Contents - Previous - Next

This is the old United Nations University website. Visit the new site at

Hunger technology and society

Post-harvest losses in tropical Africa and their prevention
An index of quantitative and qualitative food loss
Food losses through energy transfer from cereal grains to stored-product insects
Misconception of food losses
Approaches to the assessment of food losses

Post-harvest losses in tropical Africa and their prevention

G.G.M. Schulten
Royal Tropical Institute, Amsterdam, Netherlands


The reduction of food losses by 50 per cent in 1985 was set as a target by the seventh special session of the United Nations General Assembly in 1975. Since then many activities have been initiated to reach this objective.

Before 1975, however, much work to reduce storage losses had already been conducted. Data on the achievements in a number of anglophone countries can be found in Hall (1) and Commonwealth Secretariat (2, 3), and for francophone countries in Delassus and Pointel (4).

Recently, several seminars and workshops on the prevention of losses of staple foods have been organized, for example in Arusha, Tanzania (1977); Accra, Ghana (1977); Lusaka, Zambia (1978); Ouagadougou, Upper Volta (1978); Bamako, Mali (1979); Yaounde, Cameroon (1979); and Lilongwe, Malawi (1980). In 1980 the FAO held a '.travelling workshop" for the preparation of research, development, training, and information programmes for the prevention of food losses. To promote the exchange of experience among African countries, the participants visited seven countries to study jointly each other's post-harvest problems. During the past several years many training courses have been held with the assistance of various donor organizations.

In this paper the prevention of post-harvest losses in tropical Africa will be briefly discussed.

Losses and their causes

In 1978 the available information on losses was summarized by the US National Academy of Sciences (table 1) (5). Almost all data refer to cereal grains and pulses. Losses in stored maize are, in general, the highest. Processing losses are specifically mentioned for rice. Data on losses of roots and tubers are scarce, while for fruits and vegetables no data are given. Most estimates refer to storage losses. Information on losses in other components of the postharvest system is very limited.

In 1977 the FAO Action Programme for the Prevention of Food Losses became operational. The general objective of this programme is to assist developing countries in planning and implementing their national food-loss-reduction programmes at the farmer/village level. Projects of about two years' duration are conducted, consisting of direct actions to reduce losses in staple foods by, for example, the introduction of improved structures or post-harvest equipment, pest-control programmes, training, etc. In these projects loss evaluation plays an important role as well.

In 1980, 25 of these projects were conducted in tropical Africa. Most projects dealt with maize. Among the legumes, cowpeas show especially large losses during storage.

Several countries are currently developing a strategy that will guarantee a sufficient and adequate food supply for their populations. The FAO's Food Security Assistance Scheme and Food Strategy Studies, as recommended by the World Food Council, play an essential role in reaching this objective.

A major component in such strategies is large-scale storage of grains at one or more sites, often for prolonged periods. Many countries, however, do not now have enough facilities and trained personnel to store large quantities of grains. Consequently, high storage losses in warehouses or silos may occur. However, reliable quantitative data on these losses are not available.

Present constraints on effective loss reduction

From the various seminar and workshop reports it appears that in most countries there are similar constraints. The most common are:

TABLE 1. Reported Post-harvest Losses in Staples

  Country/region Weight loss Remarks
Rice West Africa 6-24 Drying, 1-2; on-farm storage, 2-10; parboiling, 1-2; milling, 2-10
Sierra Leone 10  
Uganda 11  
Rwanda 9  
Sudan 17 Central storage
Maize Benin 8-9  
Ghana 7- 17  
15 8 months' storage
Ivory Coast 5- 10 12 months' storage on cob
Kenya 10-23 4-6 months' central storage
12 Hybrid maize, hotter regions, 6 months
Malawi 6- 14 Drying, 6; on-farm storage, 8
min.10 Hybrid
Nigeria 1-5 On-farm storage
5.5-70 6 months' or-farm storage
Rwanda 10- 20 On-farm storage
Tanzania 20-100 Unspecified storage
9,14, 67 3,6, and 9 months
Togo 5- 10 6 months' central storage
Uganda 4- 17  
Zambia 9-21 On-farm storage
Wheat Sudan 6- 19  
Zimbabwe 10 On-farm storage
Barley Sudan 17  
Millet Mali 2-15 On-farmstorage,2-4;centralstorage,10-14
Nigeria 0.1 -0.2 On-farm storage
Sudan 14 Central storage
Zambia 10 On-farm storage
Zimbawe 10-15 On-farm storage
Sorghum Nigeria 0-37 On-farm over 26 months
Sudan 6-20 Central storage
Zambia 0-10 Local varieties, negligible; high-yielding varieties, 10
Zimbawe 25 On-farm storage
Legumes Ghana 7-45 Shelled beans, 1-5 months; unshelled beans, 22
Nigeria 5 4 Cowpeas
  1-2 Cowpeas stored 3 months in shell
  4.5 Ground-nuts
Kenya 30 On-farm storage
Sudan 4-27 Ground-nuts, central storage
Swaziland 5 Ground-nuts, insects and mould
Uganda 9-18.5 Ground-nuts, mainly insects end mould
Zambia 40 Cowpeas
  5 On-farm storage, ground-nuts
Roots/ Ghana 10-20 tubers Nigeria
10- 50 Yams
Rwanda 5-40  

Loss assessment and results

Notwithstanding the emphasis that has already been placed for ten years on loss assessment, data based on actual loss surveys are still very scarce. What information is available mostly refers to insect losses in stored maize (6).

Between 1968 and 1972 a total of four loss surveys were conducted in Malawi (7). The most common type of storage bin used by the small farmer is the nkokwe (fig. 1). This is a cylindrical basket of interwoven split bamboo set on a platform of laths that are raised off the ground on stilts and covered with a conical shaped roof of thatched grass during the rains. In general, the height and diameter range from 1.2 m to 1.8 m, but larger ones are frequently seen.

Maize is usually stored on the cob with the husks still intact. The normal storage period is about ten months. At the time of the last survey, which was countrywide, at least two hybrids and five improved varieties as well as traditional varieties were grown.

In 1971 about 1,150,000 short tons of maize were produced. It was estimated that about 2 per cent of this maize came from hybrids, 6 per cent from improved varieties, and 92 per cent from local varieties.

Hybrids, especially SR52, generally have husks that do not cover the cobs sufficiently, thus exposing the grains to insect infestation. In addition, the grains are very soft and easily attacked by insects. In improved varieties the grains are almost completely protected by the husks. The grains are much harder than those of SR hybrids. Traditional varieties have very hard grains and are completely protected by the husks.

The differences in insect susceptibility are demonstrated in figure 2, where the progress of infestation is shown during the storage season. The main pests were Sitophilus spp. and Sltotroga cerealella. The graphs also show that the percentage of insect-bored grains increased considerably after the onset of the rains when the grains absorbed moisture.

From data obtained with non-sophisticated survey techniques, it was estimated that the total loss due to insects in 1972 was about 1 per cent of the total crop produced. If the whole crop had consisted of improved varieties, the loss would have been 5 per cent, and with hybrid SR 52, 10 per cent. In three regional surveys conducted the year before, the mean mould damage {mainly Diplodia) ranged from 1.4 to 4.7 per cent, rodent and termite damage between 0.1 and 0.4 per cent, and insect damage between 0.4 and 1.5 per cent (8).

Fig. 2. progress of infestation in stored maize varieties and Hybrids in Malawi

Results from surveys conducted some years later suggested that the losses were higher when grain drawn from storage was sorted into edible and inedible fractions. Losses at the time of harvest could be as much as 6 per cent by weight, which could increase by an additional 8 per cent during the following seven months' storage. Factors involved in final losses were crop variety, climate, poor harvesting techniques, and consequent mould damage (2). Two regional surveys conducted in 1980 confirmed earlier data. Weight losses in traditional varieties were lower (3 per cent or less). In the hybrid SR52, however, weight losses can exceed 20 per cent when no insecticide is used. For this reason there appears to be a gradual reduction in the acreage planted with hybrids.

The survey results clearly showed that farmers growing local varieties had very few storage problems. Therefore, research and extension efforts should be directed towards farmers growing improved varieties and hybrids. With 1 per cent lindane dust, grain damage could be kept below 15 per cent for 7 - 10 months. When the farmers did not want to use an insecticide they were advised to sell their maize to the Marketing Board in August and September.

In many areas of Kenya, basically the same storage structure is used as in Malawi. In the humid coastal strip, however, maize cobs may be stored in a special loft or basket above the cooking fire in the house. It is normal practice to store the maize on the cob without the husks.

Several well designed country-wide surveys on losses of maize stored at the small-farm level were conducted between 1970 and 1976 (9). The annual losses due to insects in the period 1973-1976 were, respectively, 4.98, 5.11, and 3.53 per cent. The annual loss caused by rodents was estimated to be 1.45 per cent, making the combined loss to insects and rodents nearly 6 per cent. Significant differences in the size of the losses were found between provinces and between years. It was found that in certain areas, and for a period of up to six months, storage of maize cobs with the husks could be as effective for loss prevention as treating dehusked cobs with 1 per cent malathion dust.

In 1973-1974 in-depth studies were made in Zambia on loss-assessment methods and losses at the farm level, and cost/benefit analysis was done (10).

In a large part of Zambia the traditional storage method is similar to that in Malawi, but the storage of shelled maize is increasing. Notwithstanding its high susceptibility to insects, the hybrid SR52 is more and more replacing the traditional varieties. Most farmers sell their surpluses shortly after harvest to the Marketing Board and store only enough for their own consumption. The study was conducted on selected farms in the Central and Southern provinces and therefore is not representative for Zambia. Losses were lower than expected, which could be explained by the farmers' practice of selecting cobs with a good husk cover for long-term storage.

At the end of the storage season, weight losses ranged between 8 and 10 per cent. Taking into account the reduction of stocks throughout the season, the weight loss was 2-5 per cent over the storage period as a whole. In this study loss/benefit analysis played an important role.

The Government recommendations for maize storage were:

The costs for the farmer to follow these recommendations were calculated. Costs were higher for farmers storing their maize unshelled than for those storing their maize already shelled. For both groups, however, it paid to follow the Government recommendations. This was partly because of a reduction in weight loss. More important was the preservation of quality, so that the highest price was paid when the maize was sold to the Marketing Board.

In addition there was another advantage to following the recommendations. The farmer could grow high-yielding, hybrid maize and store a part of his crop for his own consumption without unacceptable weight loss or loss in nutritional value.

The results of the above surveys indicate that weight losses at farm level are probably lower than previously thought. The maize variety, the storage method, and the gradual decrease in the quantity of produce stored has a large impact on the final loss figure. Losses in quality can be more important than direct weight losses.

Improved storage structures for the small farmer

Various types of improved structures have been developed for the small farmer (11, 12). However, in most cases these structures are too expensive or unacceptable to the farmer. the humid tropics a small drier using firewood and a cement stave silo with a capacity of 4.5 or 2.5 tons have been introduced on several occasions.

Traditionally, the maize in this area is left in the field to dry and is thereafter stored in open, ventilated structures as cob maize with or without husks. One of the main disadvantages of this procedure is that the maize has to be left in the field to dry for a long period.

During this period high losses can be caused by insects, moulds, rodents, and birds. In addition, this field drying prevents the preparation of the field for a second crop. In spite of much extension credit and subsidies for construction, the introduction of the drier and silo has not been successful. The capital costs involved and the skill necessary to operate the drier are prohibitive. Also, a certain period of field-drying is still necessary.

Research conducted in Nigeria has shown that, in narrow cribs (fig. 3), early-harvested dehusked maize cobs can be dried and stored without excessive losses (13). The width of the crib is critical. Under very humid conditions it should not be more than 60 cm. During drying and storage the cobs have to be protected against insect infestation with an insecticide.

In eastern and south-eastern Africa, traditional cribs have been improved by raising them off the ground to make the use of rat guards possible and by plastering the bottom with mud for greater efficiency of insecticides.

Within the framework of the FAO's Prevention of Food Losses Programme, solid walled grain bins made from ferroconcrete (ferrumbu) will be introduced in Zambia. These bins will be used to store insect-susceptible hybrids such as SR52. A disadvantage is the relatively high cost. Research is being conducted to develop less costly structures

Metal grain tanks for the storage of shelled maize are widely used only in Swaziland. They are locally produced.

Control of insects

Pesticides play an essential role in the prevention of losses in stored produce.

At farm level, insecticide dusts can be effectively utilized, but the percentage of farmers using an insecticide is very small. Several reasons for this are given, such as unavailability ability in rural areas, package sizes unsuitable for effective distribution, lack of extension credit for insecticide use, and the cost of insecticides (14).

For large-scale storage, fumigants and insecticide solutions for spraying warehouses and stacks are commonly used.

About ten years ago the only insecticides that were considered suitable to protect stored produce were lindane, malathion, pyrethrins, and dichlorvos. The latter two were mainly used for a "space treatment" of warehouses against moth pests.

Lindane is now considered too toxic and too persistent. In addition, resistance to this compound is developing fast in storage pests and so it is no longer recommended. Malathion is also more and more being replaced by other insecticides because of its short shelf life, its breakdown at higher moisture contents, and the development of insect resistance to it.

Of the newly developed insecticides, pirimiphos-methyl is at present the most commonly used. The dusts are recommended for shelled or unshelled produce, while a liquid formulation is used in warehouses. However, experiments conducted in Nigeria showed that a solution of pirimiphos-methyl is also suitable for use at the small farm level to protect maize cobs against insect infestation (13).

There is general agreement among researchers that, with a suitable insecticide like malathion or pirimiphos-methyl, shelled dry grain can be protected for a long period.

Opinions differ on the efficacy of treating cob maize or unshelled produce in general. Some favour removing the husks before using insecticides and some recommend retaining the husks, while Yet others consider the use of insecticides on cob maize at all a waste of money. A definite answer for this problem cannot be given, because it all depends on local circumstances (15). When field infestation is very high, the husks prevent the insects from coming into contact with the insecticide and should therefore be removed. On the other hand, when the grains are largely protected by the husks, the insecticide can strengthen the protection already existing.

Many farmers store their maize on the cob and may have many good reasons not to remove the husks or shell the maize. Under such conditions there may be a need to make a suitable insecticide available to the farmers, even though its effectiveness may not be optimal.

There are at present very few data on insecticide use at farm level. It seems probable that in most countries not more than a small percentage of rural farmers use an insecticide in their stored produce. It has to be realized, however, that losses in traditional varieties of maize, sorghum, and millet are often very small.

There are only a few practical alternatives to insecticides for use at the small-farm level. It is a common practice to mix ash with stored grains or pulses. For small quantities of produce, this is a very effective method. There is increasing interest in using vegetable oils to protect stored pulses (16), but this method has not Yet been used.

For the time being in most countries the transportation, marketing, and distribution of grains will mainly be in sacks. Much of the planned strategic reserves will be stored in warehouses. Because of the increased quantities and longer storage periods, the development of effective protection measures for bagged produce becomes more and more important. In the past, stacks were fumigated and then sprayed at two- to three-week intervals with malathion, alternating with lindane. This gave an effective protection against beetle pests, but infestation by moths could not be prevented.

The newer insecticides are more effective against moth pests, and some of them, like pirimiphos-methyl, have a clear fumigant action and can control insects within the sacks (13).

Research has shown that insecticides persist longer on jute than on polypropylene bags, but are biologically more effective on polypropylene. Residues of insecticides are apparently higher under polypropylene than under jute. Research on this topic is being continued.

The fumigants methylbromide and hydrogen phosphide are mainly used in large-scale storage. The use of hydrogen phosphide at the small-farm level is limited to those situations where fumigable containers exist, such as in Swaziland or in the ferrumbus developed in Zambia. Liquid fumigants have found little application in Africa. At missions, research stations, and large farms, Cs2 is sometimes used.

In West Africa, plastic capsules containing 18 ml of carbon tetrachloride were introduced for the fumigation of cowpeas in plastic bags (17). About 50,000 capsules, sufficient for the treatment of 2,000 tons of grain per year, are sold annually (18). Ethylene dibromide has been used in Ghana, but is no longer recommended.

Future outlook

As a result of various seminars and workshops, many valuable data have become available on the activities in African countries to reduce post-harvest losses. Constraints were indicated by the participants, and in many countries efforts to remove these constraints were possible.

Action-oriented projects are conducted by the FAO's Prevention of Food Losses Programme and Food Security Assistance Scheme. In addition, a number of projects are being executed with bilateral assistance.

Progress has been made in regional co-operation. A proposal was introduced by the Second African Grain Conservation Seminar (19) for an eastern and southern African collaborative post-harvest programme. A regional network on postharvest technology for food grains is recommended by the FAO's Travelling Workshop (20).

Focusing on research alone, it appears that certain aspects should be give more attention.

Loss Assessment

Losses have been assessed only for maize stored at the farm level in eastern and southern Africa. Generally speaking, the results from the two areas agree. Although many uncertainties still remain, effective loss-reduction programmes can be based on the results of the surveys. The data obtained in Zambia, especially, indicate that quality aspects in loss assessment should not be overlooked.

Already much work on drying and storage of maize has been conducted in West Africa. Before the proposed improvements are introduced on a large scale, more data are needed on the losses that take place in the different drying and storage systems in order to set the correct priorities. Better quantitative data on losses in other crops are needed as well.

Loss assessment can be time consuming and expensive, but in many instances it is necessary to prevent inefficient use of funds. For the evaluation of projects it is essential. Several methodologies, each differing in cost, complexity, and reliability, have been described (21-24).

The need to assess losses in large-scale storage is in general small. In most cases unacceptable losses are very obvious without assessment.

Adaptive Research on Loss Prevention in Different Post-harvest Systems

The "traditional" method of testing, for example, insecticides or storage structures consists of first conducting experiments at the research-station level. Depending on the results obtained, the method developed will be recommended to the farmer, or a new series of experiments will be conducted.

Very often there already exists a wealth of information on the topic collected in other countries. There is a tendency for each individual country to want to repeat experiments that have already been conducted elsewhere under similar climatic conditions and circumstances. This causes unnecessary delays in making use of knowledge already available. Repeating experiments at the research-station level often yields very little information, since local conditions are often completely different from those at the small farm level. Another disadvantage is that nothing is learned about the post-harvest system of the farmer himself and the role the improvement must have in this system.

When a certain insecticide or loss prevention method has been found effective elsewhere, it seems best to test it directly at farm level on as large a scale as possible. Evaluation should consider not only efficacy but also the cost/benefit to the farmer, his acceptance of the improvement, and the consequence it has for his post harvest system and for the post-harvest policy of the country concerned. Depending on the results, the improvement can be adapted to the local situation, introduced, and evaluated. The process can be repeated until the best solution has been found. Thereafter a monitoring phase can continue until adaptive research becomes necessary again.


1. D.W. Hall, "Food Storage in Developing Countries," J.R. Soc. Arts, 142: 562-579 (1969)

2. Commonwealth Secretariat. Storege Losses of Foodgrains in Commonwealth Africa (Marlborough House, London, UK, 1977).

3. Commonwealth secretariat, Report of the Regional Workshop on Post-harvest Losses, Accra, Ghana, 25-29 April, 1977.

4. M. Delassus and J.G. Pointel, "Results of Work and Research by IRAT in Africa and Madagascar on the Protection of Stored Food Products," L'Agron. Trop., 25, nos. 10-11: 941-946 (1970).

5. Post-harvest Food Losses in Developing Countries (National Academy of Sciences, Washington, D.C., USA, 1978).

6. J.M. Adams, "A Review of the Literature Concerning Losses in Stored Cereals and Pulses, Published since 1964," Trop, Sci., 19: 1-28 (1977).

7. G.G.M. Schulten, "Losses in Stored Maize in Malawi (C. Africa) and Work Undertaken to Prevent Them," EPPO Bull.. 5, no. 2: 113-120 (1975).

8. R.A. Reader, Survey of Damage to Maize Stored under Village Conditions, Report of the Lilongwe Land Development Project, Malawi, no. 6. (1971).

9. C.P.F. De Lima, "The Assessment of Losses Due to Insects and Rodents in Maize Stored for Subsistence in Kenya" Trop. Stored Prod. Inf., no, 38, pp. 31-35 (1979).

10. J.M. Adams and G.W. Harman, The Evaluation of Losses in Maize Stored on a Selection of Small Farms in Zambia with Particular Reference to the Development of Methodology, Report of the Tropical Products Institute, G109 (1977).

11. West African Seminar on the Volunteer Role in Farm and Village-Level Grain Storage (GTZ, Eschborn FRG, 1975).

12. Manual on Improved Farm and Village-level Grain Storage Methods (GTZ, Eschborn FRG, 1978).

13. "On-Farm Maize Drying and Storage in the Humid Tropics," Agric. Serv. Bull, 40: 140 (1980).

14. R.W.D. Taylor and D.J. Webley, "Constraints on the Use of Pesticides to Protect Stored Grain in Rural Conditions," Trop. Stored Prod. Inf., no. 38, pp. 1-9 (1979).

15. G.G.M. Schulten, "The Use of Pesticides at Farmer and Village Level," in Report of the GASGA Seminar: Appropriate Use of Pesticides (in press).

16. A.V. Schoonhoven, "Use of Vegetable Oils to Protect Stored Beans from Bruchid Attack," J. Econ. Entomol. 71: 254 - 256 (1978).

17. J.G. Pointel, "Contribution à la conservation du niébé," L'Agron. Trop., 22, no 10: 925-932 (1967),

18. J.Deuse, P. Ducom, and R. Pierrot, "Note on the Use of Liquid Fumigants," in Report of the GASGA Seminar: Appropriate Use of Pesticides (in press).

19. Report of the Second African Regional Grain Conservation Seminar (distributed by Tropical Products Institute, London UK, 1980).

20. Travelling Workshop for the Preparation of Research, Development, Training and Information Programmes for the Prevention of Food Losses-African Region: Project Findings and Recommendations; Terminal Report, AG:DP/RAF/79/018 (Food and Agriculture Organization, Rome, 1980), pp.124-131

21. K.L. Harris and C,J. Lindblad, compilers, Post-harvest Grain Loss Assessment Methods: A Manual of Methods for the Evaluation of Post-harvest Losses (Amer. Assoc. of Cereal Chemists, St. Paul, Minn., USA, 1978).

22. C.P.F. De Lima, "Appropriate Techniques for Use in the Assessment of Country Loss in Stored Produce in the Tropics," Trop. Stored Prod. Inf., 38: 15-19 (1979).

23. J.G. Pointel and J. Coquard, "Le pourcantage de perte en poids et la perte spécifique, critères d'èvaluation des dègâts causés par les insectes dans les céreals et les légumineuses stockèes, "Agron. Trop., 34, no. 4: 377-381 (1980).

24. FAO, "Assessment Collection of Data on Post-harvest Foodgrain Losses," Econ. Social Dev. Paper, 13: 1-70 (1980).

Contents - Previous - Next