8. Store grain using appropriate methods

Once grain is sufficiently dry and cleaned it should be put in storage. In all cases, the moisture content of grain placed in store must be at or below the safe limit (see Section 4 Box 3). Grain may be stored on farm for different length of time as follows -

short-term (e.g. <3 months) before it is moved to the next link in the marketing chain, in this case the Collection Point of the Farmers’ Organisation, or

medium to long-term (3-12 months)where farmers keep it for household consumption or for sale at a time when prices are more favourable.

There are many options available to farmers for storing their grain and protect it against pest attack. Some of these options are presented in Table 1; note that the costs indicated in the table are only a rough guide and will vary from situation to situation. Table 1 can be used to help decide on the most appropriate store type and its associated means of preventing pest attack. You can experiment with the different options to find which best suits your needs and budget. The remainder of this section describes in more detail the options suggested in Table 1.

Storage in open weave sacks

For marketing or keeping grain on farm for periods of three months or less, open weave sacks are the most convenient option. Sacks may be made of either polypropylene, jute or sisal. The choice of bag size should meet the requirements of the Farmers’ Organisation. Typically, 50kg bags are favoured since these are more easily handled than say 100kg bags; most 50kg bags are made of open weave polypropylene. If second-hand bags are to be used then they must be thoroughly cleaned before use, this is most easily done by plunging them in boiling water and then allowing them to dry, before filling with grain. The bags should not be overfilled with grain, after filling they should be closed by hand stitching or by using a stitching machine. Before stitching, fold the mouth of the bag inwards by 5 to 10 cm, this creates a valve that helps to prevent grain being forced out of sacks when they are piled on top of each other. A 50kg bag should have at least 16 stitches across their width, larger bags proportionally more.
Before delivery to a collection point, the sacks of grain should be kept in a secure location, such as in the house. The sacks must be prevented from making contact with the floor or walls of the house, from which they might absorb moisture, causing the grain to rot. To do this the bags are placed on pallets made of sticks and/or stones so they are suspended at least 12 cm above the floor (no pallets can be constructed then a plastic sheet could be used) and away from contact with the wall. The roof above them must also be in good conditions so they don’t get wet from leaking rain water.

Sacks in a house placed on a pallet made of sticks, with good clearance from the walls

If maize or sorghum grain or beans are stored in open-weave sacks for periods exceeding 3 months, then there is a danger that insect infestation may cause significant damage, this is less likely in the case of millet, due to its small grain size and storage at moisture contents that are very limiting to insects (e.g. 10% or less), or in the case of paddy that has a seed coat that is difficult for insects to penetrate. To avoid such damage, maize, sorghum and beans that are to be stored for more than 3 months should be admixed with a suitable insecticidal dust at the manufacture’s recommended dosage rate (Box 4).

Insect-proof and hermetic stores

Besides open-weave sacks there are a variety of other store types that can be used to keep grain in the house. The efficient types are insect-proof and, even better, they may shut so tightly that they are also airtight (hermetic).

Insect proof - means that the store shuts tightly enough that insects can’t enter. If the grain is not already infested when it is put into this type of store then the grain will remain free of insect infestation during the period of storage.

Hermetic - means that the store shuts so tightly that neither insects nor air can enter. When hermetic stores are filled with grain and closed, the oxygen in the store is gradually depleted and the concentration of carbon dioxide increases. This happens due to the biological activity of the grain and any insect pests that are present will be killed. This is very convenient since pest control can be achieved without the use of insecticides that might otherwise have to be purchased.

It is important to remember that insect-proof and hermetic stored should not be place in the sunshine or close to the fire. If this happens then one side will get hotter than another. This could lead to moisture condensation on the cold side. This would lead so some grain becoming rotten. So keep such store in well shaded areas away from fires.

Table 1: Comparison of store types for safe storage of grain in smallholder households

Store type

Storage period

Pest control

Weaknesses

Costs US$ / kg

Life span

Cost/ tonne/ year

Open weave sacks (jute, sisal, polypropylene)

0-6 months

If >3 months storage then admix insecticide (Box 4).

For cowpea an option is to use solarisation if this is not for seed (Box 5)

If used >6 months, grain quality declines more rapidly than in other store types

0.1t unit

US$ 0.03

3 years

US$10 (+ pest control costs)

Improved mud silos

3-12 months

Shorter life than metal silo, very heavy so can’t be moved to new location, takes up fixed space in house whether empty of full

1t unit

US$0.1/kg

5 years

US$20 (+ pest control costs)

Metal silos

1. Make hermetic then use lighted candle or phosphine fumigation, or 2. Admix insecticide (Box 4)

Extra sealing required to make hermetic, then no access for 2 weeks.

0.18t unit = US$0.41/kg

 

1.8t unit US$0.18/kg

15 years

US$27.4 (+ pest control costs)

 
US$12.4 (+pest control costs)

Polythene bags

(1 liner + sack)

1.Solarisation if grain not for seed (Box 5)

2. Admix insecticide (Box 4)

Best for small quantities, susceptible to sharp objects and rodent attack.

0.05t unit

US$0.045/kg

2 years

US$22.5

Metal/plastic drums

Hermetic seal kills pests

Drum to be nearly full and no access for first 6 weeks of storage.

0.15 t unit US$0.26/kg
20 years
US$13.4

Triple bags

(2 liners + 1 sack)

Susceptible to sharp objects and rodent attack. No access for the first 6 weeks of storage.

0.05t unit

US$0.06/kg

3 years

US$20

SuperGrain bags (1 liner + sack)

0.05t unit

US$0.065/kg

2 years

US$32.5

Store types =

Unprotected

Insect proof

Insect proof and hermetic

 
                     
 

Box 4 Admixture of insecticidal dust to shelled grain

Insecticide dusts are recommended for use by smallholder farmers because they -

  • contain a low concentration of insecticide, making them safer to handle than more concentrated formulations

  • are ready to use

  • are supplied in small packets making the calculation of dosages easier

The instructions on the packet will tell you

·         how much powder to use, and

·         for which crop the insecticide is suitable (cereal grain, grain pulses or both) and for how long it will provide protection against insect attack.

Admixing an insecticidal dust with grain is a simple process that involves treating one or two bags at a time. The grain needs to be removed from the sack and placed in a heap on a clean surface. The insecticide is added to the grain and it is then repeatedly mixed in using a shovel. The safe use of insecticidal dusts is presented in Section 9 and the method to use for admixture to grain is shown in detail in Section 10.
 

Making a heap of grain to which the insecticidal dust is added.

Shovel the grain between two places until the dust is evenly mixed in

 
Some insect proof or hermetic stores are made of metal. If these make contact with the ground then the moisture from the ground may result in corrosion. It is there for important to raise them up from the ground by placing them on palleting.
 


Insect-proof and hermetic stores should be raised off the ground and placed inside a shelter so they are completely shaded from the sun

Farm stores that are insect-proof

Metal silos

Metal silos are insect-proof but can be made hermetic by tying rubber from a bicycle inner tube very tightly around the grain input and output ports. In order to have a quick change in gas composition, a lighted candle may be placed on the grain surface at the time the inlet and outlet ports are sealed (do not do this with plastic grain stores as they may catch fire). The candle will burn the oxygen and in so doing create carbon dioxide, this will extinguish the candle and within two weeks will kill any insects that are present. Do not open the silos until after two weeks as this will let in fresh air and the insects will survive. Alternatively, sealed metal silos can be fumigated with phosphine gas (generated from aluminium phosphide tablets) but this may not be a practical option as in many countries farmers are prohibited from purchasing and using the tablets or simply do not have access to them.
 

 

A lighted candle being used to deoxygenate a metal silo

Single layer polythene sacks

Polythene sacks (continuous sheet not open weave) can be used to store grain that is at a safe moisture content. The bag should be tightly shut by twisting then folding over the mouth of the bag and tying it shut with string (the same way as shown in Box 7). This will prevent the entry of insects (so this store is insect-proof) but a single layer polythene bag is still fairly permeable to the gases in air, so they are not hermetic. This means that when the bag is shut the gas composition does not change sufficiently to kill any insects that may be present. If this is desired then hermetic bag types such as the SuperGrain bag or Triple bag (Table 1) should be used but these are more expensive options. To prevent insect infestation in the single layer polythene sack then grain (especially cowpea) may be solarised before storage (see Box 5) alternatively insecticidal dust may be admixed (see Box 4). Polythene bags are not very strong and holes may be made in them by sharp object or by rodents. To give them further protection it is best to place them in an open weave polypropylene bag, that way there will be two layers of protection. This will involve extra expense but this will be repaid by the long life of the bag and reduced failure rate.

Improved mud silos

Improved mud silos, typically have a concrete base and concrete top with the cylinder between the base and top made of mud. As mud is porous, mud silos are not hermetic but when sealed are generally insect-proof. They can therefore be used as good stores but for storage periods exceeding three months the grain should be admixed with an approved insecticidal dust at the recommended dosage rate (see Box 4) or solarised (see Box 5).

 
Box 5 - Solarising grain to kill insect pests
When grain is placed in a solar heater, it may be heated sufficiently by the sun to kill all insects, a process called solarisation. This is usually done with relatively small quantities of cowpea (25 to 50kg), since it is labour intensive, but it could also be done with other grain. The process can reduce the viability of seed so it is better only to use it for food grain. The simplest type of solar heater consists of an insulting layer on which grain is laid to a maximum depth of about 2-3 cm, they are then covered with a sheet of translucent plastic and the edges of the sheet are weighed down with stones or other heavy items. The solar heater should be kept in the sun for at least 5 hours. After solarisation the grain should be allowed to cool before it is placed in store. If the grain is placed in an insect-proof container (see Table 1) then it will remain free of infestation. If there is free access to insects (e.g. in an open weave sack) then after 2-3 months the grain may become reinfested. To avoid this, the grain should be retreated each month. So if the grain was solarised on the 1st of June, then it should be solarised again on 1st July. More details of solarisation are given in Section 12.

Sunshinetime of soalrising

Grain being solarised in a thin layer. Beneath the grain there is matting and a layer of jute sacks while on top there is a clear plastic sheet. The grain is held like this in the sun for 5 hours.
 

Farm stores that are both insect-proof and hermetic

Metal/plastic drums

Plastic and metal drums that close so tightly that they are hermetic, make very good stores. These need to be more or less completely filled with clean dry grain, to displace as much air as possible before closure. The drums are then kept tightly shut for at least six weeks. During this time the oxygen will be depleted and the carbon dioxide rises so that any insects present will be killed. If old oil drums are to be used then they must be thoroughly cleaned (Box 6).

 

Box 6 - Important tip for using old oil drums

If metal drums are old oil drums then before use as grain stores they must be very thoroughly cleaned using a mixture of water, detergent and sand. Fill the drums with this mixture and roll them, leave for one day and repeat two more times. After that wash out with clean water and leave to dry. Make sure that after this process there is no smell of oil, otherwise repeat the process until there is no smell.

 

Hermetic plastic bags

Some plastic bags are hermetic, these are the ‘triple bags’ and ‘Superbags’. As with metal silos and metal/plastic drums, grain you are going to consume or sell in the next six weeks should not be put in hermetic plastic bags and, as with all stores, the grain should be at most 14% moisture content when entering storage. The use of hermetic plastic bags is described in Box 7.
 

Box 7 - Hermetic storage in plastic bags

Plastic bags can make good grain stores but must be kept safe from rodents that might make holes in them and so break the seal. Occasionally, insects may also make holes in plastic bags.

Triple bagging

The triple bagging is widely used for the storage of cowpea, but could be used for other pulses and cereals. There are two inner bags made of 80 micron polyethylene and one outer more durable bag to help protect against damage. The first bag is filled with grain at a safe moisture content for storage which is tied shut securely using string. The first bag is placed within a second bag and this is closed securely. A third bag is used to enclose the first two and to protect against damage; the third bag can be made of open weave polypropylene. It is recommended that the two inner bags are made of clear plastic so that the grain can be easily seen for any signs of insect attack. The bags should remain sealed for at least six weeks after they are filled and after opening they should be resealed quickly to prevent entry of pests. Triple bagging is easily adopted by farmers, provides a very high level of insect control, and the bags may be used for as long as 3 years before they become too damaged and so need to be replaced.

2.26.tif

Triple bags being prepared

Superbags

Hermetic sacks (e.g. GrainPro Superbags) are made of a multi-layer polythene material that incorporates a gas barrier that restricts oxygen and water vapour movement. These hermetic sacks are made in a variety of sizes that can hold 50kg to 3 tonnes of grain/seed. It would be normal for a farmer to place a 50kg the Superbag in an open weave polypropylene or jute bag to give it some protection and so extend its life. Studies with paddy rice stored in Superbags has demonstrated that farmers are able to maintain seed germination viability for a much longer periods, control grain pests without using chemicals and maintain grain quality for a longer period. Superbags are being traded in Africa.

 

Important points to remember

A summary of the important points to remember when using insect-proof/hermetic stores.

There are a number of general recommendations and important consideration when using insect-proof/hermetic stores. These are listed in Box 8.

 

Box 8 – Important points to remember about using insect proof/hermetic stores

1)    Grain you are going to consume or sell within six week of harvesting should not be put in the sealed store but can be stored in open weave sacks and does not need to be treated with a grain protectant such as an insecticidal dust (as few if any insects will develop in this short time).

2)    When putting grain in well sealed or hermetic stores it must be at a safe moisture content (typically 14% or less – Section 5).

3)    You must check to make sure the store has no holes in it and it is properly closed.

4)    The hermetic stores must remain fully sealed for at least six weeks if the farmer is relying on natural deoxygenation, but in the case of metal silos where a lighted candle has been used to deoxygenate the store then it should be kept closed for at least two weeks.

5)    The store should be located inside the house (or at least completely shaded from the sun) and not near a fire. It is important that the store does not get too hot. If the store gets too hot on one side and remains cool on the other then there is a danger of moisture migration, this could lead to condensation on the cold side. Condensation of water on grain can lead to mould damage. Furthermore, if the grain is to be used as seed then its viability will be lowered if it is subjected to higher temperatures (especially if over 30°C).

6)    It is better to keep any metal stores store off the floor as they may become damp and this would lead to corrosion. Place the metal store on a wooden or brick platform and this will prolong their life.