Getting to grips with maize storage

Peter Gambara
In recent weeks, farmers have been complaining after being turned away from Grain Marketing Board depots for delivering maize with moisture levels higher than those accepted.

In this article, I will demystify issues regarding moisture levels, harvesting and storage of grains, mainly maize.

The maize selling season officially starts on April 1 each year, and Government has been calling on farmers to deliver grain to the GMB.

Farmers were promised prompt payment at an attractive price of US$390 per tonne.

Tied to this was the fact that some farmers wanted to plant winter wheat before the planting deadline of May 31.

Farmers, therefore, tried to harvest and send their grain to GMB depots countrywide.

Unfortunately, though, quite a number were turned back due to high moisture levels in their grain. One of the most critical factors in successfully storing grain is moisture level.

High moisture content causes a lot of storage problems, including fungus, insects, respiration and germination.

Temperature is an important factor in the storage of grain.

If grain is stored with a high moisture level, it is still biologically active and respires. That respiration will produce heat.

Another effect of temperature is on the activity of insects and fungus. At a low temperature, the metabolic rate of insects and fungal disease is low. However, at high temperatures, their activity significantly increases, thereby causing a lot of grain rot.

Damp grain will increase the rate of respiration, and respiration also produces moisture; so it becomes a vicious circle.

Such moisture can actually spread, encouraging moulds and bacteria which also respire and give off more heat and moisture. Insect activity also tends to increase with higher temperatures.

It is, therefore, acceptable standard to set maximum acceptable levels for storage at 70 percent relative humidity and 27°C as follows:

Maize 13,5 percent; Wheat 13,5 percent; Sorghum 13,5 percent; and Millet 13,5 percent.

When most grain is harvested, it may still have moisture levels of between 18 to 25 percent, but as indicated above, that is a level too high for storage.

Farmers are, therefore, faced with a challenge of reducing this moisture level before they can sell grain or store it for future use.

It should be borne in mind that crops left standing, unharvested for long periods, start to show diminishing qualitative and quantitative returns through shattering, attacks by insects, mould, birds, rodents and, sometimes, human thieving.

Harvesting should, therefore, be completed as soon as possible, once the crop has dried to manageable levels.

Farmers resort to all kinds of drying to reduce the moisture levels of their grain. In natural drying, farmers use direct sunlight or natural air movement.

It is common during this time of the year to see farmers who no longer have grain from the previous season drying maize on rock surfaces or rooftops before taking it to the grinding mill.

However, in most cases, farmers prefer to allow most of the drying to take place whilst the crop is still in the field.

Some farmers will, therefore, cut the maize stalks and place them on “stacks”, where the maize will continue to dry.

Natural drying can be divided into three: Drying whilst the crop is still in the field before harvesting; drying of the grain when it is spread out as thin layers and drying in structures that have been built to permit air movement through the grain.

In rural and resettlement areas, it is common to see farmers constructing drying cribs that are raised from the ground and are constructed such that the length faces the prevailing wind side and the width is as thin as possible to allow air to move through the grain.

Farmers who practise field-drying leave the crop standing in the field for long periods.

However, as highlighted above, such crops are attacked by insects, birds, rodents, wild animals, humans, strong winds and an occasional rain shower.

The biggest disadvantage of field-drying is that it delays the removal of a crop where the farmer would want to use the land for the next crop as was the case with wheat farmers.

For such farmers, it makes sense to artificially dry the crop to speed up its removal and storage.

Some farmers spread the harvested crop on hard ground or roofs or on purpose-built platforms. As the crop continues to be exposed to the sun and wind, it will naturally dry. Farmers can stir the crop on a regular basis to ensure even drying.

The only disadvantage of this drying method is that the grain has to be removed or covered every night.

Some farmers use ventilated structures for natural drying.

It is common practise for the crop to be left in the field until the moisture level has fallen to around 18 percent before it is removed on the cob (with or without the husk) and placed in ventilated structures for continued drying.

With modernisation in technology, farmers have now created artificial drying structures with the use of forced air or heat or a combination of both to dry the grain.

A lot of former commercial farmers had built these structures. Unfortunately, these were not maintained by the new land beneficiaries.

Only a few such structures exist in towns, and with the over two million tonnes of grain expected this year, the few grain driers could not cope with demand.

Government had committed itself to repairing the driers on the farms so that the removal of grain, especially for those farmers wishing to venture into wheat production could be enhanced.

However, this failed to take place as anticipated and efforts should now be directed towards making them work for the next season.

The disadvantage of artificial drying though is that sometimes the grain can be over dried, resulting in a massive loss in weight. A farmer who over-dries 10 tonnes of maize at 15 percent to 12 percent will lose 340kg of grain (US$132,60 worth of maize).

It is also difficult to artificially dry a dirty crop as the chaff and dirt can block the airflow through the grain.

As a general recommendation, farmers who wish to store their grain should prevent moisture from the floor, roof and walls from reaching the stored grain.

Most farmers use grain bags to store grain from one harvest to another.

Care should be taken to create space for ease of sweeping and inspection for rodents and insects.

Farmers should also control rodents and insects by making sure buildings are rodent-proof by closing all holes and cracks using chemicals to control insects and rodents.

Farmers should read labels of these chemicals and make sure they have applied the right quantities.

Efforts should also be made to avoid storing new grain with old grain in the same room. The rush by farmers to send their maize to the GMB before it reaches the desired moisture levels could be stemming from fear that the money provided to the GMB by Treasury to buy grain might run out soon.

Unfortunately, the GMB had created a bad name for itself through late payment to farmers for delivered grain.

It is, therefore, important for Government to assure farmers that there is enough money to buy all the grain on offer and that all farmers will be paid on time.

If the above assurances are given, farmers should then be encouraged to dry their grain adequately before delivering it to the GMB.

The GMB offers free-testing of moisture levels of any grain that a farmer wishes to sell to them.

However, farmers should make sure they have taken big enough samples and different samples for each field for testing.

Mr Peter Gambara is an agricultural economist and consultant based in Harare. He wrote this article for The Sunday Mail

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