Root Crops in Nigeria (Yam & Cassava)

Root vegetables are underground plant parts eaten by humans as food. Although botany distinguishes true roots from non-roots, the term “root vegetable” is applied to all these types in agricultural and culinary usage. Root vegetables are generally storage organs, enlarged to store energy in the form of carbohydrates.

In this article, we want to concentrate on root crops- namely yam and cassava. These two crops have dominated the activities of small scale farmers in agricultural food production. You will agree with me that the two root crops have constituted a large percentage of food requirements for this country.

In actual fact, the two cater for the main carbohydrates content of our food needs. There are many root crops grown all over the country and several varieties of each root crop exist.

The main Food Crops of Nigeria

Yam (Discorea spp. Family)

Yam is one of the most important root crops. It is the staple food for many Nigerians and is classified on the level of specie. They are obtained from several species of Dioscorea, a very large climbing tropical plants. They are monocotyledonous plants.

External Morphology

A description of the habitat of the crop will be based on the species- for purposes of easy identification. i. Dioscorea rotundata (White yam): Cultivated mostly in West Africa, and the West Indies, D.rotundata has cylindrical vines with leaves which are longer than they are broad. Their stipules and prickles are triangular in cross section. The tuber is white and requires about seven to twelve month to mature. Also it is generally thin skinned and the vines twine anti-clockwise.

ii. D. cayensis (Yellow Yam): This has leaves that are broad and long, with cylindrical vines. Prickles, where not joined are long and slender, and cylindrical in cross section. Stipules are generally narrow and constricted towards the base. 

The tubers take about a year to mature and the flesh is yellow. The tuber skin is thick, and the brittle vines twine anti-clockwise. This specie has a male inflorescence which occurs singly or in pairs, but in groups.

iii. D. alata (Water yam):Leaves are broadly ovate and are borne opposite each other on the stem and the vines are angular. There are no pricks or stipules. The tuber flesh is watery and creamy white and the tuber is unevenly shaped. The skin is thick and there is a creamy or bright purple epidermal layer immediately below the skin. Maturity is in about 8 to 10 months. The vines are weak and twine in a counter clockwise direction. The crop has both male and female inflorescences and produces fruits which are three-celled capsules containing two seed in each cell.

iv. D. dumentorum (Three-leaf Yam): For this specie, stems are cylindrical with compound leaves of 3, 5, or 7 leaflets. Twining is clockwise. Tubers are bunched and the flesh colour varies from white to creamy white or yellow. Tubers may be bitter and sometimes they are soaked for about three days before being prepared. Some varieties are poisonous. Tubers reach maturity in about 10 months.

v. D. bulbifera (Aerial Yam): Vines twine clockwise and are cylindrical. Vines carry cordite alternate leaves. There are buds which enlarge into bulbils which vary in size and shape, but are mostly bean-shaped. There are both male and female inflorescences. Maturity is in about 9 to 10 months.

vi. D. esculena (Chinese Yam): Here, twining is in clockwise direction and bear alternative, pale green relatively small cordite leaves. The plant produces a bunch of soft, sugary tubers at the base of the stem. Tubers are small rounded structures, never attaining any large size. Maturity is in about eleven months. The tuber bruises easily and do not store well and sprout again within a short period.

Climatic and Soil Requirements

Yams, generally, require well drained loamy soils, rich in humus or alluvial soils.

D. rotundata is grown more along the riverine areas, i.e. on alluvial soils. Yams like high temperature, direct simple sunshine and rainfall of about 75cm- spread over the first six months of its rapid growth.

Land Preparation

The first step is the clearing of the land. Controlled and light burning is advisable where it is difficult to clean the site. In grassland areas, the grass is pulled from the root and it may be used as mulch when dry.

Locally, holes are dug at intervals or heaps or mounds can be made; if erosion or flooding is anticipated, the mounds are made bigger. Also to check erosion, bonds are built in-between the heaps. The bonds also help in retaining moisture when the areas are dry. The standard practice is however the making of ridges with hoes or tractors. Crossbars are made at intervals to prevent erosion. On sloping land, ridging is most advisable along the contour in order to minimize soil erosion.

Sowing

Sowing is normally during the early rains, but in areas where there are heavy soil or loose alluvial soil, yam can be sown at the end of the rains around November to December, and otherwise sowing should be from February to March. Planting can be done with yams sets or seed. Seeds rate should be about 2.5 to 5 tonnes per hectare with each yam set weighing 0.8kg to1kg; but for seed production, set should weigh between 0.2 to 0.5kg each. The sets should be treated against yam beetle by rolling them in Aldrin 2.5% dust before planting.

Planting Materials

It is true that it is possible to propagate yam through seed, but this has not been very easy and fast with those varieties that have viable seeds. It is also true that there have been some reported cases of producing tubers from vine cuttings, but this may require a long time of management before it can produce yams. Yam is usually propagated by vegetative means, using tuber which is the edible part and which is in high demand for consumption.

As a result, the demand on the tuber is too much. Since the use of tuber, most often, does not give more than 4 times the weight planted, it is necessary to find ways of reducing the use of the tuber as propagation material. In their search for ways of doing this, the National Root-Crops Research Institute, Umudike, Umuahia developed the Minisett Technique for multiplying planting materials.

Weeding

Weeding is done 2 to 4 times; early weeding is necessary. The first weeding is done when maize is being inter-cropped and this serves as seed-bed preparation for maize. The second weeding is for the yam and maize planted; and at the third weeding, vegetables are planted. By the time of the forth weeding, cassava is inter-planted and by this time, the yam is almost ready for harvesting. At each weeding, the mounds or ridges are moulded up.

There is the provision of some support for yam vines. Such staking materials as the stems of grasses, bamboos, raffia and palm fronds and the stems of erect plants are used. Crops are also used, in some cases. These are tied to trees and the vines directed to trail up the ropes. Yield can be reduced by about 50% if there is no staking.

Fertilizer Application

Apply 10, 10:20 mixed fertilizer at the rate of 260 – 500kg per hectare. Apply 10cm away from the stand and 10cm deep around each stand at 2- 3 months of planting.

Harvesting Storage and Yield

Harvesting can start when the tuber is still growing at about 6 to 7 months of planting. Topping is done to produce seed yams. The harvest, at this time, cannot be stored because it contains too much water, so it is eaten or sold.

Early harvest provides food during the period of scarcity. The process of early harvesting while tuber is still growing is called detuberisation of topping and is most common with rotundata. The main harvest starts about 7 to 9 months from planting when the leaves begin to weather and the vines also begin to turn yellow.

However D. esculentum stays longer than all the other species before it is ready for harvesting. Harvesting is done carefully, to avoid bruising the tuber for this will shorten the storage period. The tubers are tied in a shaded barn allowing free movement of air, after they have been well cleaned. In tying the tubers, space is left at the base of the stick to keep off rodents; and also no tuber is allowed to touch the other.

The barn is inspected regularly and diseased tubers are removed. Sometimes, the yams harvested early may be buried for 1 to 2 months before tying in barns Yield may reach 7 – 17 tonnes per hectare in case of sole crop, depending on the type of soil, variety of the yam and other environmental factors. However, where yam is grown under mixed cropping, yield varies between 5 – 12 tonnes.

Pests of Yam

One of the most important pests is yam beetle (Hetroligus sp.) These species have their breeding areas around rivers banks and low lying riverine areas. In the case of Hetreligus rneles, breeding usually starts in November to December.

The eggs hatch in these areas and there are three larva stages. The first larvae feed on decaying matter and the second and third feed on grass roots. After a long prolonged larval life of about 20 weeks, the adults emerge around March and embark on the first flight, which is called the feeding migration. This occurs between April and June.

Conclusion on Root Crops in Nigeria (Yam & Cassava)

You have learnt about root crops- namely yam and cassava, with special emphasis on yam. These two crops have dominated the activities of the small scale farmer in the agricultural food production. 

So far in this post, you have been exposed to a major two food crop in Nigeria. You have also been taken through the varieties of the crop; the cultivation method and the economic importance of the crop.