumption, it is usually harvested 9-12 months after planting. It is sometimes harvested earlier if needed for food. When grown for starch production, it may be harvested after 18 months or even later. Optimum harvest period depends on root quality, yield and climatic conditions. Storage roots become too woody if harvesting is delayed.
Average world yield of fresh roots is 10 t/ha. There is much variation between countries; in Asia, yields are higher (on average 13 t/ha) than in other continents. Nevertheless, actual yields on the farm are far below potential yields. Under optimal conditions, a yield of 30 t/ha of dry storage roots, i.e. 90 t/ha of fresh storage roots, is possible. Much depends on climate, soil fertility and the inputs. Annual yields of fresh roots of 30-40 t/ha are not difficult to achieve. But, as cassava is often grown on poorly fertile soils with low inputs, such yields are quite rare in practice.
Leaves can be taken from plants grown for roots or from plants specially cultivated for their leaves. If the leaves of a crop intended for root production are harvested, there is a reduction in the storage root yield. If cassava is grown specially for its leaves the first harvest can be 50-70 days after planting; yields of 20 000 kg/ha per year have been reported.
Handling after harvest
Once harvested, cassava must be consumed or processed within a couple of days. Physiological changes cause blue or brown vascular streaking in the roots, just below the peel, already 2 days after harvest. In addition, a microbial deterioration normally starts after the onset of physiological deterioration but often within a week after harvest. First symptoms are blue or brown streaks throughout the root. Spoilage is fastest in damaged roots.
For danger of toxicity on consumption directly after harvest, see under Properties.
Some progress has been made with storage of fresh roots. One method is to pack fresh, undamaged roots in moist sawdust in boxes; it can be used for marketing roots in urban areas. Storage for up to two months is possible. Microbiological deterioration, however, still occurs. Another method of storing cassava is to dry pieces of roots, called "gaplek" in Indonesia. These chips should be dried quickly to avoid deterioration. Sundrying is quite common. The shape of the chips is important for quick drying, a cube 1 cm across is recommended. For commercial production, as in Thailand, chips are dried on concrete floors. Afterwards they are converted into pellets, which are denser than chips and easier to transport. In smallscale drying on the farm, cassava chips are often dried on woven bamboo mats.
Cassava can also be stored as flour. For this purpose, roots are peeled, grated, squeezed, and then slowly roasted and dried. This product is called "farinha de mandioca" in Brazil and "gari" in West Africa. Cassava starch is usually prepared in special factories. Roots are washed, crushed and further processed. The starch is usually separated by centrifuging.
In SouthEast Asia, only limited cassava germplasm banks occur, e.g. in Thailand 250 germplasm accessions have been registered, in Indonesia 208, in the Philippines 338, and in China 72. The largest cassava germplasm bank is at CIAT (Centro Internacional de Agricultura Tropical) in Colombia, containing about 3000 accessions from large parts of Central and South America.
Breeding of cassava started seriously in Indonesia in about 1908, largely with genotypes imported directly from South America. The most extensive breeding programmes are at CIAT in Cali, Colombia and at IITA (International Institute of Tropical Agriculture) in Ibadan, Nigeria.
In 1983, a Thai-CIAT cassava breeding programme was established in Thailand with the dual function of developing cultivars for Thailand and generating promising breeding material for other Asian programmes. Transfer of selected clones and seed (from crosses) from Thailand to other cassava breeding programmes in Asia has recently been initiated. In Thailand, five cultivars have been released so far: Rayong 1, 2, 3, 60 and 90. Breeding in Thailand is especially oriented towards higher yielding ability, higher storage root dry matter content and early harvestability, compared to the traditional cultivars. This breeding programme has stimulated the release of new cultivars in other Asian countries too. Local breeding in Indonesia has resulted in the release of Andira 1, 2 and 4, the latter in 1986. As a result of cooperation with the Thai-CIAT breeding programme some new clones will be released in the near future. New cultivars, some of them CIAT-introduced materials, have also been released recently in the Philippines.
Additional objectives of the breeding programme are low glucoside content, broad adaptation to various soil and climatic conditions, good eating quality and resistance to the main diseases and pests.
Because of population growth, cassava production for human consumption, especially in Africa, is steadily increasing. In Asia, however, production remained rather stable during the last decade. It appears that if average income in society increases, the relative importance of cassave for human consumption decreases. Cassava may become more important as a feedstuff if the use of storage roots in local animal feed receives more attention. Production in Thailand and Indonesia for the feedstuff market of western Europe is expected to remain quite important.
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H.J. Veltkamp & G.H. de Bruijn