4 Yams

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4.1 The environmental requirements of yams
4.2 The yam tuber
4.3 Farm-economic aspects of yam production
4.4 Yam harvesting
4.5 Causes of storage losses for yams
4.6 Traditional storage systems for fresh yams
4.7 Measures to improve traditional yam storage

Yams are widespread in the humid tropics throughout the world and in a wide variety of species. Of particular significance are the white Guinea yam (Dioscorea rotundata Poir), the water yam Dioscorea alata L.), the yellow yam Dioscorea cayenensis Lam.) and the Chinese yam Dioscorea esculenta (Lour.) Burk.)

The white yam originates in West Africa, however, without a wild variety being known of it is the most important variety of yam cultivated for human nutrition not only in this region, but throughout the world.

Most widespread, but not of the greatest economic status, is the water yam. This variety is native to South East Asia and probably originates in Burma, today's Myanmar. The water yam, with the white yam, is the most important variety of yam cultivated in West Africa today.

The appearance of the yellow yam is very similar to that of the white yam. Many authors also speak of a subspecies here (ONWUEME, 1978). Apart from some morphological differences, the yellow yam has a longer period of vegetation and shorter dormancy than the white yam. The yellow yam is native to West Africa where wild varieties also exist. Apart from the region of its origin, the yellow yam is only found with economic significance in the West Indies.

The Chinese yam comes from Indo-China. Nowadays it is most widespread in South East Asia, in the South Pacific and in the West Indies. The Chinese yam has only recently been introduced to Africa and has only played a subordinate role here so far.

The bitter yam (Dioscorea dumetorum (Kunth) Pax) is marked by the bitter flavour of its tubers. Some sorts are highly poisonous. The cultivation of the bitter yam is mainly limited to the West African region where wild varieties cam be encountered.

There are many other varieties of yam, some of these are only of regional significance and do not occur in West Africa For this reason, a more detailed description of these can be dispensed with here.

4.1 The environmental requirements of yams

The yam is a plant of tropical climates and does not tolerate. Temperatures below 20°C impede the growth of the plant which needs temperatures between 25 and 30°C to develop normally.

Most varieties of yam have a growing phase of 7 - 9 months up to maturity. The yam requires an annual precipitation of over 1,500 mm distributed evenly over the vegetation period to take full advantage of its production potential. For this reason, a long rainy season during the growth period has a positive effect on the yield of yams Off the other hand, the plant is able to survive longer dry periods which, however, reduce the yield considerably.

The yam makes high demands on soil fertility. Soils with a high humic content correspond best to the requirements of the yam. On soils which are low in nutrients and which are predominant in the humid tropics, the yam is often the first member in crop rotation so that its high demand on nutrients can be fulfilled In addition to a high concentration of nutrients, the soils should have good water-bearing properties as yams are not able to tolerate stagnant water. They also need deep soil which is free of stones. Shallow or soils impede the formation of the tubers and or result in deformity.

Although not completely analysed, there are many indications of light intensity affecting growth - in particular of the tuber. Thus staking for the tendrils promotes the yield Semi-shade, e.g. in a greenhouse or under trees, leads to a noticeable loss in yield (ONWUEME, 1978).

4.2 The yam tuber

Economically the most important part of the yam is its tuber. This can vary greatly in shape and size and makes manual harvesting very difficult and has so far prevented any kind of mechanisation in harvesting. Cultivated forms of yam mostly produce cylindrical tubers which cam be very heterogeneous in size and weight.

The outer part of the tuber forms several layers of cork. These layers constitute effective protection from lesions, water loss and against the penetration of pathogens in the soil as well as in storage after the harvest The inner part of the tuber is formed by a tissue which is interwoven with vascular channels Carbohydrates, mainly in the form of starches, are stored in this tissue. Apart from the most important constituents of the tuber, water and carbohydrates, this also contains small quantities of proteins, fats and vitamins. As can be seen from Table 6, the tubers of various varieties of yam differ in the relative composition of their constituents.

Table 6: The composition of various species of yam tubers

NB. The figures have been rounded. The results for D. rotundata correspond to those for D. cayenensis which was not included in the table

Source: Coursey, 1967 (modified)

The yam tuber is primarily for vegetative propagation if complete tubers are used for propagation, germs will form in the region of the head. Also segments of the tuber can germinate as long as these include a piece of the outer surface of the tuber. The ability of the tuber to form germs at any point on its surface is made use of by the "Miniset Propagation Method" (INPT, 1988). Using this method, the plants required per hectare can be reduced from approximately two tons to approx. 400 kg.

4.3 Farm-economic aspects of yam production

The yam is a demanding plant in every respect. Its demands on the soil fertility mean that it is mostly the first member in crop rotation.

The preparation of the fields, ridging, vegetative propagation mulching, weed control and harvesting mean a great input of work. About 500 working days have to be calculated per hectare with a harvest yield of 10 tons of tubers (COURSEY, 1966) There is also little indication of relief through mechanization, even for parts of activities (ONWUEME, 1978).

According to the variety and sort of yam, the production potential to 20 - 50 tons per hectare. The average yield in Africa however, only amounts to around 10 tans per hectare (FAO, 1985). Of these, 2 tons per hectare have to be reserved for traditional propagation, leaving only 8 tons for consumption.

The output per unit area for the yam is very high. However, it must be considered that yams can only be grown in primary locations. Labour productivity is low. This may be a reason why yam production is stagnating m many places or is even declining. In Togo between 1911 and 1986 yam production fell from 807,000 tons to 409,000 (INPT, 1988). This is mainly due to a shortage of manpower m rural regions. The stagnating production must also partly be seen as a result of an increasing concentration of the population. This effects a higher and higher production index for arable land and the cultivation of boundary locations. Both restrict the cultivation of yams; but promote the cultivation of cassava (cf. Chapter 5.3) which is gradually replacing the yam in many places.

4.4 Yam harvesting

There are two processes of harvesting yams:

- single harvest
- double harvest

The single harvest involves harvesting all the tubers of a plant in one working procedure. The time for harvesting is not a critical date since one month prior to wilting point (sign of physiological maturity of the tuber) the growth of the tuber is extensively completed. Harvesting should however be finished within 1 - 2 months of the wilting point, or otherwise losses due to tuber rot must be expected (ONWUEME, 1978).

The double harvest is divided into a first and a second harvest. Depending on the sort of yam, the first harvest takes place about 4 - 5 months after emergence of the plants. The tubers are carefully uncovered and separated from the plant without damaging it. After the harvest, the bed which has been dug open is re-prepared. The plants react to this interference with increased production of tuber tissue so that a second harvest can take place after the wilting point.

The double harvest the properties of the tuber. The tubers from the second harvest have pronounced "planting features" and are less suitable for eating. Thus the high work input in the process of double harvesting is mainly for the purpose of producing plants for vegetative propagation.

The tubers from the first harvest are available early. They are highly estimated and attain correspondingly high prices on the markets.

The double harvest is a process with a very high input of labour. Mechanisation is very difficult which means work relief through the use of technical progress is hardly possible (ONWUEME, 1978).

4.5 Causes of storage losses for yams

4.5.1 Dormancy
4.5.2 Transpiration
4.5.3 Respiration
4.5.4 Germination
4.5.5 Rot due to mould and bacteriosis
4.5.6 Nematodes
4.5.7 Insects
4.5.8 Mammals

Losses which can occur during the storage of fresh yams have very varying causes. Some of the losses are endogenous, i.e. physiological. These include transpiration, respiration and germination. Other losses are caused by exogenous factors like insect pests, nematodes, rodents, rot bacteria and fungi on the stored produce.

4.5.1 Dormancy

The possibility to store fresh yam tubers is decisively influenced by their dormancy. Dormancy occurs shortly after physiological maturity of the tubers (wilting point). During dormancy, the metabolic functions of the tubers are reduced to a minimum. Dormancy evidently serves to facilitate the tuber, as an organ of vegetative propagation, to overcome an unfavourable climatic period. Consequently, varieties of yam native to regions with marked arid seasons have a longer period of dormancy that those native to regions with shorter dry seasons. The duration of natural dormancy fluctuates according to the variety of yam between 4 and 18 week (cf. Table 7).

Table 7: Comparison of dormancy duration for different varieties of yams in selected locations

Source: PASSAM, 1982 (data from various sources compiled by the author)

The varying length of natural dormancy determines that the different varieties of yam have more or less good natural storage properties. The duration of dormancy however does not only depend on the plant but is also influenced by physical factors. A fall in temperature, even if this is only a few degrees Centigrade, prolongs dormancy. Vice versa, a rise in temperature reduces dormancy (PASSAM, 1982). Relative humidity also has a similar effect. High humidity e.g. at the beginning of the rainy season, promotes germination Low humidity on the other hand, prolongs dormancy (WICKHAM, 1984)

4.5.2 Transpiration

Depending on the variety, yams have a water content of 60 - 80%. During storage, the water content of the fresh tubers reduces continually. Water loss vanes depending on the phase of storage During the first weeks after harvesting, a reduction in the water content of the tuber is hardly noticeable in some cases, water content will even rise slightly during this phase (COURSEY, 1961).

During a storage period of five months, the weight of the tuber falls by up to 20% due to transpiration (COURSEY and WALKER, 1960). Data concerning the loss of weight due to transpiration show some difference. The reason for this is that the intensity of transpiration is considerably influenced by the predominant climatic conditions (temperature and relative humidity).

Loss of weight due to transpiration has no influence on the nutritional value of the tubers can even rise in relation due to transpiration. Despite this, a great loss in weight from transpiration is not desired. Due to this, the tubers lose their viability (germination), shrink, become unattractive and undergo a change in flavour which is not wanted (ONWUEME, 1978). As yams are mainly sold according to fresh weight and appearance, it is in the interest of the farmer to preserve the water content of the fresh tuber as much as possible (ONWUEME, 1978).

4.5.3 Respiration

The yam tuber is a living organ. This is why metabolic functions continue during dormancy to preserve its viability. The energy essential for this is taken by the tuber from its store of carbohydrates. Carbohydrates are burned to gain energy during which process CO₂ and H₂O are emitted to the environment as gases.

In contrast to transpiration which only causes water loss in the tuber, respiration involves the use of stored energy. This consequently is lost for human nourishment. During dormancy, one kilogram of tubers stored at 25°C loses the equivalent of 3 ml CO₂ per hour (PASSAM and NOON, 1977).

Table 8: Proportion of respiration in the total loss of weight during storage using the example of 1). rotundata

Source: Passam, 1982 (modified)

The weight losses occurring during storage due to respiration and in general are shown in the table. It becomes clear here that the weight losses depend on the storage phase and the storage temperature. It is also clear that respiratory losses are not as strongly influence by the as the transpiratory losses.

4.5.4 Germination

Germination marks the end of dormancy for the yam tuber. Germination does not occur at the same time for all tubers of one variety which are stored together. Germination is more a dynamic process and takes place gradually.

Environmental effects, in particular relative humidity and temperature, affect germination It was observed by PASSAM (1977) that tubers of Dioscorea rotundata already germinated after 20 days at a humidity of 100% and a temperature of 25°C. At the same temperature and a relative humidity of 60 -70%, germination began after 40 days, and at 17-C and a humidity of 100% after 30 - 40 days (ibid.).

Whether in addition to humidity and temperature, of her factors e.g. direct sunlight, affect the beginning of germination, has not yet been clarified. Whether the plants own growth hormones affect germination is also not clear.

Energy required to form the germ is taken from the carbohydrate reserves. During the process of germination, the tuber quickly loses nutrients, dries out and rot pathogens penetrate it so that further storage becomes impossible (PASSAM and NOON, 1977).

4.5.5 Rot due to mould and bacteriosis

Tuber rot caused by various pathogens is one of the most significant causes of loss during the storage of fresh yam tubers.

The fungi causing rot are normally lesion pathogens. They can only actively penetrate the tuber through lesions, cuts, holes bored by nematodes or where rodents have bitten the tubers (COURSEY, 1967). Frequently only one variety of fungus penetrates the tuber initially and is then followed by others.

There are various types of rot on the yam tuber. Depending on the consistency these are distinguished by "dry", "watery" and "soft" rot (CENTRE FOR OVERSEAS PEST RESEARCH, 1978). Rot can infest only parts or the complete tuber. "Dry" rot can often not be observed externally. Rot effects changes in consistency and flavour frequently the tubers no longer suitable for consumption or causing a considerable loss in market value. Bacteria can also cause rot However, these are not as aggressive as mould fungi.

There are numerous species of mould fungi which infest yam tubers but often these are only of regional importance. The following are among the most significant species:

- Botryodiplodia theobromae,
- Penicillium.spp,
- Aspergillus spp.,
- Fusarium bulbigenum (COURSEY, 1982).

4.5.6 Nematodes

Nematodes occur on yams as root and tuber parasites. The nematodes mostly infest the plant during the vegetation period and remain in the tubers after the harvest. They damage not only the tubers themselves but also create entries for other pests, in particular for mould fungi. For this reason infestation by nematodes is often accompanied by tuber rot which mostly causes greater economic damage than infestation only by nematodes.

The yam worm (Scutellonema bradys) is one of the most important nematode parasites of the yam tuber. The yam worm particularly damages the periderm and subperiderm, cell layers which are directly under the cork shell. The beginning of infection can be detected by narrow, yellow wounds which are directly under the shell. In the course of time these wounds become brown. On the exterior, deep cracks indicate infection. The yam worm can cause symptoms of dry rot if other pathogens are missing (CENTRE FOR OVERSEAS PEST RESEARCH, 1978). As the yam worm destroys the meristem, the tuber often loses its germination capacity as a result of infection (ibid.)

The root-knot nematode (Meloidogyne spp.) is a widespread pest in the tropics. Several varieties of this pest also infest the roots and tubers of yams. The root-knot nematode lives freely in the soil and can penetrate softer parts of the tuber. The larvae grow quickly in the adult phase only the females are parasites. These lay their eggs in the tuber as well as in the earth surrounding it. After harvesting, the larvae and eggs continue to live in the tuber. The root-knot nematode causes nodulated and often wrinkled and shrunk yam tubers (CENTRE FOR OVERSEAS PEST RESEARCH, 1978).

The root-lesion worm (Pratylenchus spp.) infests the tubers as a larva or as an adult worm. It causes dark-brown dry rot which penetrates the tuber irregularly In some cases, the shell of the tuber is tom open by the infection leaving the way free for secondary infections (CENTRE FOR OVERSEAS PEST RESEARCH, 1978).

In addition to the nematodes mentioned above there are a number of others which are parasites to the yam tuber. However, these are only of secondary importance.

4.5.7 Insects

There are varying statements in literature about damage caused by insects to stored tubers (incl. COURSEY, 1967; ONWUEME, 1978). According to investigations carried out by SAUPHANOR and RATNADASS(1985), it can be assumed that the pressure of pests will become regionally more important due to pests which are introduced accidently.

Insects damage the yam tubers in two different ways: on the one hand they cause losses of substance due to injury and in addition, can reduce germination capacity. On the other hand they damage the epidermis allowing rot fungi in particular to penetrate the tuber and cause secondary damage.

The yam beetle (Heteroligus spp.) according to details stated by ONWUEME (1978), is the insect which causes the most damage to yams in West Africa it attacks the tuber during the growth phase which then only rarely dies. The epidermis is destroyed during eating leaving the way open for secondary infections leading to mould, which can cause high storage losses

Other extensively widespread pests which infest the yam tuber during storage are mealy bugs and yam mealy bugs (Aspidiella hartii and Planococcus dioscorea). These from whitish colonies which can cover the whole tuber. The insects suck the juice out of the tuber leading to a certain loss in weight. However, what is more significant is that the tubers which are infested are not suitable for sale and the mealy bugs have a negative effect on germination capacity (SAUPHANOR and RATNADASS, 1985).

The most important insect pests of stored yam tubers are a pyralid moth (Euzopherodes vapidella) and a moth (Tineidae sp). The pyralid moth normally infests the tubers shortly after the harvest it lays its eggs in existing wounds but can also penetrate the epidermis for this purpose. The pyralid moth prefers D. alata varieties, which in to other varieties have a high water content Infestation causes a loss of substance in the tuber.

The tineid moth prefers D. cayenensis varieties as these contain comparatively more starch. The tineid often occurs as a secondary pest after the pyralid moth when the plant has already lost moisture due to the pyralid moth. The moth's larvae can eat out the infested tuber within a month leaving only the corked epidermis Both species seem to be gaining in importance in the region of West Africa although in the past the pyralid moth was only widespread in Nigeria. Since the seventies it has also appeared in the Ivory Coast (SAUPHANOR and RATNADASS, 1985).

Other groups of pests are termites which cam penetrate storage. These voracious insects penetrate the epidermis and set up corridors in the tuber. Termites can eat out whole yam tubers within only a few weeks.

Losses in storage due to insects are difficult to quantify. Investigations carried out in the Ivory Coast came to the conclusion that 25% of losses after four months of storage were caused by insects. Secondary infections were not taken into account in the calculations (SAUPHANOR and RATNADASS, 1985).

4.5.8 Mammals

Among mammals, rodents are the most important pests for stored yam tubers. In the region of West Africa most damage is caused particularly by the giant rat (Cricetomys) and the common rat. (Rattus) (ONWUEME, 1978). Stored yam tubers are also popular with monkeys and warthogs as well as with domestic animals like goats and sheep.

Mammals primarily cause quantitative losses by gnawing. However, they frequently contaminate the stored produce with their excrements. By eating, mammals damage the epidermis of the yam tubers which promotes rot infection. Tubers showing only slight damage from gnawing cam thus be completely destroyed by a secondary infection.

Mammals cam cause damage in all kinds of open storage facilities. Particularly at risk are stores where the tubers lie directly on the ground.

4.6 Traditional storage systems for fresh yams

4.6.1 Leaving the yam tubers in the ridges after maturity
4.6.2 Storing the yam tubers in trench silos
4.6.3 Storage of yam tubers in heaps on the ground
4.6.4 Storage of yam tubers in clamp silos
4.6.5 Storage of yam tubers under a conical protective roof made of maize or millet stalks
4.6.6 storage of yam tubers in mud huts
4.6.7 The storage of yam tubers in the yam barn.

Climatic conditions m humid and semi-humid tropics promote continuous methods of production. Despite this, the yam is a seasonal fruit and can only be harvested at certain times throughout the year. Even if several yam varieties are included in crop rotation, a continuous supply of fresh yams cannot be provided over the whole year. For this reason, they have to be stored so that bottlenecks in supply cum be avoided. Storage is also necessary for the purpose of preserving plants for vegetative propagation.

For appropriate storage, very varied systems of storage for yams have been developed m West Africa, the centre of yam cultivation. These systems are mostly marked by simple technical solutions and frequently have existed since time immemorial without having undergone any substantial changes.

The types of storage structures are influenced by various factors. These include climate, purpose of the yam tubers in storage and socio-cultural aspects of storage (symbols of prosperity, use for cult purposes). However, the storage structures are also influenced by the type of building materials available and the resources of the farms, in particular, the availability of labour and capital (FAO, 1990).

The storage systems existing in West Africa have only been mentioned rudimentarily m literature so far. Many determinants and interactions concerning these systems have to be considered unknown (CHINSMAN and FIAGAN, 1987). All systems are in need of further analyses to define the features relevant to storage. In the following chapters a number of storage systems widespread in West Africa, will be described. Due to the limited amount of literature on this subject the descriptions cannot be seen as complete.

Statements on possible storage periods and storage losses are very varied (COURSEY, 1967; N'KPENU and TOUGNON, 1991). Apart from this, a standardised method of defining storage loss does not yet exist. This means that the methods and approaches in analysing and defining the losses are not standardized. Furthermore, it must be remembered that the farmers under some circumstances may judge the losses in a different way from us as their assessment is primarily oriented to quantity. In view of this uncertainties the small amount of data illustrating storage periods and losses is to be dispensed with here.

4.6.1 Leaving the yam tubers in the ridges after maturity

The yam tubers are ripe for harvesting when the foliage has died. Without having to fear any great loss in yield, the harvest cum then take place some time afterwards and the tubers can simply be left in the ridges. The duration of this type of storage depends on the particular variety of yam and cum extend over 1 to 4 months (COURSEY, 1983).

From an economic point of view, this method of storage is quite feasible since no costs are incurred in erecting a store. However, opportunity costs have to be allocated to this method as the field cannot be, or only partly, used otherwise due to the yam tubers remaining there. This method provides no protection from pests (insects, nematodes and rodents) or rot (COURSEY, 1967). Neither does this method allow a periodic check of the condition of the stored produce. During the dry season when the ground dries out and becomes as hard as rock, harvesting without greater losses becomes almost impossible (NWANKITI and MAKURDI, 1989).

4.6.2 Storing the yam tubers in trench silos

The yam fields often have to be located a considerable distance away from the settlements. As particularly during harvest time labours is only available to a limited extent, the farmers make silos in the fields or on the edges of the fields. This saves on labour necessary for transportation during the harvest.

A typical storage facility made in the fields is the trench silo. To make this, a pit approximately corresponding to the expected volume of yams to be harvested is excavated. The pit is lined with straw or similar material (NWANKITI and MAKURDI, 1989). The tubers are then stored on the layer of straw either horizontally on top of each other or with the tip vertically downwards beside each other So far it is not known whether the method of storing - horizontally or vertically - influences storage behaviour.

The trench silo cum be built underground or so that put of the store is above the ground. It is covered with straw or similar materials. In some cases a layer of earth is also added. This type of storage system cum mainly be found in regions with a pronounced dry season

Fig.1: Storage of yam tubers in trench silos (Source: NWANKITI and MAKURDI, 1989)

The trench silo provides protection from respiration and transpiration weight losses of the tubers. A disadvantage is the lack of ventilation and the direct contact of the tubers. This causes the stored produce to become warm and thus promotes the formation of rot (NWANKITI and MAKURDI, 1989). The contact existing between the tubers promotes the spread of rot within the silo. The closed structure of the trench silo does not allow regular checking of the produced stored. Apart from this, the silo offers good refuge for rodents who cum cause the corresponding damage to the stored produce (ONWUEME, 1978).

4.6.3 Storage of yam tubers in heaps on the ground

According to this method of storage the yam tubers are piled on a carpet made of dead yam climbers into a heap. This normally happens under a tree providing shade and the heap is covered with maize or millet stalks or similar materials (FAO, 1990).

This method of storage can be erected without any costs. The shady tree somewhat balances out the temperatures occurring throughout the day and provides certain protection against overheating of the produce.

This storage is badly ventilated. As it is closed, the produce cannot be checked regularly. This promotes rapid spreading of rot which means that storage duration is strictly limited. The stored produce is also damaged by insects and rodents which can hide themselves very well in the store (NWANKITI and MAKURDI, 1989)

4.6.4 Storage of yam tubers in clamp silos

In Nigeria, attempts have been made to store yam tubers in clamp silos. The technique of building the clamp silo was oriented to experience gained in northern Europe (WAITT, 1961). The results of storage in clump silos in Nigeria were contradictory. They were better for some varieties of yam in comparison to the traditional yam ban but were worse for others. The clump silos met with little acceptance for the storage of yams among the local population for socio- cultural reasons (COURSEY, 1967).

4.6.5 Storage of yam tubers under a conical protective roof made of maize or millet stalks

This type of storage is often erected under a shady evergreen. It consists of a conical protective roof which can also be lengthened as e.g. in Fig. 2. The tubers lie on top of each other under this protection (N'KPENU and TOUGNON, 1991).

This method requires no financial investment. The additional work input required is also limited. The shady tree makes temperature fluctuations throughout the day milder and the light protective roof allows sufficient ventilation (ibid.)

Problems arise with the possible entry of insect pests and rodents in addition, there is also the risk of wild and domestic animals damaging the roof construction in their search for food and causing damage by feeding on the tubers which can lead to rot. As the tubers are piled on top of each other and the roof completely covers the tubers, it prevents regular visual checking of the produce stored.

Fig 2: Example of storage for yam tubers with maize and millet stalks (Source: ASIEDU, 1986)

4.6.6 storage of yam tubers in mud huts

This type of storage is often encountered in the savanna areas of the Yam Belt - i.e. in regions with a pronounced dry season (NWANKITI and MAKURDI, 1989). They have firm walls erected in the traditional mud style The roof consists of grass or other plant materials The construction is generally oriented to the particular regional architectural customs.

The yam tubers are piled on top of each other in the hut. The mud hut provides very good protection from rain and direct sunlight. With the roof made out of plant materials, this method of mud construction evens out temperatures.

The lack of ventilation and the piling of the yams are problems here. Both promote the formation of rot and the stored yams can only be checked with difficulty (ibid.).

To build the mud hut requires a relatively high input of capita and labour. However, the hut acknowledges this by having a low degree of maintenance need and a service life of 20 - 30 years (N'KPENU and TOUGNON, 1991).

Fig 3: Traditional mud hut for the storage of yam tubers (Source: NWANKITI and MAKURDI, 1989)

4.6.7 The storage of yam tubers in the yam barn.

This system of storage is the most widespread among traditional yam farmers in West Africa. A yam barn consists of vertically erected wooden posts of about 3 meters in length and set at a distance of 50 cm to each other. The vertical posts are stabilised by attaching horizontal posts to them. Frequently trees which are still growing are integrated into the storage system for static reasons and also to provide natural shade (NWANKITI and MAKURDI, 1989).

The yam barn is erected in the open air and it is important that there is sufficient shade available. To provide this, a roof is sometimes made of palm leaves, or evergreens are used as natural shade. The bum has to be constructed in an airy spot so that the surplus humidity in the air occurring from respiration and transpiration of the tubers can be emitted. Sufficient ventilation also reduces the risk of the tubers heating and thus limits weight loss due to respiration and transpiration (ONWUEME, 1978).

The yam tubers are tied above each other to the vertical posts - mostly using plant fibres - starting from the bottom. The farmers use a particular method of tying for this (NWANKITI and MAKURDI, 1989).

Fig. 4 yam bum with living trees to provide shade (Source: WILSON, undated)

The yam bum is a well-aerated storage system which is easy to check. Germs and rotting tubers are easily removed. This system shows no problems during the dry season. During the rainy season the high humidity however leads to rapid rotting of the tubers (ONWUEME, 1978).

The construction of the yam barn for use over several requires not only a high input of costs (wood for construction) but also of work. Repair work normally occurs annually Putting the tubers into storage, i.e. tying each individual tuber up, is a great amount of work. The tubers are often injured during tying which promotes the formation of rot (NWANKITI and MAKURDI, 1989). The traditionally open method of building provides no protection from insect pests or termites. Often no measures are taken to protect the produce from rodents.

Fig. 5 The technique of tying the individual yam tubers up in the yam barn (Source: ASIEDU, 1986)

4.7 Measures to improve traditional yam storage

4.7.1 Care in harvesting transport and storage
4.7.2 Curing
4.7.3 Influencing dormancy
4.7.4 Influencing the storage climate
4.7.5 control of rot
4.7.6 Control of nematodes
4.7.7 Control of insects damaging stored produce
4.7.8 Measures for protection from mammals
4.7.9 The improved traditional yam barn

Any measures to improve existing storage structures have to be in harmony with the relevant reasons and purposes for these improvements must not have a negative effect on the socio-cultural symbolic character which many storage systems have in addition to their purpose of providing protection. Furthermore, measures towards improvement have to be economic from viewpoint of the farmers and must not place excessive demands on his resources (e.g. work and capital).

The suggestions made below primarily serve to improve the traditional storage structures and methods. The basis for the suggestions towards improvement derives mostly from experience gained by the yam farmers themselves or experiences shared by these. The results of research are also taken into consideration as far as these appear suitable for use by smaller farmers.

In addition to measures towards improving traditional storage structures new, extensively technical solutions were worked upon. These include systems like storing the yam tubers m refrigerated storage facilities or in a controlled atmosphere and the use of radioactive radiation to inhibit germination and to prevent rot (DEMEAUX and VIVIER, 1983).

These processes are not to be discussed in greater depth here. Nevertheless, these technically extensive processes offer bases for the reduction of storage losses caused by germination transpiration and respiration and thus involve the central problems of storing fresh yam tubers. The high degree of technical requirements and the investments required of the farmers do not allow these processes to be successfully applied to the level of the small farm producers at present. In view of a demand which is becoming more and more centralised in African countries due to advancing urbanisation, these systems which could contribute to food self-sufficiency should not be completely disregarded.

4.7.1 Care in harvesting transport and storage

Although the yam tuber looks very hardy, the epidermis car be easily injured. Each injury, regardless of its size, increases the risk of infection and thus early deterioration due to rot (FAO, 1981). For this reason, it is absolutely essential to keep the risk of injury as low as possible if storage is to be long-term and successful (PLUMBLEY, 1982).

To reduce the risk of injury, the yam tubers have to be harvested with great cue and caution. This is indubitably made more difficult by the size and irregularity of the tubers (SADIK, 1987). Tubers are often also damaged during transport For this reason, the tubers should be moved very carefully and not thrown. High piles on transport vehicles increase the risk of injury stemming from pressure and should consequently be avoided. A further cause of injury is when they are heaped and tied when the tubers are stored in the yam barn.

Many farmers are not aware of the relationship between injury and tuber rot. For this reason, the farmers should be sensitized to this. It should also be made clear how the success of storage quite decisively depends on the condition of the stored produce at the time of puffing these into storage (SADIK, 1987).

Fig. 6: Example of how yam tubers can be injured during harvesting (Source: WILSON, undated)

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