No continent depends as much on root crops in feeding its population as does Africa. Cassava (Manihot esculenta Crantz) occupies an important position in Ghana's agricultural economy. Very little documentary evidence is available on the evaluation of current cassava post-harvest techniques and the magnitude of storage losses in the Northern Region of Ghana. This information is needed for the decision-making of small holders, as well as on the national level for the allocation of natural and financial resources.
Traditionally, the cassava root is simply left in the ground, but with population growth and decreasing agricultural land availability, the highly perishable cassava root is now harvested and immediately processed into dried cassava chips (kokonte) in the North of Ghana. Processing comprises peeling, slicing into pieces and sundrying for two to three weeks. The dried chips are stored for six to eight months depending on the consumption rate of the farm-family. The chips are particularly susceptible to attack by storage pests and to a lesser degree to by microbes and rodents. Very little is known about storage losses of cassava chips; consequently, well-established methods to assess storage losses, do not exist nor are there any reliable data on losses published in Ghana.
The quantification of storage losses was assessed with previously known methods: (1) GM, (2) VGM and (3) VDSM. The MM was developed in the laboratory in Berlin because of the shortcomings of already existing methods. During the research in Berlin and Tamale, Ghana, all methods were compared based upon certain criteria, such as suitability, apparatus and costs and field practicability. The GM was chosen as a reference method against all other methods because of its precision in measuring weight losses. With this method, a very good correlation was measured with boreholes on chips of r = 0.9 for the trials under controlled conditions and for the field study in Ghana, but it was recognised that the method is laborious and time-consuming because of the need to obtain baseline data. The other quantitative method, the VGM, has its great advantage in being able to assess losses without collection of baseline data from undamaged chips, but during our investigations the VGM either overestimated weight losses at low ranges of post-harvest losses or underestimated weight losses at higher ranges of damage. The correlation of the VGM with boreholes was only r = 0.4 under controlled conditions. The newly developed MM overcame the shortcomings of the above-discussed conventional loss-assessment methods. The method was able to measure weight losses of chips accurately over a storage period, and it does not depend upon use of baseline data. This method produced precise figures which positively correlated with boreholes and r = 0.9 for field studies and studies under controlled conditions.
Two quantitative methods, GM and MM, assessed post-harvest losses quite similarly, whereas the VGM was less reliable. All three technical methods risk underestimation of losses due to hidden infestation: when insects or flour remain in boreholes and block frass tunnels, volume and weight measurements may be inaccurate. Difficulty arose when severely damaged chips became crumbly and absorbed water, making measurements of the outer and inner volume impossible, especially by the VGM and MM. All technical methods are able to produce objective results which make comparisons with other studies in other regions possible. The technicality of operations might limit the use of the GM, MM and VGM to researchers and trained project and field staff.
The VDSM is a compromise between qualitative and quantitative methods, giving less precise estimations of losses but quite easy to operate. The method has the advantages of requiring no sophisticated equipment and uses reference photographs or drawings for grading chips in different classes. The VDSM also uses the relationship of boreholes-to-weight-losses as one factor for assessing damage scales on cassava chips, but this could result in inaccuracy because not every hole is the beginning of a frass tunnel and results in weight losses. When calibrating damage classes with different loss-assessment methods during our studies, the range of weight losses for each scale varied with the quantitative methods chosen for calibration. The accuracy of determining weight losses with visual scoring is quite poor; however the possible usage of the method by different groups i.e. farmers, traders and extension staff, accounts for its future wide use for general estimation of losses sustained by the farmer.
The variability of local storage techniques indicates that no complete loss-assessment methodology for all situations would be practicable. There can be no single best method for assessing losses; the methodology must be prescribed for each situation to meet local constraints.
Post-harvest weight losses measured in Ghana were about 8% on the farm and about 21% in the station after eight months of storage. The cumulative losses are about 2.5% on-farm and about 5% on-station. The cumulative weight losses were calculated by applying the weight losses to the quantities of chips originally stored and the pattern of family consumption, providing an indicator of overall losses throughout the year. The quite low weight loss figures on the farm contrast with global estimations of 30 to 50%, or weight loss estimations for stored chips in Ghana of about 15 to 20%. The much higher weight losses in the station are due to (1) infestation pressure from a nearby large-scale maize store, and (2) lower quality chips bought from farmers for our in the station trials.
Also qualitative aspects were considered, such as HCN and starch content of cassava roots. The starch content of the dried chips decreased slightly by about 7% under controlled conditions; 72% was measured at the beginning, and this decreased in the end to 68%. During the field trials the starch content dropped by 0.4%. The HCN content under controlled conditions dropped by 57%, whereas under field conditions a decrease of 44% was measured. Although the qualitative starch losses are minimal on infested chips, other waste produce that decreases the possibility for human consumption should be considered. Insects produce uric acid, inoculate fungi and bacteria, and leave faecal matter and castoff skins in and on the chips, creating a foul odour. The potential harmful effects of fungal and bacterial metabolites on consumers are so far not really appreciated in developing countries (Majumder, 1982; Coker, 1994).
Main post-harvest losses on chips are attributed to insects, especially Bostrichids. During the trials under controlled investigations and with species of the Bostrichid family, P. truncatus caused the highest losses in single populations compared to D. bifoveolatus and R. dominica In individual populations, P. truncatus caused weight losses between 39% and 57%, depending on the climatic conditions and start populations of insects, whereas D. bifoveolatus and R. dominica caused losses between 2 and 6%. The high weight losses caused by P. truncatus are attributed to its enormous frass and development potential. Also in mixed populations with two different insect species, P. truncatus outcompeted all other species for food and space, but compared to single populations the development rate was low due to interspecific competition. Experiments with different insect species exposed to 100 g dried chips were set up in the laboratory because always a range of insect pests is found infesting dried chips in Africa.
Causes of storage losses during the field studies in Ghana were mainly due to D. minutus S. zeamais, and R. dominica which amounted to 79%, 10% and 6% of the total insects found during the studies. The changing insect population during the on-farm and on-station studies and possible interactions between the species made it difficult to assess relationships between damage level and numbers of insects. In some cases, only small numbers of Bostrichids Sinoxylon sp. (Duftschmid) or Heterobostrychus brunneus (Murray) caused the complete weight loss of the stored cassava produce under field conditions.
Post harvest weight losses in cassava also occurred shortly after harvest. Data gathered in Ghana during the general survey in 1994 and the multiple visits in 1995 revealed that between harvest and processing of cassava and even before storage, about 46% and 54% of the produce is. About 46% to 55% of the harvested produce is actually stored.
Quantification of losses on stored produce is only source of data for evaluation of post-harvest storage techniques. The perceptions of farmers towards losses also contribute to an accurate evaluation. The farmers perceived their losses very differently, ranging between 3 -27%, in some cases up to 40%. This higher estimation of losses compared to our studies is due to the importance the farmers attach to cassava chips for their own survival. The timing of the survey coincided with a period when nearly all storage structures had been depleted, and the time interval before the next harvest of other produce such as maize was to be quite long. Losses at that time were severely felt.
Another consideration in the appraisal of the current storage techniques of cassava is the relationship of costs of storage and losses to gross returns of chip sales. The calculations of the gross returns showed that the storage of chips up to 6 months and even for a longer time was beneficial, but storing for a longer time exposes chips to insects, and in any case most of the farmers have depleted their stores after six to eight months of storage. The high inflation rates in Ghana in 1995 of 60% and also difficulties attaching values to storage structures, labour and maintenance made an economic evaluation of chip storage difficult. Moreover, the farmers rarely considered costs for storage or labour, because they collect most of their materials in the surroundings and friends supply labour.
The evaluation of the current post-harvest system for dried chips depends on many variables of which the mere quantification of post-harvest losses is just one. Considering the future of cassava chip production in light of the present study, it is important to note the potential of cassava chips as fodder for cattle within the country and for export to Europe. In fact, the creation of marketing opportunities as well as infrastructural improvements may be source of more significant change to the post-harvest system - than marginal recommendations for improvements of storage techniques.