|Better Farming Series 16 - Roots and Tubers (FAO - INADES, 1977, 58 p.)|
Yam is the name given to many plants with tubers belonging to the family of Dioscoreaceae. Yams, or Dioscorea, are herbaceous plants. Their stem consists of two parts: an aerial stem which climbs bv winding round a stake and lasts only a year; and an underground stem that can live a long time.
The underground stem thickens to produce one or more tubers called yams
Cross section of a yam
The tubers contain reserves to feed the plant and enable it to produce fruits and seeds.
But the tubers are lifted before the plant makes seeds.
When they are ripe, the tubers are brown in colour on the outside, but the flesh is white, yellow or red. Their weight varies between 2 and 5 kilogrammes.
In rich, well- worked, deep soil and on mounds, yams can reach weights of 15 to 20 kilogrammes and more.
The aerial stem may be smooth, may bear thorns, or may be covered with little hairs.
Depending on the variety, the aerial stem of a yam may be round in section, or square.
The leaves are alternate or opposite, smooth or hairy. They are usually heart- shaped. In certain species small tubers called bulbils are found in the axils of the leaves
Piece of yam stem
The flowers, white, green or red, are arranged in clusters or in spikes; the male flowers are separate from the female flowers. Some varieties of yam bear male and female flowers at the same time; others bear only male or only female flowers.
The fruits are divided in three parts and each part contains two seeds.
To recognize them we look at:
- the section and appearance of the aerial stem;
- the direction in which it winds round the stake;
- the shape of the leaves and their position on the stem;
- the colour, shape and taste of the tubers;
- the presence or absence of bulbils.
Yam stem winding round a stake
Yams may be classified in six groups:
- Dioscorea alata
The stems wind in a counterclockwise direction. They are smooth and thornless. They are four- sided. The leaves are simple and opposite. The aerial stems and the leaf- stalks are winged.
Each plant of Dioscorea alaa often produces only one tuber, more rarely two tubers. The tubers are covered with rootless.
This variety is quite robust, and gives a big yield. The tubers stand transport well and keep well.
This is a late or medium early variety of yam. The growing period is 8 or 9 months.
Leaf and tuber of Dioscorea alata
This variety is generally called the water yam.
Other names are:
b- b and nza
gbra- guMalinkor khabi- gbouel; (Soussou)
Mali and Senegal:
anda- ba (Bambara)
- Dioscorea cayenensis (Guinea yam)
There are great differences among the varieties of Dioscorea cayenensis.
Some are early varieties harvested only once; they are usually planted when the rainy season has already begun. In west Africa these varieties are harvested between November and January. In Ivory Coast they are called lokpa.
Other varieties, late or medium early, are harvested twice. These yams are planted early, often before the rainy season has begun.
The first harvest is about 6 months after planting (August- September). The mature tuber or tubers are removed carefully, and the roots left undisturbed.
The second harvest is taken 4 to 6 months later ( December- January ). Only the tubers from this last harvest are used for planting.
The medium early varieties are: gnan and klinglr krenglwhich grow in 6 to 7 months.
The late varieties are: sepelo and kangba, which cannot be harvested before 8 or 9 months.
The stems of Dioscorea cayenensis wind in a counterclockwise direction. They are round and often have thorns.
As a rule, each plant produces one yellow fleshed tuber, the shape of which is very varied.
- Dioscorea dumetorum
The stems wind in a clockwise direction. They are oval and are generally covered with hairs. The leaves are alternate; they have three leaflets.
This variety is well suited to conditions in savanna country; it withstands drought well and even sometimes comes through brush fires without much harm.
Each plant of Dioscorea dumetorum may have several tubers. The tubers have no rootless, but are smooth except for wrinkles running across them.
- Dioscorea trifida (cush- cush yam)
This yam is still little known in Africa.
The stems wind in a clockwise direction. They are four- sided.
The leaves are alternate and deeply divided into three to six lobes.
Each plant produces several small, elongated tubers.
- Dioscorea esculenta
The stems wind in a clockwise direction and have thorns.
The leaves are alternate and are entire, or deeply divided into several lobes.
This is a late variety that grows in 9 to 10 months.
Each plant produces a large number of small tubers between 30 and 40. It is popularly called the white man's yam. Names for it are:
Togo and Benin:
- Dioscorea bulbifera
This variety of yam grows in 9 months. It is chiefly grown in western Cameroon. Names for it are:
danr dana (Soussou)
The stems wind in a clockwise direction and are thornless.
The leaves of Dioscorea bulbifera are alternate, large and hairless.
Leaf and aerial tuber of Dioscorea bulbifera
Little aerial tubers, called bulbils, are to be seen in the axils of the leaves.
These bulbils develop by the transformation of buds.
They may be as much as 10 centimetres long. They have white, firm flesh and are good to eat when cooked.
These bulbils store food reserves, just like underground tubers. The underground tubers are smaller.
To grow well, yams need a warm, humid climate, with abundant, prolonged rain.
Yams cannot be grown in very dry regions, or where the sunlight is too strong. Yams need shade during the early stages of growth.
This is why in Africa, yams are grown in regions between the dense forest and the dry, treeless savanna.
Yams grow well in rich, deep, permeable soil that is not too sandy.
The tubers do not grow well in heavy soils.
Swampy land that is flooded for several days during the rainy reason is not suitable for growing yams.
It is best to plant yams at the beginning of the rotation, as a first- year crop after clearing the land.
If yams are grown after a long fallow, they find plenty of mineral salts in the soil, and yield many good tubers.
Before planting yams, the soil must be well prepared.
- Clear the land before the rainy season. Cut down the trees, cut the branches. Stack the trees and branches and burn them.
Do not cut all the trees. Leave some of the little ones. They can be used as supports for the aerial stems of the yams. These natural supports will later be supplemented by stakes.
- Till the land to a depth of 20 to 40 centimetres.
- At this time add organic manures, well- rotted farmyard manure, compost or green manure, at 10 to 40 tons a hectare.
Inorganic fertilizers may be used to get a greater yield.
The amounts vary according to the country, region, or even the soils in the same field.
Research stations like IRAT' make a special study of food crops,
and advise farmers.
In Liberia, it is known that the application of potassium (K) fertilizers is valuable in increasing yields.
In Nigeria and Ghana, the agricultural service advises that yams should be given the following fertilizers:
250 kg/ha ammonium sulphate;
65 kg/ha single superphosphate;
215 kg/ha potassium chloride.
The farmer who wants to make progress should all the time ask for advice from the agricultural service.
- In many African countries, yams are planted in mounds 30 to 40 centimetres high and 1 or 2 metres apart. These mounds are made at the beginning of the rainy season. The soil which has thus been well loosened holds plenty of water.
Sometimes the mounds are only made 2 or 3 months after planting. This earthing up encourages the development of tubers but takes a lot of work from the farmer.
If the soil is fairly deep and is deeply tilled, it is not always necessary to make mounds. In that case, more tubers can be planted and the density is greater.
Yams are planted at the beginning of the rainy season. Plant them 5 to 10 centimetres deep 1 metre apart in all directions or 90 centimetres by 1 metre. This gives the tubers plenty of room to fatten up, and the plant makes use of all the rainy season water.
Many kinds of yam bear flowers which fruit and produce seeds. So it is possible to obtain new yam plants by sowing these seeds.
But this way of propagating is no use to the farmer. The new plants grown from seed are not always like the parent plants. Often the yield is less, the tubers are much too small and of bad quality and contain a poison called dioscorine.
For all these reasons, it is better to propagate by cuttings. But here care is needed Take cuttings from ripe tubers, and not from the aerial stems, as is done with cassava. These root cuttings make plants which are like the parent plant, and give good yields.
For the cuttings use pieces of tuber or small whole tubers. To get regular sprouting and good yields, the cuttings "whether whole tubers or pieces) should weigh between 250 and 400 grammes.
The amount of yams planted represents a considerable part (about a quarter) of the harvest. That much of the harvest must be set aside and well stored for use in planting later.
Plant only fully ripe tubers. It is best to use the part of the tuber nearest the crown. This top of the tuber contains many growth buds and shoots more quickly than the rest of the tuber. For this reason, tops of tubers must all be planted in the same field.
The remaining yam tubers are planted in another field. They sprout less quickly.
With the Dioscorea bulbifera variety of yams, the bulbils can be planted in the same way as tubers. Wait until they are quite ripe, when they are easily removed from the stem.
Do not plant tubers or bulbils that are damaged, rotten or diseased.
Small yam tuber used for planting
The bulbils, pieces of tuber or small tubers are planted in the top of the mound at a depth of 5 to 10 centimetres, and covered with soil. When there is too much sun or the light is too strong, cover the mound with grass, so that the sun will not dry out the young plant and the rain will not wash away the soil and the tubers
- In savanna country where there is a long dry period, stakes are not used.
The aerial stems trail on the ground. By covering it, they prevent weeds growing, and protect it against dryness.
A yam mound
CONTROL OF WEEDS
For a good harvest, hoeing must be done two or three times during the early stages of growth.
When this cultivation is being done, the mounds are remade at the same time.
Later, the abundant vegetation of the yams prevents the growth of weeds.
It is then not necessary to hoe.
CONTROL OF DISEASES AND PESTS
Yams have few diseases.
However, rodents, some insects and fungi cause damage.
Damaged tubers rot quickly and cannot be kept for long.
Depending on the variety, yams are harvested 6 to 12 months after planting. Lift the tubers when the leaves and stems turn yellow and dry.
Do not leave the ripe tubers too long in the ground, otherwise they become bitter and may rot.
With some varieties, only one crop is harvested. Others are harvested twice.
At the first harvest, after 6 months, the biggest tubers are lifted.
The second harvest is taken 3 to 6 months after the first.
Or the crop may be harvested as and when needed.
Early varieties, such as lokpa, do not store well. These yams should be eaten immediately after lifting.
Late varieties, such as Dioscorea alata, may be stored for 5 or 6 months.
But they must be kept dry and protected from rats and other
They should be under a roof, on dry ground or on boards supported on posts.
To prevent rot, the tubers should not be heaped up too much.
Yams are the staple food of many peoples of Africa.
Yams are eaten fresh, or are treated and preserved.
Most varieties of yams, especially the wild species that are not cultivated, contain a poison (dioscorine). But this poison is removed by washing the yams several times in salt water and by cooking them well.
This is why yams must never be eaten raw, but only when they are thoroughly cooked.
When yams are eaten fresh, either boiled or fried, peel them first, cut the tubers into pieces and wash them carefully. The boiled yams are pounded to make mashed yams.
The fresh tubers are peeled, sliced, and dried in the sun.
Sometimes yams are steamed before being dried in the sun. And sometimes after a meal, the remains of foutou (mashed fresh yams) are carefully gathered up and made into little balls which are then dried in the sun.
Like cassava, the slices or yam and the balls of foutou can be kept for a long time once they have been well dried.
To make flour, the slices or the little balls are pounded in a mortar, or ground in a mill. The flour thus produced is used to make a dough.