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1. Introduction

1.1 Importance of cassava
1.2 Problem analysis
1.3 Objectives

1.1 Importance of cassava

No continent depends as much on root and tuber crops in feeding its population as does Africa. Cassava (Manihot esculenta), yams (Dioscorea sp.) and sweet potatoes (Ipomea batatas) are important sources of food in the tropics. The importance of cassava to many Africans is epitomised in the Ewe name for the plant, Agble, meaning "there is life". The production trend world-wide is positive for cassava over the last years, and the production increased by 12.5% between 1988 and 1990 with Nigeria becoming the largest Cassava producer in the world. Cassava and yams also occupy an important position in Ghana's agricultural economy and contribute about 46% of the agricultural Gross Domestic Product (GDP). Cassava accounts for a daily calorie intake of 30% in Ghana and is grown by nearly every farming family. Cassava is the most favoured among all root crops and even all food crops by Ghanaian consumers.

1.2 Problem analysis

Traditionally the Cassava root, after maturing, was left in the ground and harvested when needed. This "underground storage practice" has many disadvantages because it makes land unavailable for further cultivation, and the quality of the roots diminishes with storage in the soil and leaves roots unsuitable for many types of processing. Increasing land pressure, population growth, and expansion of area under cultivation resulted in the evolution of storage of dried Cassava chips in the Northern Region of Ghana. Changes in farming systems have affected harvesting and storage patterns and caused farmers to store Cassava in large amounts in storage structures with increasing susceptibility to attack by insects and fungi. A 1992 study (Nweke et al.) revealed that about 42% of harvested cassava roots in West and East Africa are processed into dried chips and flour, but data on post-harvest losses of cassava are scarce; this is probably related to the fact that cassava is regarded as of low commercial value and loss studies are too cost-intensive. Most data on local or national post-harvest losses result from casual estimates, as serious studies are rarely undertaken for roots and tubers (Dick, 1988; R. Al-Hassan, 1991; Compton, 1991).

In the early 1970s there was a developing awareness that total food availability could be improved through reduction of post-harvest losses and attention was focused on this neglected area. A 50% reduction in post-harvest food losses by 1985 was called for by the United General Assembly in 1975 (Schulten, 1982). The greatest emphasis was placed on cereals, and only recently root and tuber crops are being given more attention. Serious attempts have been made to establish reliable and replicable methods of assessing post-harvest losses during the last decade to evaluate the impact of insect pests and the consequent effects on food security (Adams and Harman, 1977; Harris and Lindblad, 1978; Boxall, 1986).

Isolated estimations of loss, for example, the much-quoted global figures of 30% for post-harvest losses of cereal grains or roots and tubers to insects after harvest, may serve as a preliminary indicator to draw the attention of administrators and to others responsible for post harvest matters to the fact that some losses are occurring, and to the need for more detailed studies. There has been a tendency to overestimate losses, and to base estimates on extreme cases rather than on sound empirical testing. By contrast, the results of detailed field studies suggest that under traditional storage systems in tropical countries, losses are typically around 5% over a storage season for grains studied. There has been concern in recent years about unreliability and lack of standardisation of observations on post-harvest losses, particularly in tropical countries and in the root and tuber field. For many years the estimation of such losses has been based on extrapolation of comparatively nonstandardised studies together with subjective assessment (Boxall et al., 1979; Tyler and Boxall, 1984; Boxall, 1986; Greely, 1987; Katere and Giga, 1990).

The major cause of losses during cassava chip storage is infestation by insects. A wide range of species that feed directly on the dried chips have been reported as the cause of weight loss in the stored produce. Some loss assessment studies and estimations on dried cassava chips have been carried out in different countries. Hirandan and Advani (1955) measured 12 - 14% post-harvest weight losses in India for chips stored for about five months. Killick (1966) estimated for Ghana that 19% of the harvest cassava roots are lost annually, and Nicol (1991) estimated a 15 - 20% loss of -dried chips stored for eight months. Pattinson (1968) estimated for Tanzania a 12% weight loss of cassava chips stored for five months, and Hodges et al. (1985) assessed during a field survey post-harvest losses of up to 19% after 3 months and up to 63% after four to five months due to the infestation of Prostephanus truncatus (Horn). In Togo, Stabrawa (1991) assessed post-harvest weight losses of 5% after one month of storage and 15% after three months of storage due to insect infestation, and Compton (1991) assessed weight losses of about 9% for each store in the survey area in Togo. Wright et al. (1993) assessed post-harvest losses of chips of about 14% after four months of storage, about 20% after seven month of storage and up to 30% when P. truncatus attacked the dried chips. In addition, Wright et al. (1993) estimated that about 4% of the total national cassava production in Togo is lost during the chip storage. This was about equivalent to 0.05% of the GNP in 1989.

An area of controversy lies in the calculation of storage losses which could subsequently be expressed in economic terms. Losses may be measured in terms of quantity and quality. It is difficult to incorporate different types of losses, e.g. nutritive deterioration or reduced processing quality, into a single index of food loss. Because of these difficulties there is so far a general consensus that the major emphasis in loss assessment studies should be upon physical loss (Katere and Giga, 1990). Instruments in assessing losses are mostly quantitative methods which consider only one aspect of post-harvest losses. But for a complete appraisal of the post-harvest storage techniques it is also necessary to incorporate other aspects such as the perception of farmers towards the extent of losses or the socio-economic environment of farm households, which are rarely taken into account when assessing losses and recommending improved storage management or investment in pest control for farm-stored roots. The specific economic, technical and, especially, socio-cultural environment of subsistence farm-households have important implications for peasants' decision-making behaviour (Stumpf, 1989). Weight losses in a range of up to 5% appear to be accepted by many farmers in African countries because it is God's will, and additional efforts are often not undertaken because there are other limitations such as financial problems that have to be taken into account. Thus it is very important to obtain information on farmers' view of storage management and constraints affecting certain options and objectives (Ashimogo, 1994).

1.3 Objectives

Because the extent of post-harvest losses of stored dried cassava chips in the Northern Region of Ghana has not been determined yet, it is difficult to appraise the present post-harvest system and outline improvements in this area. "Post harvest systems are defined as the set of operations and functions between production and consumption of agricultural commodities that are fulfilled by different agents in order to achieve an effective and efficient food supply, and the interaction between them" (Fleischer, 1996).

The major objective of this thesis is to critically evaluate the current storage technique for dried cassava chips in small-scale farm households in Ghana and measure the extent of post-harvest losses. The critical assessment of the post-harvest system should lead to an appraisal of the current situation and functioning of the system and, if necessary, suggesting possible improvements. But the effectiveness of any action undertaken to reduce post-harvest losses must be practical within the prevailing post-harvest system and economically justifiable.

For further investigations of the post-harvest system in terms of post-harvest losses, it is necessary to make reliable estimates of weight losses and to take the necessary steps to improve the situation for the overall development of an economy. Loss assessment studies are an useful tool for evaluating the impact of insect pests in stored chips and current effects on food security. It can be also used for direct comparisons between different systems, such as storage techniques or insecticide treatments, and to measure the relative merits of each. Loss assessment methods were reviewed with respect to relevant aspects such as suitability and limitations. Furthermore, an improved loss assessment method was developed and tested because of the shortcomings of the existing methods. For the critical assessment of loss assessment methods under field conditions it was necessary to undertake experiments under controlled conditions. These experiments in the "Institute for Stored Product Protection of the Federal Biological Research Centre for Agriculture and Forestry" in Berlin provided the basis for further experiments in Ghana.

Another important objective was to consider other post-harvest losses such as qualitative and economic ones and to relate them to the specific socio-economic environment of farm households in the study area in Northern Ghana. A socioeconomic study gathered background information on post-harvest systems. In addition, a price study supported the economic assessment of chip storage in the Northern Region of Ghana. A short description of the importance of cassava for the population, the study area and storage structures in Ghana is outlined in the following section.