HomeStandard StyleMinimising post-harvest losses in stored grain

Minimising post-harvest losses in stored grain

Two weeks ago on local television they showed a story of a farmer who had 120 tonnes of maize destroyed by fire. Some farmers had their drying maize affected by the rain that fell two weeks in some parts of the country. These are stories of painful post-harvest losses. I caught up with the pesticides registrar in the department of research and specialist services in the ministry of Agriculture, Mechanisation and Irrigation Development, Kwadzanai Mushore, who listed ways of minimising losses as well as being my guest writer this week. Below is his contribution.

farm & garden with Doreen Badze

A maize stook
A maize stook

Post-harvest storage losses in stored grain can amount to as high as 30%. This presents not only a loss of food and nutrition, but also all the inputs used along the crop production and storage chain. Examples of these are costs associated with land preparation, fertilisers, seeds, pesticides and transportation, among other costs. The main causes of storage losses are insect pests, rodents and rotting. Zimbabwe has been hit by droughts in recent years and very few farmers have been able to harvest significant grain yields. However, this year, a lot of farmers expect a good harvest because of good rains which fell in the country and the inputs which were provided by the command agriculture initiative. This brings about the need to try by all means to minimise post-harvest grain losses. The following are ways of minimising such losses.

Early harvesting before the grain is infested by insect pests

It is very important for farmers to remove their grain from the fields as soon as it is dry enough. This will reduce its exposure to insect pests. A lot of grain infestation by insects starts in the field. Examples of insect pests causing serious problems in Zimbabwe are the larger grain borer (LGB), Prostephanus truncatus and maize weevils (Sitophilus zeamais). LGB is not native to Africa and it came because of grain trade. Conducive conditions here allowed it to quickly establish itself. Trees, wooden furniture and even carpets can be destroyed by LGB. Due to its wide variety of hosts, its presence is in many places which previously were not imaginable to be a habitat for grain pests.

It is, therefore imperative that its access to grain be minimised. Removal of grain from the field before infestation is of paramount importance.

In comparison, maize weevils do not have a wide variety of hosts like LGB and are easier to control. Nevertheless, early crop removal from the fields will help minimise grain damage by weevils.

Drying of grain before storing

In order to avoid grain losses due to rotting in storage, grain must be dried to the recommended 12,5% moisture content. Anything higher than this may provide conditions favourable for fungi, which cause rotting.

Hygiene in storage structures

Before bringing in new grain, it is very important for farmers to clean the storage structures. This entails general cleaning and removal of previous grain which may be harbouring insect pests. Additionally, all possible entry points for pests, both insects and rodents, must be closed. Where farmers can afford, the structures should be fumigated using fumigants like aluminium phosphide.

Non-chemical control methods

Hermetic bags: These are woven polypropylene bags lined with one or two layers of plastic inside which enable the total exclusion of air. The plastic lining in Hermetic bags prevents the entry of air into and outside of the bag, thereby creating a vacuum that starves insect pests of oxygen and suffocating them since they cannot survive in the absence of air.

Metal silos: These are cylindrical bins made of zinc sheets. These have a special design for a provision for burning a candle inside without burning the grain. They are sealed and the burning candle uses up all the oxygen. This kills by suffocation since no insect pest survives in places without oxygen. Additionally, insect pests will not be able to bite through the metal. Metal silos should be kept under cool conditions and should be raised to avoid damage by ground moisture. They can be made with varying capacities from 50kg up to three tonnes. More information concerning metal silos can be obtained from the department of mechanisation in the ministry of Agriculture, Mechanisation and Irrigation Development.

Chemical control

Chemical control is the most widely used method of storage pest control. It involves the use of synthetic pesticides and various types exist on the market.

Grain dust insecticides: These insecticides are based on one or a combination of any of the following active ingredients: deltamethrin, thiamethoxam, pirimiphos-methyl, permethrin and fenitrothion. They are the most common grain protectants in Zimbabwe and are relatively easy to use. They have been in use for a very long time and are also easily available. Their use involves mixing the grain protectant dust with grain and any insect which gets into contact with the chemical gets killed. While the use of chemical dusts has been in use for a long time against maize weevils, some of the commonly available dust is failing to control the LGB. This brings about the need for farmers to be able to determine the pest spectrum of their area. There is on-going research to determine if resistance has developed to the old group of pesticides and some companies are now reconstituting their grain protectants in order for them to control the current storage insects.

After treating with dusts and before consumption, grain must be washed to remove pesticide dust particles and then dried before processing.

Pirimiphos-methyl 50 EC formulation: This method is suitable for commercial seed production set-ups. The pesticide is mixed with water and is sprayed over grain. The grain is then dried and stored.

Treated grain bags: Grain bags are treated or impregnated with pesticides like deltamethrin which kill insects when they try to enter inside to eat the grain. This method is relatively new to Zimbabwe and its effectiveness needs to be ascertained.

Aluminium phosphide tablets: These are commonly referred to as mapiritsi echibage. Phosphine gas is liberated when tablets are exposed to air and it kills insects and rodent pests. However, aluminum phosphide is classified as a restricted use pesticide. This means it must be used by people who are trained to handle pesticides and must not be used in the home. The correct application rate is about five or six tablets per tonne and not a tablet per bag. The tablets are not to be mixed with grain. Rather, they must be put in a plate away from grain and the gas liberated will kill pests. Grain must be treated under sealed conditions for five to seven days in a place far away from human dwellings. Grain must be ventilated for five days after fumigation. Aluminum phosphide has no residual effect and it kills pests which are present at the time of fumigation and any opportunistic infestation after treatment must be avoided.

In using pesticides, farmers are encouraged to buy and use registered pesticides from registered and reputable dealers only. They must desist from buying from backyard shops since there are a lot of adulterated pesticides around. These may not be effective or may have some dangerous substances. They are also encouraged to read and understand labels before use or get assistance to understand these. Farmers also need to apply the correct dosages as recommended by the distributor on the label. Before consumption, farmers are encouraged to observe safe periods. Safe period refers to the time from application of the pesticide to the time when the pesticide is adjudged to have disintegrated so that it will not pose any health threats to the consumer. This is stated on labels. Consumption before the lapse of this period will cause health problems.

In the past, people used methods like tree leaves to protect grain from insect pests. However, these are not effective against modern-day insect pests like LGB.

It must be noted that regular inspection of the grain in storage is important. This must be done at least once a month to determine if there is infestation. Signs of infestation may include, but are not limited to grain damage, live insects, white powder from feeding action of insect pests, high humidity and high temperatures due to respiratory activity by the pests. Regular inspection will enable corrective action to be taken before huge losses happen.

More information concerning the best pesticides or methods for preserving grain can be obtained from the pesticides registration office in the ministry of Agriculture, Mechanisation and Irrigation Development as well as extension staff from the department of agricultural technical and extension services.

This week in our nursery, we have the following seedlings; lettuce commander, rape rampart, covo hybrid, eggplant, onion tx grano, tsunga paida, and broccoli
Happy farm and garden week!

Doreen Mutobaya Badze is a retired nurse and passion-driven gardener. She can be reached on Cell: 0779730880 or 16 Metcalf Road, Greendale. Email: or Facebook Page: Badze Garden Nursery

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