1 Introduction

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In addition to cereals, roots and tubers are the second most important group of food crops. This particularly applies to the part of Africa south of the Sahara where, of the total foodstuffs produced, roots and tubers make up 31 % and cereals 53% (PAULINO AND YEUNG, 1981). In western African countries (Ivory Coast, Ghana, Nigeria...) more than 50% of the supply of calories is derived from the cultivation of roots and tubers (OKIGBO, 1987).

The most significant root and tuber crops produced in Africa are cassava and yams with annual quantities of 51 and 24.4 million tons respectively which correspond to around 40% and 96% of world production.

Table 1: Cultivation area (in 1,000 hectares - 1984) for various tropical roots and tubers according to regions

Source: FAO, 1985 (modified)

In Africa, roots and tubers are cultivated almost exclusively by small farmholder to secure their own subsistence. Only about 20% of produce is marketed (FAO, 1986). The quota sold varies from region to region and is far lower in rural areas than in the catchment areas of conurbations. The reason is to be found at least partly in the high transport costs in comparison to cereals which makes production near to consumers more favourable (LYNAM, 1991).

Table 2: Production (in million tons - 1984) of various tropical roots and tubers according to regions

Source: FAO, 1985 (modified)

A steady decline in the production of yams amounting to an annual average of over 1% has boon observed for Africa over a decade. During the same period, the production of cassava increased annually by 1.7% but remained well behind the population growth rate of Africa (OKIGBO, 1987)

The production of roots and tubers especially in Africa is in an area of conflict involving various forces. Roots and tubers indubitably have considerable production potential which can lead to an improvement in nutrition on the African continent. The output of roots and tubers per unit of area is superior to cereals grown under the same agroclimatic conditions. Particularly cassava provides acceptable yields even on very poor and acidic soils which are typical for the tropical areas of Africa. Roots and tubers survive longer dry periods and are far less susceptible to mass pests (locusts and bird pests etc.) than cereals. This has led to cassava particularly becoming widespread on small farms in Africa since the beginning of the 20th century (LYNAM, 1991).

The high water content in roots and tubers is on the one hand the reason for the low price per unit and, secondly, makes the fresh roots and tubers unsuitable for transport ((HAHN, 1989). The high water content makes storage difficult which frequently involves high losses. These two factors constitute an obstacle for further increases in the production of roots and tubers.

Table 3: Average yields (tons per hectare - 1984) for various tropical roots and tubers according to regions

Source: FAO, 1985 (modified)

The labour productivity of roots and tubers is mostly low, particularly when this includes the necessary processing of products to preserve them. In rural regions where there is a high migration rate, the low labour productivity seriously restricts production.

The production of roots and tubers is often impeded by national agricultural policy in many areas, the consumption of imported cereals is subsidised and has a negative effect on the demand for traditional food crops (VEELBEHR, 1991). The substitution in cities of roots and tubers by other staple foods containing starch is not price-induced but is a result of labour management factors. The opportunity costs for women's work in urban areas are far higher than the country. This encourages the demand for foods which require only a low rate of work input for their preparation (LYNAM, 1991).

Although national and international agricultural research institutes have also concerned themselves in the meantime with roots and tubers, the expenditure involved in research is very low in comparison to the significance these crops have for human nourishment (LEIHNER, 1991). Research so far has mainly been centred on cassava which is the only tropical root and tuber which plays some role in world trade. Research focussed mainly on aspects concerning cultivation, phytopathology and increases in yield.

Aspects of storage and post-harvest protection have rarely boon dealt with in the past. This particularly applies to storage for small farmers, although this group of producers is the one which cultivates and stores the most roots and tubers especially in Africa.

Table 4: Investments by developing countries research into various food plants containing starch (in US dollars, 1975)

Source: acc. to COCK, 1985 (data refers to 1975)

According to LANCASTER AND COURSEY (1984) it must be assumed that die losses of roots and tubers in post-harvest handling amount to around 30% of production volume Even a slight decrease in storage losses could lead to:

- the supply of foodstuffs being increased and seasonal fluctuations being evened out,
- natural resources being conserved, since neither more area nor additional production input towards increasing the crop are involved,
- an improvement in the country's own supply of foodstuffs and thus a contribution to balancing the movement of foreign exchange,
- living conditions in the country being improved by better supplies of staple foods and a simultaneous increase in the proportion available for sale, and thus to restricting rural-urban drift

Knowledge of traditional storage systems and likewise, their socio-economic and cultural background, only exists to a limited extent. In view of this, and of the significance of an improved storage system from a microeconomic as well as macroeconomic aspect, it is the purpose of this investigation to:

- systematise the knowledge available on traditional storage systems for roots and tubers,
- make gaps in knowledge visible, and
- to define approaches for possible improvement measures.

This investigation cannot deal with all tropical roots and tubers; it must be restricted to cassava and yams which are the two most important representatives of this group of food crops in Africa.

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