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CLOSE THIS BOOKTraditional Storage of Yams and Cassave and its Improvement (GTZ)
4 Yams
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT4.1 The environmental requirements of yams
VIEW THE DOCUMENT4.2 The yam tuber
VIEW THE DOCUMENT4.3 Farm-economic aspects of yam production
VIEW THE DOCUMENT4.4 Yam harvesting
4.5 Causes of storage losses for yams
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT4.5.1 Dormancy
VIEW THE DOCUMENT4.5.2 Transpiration
VIEW THE DOCUMENT4.5.3 Respiration
VIEW THE DOCUMENT4.5.4 Germination
VIEW THE DOCUMENT4.5.5 Rot due to mould and bacteriosis
VIEW THE DOCUMENT4.5.6 Nematodes
VIEW THE DOCUMENT4.5.7 Insects
VIEW THE DOCUMENT4.5.8 Mammals
4.6 Traditional storage systems for fresh yams
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT4.6.1 Leaving the yam tubers in the ridges after maturity
VIEW THE DOCUMENT4.6.2 Storing the yam tubers in trench silos
VIEW THE DOCUMENT4.6.3 Storage of yam tubers in heaps on the ground
VIEW THE DOCUMENT4.6.4 Storage of yam tubers in clamp silos
VIEW THE DOCUMENT4.6.5 Storage of yam tubers under a conical protective roof made of maize or millet stalks
VIEW THE DOCUMENT4.6.6 storage of yam tubers in mud huts
VIEW THE DOCUMENT4.6.7 The storage of yam tubers in the yam barn.
4.7 Measures to improve traditional yam storage
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT4.7.1 Care in harvesting transport and storage
VIEW THE DOCUMENT4.7.2 Curing
VIEW THE DOCUMENT4.7.3 Influencing dormancy
VIEW THE DOCUMENT4.7.4 Influencing the storage climate
VIEW THE DOCUMENT4.7.5 control of rot
VIEW THE DOCUMENT4.7.6 Control of nematodes
VIEW THE DOCUMENT4.7.7 Control of insects damaging stored produce
VIEW THE DOCUMENT4.7.8 Measures for protection from mammals
VIEW THE DOCUMENT4.7.9 The improved traditional yam barn

Traditional Storage of Yams and Cassave and its Improvement (GTZ)

4 Yams

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 lndo-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

Variety

Moisture

Carbohydrates

Fats

Crude protein

D. alata

65-73

22-29

0,1-0,3

1,1-2,8

D. rotundata

58 - 80

15 - 23

0,1 - 0,2

1,1 - 2,0

D. esculenta

67-81

17-25

0,1 -0,3

1,3- 1,9

D. bulbifera

63-67

27-33

0,1

1,1-1,5

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 m 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, 1 978).

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

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

Varieties

Location

Dormancy (in weeks)

D. alata

Caribbean

14 -16


West Africa

14-18

D rotundata

West Africa

12 -14

D. cayenensis

West Africa

4 -8

D esculenta

West Africa

12 -18


Caribbean

4 - 8

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

Age of tuber weight loss in %

weight loss per day in %

Proportion of respiration in


25°C

35°C

25°C

35°C

After harvesting

0,22

0,36

27

30

During dormancy

0,15

0,28

7

10

During

0,21

0,34

35

20

germination





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 hommones 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 tinned 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

Climatic conditions m humid and semi-humid tropics promote continous 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 d 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; NKPENU 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, 1 989)

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 bum 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

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 d 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 d 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, 1 982).

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)

4.7.2 Curing

Curing allows injured fruits marked by a high water content to heal themselves. The process was initially tested on potatoes and sweet potatoes but a positive effect was also shown for yams (DEMEAUX, 1984). So that the healing process for the wound car occur and the wound is not only dried out it is essential for temperature and humidity to be increased

Increased temperature and humidity stimulate the yam tubers to from cork cells which can hermetically close the lesions The cork cells are formed in the cork cumbium and then make their way to the wound areas which they close with several layers of wound periderm (BAUTISTA, 1990)

To form the wound periderm certain metabolic processes are necessary These processes use energy which is gained by expiring starch stored in the tuber. During the respiration processes, water, carbon dioxide and heat are released into the environment (BAUTISTA 1990) Thus the healing of wounds is always connected with a certain loss in tuber weight.

The losses in weight depend on the "curing conditions", i.e. on temperature, humidity, duration of the process and the size of the wound Experiments in Togo (temperature 35 - 40°C, relative humidity 80 - 95%, duration of treatment 3 days) showed losses in weight due to curing of approx. 1 % of he fresh weight of the tubers (FAO, 1990).

Curing car seal a wound so that neither the water in the fruit can emerge (certain weight loss and shrinking of the tuber) nor can rot enter the tuber. The germination capability of the tuber is not affected by this process so that tubers which have been treated can be used for vegetative propagation.

Healing the wound should be carried out directly after harvesting the tubers (BOOTH, 1978) Clean and smooth cuts heal best of all. injuries due to squashing do not normally heal but remain as a centre of infection (COURSEY, 1982) All wounds, squashed areas and other injuries should consequently be cleanly cut out.

There are various processes for healing wounds. These differ in their technical methods, their demands on the climate and in the duration of treatment (FAO, 1990; DEMEAUX, 1984; BEEN et al, 1977).

Curing under a jute sheet or under jute sacks is a process developed by the FAO in Togo (FAO, 1990). The tubers, after the appropriate preparation are piled horizontally over each other and covered with a thick layer, about 15 cm, of straw. The whole pile is then covered with a jute sheet or with jute sacks.

This process is very costly since approx. 50 US Dollars have to be estimated for the sacks (FAO, 1990). If the process were carried out with a sheet, the costs would be even higher. The management of this process is also demanding since keeping the temperature (35 - 40°C) and the humidity (80 - 95%) is quite difficult. Cost and management requirements give rise to the question as to whether this process can at all be adopted by the farmers.


Fig 7 Healing wounds under a sheet made of natural fibres (Source: WILSON, undated)

Another process is the so-called "pit-curing system" which is widespread among yam farmers in Bendel State in Nigeria. For this process, a pit of approx. 2.5 x 1.5 x 1 metre is excavated and the bottom is covered with sawdust. The tubers are put in and covered with a thin layer of soil (NNODU, 1987).

This process shows its best effect at a temperature of 26°C and a relative humidity of 92%. Duration of treatment amounted to 11 and 15 days. In comparison to untreated tubers which were all affected by rot after 4 months of storage in a yam barn, the cured tubers showed only 53% and 40% rot (duration of curing: 11 days and 15 days respectively) (NNODU, 1987).

To define optimum "curing conditions" is very difficult and is influenced amongst others, by the type of yam, the type of wound and the degree of tuber maturity (BOOTH, 1978). It is thus not surprising that the statements on temperature, relative humidity and duration of treatment greatly vary in the relevant literature (DEMEAUX, 1984; FAO, 1990).

Farmers prefer curing processes with low additional costs and a low input of extra work which effect a substantial improvement in the storage behaviour and which are simple to handle. Future activities regarding improvements in the curing process should be oriented to these requirements of the farmers.

The work of BEEN et al. (1977) also goes in this direction. These determined that tubers which had only been placed in direct sunlight for a certain time showed similar storage behaviour to tubers which had been subjected to more extensive treatment. It must be remembered here that the extensive curing processes require a climate which also promotes the reproduction of pathogens and which consequently, under some circumstances, could have a counterproductive effect (ONWUEME, 1978).

Table 9: The influences of curing processes on the storage losses of D. rotundata

Climatic

Weight loss after

After 70 days uncontrolled storage

conditions

treatment in %

Weight loss %

Germination %

Direct sunlight


11,0 22,5

77

26°C/66% RH


9,1 35,5

33

30°C/91% RH


2,1 36,1

50

40°C/98% RH


4,3 20,9

73

Source: BEEN et al., 1977
(modified)

4.7.3 Influencing dormancy

As already stated earlier, the length of time the tubers can be stored strongly depends on the length of dormancy (cf. 3.5.1). Prolonging dormancy is thus essential in extending the storage of fresh yam tubers.

The duration of dormancy can be influenced to a certain extent by temperature and relative humidity. Low temperatures and low relative humidity rates prolong dormancy (PASSAM, 1977). The possibilities of changing the temperature and humidity to influence dormancy are limited as the tube tissue is destroyed when the temperature falls below 15°C (ibid.). A humidity which is too low also hinders storage quality as early drying of the tubers is induced by this.

Influencing the storage climate by external energy (refrigeration) is restricted economically due to the low product value of yams and the high energy costs.

Another possibility to influence dormancy is by using chemical agents to inhibit germination like are used, for example, in successfully storing potatoes (PERLASCA, 1956). When applied to yams, the substances used for potato storage showed no effect. The reason is that yams, in contrast to potatoes, do not germinate until late and then not in the epidermis but in the cell layers below this. The agents applied in storing potatoes could have a counterproductive effect on yams as they impede the healing process and can promote the formation of rot (DEMEAUX, 1984).

Experiments to influence germination with natural and synthetic growth hormones showed positive beginnings. Amongst others, gibberellic acid, a synthetic hormone available in several compositions, and batatasins were tested. The latter are natural growth hormones which occur, amongst others, in different Dioscorea varieties. Batatasins applied endogenously showed no or a very limited effect on dormancy (PASSAM, 1984) so that further progress in this direction is doubtful.

Experiments with gibberellic acid were positive in some cases, i.e. dormancy was clearly prolonged by the effect of this hormone. If gibberellic acid is applied to the foliage prior to harvesting, dormancy is only extended for Dioscorea esculenta. Applied to Dioscorea alata, gibberellic acid showed no effect at all (WICKHAM, 1984,a). If gibberellic acid is applied after harvesting the dormancy of Dioscorea esculenta as well as Dioscorea alata can be extended. Use on Dioscorea bulbifera remained without effect (WICKHAM,1984,b).

Table 10: The effect of various chemical growth regulators (germination inhibiting agents) on the storage quality of yam tubers

Yam variety

Chemical agent

Effect on the storage quality

D.alata

methyl-a-NAA

+ 1,5 - 2 months


chlorethanol

promotes germination


gibberellic acid

+ 4 weeks

D. rotundata

methyl-a-NAA

no effect


gibberellic acid

no effect


IAA

no effect


kinetin

no effect

D. esculenta

gibberellic acid

+ 6 weeks

Source: PASSAM, 1982 (modified)

According to WICKHAM (1984,b) the best effect occurs when the tubers have been treated for 22 hours in a solution of 150 mg/litre gibberellic acid. Other authors recommend other concentrations in some cases for the same agent (MARTIN, 1977; DEMEAUX and VIVIER, 1984). According to OSIURO (1992), dormancy can be extended for longer, the higher the concentration of the agent is.

Apart from the concentration of the agent, the point of time when it is applied is a critical factor in influencing the hormones for dormancy. MARTIN (1977) defines application towards the end of natural dormancy to lengthen this, a fact which is disputed by WICKHAM (1984,a). For PASSAM (1985), the condition of the tuber is a decisive factor in the effect of gibberellic acid. If gibberellic acid is applied to freshly germinated tubers this will promote the formation of germs. If the germs are removed prior to application it will delay re-formation of germs. The most favourable time for the application of gibberellic acid according to DEMEAUX and VIVIER (1985) is just after harvesting.

According to research findings so far, it can be assumed that gibberellic acid delays the formation of genes, i.e. prolongs dormancy However, there is a necessity for application methods, times and agent concentrations to be clarified. Only when these points have provided precise results and the economic efficiency of the process has been proven can recommendations on practical application be expressed.

Until such information is available, the germs should be removed manually. Since too frequent removal of the germs stimulates re-growth, the germs should not be removed until these have attained a length of approx. 50 cm.

4.7.4 Influencing the storage climate

During storage, certain metabolic processes have to take place so that the tuber retains its viability and reproductive quality. The intensity of respiration and transpiration is partly dependent on the "storage phase" at which the tuber is (cf. Chapters 3.5.2 and 3.5.3). In addition, the storage climate, i.e. temperature and humidity, have an effect on this. These two determinants in storage behaviour are not given quantities but can be manipulated by means of certain methods.


Fig.8 Respiration of yam tubers during storage at varying temperatures (0 = at 25°C, X = at 35°C storage temperature) (Source: PASSAM et al., 1978)

4.7.4.1 Influencing the storage temperature

In general, it can be determined that a longer storage period is possible at lower temperatures. At lower temperatures, respiration is lower and simultaneously, the formation of germs is delayed (DEMEAUX and VIVIER, 1984).

For many tropical fruits there is a "critical" temperature. Below this, an irreversible change in tissue occurs resulting in rapid deterioration of the fruit. The critical temperature for tropical fruits, also referred to in literature as causing the irreversible "chilling injury" alteration to the tissue, is well above freezing point. For yams, depending on the variety, it is between 13 and 15°C (DEMEAUX and VIVIER, 1984) Other authors state the critical range as being between 10 and 12°C (DEMEAUX and VIVIER, 1984).

Consequently, a reduction in temperature to improve the storage quality of yams is very limited and this should not fall below 15°C. Without using external energy for cooling even this value can hardly be retained under tropical conditions. The use of external energy also making the construction of closed and insulted storage structures essential, cannot be considered a possibility to improve the storage of yams on a small-farm level for reasons of cost.


Fig 9: The influence of temperature and air humidity on the losses in yam tuber storage (Source: DEMEAUX and VIVIER, 1984)

Even low reductions in temperature lengthen the period of storage for yams. For this reason, all possibilities available for this which are economically feasible should be made use of Initially and primarily to be thought of here are simple changes in the construction of traditional storage structures to take advantage of the natural temperature fluctuations between day and night. Planting shady trees and the use of air currents can also lead to a noticeable reduction in storage temperatures and thus provide a contribution to improving the storage climate.

4.7.4.2 Influencing humidity of the air

There is an exchange of water vapour between the stored produce and their environment for the purpose of balancing out the moisture content of the produce and its surroundings. Dried crops e.g. cereals, tend more to re-absorb moisture from the surrounding air. Crops like yams which have a high moisture content tend to emit moisture to their environment during storage.

Loss of moisture from stored yams is not desired since this leads to economic loss (loss in weight and shrinking of the tubers) without improving storage quality. Consequently, the air in storage should have a humidity rate at which the exchange of water vapour is minimal. At a storage temperature of 26 - 28 °C which can be assumed typical for West Africa, a relative humidity of 70 - 80% leads to an equilibrium, in which the exchange of air between the tuber and its surroundings is very low.

With these storage conditions, the tuber retains the properties which define its quality like colour, aroma, flavour and chemical composition. At a higher relative humidity, there is the risk of water vapour condensing which promotes the formation of mould on the tubers.

When considering measures to influence humidity, only those should be taken into account which are technically and financially feasible for the target group of farmers. In the foreground here are alterations in construction which promote the exchange of air and thus remove superfluous air moisture from storage. The changes in construction can be supported by selecting a location which encourages air exchange.

4.7.4.3 Promoting ventilation

Normal atmospheric air consists of 78% hydrogen, 21% oxygen, 0.03% carbon dioxide and a varying content of water vapour.

The supply of oxygen from the air is essential for the metabolic functions to preserve the life of the tuber. At the same time the tuber releases water vapour and carbon dioxide. If the composition of the atmosphere in storage deviates from the normal state of the air as a result of the metabolic functions this can have an unfavourable effect on the condition of the stored produce.

Excessive air moisture which can condense if the temperature falls, promotes the formation of rot. Very low concentrations of oxygen prevent respiration and promote an undesired fermentation of the tubers in storage. Increased carbon dioxide and ethylene concentrations where yams are stored are not desired either. Increased carbon dioxide concentrations cause destruction of the tuber cell structure. Ethylene is a growth hormone which promotes germination (BATISTA, 1990).

For the above reasons it becomes clear that changes in the composition of the atmosphere in storage are not desired as these can have a negative effect on storage. To avoid undesired changes in the atmosphere the store must be sufficiently ventilated. Ventilation is not only for the purpose of gas exchanges between the store and the environment but also affects the temperature in storage.

Controlling ventilation is not simple and easily leads to counterproductive effects. If, e.g. the store is ventilated during the day, this can, at raised temperatures, lead to undesired heating of the stored produce. Inadequate ventilation at very low humidities promotes drying out of the tubers in storage. The store should consequently be ventilated at night as far as possible since temperatures are lower during this time and the relative humidity is normally higher (SADIK, 1987).

As with other improvement measures, the improvements in ventilation should be as simple as possible to carry out and not incur any additional cost. Where storage facilities are to be newly erected, locations allowing natural ventilation by means of air currents should be selected. Apart from this, the tubers should be stored so that ventilation is not hindered. Storage in huge heaps and in trench silos are consequently not suitable to meet the demands of sufficient ventilation.

4.7.4.4 Providing shade for storage facilities

On the one hand, the direct effect of sunlight on stored produce increases storage temperatures. On the other hand, the formation of germs is promoted by this. For this reason, the store should be sufficiently m the shade.

Sufficient shade can be attained by constructions where storage structures are covered by a roof. Roofs should be made of plant materials available locally for cost reasons but also due to the high heat insulation provided by these. A roof not only keeps the rays of the sun out but also protects the stored produce from rain showers which promote the formation of rot.

In addition to building roofs, natural shade should also be made use of, as protection for the produce e.g. evergreen trees. When mounting roofs for shade and taking advantage of natural sources of shade, it must be observed that the ventilation of the store is not affected negatively.

4.7.5 control of rot

As already stated, rot is caused particularly by fungus and bacteria pathogens. These can however only penetrate the skin of the tuber through damaged spots, like injuries, lesions and holes made by nematodes.

An important precaution is consequently to minimise the risk of injury to the tuber during harvest, transport and storage by treating it carefully. Tubers already showing rot at the time of being stored should be put to some other purpose.

The danger of rot can be reduced by curing processes (cf. 4.7.2). In this way wounds are closed so that agents causing rot can no longer enter the tuber. In addition to curing, the wounds can be treated with traditional means like ash and limedust (ONWUEME, 1978).

Since rot can be passed from tuber to tuber the stored produce must be checked on a regular basis so that infested tubers can be removed from the store in good time.

Treating the tubers with fungicides is also a measure which can be used to control rot. Satisfactory results have only been achieved with thiabendazol and benomyl (DEMEAUX and VIVIER, 1984). These substances only have a low degree of toxicity and remain locally in the tuber skin, i.e. they do not move into the flesh of the tuber (DEMEAUX and VIVIER, 1984).

Treatment with fungicides is recommended as a bath. The concentration of the agent is stated as 250 - 2500 ppm at a treatment duration of 2 - 30 minutes (ibid.). It is considered necessary that further experiments to define the treatment with fungicide be carried out in view of the wide range of agent concentrations.

To avoid subsequent damage but also to achieve the appropriate effect, the treatment with fungicide requires a very precise procedure. This necessitates a high degree of extension and backstopping for small African farmers in the use of fungicides.

4.7.6 Control of nematodes

The control of nematodes is simultaneously also a precautionary measure against rot agents who follow the nematodes and often cause greater damage than the nematodes themselves.

Nematodes as parasites on roots and tubers are spread by plants which are infested. For this reason, only plants which are free of nematodes should be used for vegetative propagation.

As nematodes are also freely existent in the soil the relevant crop rotation (long periods between the planting of two yam crops) can reduce the pressure of the pests. To qualify this, it must be said mat most nematodes which are parasites on yams also have other host plants. Control of the nematodes by appropriate rotation is thus made more difficult.

Measures like chemical control or the treatment of tubers with hot water (CENTRE FOR OVERSEAS PEST RESEARCH, 1978), seem less suitable for use on small farms. On the one hand the processes are not yet mature, and on the other the essential financial and labour inputs are too high to encounter sufficient acceptance.

4.7.7 Control of insects damaging stored produce

Measures to control insects causing damage within yam stores basically have two purposes: firstly the damage caused by insects (eating and loss of quality) are to be avoided or at least reduced. Secondly, control measures are to avoid secondary damage caused by rot pathogens which can penetrate the tuber through the injuries to the epidermis caused by insects.

As precautionary measures, separate storage of infected and healthy tubers can be considered. In some cases, e.g. with the yam moth, this is difficult since infestation cannot always be observed externally. For hygienic reasons, all parts of the tuber which are infested by insects should be burned and not kept in the proximity of the store (WILSON, undated).

The types of storage also have an influence on the infestation of the produce by insects. SAUPHANOR and RATNADASS (1985) report that tubers stored in trench silos are not infested by moms or pyralid moms which can cause great damage in storage above ground. As storage structures are also selected on the basis of other criteria no particular type of storage can be recommended at this point to reduce storage losses due to insects.

The control of scale insects can be carried out with pyrimiphos-methyl in a concentration of 25 g per litre water. The tubers remain in this solution for 10 minutes and are subsequently dried (SAUPHANOR and RATNADASS, 1985).

Deltamethrin is recommended for the control of tineid and pyralid moths. The product is applied in concentration of 2.5 grammes per 100 litres water as a dip with a duration of 10 minutes. If infestation occurs this should possibly be repeated. For economic reasons, the agent can also be sprayed onto the stored produce (ibid.).

The statements on chemical control of insect pests on stored yams in the literature available are very limited. Further investigations are necessary or should be repeated to define suitable insecticides, concentrations, application techniques and the time of application.

Farmers tend to take the problem into their own hands when a certain intensity of damage is evident. When this happens, chemical products which are not appropriate are often used, and which can lead to food poisoning in some cases if the yams treated are eaten. The products are often improperly concentrated and applied. In view of possible mistakes in application but also in view of the possible economic damage the insects can cause to yam storage, clear recommendations should be compiled for application during chemical control.

The matter of biological control of pests has hardly been mentioned so far. According to SAUPHANOR and RATNADASS (1985), Phanerotoma leucobasis Kriech is a natural enemy of E. vapidella whose eggs it eats. In how far there is a basis for biological control here will have to be a question for future research.

4.7.8 Measures for protection from mammals

To protect stored produce from mammals, measures depending on the species and on the type of storage have to be undertaken.

Domestic animals can mostly be kept away from stores by fences. Stores erected on stilts, e.g. a platform store, due to their construction provide good protection from domestic animals which could damage the stored produce. These stores can be quite easily protected from rats. Metal funnels are mounted on the stilts with the wide end downwards at a height of approx. 100 cm Rats and other rodents are not able to get past these obstacles.


Fig 10: Material for and mounting of a device to provide protection against rodents (Source: PROJET BENINO-ALLEMAND undated)

4.7.9 The improved traditional yam barn

In view of reducing losses and long storage, the yarn barn shows the best results in comparison to other storage systems widespread in West Africa. This is one of the reasons for the yam barn frequently being selected as the basis for improvement measures to traditional systems of storage on which this work is based. Without changing the type of storage some measures and extensions to the construction can be carried out and can lead to a considerable improvement of the barn.

Improved storage of fresh yam tubers begins during harvesting. Injuries should be avoided as much as possible as these constitute doors for rot viruses. For this reason, harvesting, transport and storage have to be carried out with as much care as possible (NWANKITI et al., 1989). When transporting over longer distances, the tubers should not be piled up too high or this will quickly lead to injury to the epidermis and the formation of bruises.

Immediately after harvesting, the tubers should be subjected to curing (cf. Chapter 4.7.2). Bruises and lesions on the tubers should be cut out as smooth wounds heal better. For hygienic reasons the soil clinging to the end of the tubers should be removed. In how far treatment of wounds with ash or other traditional means improves storage ability will have to be clarified by further experiments. Prior to storage, the remains of the previous year's harvest should be removed and burned as this can constitute a source of infection.

The traditional yam barn has some disadvantages. Consequently, the following improvements should be made.

- A roof construction similar to a hut and made out of local materials like straw, palm leaves etc. should cover the barn. A roof made of plant materials not only provides sufficient protection from sunlight or rain but also regulates temperature fluctuations due to its insulation features. The roof should have a height of at least 2.50 metres so that ventilation of the barn is not restricted (FAO, 1990).
- The barn should be made safe from rodents and domestic animals. There are several possibilities here. It can be surrounded by a fence made of oil barrels which have been cut open. Possible would also be a wall which, however, would have to be at least one metre high. As rodents can easily overcome a wall (in contrast to an oil barrel barrier) the space between the top of the wall and the roof should be protected with fine wire mesh. It is important that the barn is fitted with a door which closes well and will also prevent theft.
- In the modified yam barn the tubers are stored on multi-level shelves. The shelves can be constructed of various locally available materials as far as these provide sufficient support. The lowest board should be about 50 cm above the ground so mat no moisture is taken up from the ground. The shelves should be arranged so that a visual control of the tubers is possible quickly and all around. This is facilitated by the tubers only being stored in two or three layers on each shelf. It will also prevent too much weight exerting pressure on individual tubers and thus reduces the risk of bruising.
- The selection of the site is very important in making use of the advantages for the system. This should be chosen so mat natural air movements can be used for ventilation. The store should be set sideways to the main wind direction so that the natural movement of the air can be used to its full effect. Existing natural sources of shade, e.g. evergreens, should also be taken into account during selection of the site as the temperature in the interior of the barn can be considerably reduced by these.

The natural shade and its temperature reducing effect can mean too strong ventilation during the day. Consequently it must be ascertained mat not too much hot air enters the store as ventilation during the day.

The size of the store can be adapted to individual needs. There is no documented experience on costs for construction and maintenance of the yam barn. As local materials are mostly used, the extra financial means necessary should be limited in comparison to the traditional yam barn.


Fig 11 Example of a rodent-proof fence for storage of yam tubers in the yam barn and in similar storage systems (Source: Wilson, undated)


Fig 12 Simple shelves made of local materials for the storage of yam tubers (Source: NIGERIAN STORED PRODUCTS RESEARCH INSTITUTE 1982)

The use of germination inhibiting agents like gibberellic acid, treating the tubes with fungicides and insecticides can be considered as complementary means of improving storage systems. The lack of practical experience in the application of these prevent any concrete recommendations on the use of such products at this stage.

Regular inspection of the stored products is important for the success of storage systems. Rotting tubers must be sorted out and removed. Germs have to be removed regularly. The INPT (1988) recommends removing germs when these are approx. 50 cm long. Removing the germs too frequently induces the tuber to produce more germs.

According to investigations by NWANKITI et al. (1988), the improved yam barn can contribute considerably to reducing losses. The weight losses observed after six months storage in the traditional yam barn were 41.7%, in the improved yam barn these were 13.3% and with the improved yam barn with extra protection from rodents, 10.8%.

The results of investigations by NWANKITI et al. (1988) indicate that even simple improvements to the traditional yam barn can substantially reduce losses. For this, not all of the improvements mentioned above have to be carried out. Also individual improvements can clearly reduce losses. This means that improvements can be oriented to particular local conditions and requirements of farmers.

Considered macroeconomically, the improved yam barn leads to an increase in the supply of foodstuffs which can be produced on the domestic market. A contribution can be made to the balance of trade if the foodstuffs produced substitute food imports.

For the farmer, improved storage means an increase in subsistence security. At the same time he gains larger scope for decisions on selling and is better able to take advantage of price movements to improve his income.


Fig. 13: Model of an improved yam barn with protective roof and walls (Source: NWANKITI and MAKURDI, 1989 (modified))

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