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5 Drying and storage

Main points in Chapter 5
Drying and storage

The usual post-harvest stages for maize

Principles and practice of drying

Principles and practice of storage

Calculating storage viability

The Larger Grain Borer

Hybrid maize has shorter and looser husks than traditional varieties of maize.
Because of this, hybrids are not as well protected and are best stored shelled and not on the cob.

Since traditional storage is not suited for shelled maize, farmers may need to consider a new kind that is.


Prior to liberalization, farmers only stored marketed maize for a short period, until they could arrange delivery to the marketing board. As we have seen, since liberalization farmers have been faced with a wider variety of marketing situations. If there is a buyer they can sell their maize immediately and not worry about storage but if there is no buyer then they have to store their maize until such time as they can arrange sale. Moreover, farmers may deliberately want to hold on to their maize for some months in the hope that the price will rise.

Storage requirements have therefore changed significantly from the days when the farmer only had to worry about storing his own food requirements and some seed for the following planting season. This was nearly always a local variety of maize and was stored on the cob, without use of pesticides, in an open store. However, maize for marketing is now mainly hybrid maize which, because of its shorter and looser husks, should ideally be stored shelled in a closed store with the use of pesticide. In fact, the spread of the Larger Grain Borer in the region makes the use of good storage essential.

The extension officer therefore needs to be able to advise farmers on:

This chapter first reviews basic principles of drying and storage and the practice in the region and then considers how farmers can calculate the viability of an investment in a new store. Finally, it looks at what extension workers can do to help stop the spread of the Larger Grain Borer.

Note: A detailed description of different types of on-farm drying and storage is beyond the scope of this Guide, although a few are illustrated in this chapter. However, national extension services should recommend drying and storage methods based on local conditions, and make these recommendations; together with information on construction methods and costs, available to all extension officers.


Figure 6 illustrates the stages that maize can go through after reaching maturity in the field. Maize reaches physiological maturity between 130 and 160 days after planting, depending on the variety. At this stage the crop has a moisture content of about 30 percent, and can be harvested provided that adequate drying facilities are available. However, small-scale farmers often harvest their maize long after it has reached physiological maturity, largely because they lack suitable drying facilities. Harvesting may be delayed by up to two months in order to allow the maize to dry. In some countries it is also common for the cobs to be left in the field for a further period, on piled, cut maize stalks.

Delayed harvesting leads to many problems. In general it can be said that the longer maize stays unprotected in the field after it has reached full maturity, the higher the losses. Reasons for this are:

Figure 6
Usual post-harvest stages for maize

Figure 6

Box 3
Reasons for drying

  1. Maize is physiologically mature, that is it ceases to grow and accumulate more dry matter, when the grain moisture content ranges between 30 and 35 percent. At this stage, the grain is still a living organism and, as such, it respires.

  2. Respiration causes the production of heat, carbon dioxide and water. The higher the temperature and moisture content, the higher the respiration rate and the higher the loss of dry matter.

  3. The ideal moisture content for storing maize is below 13 percent. For every 1.5 percent of moisture content above that grain deterioration doubles.

Note: It is thus essential that farmers dry their grain to recommended rates if they are to store it for any length of time.

Box 4
On-farm drying

An appropriate drying facility for use by small farmers should:

  1. Enable the farmer to avoid field drying by permitting adequate drying of maize with a starting moisture content of around 30 percent.

  2. Utilize locally available building materials where possible.

  3. Protect the maize against termites, rodents and other animals.

  4. Protect the crop against rain.

  5. Not require too much skill to construct.

Two simple methods that can be use to sun-dry shelled maize after it has been partially dried on the cob are:

1Spread shelled maize no more than 5 cm deep on the plastic sheet2 Rake the maize from side to side while it is drying
Sun-drying shelled maize on a plastic sheet on the ground
 3Protect the maize at night by folding the sheet in half
4 You can also gather the edges of the sheet to form a sack5 Tie the sack and carry the maize inside at night

1Build a mound of hard-packed earth and lay out a sheet of black plastic to dry your maize on
2Make a frame of wire, wood or bamboo and prepare a clear plastic sheet large enough to cover it
Sun-drying shelled maize in a drying tunnel 
3Spread shelled maize no more than 5 cm deep on the black plastic, cover the frame with the clear plastic and weight edges with earth

When maize is brought in from the field it is often divided into two. Maize reserved for family consumption is placed in traditional stores, unhusked. The maize the farmer intends to sell is generally husked and placed on raised platforms or fenced bare ground, where additional drying is done before shelling and storage in bags. Existing drying methods are, however, often unsuitable for handling large quantities of maize cobs at high moisture levels. Reasons for this include:

An example of a suitable drying structure is the improved rectangular crib. Such a crib could allow for maize harvested at 30 percent moisture content to be dried on the cob to recommended rates in about six weeks. At this time it would be ready for shelling, treatment with insecticide and safe storage. An advantage of this crib is that it can also be used to store bagged and treated maize in quite good condition. Experiments in the region suggest that the storage of maize in bags in this way compares favourably with the storage of maize in solid-walled bins.

A drying and storage crib, used normally to store maize on the cob, can also be used to store shelled maize in bags.

Improved maize crib after removal of stored cobs-Madagascar

Box 5
Using a rectangular crib for drying

Pre-loading. Maize should be put in the crib only after the crib has been well cleaned and repaired. Preferably, a liquid insecticide should be applied to the inside.

Loading. The maize should be harvested soon after maturity at a moisture content of around 30 percent. The cobs should be husked and any affected by insects or mould should be discarded.

Drying. In a well-designed and built crib the moisture content should go down to around 14 percent within about six weeks.

Emptying. Maize should not be kept on the cob for more than two months as this will lead to heavy insect infestation and losses. The crib should be emptied after 6–8 weeks through a door at floor level or by removing a couple of floor boards to allow the cobs to drop into a container placed below the crib. (The maize can then be shelled, treated with insecticide, placed in bags and put back in the crib.)


A store serves the following purposes. It provides protection against:

As previously noted, farmers' storage requirements have changed in recent years, as a result of marketing liberalization. But there are other changes affecting storage, which are not so obvious but can have a significant effect both on farmers' storage needs and on their ability to meet these needs.

Such changes include:

There are many types of store in the region. They will not be described in great detail here as it is important that extension workers follow national or local recommendations. The following general types of maize storage can, however, be identified:

The drying and storage crib, and the mud or cement-plastered basket are examples of the more traditional storage common to the region. The brick bin and the ferrocement bin are examples of improved storage. While improved storage is better, it is also more costly. If improved storage is beyond the reach of the small-scale farmer, traditional methods can be improved upon. Some photographs of these kinds of storage are shown on the following pages. Brief descriptions of their construction and some illustrations of improved storage methods are also shown.

Traditional cribsTraditional baskets
Storing maize in a crib on the groundOpen-basket storage
Maize crib roofed with thatchClose-up of basket mud-plastered inside and on top
Brick bins 
Bins roofed with thatchForming a brick grain chute
Bins roofed with brickBeginning to inset top bricks to form roof

Mud or cement-plastered basket

The traditional basket storage is made with walls of light branches, split bamboo or other woven material. It can be improved by plastering both the inside and the outside with either mud or cement mortar. A plastered basket provides additional protection against the Larger Grain Borer. The normal base for this kind of storage is a wooden platform at least 75 cm above the ground. If a wooden platform is used, the posts should be treated against termites and fitted with rat guards. However, the storage can be further improved by using a stone base topped with either mud or cement mortar. The entire basket should be protected by an overhanging detached roof or even a permanent roof structure. See page 68 for illustrations of various details for the improvement of a plastered-basket storage.

Rectangular drying and storage crib

As noted earlier this kind of crib can be, and often is, used for storage of shelled maize in bags, after that maize has been dried on the cob. Ideally, the crib should be rectangular with an interior width of 60–70 cm, but never more than one metre. It should face the prevailing wind during the drying season and have a floor structure at least 75 cm above the ground. The posts should be treated against termites and fitted with rat guards. Cribs should be erected a good distance from the fields to reduce insect infestation and their location should be free of grass, bushes, waste maize and water. See page 69 for illustrations of various details in the construction of an improved rectangular drying and storage crib.

Brick bin

The walls of this kind of bin are usually built with burnt clay bricks and plastered with either mud or cement mortar, although mud bricks may also be used. The foundation is made of stone topped with cement mortar or a concrete slab and, if it is available, covered with polythene sheeting. A chute to remove grain is set into the first rows of bricks as the walls are built. When finished the structure is plastered with mud or cement mortar both inside and outside. This bin is closed with a poured concrete lid which has a manhole cover in the centre. When the bin is finished and ready for use, it should be further protected by an overhanging detached roof or even a permanent roof structure. See page 70 for illustrations showing various details in the construction of a round brick bin.

Mud or cement-plastered baskets

1 Examples of traditional basket storage which can be improved by raising the floor at least 70 cm above ground, providing rat guards and …
… plastering the walls with mud or cement mortar inside and out

Improved rectangular  drying and storage cribs
1Examples of rectangular drying and storage cribs
Note: Angle braces are needed between each of the long posts to insure stability of the crib (see section).

1 Stone base topped with cement mortar and a chute to remove grain set at floor level2Detail of a wooden chute with sliding door (a photograph on page 66 shows a chute built using bricks)3Plaster inside and out and position concrete lid with manhole cover
4 Forms for pouring concrete lid5 Section through lid showing forms in place after pouring concrete

Ferrocement bin (Ferrumba)

This bin is built using chicken-wire reinforcement and cement mortar. A wooden frame built of light, straight tree branches or used wooden strips forms the shape and holds the chicken wire in place until the outside cement mortar is applied. The foundation is ideally made of stone, but the bin can also be supported by a termite-resistant wooden platform. The top opening of the finished structure, which is wide enough for a man to enter, can be covered with either a cone-shaped lid (also made using chicken-wire reinforcement plastered with cement mortar) or a flat cement lid. When the lid is in place on top of the bin, a bicycle tyre or a strip of rubber around the opening will ensure a tight fit. See page 72 for illustrations showing the steps in the construction of the ferrocement bin.

Ferrocement bin with grain chute - flat cement lid in foreground

NOTE: In Africa this bin is often called the “Ferrumba”, which was taken taken from the construction material “Ferrocement” and the word “Rumba”, a traditional storage structure from Northern Nigeria. This bin has been widely promoted by technical assistance projects in Eastern and Southern Africa. However its use by farmers in the region has been limited, partly because of cost and partly because until recently such structures were not required. Marketing liberalization and the spread of the Large Grain Borer may make all types of solid-wall bin more attractive to farmers.

Ferrocement bin (Ferrumba)*
1Stone base topped with cement mortar and a spout to remove grain2Two pieces of chicken wire on top of the base to reinforce the bottom3 Position wooden frame to shape the bin and fold up the chicken wire
4Cover the entire frame with chicken wire leaving an opening at the top5 Plaster the outside with cement mortar and when it is dry remove the wooden frame and plaster the inside6 Make a lid, either cone-shaped or flat7 Use a bicycle tyre or a piece of rubber to ensure a tight fit

* Designed by K.N. Østergaard with help from the Danish International Development Agency (DANIDA)

Box 6
Managing storage in a solid-wall bin

  1. A new store should not be used for at least six weeks after completion of construction, in order to dry out the cement and to repair minor cracks that may develop.
  2. The inside floor and walls should be carefully cleaned to remove all dust before loading the grain into the bin. This should be done even if the bin is new.
  3. Infestation can be reduced by early harvesting and only selecting good ears.
  4. Insecticides can be used, but should be used with care. Recommendations of the relevant government authority should be followed closely.
  5. When loading and unloading the bin, care must be taken not to damage the structure.
  6. If grain unloaded from the bin shows any sign of mouldy odour or high insect infestation, the bin should be immediately emptied and treated or dried, as appropriate.
  7. All bins require maintenance. Mud-plastered structures should be inspected from outside every two weeks when grain is in the bin and monthly otherwise. This is to avoid losses from insects which can penetrate the walls through cracks, as well as to reduce the risks that the whole structure will disintegrate. Cement-plastered structures should be inspected at least once a year and repairs carried out two months before the bin is to be loaded with grain again. Foundations should be kept firm to avoid leaning of the floor and wall structure, which will cause considerable cracking. The foundation should be checked every year and wood preservative should be added to poles every year.


Calculating the benefits of improved storage for the farmer is not easy. Benefits could be measured in terms of reduced losses. However, if the farmer uses unimproved, traditional maize storage structures for the long-term (that is, up to one year) storage of hybrid maize meant for the market, his losses through insect infestation, rodents and other damage are likely to be far greater than his benefits through getting a higher price. If he is able to, he would be better off selling the maize immediately. This will certainly be the case in areas affected by the Larger Grain Borer.

The questions the farmer has to ask himself are …

If the farmer is considering keeping back some maize for later sale at a higher price or if he is not sure that he will find a buyer early in the season, then he will have to make some investment in improved storage. It may be that improvements to a traditional crib will be adequate. In other circumstances, however, the farmer may have to invest in a solid-wall bin. The extension worker needs to be in a position to advise the farmer whether it is worthwhile to make that investment or not. The price of a new store may seem very high to farmers and they may be reluctant to make the investment, even though it will benefit them in the long run. To advise farmers on the viability of storage, extension workers need to:

Costs of storing

Assuming that the farmer does not have any labour costs to prepare maize for storage and to load the maize into the bin and take it out again, then his storage costs are: insecticides (if used), bags (where the maize is stored in bags), maintenance of the store.

Extension workers need to know the recommended rate of application for insecticides and ensure that farmers follow the recommended procedures. They also need to know the price of insecticide in the quantity required by the farmer.

Figure 7
Calculating costs of storage construction

Costs for storage construction have to be calculated on the basis of local construction methods and construction material prices. A well-informed extension officer will be able to assist the farmer in determining costs. The following outlines will be of help …
Brick binCosts
Number of bricks required × cost per brick 
Bags of cement required × cost per bag 
Labour (bricklayer): Number of days × daily cost 
Total cost 
Ferrocement binCosts
Number of bags of cement required × price per bag 
Number of metres of chicken wire required × price per metre 
Labour (cement work): Number of days × daily cost 
Total cost 

Profitability of storing It is important to realize that a farmer is not building a store for just one season. He would be unlikely to get his investment back in one year. A well-constructed bin, regularly maintained, can last for many years. Thus, when deciding to construct a store he needs to be asking himself …



In countries where markets have been liberalized for some time and where there are functioning market information services, information should be available concerning seasonal price movements. The Market Information Service, if it has not already done so, should analyse annual price trends and make this information available to extension workers. As an example, Figure 8 illustrates price trends in Zambia in recent years. The extension worker will, of course, have to translate prices in urban areas into farm-gate prices.

On the basis of such information an informed guess needs to be made as to how much the maize price is likely to rise in an average year. That does not mean storage will always be viable because in some years the drop may be so large that storage in hope of a higher price later in the season will not be justified.

Figure 8
Seasonal price trends in Zambia

Figure 8

A farmer who sells his crop immediately after drying could put the money in the bank and earn interest. If he is to store then he clearly needs to earn more from storing than he would by banking his money. This has to be taken into account in calculating storage profitability, as does the impact of inflation (see Chapter 4).

If the information mentioned above is available it is possible, ignoring for the time being the cost of constructing the store, to assess the profitability of storage, as shown in Figure 9. The calculation assumes that correct storage techniques are used and that no maize is lost through insect damage.

In the brief discussion on the Larger Grain Borer (LGB) in the next section, we not that farmers in the region who want to store maize for any length of time will have to invest in new stores. The potential losses from the LGB are so high that farmers cannot consider long-term storage using traditional methods unless those methods are greatly improved.

Thus, farmers will have to answer the question …


From the calculations illustrated in Figure 9, we can see that storage in this example is profitable when the farmer keeps his maize until at least January of the following year. However, that calculation does not include the cost of the store. Farmers who do not have to store, that is they are able to sell their maize fairly soon after harvest, therefore need to do one further calculation. See Figure 10 for an example of this calculation.

In this, hypothetical, example constructing a store represents a good investment for farmers. However, the actual calculation will depend on local building costs and local maize prices. It should also be noted that as more and more farmers build stores in order to sell their maize later in the season, the amount by which prices rise over the year is likely to become smaller, so reducing the potential profitability of storage.

Figure 9
Calculating storage profitability per ton in $

Annual maintenance and Insecticide202020202020202020
Bank interest lost24681012141618
Crop value in July100100100100100100100100100
Total storage cost122124126128130132134136138
Selling price100100105115130145160165160
Storage profitability-22-24-21-13013262922

Note: Bank interest on deposits assumed to be 2% per month

Figure 10
Calculating storage viability

1Estimated cost of store construction$120.00
2Cost per year over five years
($120.00 ÷ 5 years)
3Cost per ton stored per year
(2 ton/year in this example)
4Maximum benefit from storage per ton/year
(Figure 9/March storage profitability)
Potential storage benefit per ton/year (4 minus 3)$17.00


The Larger Grain Borer(LGB), Prostephanus truncatus, was accidentally introduced into the region, probably in a consignment of food aid from Central America. In its natural environment the LGB does not in general present a major threat for stored maize. This is because of different storage practices and because of the presence of natural enemies, in particular a predatory beetle that preys only on the LGB. In Africa there is no predator and the LGB has been spreading rapidly in East Africa. By the end of 1997 LGB had been confirmed in Kenya, Tanzania, Zambia, Malawi, Burundi and Rwanda and the neighbouring countries of Zimbabwe, Botswana, Namibia and Mozambique were at extreme risk.

Maize cob reduced to powder by the LGB

The LGB is a major pest for stored maize, in particular maize on the cob, and for dried cassava. However, it also develops populations in forests, where it feeds on wood. It is improbable that the LGB can ever be entirely eradicated in the region, but farmers can protect their stored produce against it and other storage pests by following recommended practices. The predatory beetle, Teretriosoma nigrescens, has been released in most affected countries and this is expected to keep LGB populations at tolerable levels.

Sack of maize totally infested by the LGB

The Larger Grain Borer can be contained but this will require a major communication and extension effort which will involve every extension officer. Each affected country in the region has or is developing its own programme of control. Such a programme is likely to involve extension workers in the following ways:

Note: The danger posed by the Larger Grain Borer cannot be underestimated. The damage it can cause to stored crops is much higher than other common storage pests. LGB can cause losses of between 30 and 40 percent of dry weight of maize stored over a three to six-month period and up to 70 percent in the case of dried cassava.

Prostepanus truncatus
What it looks like
• Shiny, dark brown
• Head turned down under body
• Strong jaws which cut wood
• 3–4 mm long
Enlarged adultActual sizeWhere it lives
• In MesoAmerica and Africa south of the Sahara
• In maize, dried cassava and sweet potatoes
Life cycle
• Life cycle is completed in about 25 days depending on temperature and moisture content
• Each female lays 300–500 eggs
Larva in kernel of maizeEggs
• Laid inside insect-damaged kernels
• Hatched in about 3–7 days
Pupa in kernel of maizeLarvae
• Crawl inside damaged kernels eating flour left by boring adult beetles
• Finish growing inside kernels
• See illustration of larva inside kernel of maize
• See illustration of pupa inside kernel of maize
Adults eatingAdults
• Cut their way out of kernel
• Feed on the kernel
• See illustration of adults eating

Two existing characteristics of rural maize storage in the region contribute to the spread of the Larger Grain Borer:

Wood-based storage structures: LGB belongs to the woodborer family of beetles. This enables the survival of the insect away from stored grain and between harvests. It also means that stores constructed from wood offer no protection against the LGB. An important role for extension workers is therefore to encourage farmers to construct brick bins or other non-wood structures.

Storing Maize on the Cob: The traditional practice of storing maize on the cob has two disadvantages which make it unsuitable when trying to overcome the Larger Grain Borer. As noted earlier, traditional storage is not suitable for hybrid varieties. These are susceptible to infestation due to having shorter and looser husks which do not offer the same protection as the husks of traditional varieties and allow access to the grain immediately after the cob has reached physical maturity. Moreover, the grain of hybrid maize is softer than that of traditional varieties. Further, storage on the cob means that the maize cannot be effectively mixed with insecticides, which are now almost essential to provide protection against the LGB.

Extension officers need specific information to help farmers combat the LGB. This information should be available from national authorities such as ministries of agriculture and includes:

Governments in all affected countries are mounting publicity campaigns, using radio, videos, posters and other means of communication. There is, however, no substitute for the farmer learning about how to control the LGB from his extension officer. Personal contact is very important and extension workers have a vital role to play if this pest is to be controlled. The illustrations shown on page 81 may be of value to extension workers in helping farmers to recognize the LGB.

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