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There are numerous publications in FAO dealing with this subject, in particular FAO Bulletin N° 53 - Processing and storage of foodgrains by rural families (1983); FAO Bulletin N° 93 - Post-harvest operations and management of foodgrains (1992) and FAO Bulletin N° 109 - Grain storage techniques (1994). This chapter is based on these publications.
Traditional means of grain storage at village level are the result of the evolution of ingenious empirical systems. Through the generations, farmers have developed their own techniques, often very elaborate and masterly (see Chapter 5). Apart from the storage function, the granaries and other traditional structures are designed to reduce to a minimum the losses caused by the principal enemies of the harvest: insects, moulds, rodents, and fire. Proliferation of insects and moulds depends on climatic factors (humidity, temperature and the interstitial environment of the grain); the presence of rodents, termites, fire and theft is linked to the techniques of construction of the granary (location, materials and type of architecture). Evaluation of the wastage through storage in traditional granaries has only been subject to precise measurement recently. Data available is fragmented and cannot be extrapolated to represent a complete country or sub-region (Table 6.1).
Table 6.1 - Rate of loss through storage of millet and sorghum in traditional granaries in Africa
| Country | Product | Loss (%) |
| Senegal | Millet in ear | 2.2 |
| Sorghum in ear | 5.3 | |
| Sorghum as grain | 9.5 | |
| Northern Nigeria | Sorghum in ear | 4.0 |
| Sorghum as grain | 4.0 | |
| Mali in ear | 2 to 4 | |
| Niger | Millet in ear | 10.1 |
| Millet as grain | 3.4 | |
| Burkina Faso | Millet in ear | 6.9 |
| Millet as grain | 8 months storage |
Although peasant farmers find it difficult to estimate losses caused by insects, on the other hand they can precisely identify the infestation period. The FAO project in Benin studied interest in how villagers perceive and estimate losses due to insects according to several criteria: quality of conservation between varieties of cereals and period of infestation. Generally, it can be affirmed that insects cause the greatest losses (Boxall, 1994).
In the Guinean zone of Benin, Togo and the Côte d'Ivoire, high humidity, even during the short dry season, favours development of the principal storage pests: Sitophillus spp (weevils) and Prostephanus truncates (Larger Grain Borer). According to information received from the farmers, losses caused by insects after six months storage are 2% to 3% for husked maize cobs. To that must be added losses (up to 15%) in threshing from grains spoiled by insects after three months storage. The appearance of the Larger Grain Borer has raised doubts about the effectiveness of granaries and traditional techniques.
In other zones of the sub-region, climatic conditions do not favour the proliferation of common insects during the first five months of storage (December to April). In May, higher humidity provokes the appearance of insects. Species observed on sorghum and millet in ear are Corcyra cephalonica (Rice Moth), Rhizopertha dominica (Lesser Grain Borer) and Tribolium castenum (Rust Red Flour Beetle). The wastage caused by the Rice Moth is small since it is limited to the upper 20cm of the stock.
The development of moulds is linked with specific atmospheric conditions (temperature and humidity) as in the southern zones bordering the Gulf of Guinea where humidity is very high. In the rest of the sub-region the climate does not favour the growth of moulds, and granaries installed on platforms are conveniently isolated from the soil. Except where water infiltrates through a defective roof, farmers do not report any cases of moulding in their granaries.
In the region of Mopti in Mali and Ayorou in Niger, serious losses have been revealed following the transfer of humidity through the mud floor of granaries, the only screen between the soil and the grain. Where the storage of maize cobs, millet or sorghum is on the ground in the new warehouses (among the Guidimakha in Mauritania and the Borgou in Benin) moulds appear in the lower layers of the stock.
The case of mycotoxins
Mycotoxins are highly toxic metabolites produced by various moulds. The toxicity of mycotoxins causes chronic illnesses affecting the nervous system, the cardio-vascular system and the digestive and pulmonary systems. Certain mycotoxins are carcinogenic and others are immunodepressives (lowering resistance to disease), (Cooker, 1994).
The principal mycotoxins are produced by certain species of Aspergillus, Fusarium and Penicillium. The worst are aflatoxins including the Aflatoxin M1 in milk (produced by A. flavus and A. parasiticus), Ochratoxin A (A. ochraceus and P. verrucosum), Zearalenone and Deoxynivalenol (F. graminearum) and Fumonisins (F. monoliforme).
For most villagers, the presence of mice in granaries is almost permanent. Present in straw granaries, they equally find their way into clay granaries through the roof or by making holes in the base and can thus cause the granary to collapse.
In the village of Missira (Senegal) rats have appeared in large numbers after some ten years. They cause serious wastage of the stored produce (as much as 20% loss per year) both by their consumption and faecal contamination.
The increasing importance of theft is a consequence of the lack of food after several years of drought. The fear of theft is a factor determining the choice of storage technique. Farmers increasingly see themselves forced to completely thresh the whole of their crop to store as grain in the security of a warehouse. In the region of Tillabery (Niger) some people prefer to store paddy in their huts or shops, even if they know that this method is not favourable to conservation.
Straw granaries are subject to fire risk and are therefore constructed away from dwellings. In Missira (Senegal) villagers have adopted bulk storage in clay warehouses with metal roofs for fear of fire, even if this type of storage is less effective.
Under socio-economic pressure granaries may be built near habitations or in improvised locations to the detriment of storage conditions and despite the fire risks.
Numerous studies during the last 20 years have shown that in traditional post-harvest systems, losses are generally maintained at a low level, around 5%. This value constitutes an optimal threshold since the evolution of storage systems does not always offer an adequate protection for produce.
As has been shown in Chapter 5 (Paragraph 5.1.3), activities in preparation for storage (cleaning the granaries, destruction of infected residues, selection of healthy ears at harvest) permit a substantial reduction of insect attack. Also, hygiene is absolutely necessary to ensure effective control with chemical products.
Certain high yielding varieties are often more vulnerable to insect attack than local varieties which are naturally selected for aptitude for storage and resistance to insects. In agronomic research, selectors take into account factors linked to production (yield, drought resistance) and to use (processing and consumption). Research into varieties resistant to insects is perhaps judged to be secondary since control may be achieved with the aid of insecticides.
This term, habitually synonymous with conventional insecticides, is applied also to fumigants, local insecticides and biopesticides. Pesticides are considered as the most effective technology for protection of stored products. They have gained the approval of peasant farmers, they are adaptable to storage systems and need little effort in application. Moreover, they are economically profitable: the value of grain saved representing 10 to 25 times the cost of the insecticide. More important benefits may be obtained in zones infested with the Large Grain Borer. However, despite the employment of pesticides, considerable storage losses are still reported.
Chemical insecticides
The most common insecticides used in grain storage are the organophosphorous compounds pirimiphos methyl, chlorpiriphos methyl, fenitrothion and malathion. In general they are very efficacious against common pests, they are less so against the Bostrichidae or Grain Borers which include the Larger Grain Borer. Synthetic pyrethroids such as deltamethrin and permethrin are used against this insect, either alone or together with organophosphorous compounds.
The utilisation of insecticides unsuitable for the system of storage is a major problem. Where maize is stored on the cob because of lack of space or labour, the farmer is forced to shell it just before consuming or selling it. However, to prevent insect infestation the treatment should be applied to the grain immediately after harvest. Sometimes, the use of insecticides can be dangerous. Farmers are using, fungicides, acaricides and insecticides in powdered form, for stored foodstuffs, which should be reserved for the protection of seed, for example HCH, Thioral, Thirame or, worse, Dieldrin and Endrin.
In the face of this situation several actions should be taken:
Inspection methods permit the detection of indicators or levels of infestation corresponding to application of a treatment in order to avoid serious losses and wasteful, superfluous insecticide treatments.
Efforts at information extension on pesticides are undertaken principally in the zones dedicated to cotton. Extension services normally advise the employment of insecticides recognized as efficacious but less noxious, notably pirimiphos methyl. It is recommended to invest in training commercial pesticide distributors on safety and use of pesticides for storage, who can then provide this information to the farmers. Moreover, improvement of the labelling of pesticides (correct formulation and mode of employment) should be done. Labelled packages should already be in the appropriate size in order to avoid having distributors subdivide them into smaller packages, which is a dangerous procedure.
- Distribution
For some time, public bodies were able to support the provision of inputs and to provide advice on their utilization. Economic reform programmes have reduced budgets and have led state bodies to retire from input distribution with a reduction in extension services, leaving in their place the private sector. It is essential to ensure timely distribution of sufficient quantities of recommended insecticides in order to limit the inappropriate use of certain other products.
Legislation
Few countries have proper legislation permitting the control of the composition of phytosanitary products, their importation and their marketing. In most cases legislative measures are non-existent or not accompanied by the means of enforcement.
The range of insecticides available for stored grain is limited and introduction of new insecticides is slow. The application of insecticides to food products is regulated by the International Commission of the Codex Alimentarius. New chemical products thus have to pass toxicological and environmental tests. The market being relatively restricted, manufacturers do not show much inclination to develop new products.
Fumigants
Another risk with pesticides is the resistance that pests develop in response to chemical products. The phenomenon of the resistance of insects to the fumigant phosphine is more serious than resistance to insecticides because there is no other alternative for fumigation on a small scale.
Fumigation with phosphine is attractive for farmers because its formulation as tablets permits easy distribution and simple application. However, the dangers represented by inappropriate packaging, high toxicity and risks of re-infestation are often under-estimated. Airtight enclosures should be adopted to achieve completely safe fumigation. Metal or plastic drums or double-thickness plastic sacks, silos of non-porous cement, of metal or of baked earth represent a number of possibilities. The debate on the use of fumigants continues - it seems unrealistic to prohibit their use. Training farmers and distributors is necessary.
To prevent contamination by mycotoxins, it is necessary to bring stored products down rapidly to safe moisture content and to maintain that level, to combat pests and rodents and to reduce physical damage to the grain.
A strategy for control of mycotoxins should include the following actions and should be applicable by semi-qualified operators:
Up to now, the successful implementation of such a strategy has been achieved by the Mycotoxin Centres in Pakistan, Bangladesh and Philippines and partially in Zambia. The establishment in Africa of Regional Work Groups on mycotoxins should be envisaged in order to determine the needs in terms of studies and the strategy to follow as a function of the needs.
Concern relating to the cost, availability, high toxicity and resistance to insecticides has encouraged research into alternative compounds.
Peasant farmers have traditionally used a wide variety of local products with insecticidal properties: minerals, oils, and vegetable extracts from plants and trees.
Minerals
Minerals such as sand, chalk and ash are used on grain as a physical obstacle to the insects. Large quantities are needed, from 10g per kilo of grain up to 50% of the weight of the grain to be treated. In the conservation of Niébé in large sealed jars, these products are employed in finely powdered form to fill interstitial spaces and eliminate the air (oxygen). This method of preservation of leguminous grains is common among the Hausa (Niger), the Bwaba (Mali) and the Dagari (Burkina Faso).
Very fine powders are more effective as insecticides although difficult to produce locally. Industrial processes are needed to control the effect of powdery sub-products on the workers. These inert, absorbent or dehydrating powders act on the cuticle of the insect, causing loss of body fluids, desiccation and death. However the action of these powders is ineffective unless the microenvironment of the grain is dry enough to permit rapid desiccation.
Oils
The effect of oil on the grain is complex. Certain oils form a physical barrier, others are repulsive while others have true insecticidal properties. Their use is costly because of an application rate of 5 to 10ml per kilo of grain and this can affect germination. The use of oils is a promising alternative, and also effective against borers, but methods of oil extraction and grain cleaning will have to be improved.
Plant products
Farmers use products of vegetable origin (leaves, stems,, roots, flowers, fruits) against storage insects and termites. Among the plants whose use has most often been reported is Hyptis spicigera. This is used for the protection of legumes in pod (niébé, voandzu, groundnut), in the form of powder on cereals or as a rough cast lining in granaries.
The neem (Azadrichita indica) is the most popular and promising vegetable insecticide. Extracts of neem and products derived from it have even been manufactured and marketed as insecticides. Other promising materials include sweet flag (Acorus calamus), wormseed (Chenopodium ambrosioides) and peppers (Piper spp. ) with repulsive aroma and taste.
Other plants have been surveyed but their scientific names have not been identified - in particular, among the Dagari of Burkina Faso, a plant called "Napkaw". The stems and the dried and pounded leaves are reduced to a powder and mixed with ash for support. Application is made by successive layers on stocks of sorghum, rice and groundnut. Napkaw, whose pods resemble those of soya beans is known among the Lobi of Burkina Faso under the name of "Tingtingou" or "plant which kills flies" and is used for protecting animals.
Among the Gourounsi of Burkina Faso the flowers of Cymbopongo giganteus are used against insects and among the Gourmantché a plant called "Jumfani" protects the niébé in pod.
The local production of natural insecticide materials has several beneficial effects:
Despite the wide interest, few institutions have occupied themselves with the production, collection, processing and application of these compounds. The following information is still needed:
Growth regulators are compounds which act on the biochemical processes of insects and prevent their development. The modes of action of growth regulators vary. Some inhibit production of the chitin necessary to the formation of cuticle. Others disturb the production of hormones controlling metamorphosis and development and control is thus slow. Species-specific, they are not neurotoxic and do not risk being harmful to humans or other vertebrates.
Although known and available on the market, the development of growth regulators has been slow. It is too soon to suggest that they will become viable alternatives. Registration procedures for each application are long and costly which reduces their competitivity in terms of cost.
Biological control consists of introducing a natural predator specific to a pest with the objective of destroying it in a lasting manner. Biological control, which is still rare, requires little or no recurrent cost and little effort on the part of the farmer. It has been tried in Africa for control of the Large Grain Borer, using Teretrisoma nigrescens, its hereditary enemy originating in Central America. The results of this means of control of the Large Grain Borer in Kenya, Togo and Ghana are not yet known but are in the course of evaluation (monitoring and impact analysis).
The control of storage pests needs a specific evaluation for each particular situation.
Traditional methods of combat as well as pre-storage practices reduce insect infestation significantly. Use of natural insecticides should be encouraged actively, applying more field research and assistance for the development of technology for extraction and processing of natural compounds. However, it will be necessary to estimate as accurately as possible the cost of using these alternative compounds (development costs, toxicity and registration procedures).
The employment of conventional insecticides, already in use for some time, should be improved (training, supply), in order to avoid problems such resistance of insects to chemical products. This activity, up to now developed by the public sector, will need greater involvement of the private sector in future years.
Lastly, the introduction of new storage structures and the employment of pesticides should be technically valid and socio-economically acceptable. This will be best accomplished if the active participation of the peasant farmers is sought at every phase in the process of technology development.