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Root Crops: YAM (Dioscorea spp.)
| By goGreen | October 2, 2011 |  |
Common name
YAM.
Botanical name
Dioscorea spp.
Family name
Dioscoreaceae.
Other names
Car (Braz.); Gname (It.); Ighnam (Ar.); Igname (Fr. and It.); Ignamekolle (Ger.); Inhame (Port.); Iniamas (Nether.); Name (Sp.); Nyambi (Fr.); Nyami (Sen.); Yamswurzel (Ger.).
Botany
Dioscorea is a large genus of over 600 species with subterranean tubers or rhizomes. The tubers are storage organs and often grown to a considerable size; they produce short, fibrous roots and annual shoots, which are twining (except in dwarf species), the direction of twining being specific.
The leaves are petiolate, often cordate, with strongly marked reticulate veining (unusual for a monocotyledon), sometimes lobed, occasionally palmately compound. Many species produce bulbils in the axils of the leaves which have the morphology and appearance of condensed stems and in a few instances are relatively large and tuberous. The flowers are small, and borne in long racemes, with male and female separate and usually borne on different plants. The female flowers are followed by dehiscent capsules, usually trilocular, with 6 seeds, usually winged for wind dispersal, though many of the cultivated forms have become partially or highly sterile.
The genus Dioscorea is divided into a number of taxonomic sections; the important food yams are grouped in the following: Enantiophyllum-D. alata, D. rotundata/D. cayenensis complex, D. opposita and D. japonica; Lasiophyton-D. dumetorum and D. hispida; Combilium-D. esculenta; Macrogynodium-D. trifida; Opsophyton-D. bulbifera.
Origin and distribution
The genus Dioscorea is considered to be among the most primitive of the Angiosperms and was present and well diversified in part of the southern world at the end of the Cretaceous period (approximately 75 million years ago), and the early spread appears to have been via an antarctic continent (whose climate was totally different in early geological times). The occurrence of Dioscorea spp. in southern Asia, Africa and South America long pre-dates human history and domestication of the different species in these areas appears to have been by aboriginal man. Wild yams and domesticated cultivars occur throughout the tropical and subtropical world, with one dwarf species (D. pyrenaica Bub. and Borders) as far north as the Pyrenees. West Africa is the most important cultivation zone, where yam is a major staple, producing about 93 per cent of the world’s edible yams, but the crop is also of considerable importance in parts of eastern Africa, the Pacific area (including Japan), the Caribbean and tropical America.
Cultivation conditions
Temperature-most edible yams cannot withstand frost and make poor growth below 20°C. Optimum growth occurs at about 30°C; temperatures much above this have an adverse effect, especially if associated with drought. An exception is D. opposita (see Chinese yam).
Rainfall-although generally considered drought resistant, yams require adequate moisture throughout their growing period and there is a positive correlation between high and regular rainfall, vine growth and tuber yield. For optimum yields adequate moisture between the 14th and 20th weeks of growth is of great importance. The major areas of production are centred where there is a sharply demarcated dry season of 2-5 months and a rainfall of 120-150 cm or more during the growing season. In parts of West Africa yams are grown where the rainfall is as low as 60 cm per year, but yields are very poor, while crops are also obtained where the annual rainfall reaches 300 cm.
Soil-good drainage is essential and for optimum yields a deep well-drained sandy loam is required. On heavy, waterlogged soils the tubers are liable to rot, while on poor soils the weak root system is unable to obtain sufficient water or nutrients to produce reasonably-sized tubers. Most yams are grown on land after it has been cleared from bush; fallow mulching is often practiced and FYM at the rate of 17.5-25 t/ha gives greatly increased yields. Fertilisers are not widely used but there is a wide response to treatment, particularly to the application of phosphorus and potassium. The use of potassium sulphate at the rate of 125-370 kg/ha or a 12:12:8 NPK mixture at the rate of about 60 g per mound has been recommended in Nigeria; while in Trinidad potassium muriate at 376 kg/ha and superphosphate at 376 kg/ha are used. In Barbados yams grow well as a rotation crop in sugar cane land that has residual nitrogen and potassium from the previous cane crop, but yields have been substantially increased by the application of 225 kg/ha of a 9:10:23 NPK fertiliser. Application should be 2-3 months after growth commences. The application of potash alone has given yield increases in Nigeria, and it is also reported to increase the storage life of the tubers, while chlorine in the fertiliser adversely affects the starch content.
Altitude-most yams can be grown successfully at low or medium elevations and some, such as D. alata, are reported to be grown at altitudes up to 2 700 m in the Himalayas, but in general yields are considerably reduced above 900 m.
Day-length-the majority of Dioscorea spp. exhibit a photoperiodic response and although day-lengths greater than 12 hours favour the growth of the vine, tuber development is normally most satisfactory under short-day conditions (10-11 hours of daylight).
Cultivation conditions
Temperature-most edible yams cannot withstand frost and make poor growth below 20°C. Optimum growth occurs at about 30°C; temperatures much above this have an adverse effect, especially if associated with drought. An exception is D. opposita (see Chinese yam).
Rainfall-although generally considered drought resistant, yams require adequate moisture throughout their growing period and there is a positive correlation between high and regular rainfall, vine growth and tuber yield. For optimum yields adequate moisture between the 14th and 20th weeks of growth is of great importance. The major areas of production are centred where there is a sharply demarcated dry season of 2-5 months and a rainfall of 120-150 cm or more during the growing season. In parts of West Africa yams are grown where the rainfall is as low as 60 cm per year, but yields are very poor, while crops are also obtained where the annual rainfall reaches 300 cm.
Soil-good drainage is essential and for optimum yields a deep well-drained sandy loam is required. On heavy, waterlogged soils the tubers are liable to rot, while on poor soils the weak root system is unable to obtain sufficient water or nutrients to produce reasonably-sized tubers. Most yams are grown on land after it has been cleared from bush; fallow mulching is often practiced and FYM at the rate of 17.5-25 t/ha gives greatly increased yields. Fertilisers are not widely used but there is a wide response to treatment, particularly to the application of phosphorus and potassium. The use of potassium sulphate at the rate of 125-370 kg/ha or a 12:12:8 NPK mixture at the rate of about 60 g per mound has been recommended in Nigeria; while in Trinidad potassium muriate at 376 kg/ha and superphosphate at 376 kg/ha are used. In Barbados yams grow well as a rotation crop in sugar cane land that has residual nitrogen and potassium from the previous cane crop, but yields have been substantially increased by the application of 225 kg/ha of a 9:10:23 NPK fertiliser. Application should be 2-3 months after growth commences. The application of potash alone has given yield increases in Nigeria, and it is also reported to increase the storage life of the tubers, while chlorine in the fertiliser adversely affects the starch content.
Altitude-most yams can be grown successfully at low or medium elevations and some, such as D. alata, are reported to be grown at altitudes up to 2 700 m in the Himalayas, but in general yields are considerably reduced above 900 m.
Day-length-the majority of Dioscorea spp. exhibit a photoperiodic response and although day-lengths greater than 12 hours favour the growth of the vine, tuber development is normally most satisfactory under short-day conditions (10-11 hours of daylight).
Planting procedure
Material-edible yams are normally propagated by the use of small tubers (seed yams), cuttings off the tubers, setts (pre-sprouted tubers or pieces of tuber), or bulbils. It is possible to use vine cuttings, but tuber production by this method is generally uneconomic. (All types of vegetative planting material other than vine cuttings are commonly referred to as setts.) The best planting material is the small whole tuber and species such as D. esculenta and D. trifida, which produce a fairly large number of tubers, can be propagated very easily by reserving a few of the tubers and planting these at the beginning of the next season. Other species such as D. bulbifera and some forms of D. alata produce aerial bulbils, which can be used, but the majority of the more important food yams only produce 1-3 tubers a season and in this case setts cut from the tuber are often used and are referred to as tops or ‘heads’ (proximal), middles, and bottoms or ‘tails’ (distal). In general, tops are preferred and the larger the sets, the earlier and greater is the rate of germination. The weight of sett used varies from about 0.25 to 2 kg but occasionally whole tubers weighing up to 4.5 kg are used, especially when extra large yams are required for ceremonial purposes. Sometimes the body of the yam is cut off and the head left in the soil to grow and produce seed yams for propagation; this practice is known as ‘topping’ or ‘milking’. In addition, planting material may be produced in specialised yam nurseries, where one to four small setts of 85-150 g are planted in small heaps of soil, normally up to 12 500/ha, and these yield small yams suitable for use as seed. Most yams have a definite period of dormancy, but this may be broken by the use of a chemical such as ethylene chlorhydrin, where production of out-of-season tubers is required.
A recent development has been the production in Barbados of virus-tested planting material, in which yams grown from virus-free meristem tip cultures are being multiplied in the field and, after inspection, distributed for planting. This material has been tested in a number of Caribbean islands and has given approximately double the yield obtained when conventional seed yams are planted, and the operation is now commercial.
Method-yams are usually intercropped with maize and vegetables, such as cucurbits, pumpkins, peppers and okra, but mono-culture, normally on small plots, is increasing in certain areas of West Africa and the Caribbean. Three types of planting systems are practiced: the setts may be planted on the flat, they may be planted in trenches or holes, or they may be planted on mounds, ridges or raised beds. The last method is the most widespread and the mounds can vary from about 50 cm high and perhaps twice as wide at the bottom, to nearly 100 cm high and twice this width at the base. In the smaller mounds one sett is normally planted and in the larger ones three or four, or even eight to ten setts. In general, larger mounds are preferred and the setts are planted in holes dug in the sides near the natural ground level. They are planted deeply to avoid drying out of the young shoots and for this reason the head of the sett is also placed downwards. Sometimes, instead of individual mounds, ridges are used and the setts are planted along the sides of the ridges. Planting on the flat is only practiced in areas such as river flood plains, where the soil is deep and soft. In this system, the setts are planted in holes just below the soil surface. Support for the growing vines is usually provided, most often stakes or trellises, or strings attached to horizontal ropes or trees, sometimes corn stalks left from an intercrop of maize, or even bushes: there is evidence indicating that such support is necessary for satisfactory plant and tuber development. However, a few cultivars, notably of D. alata and D. esculenta, are adapted to trailing on the ground without support, and in Barbados D. alata is cultivated, without staking, on the plantation scale. Recent work suggests that with close planting other species may also give satisfactory yields without support.
Planting is normally by hand, but a mechanical planter is now being used in Barbados. For optimum yields yams must be kept free from weeds, at least for the first three months of growth, and the following herbicides have been used successfully: diuron 2.6 kg/ha, together with TCA at 4.4 kg/ha, and atrazine at 1.5-2.9 kg/ha.
Time of planting-yams are not normally grown under irrigation and in areas where the rains last 8-10 months planting normally takes place just before or at the beginning of the rains. Where the rainy season is less than 8 months it has been found that early planting, up to 3 months before the rains, can give a 30 per cent increase in yield.
Field spacing-a wide range of planting distances is used, depending upon the species, the soil type and the water table and whether intercropping is practiced; mounds are often irregularly spaced and planting distances ranging from 0.7 to 2.3 m2 have been reported. Generally, the wider the spacing the lower the yield and common spacings are 1.2 x 1.2 m, 1.2×0.9 m and 1.8×0.6 m.
Seed rate-the number of setts used obviously varies according to the species and the cultivar, but for most large-tubered yams 10 000-15 000/ha are used, requiring at least 2.5 t/ha of setts.
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