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Maize seed management – Post-harvest treatment and storage

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In the last two decades, researchers have come up with many new production technologies for different cropping systems. The growing of improved varieties, mineral fertilizer use, rotations, and intercrops, have all boosted production considerably. Nevertheless, the gains of technological advancement are threatened by poor post-production techniques to process, handle and store the increased production. Due to high levels of investment in crop production, post-harvest losses, in the form of quantity or quality, should be kept at a minimum.

Quantity losses can occur because of inconsistent harvest methods, spillage during transportation, or damage by pest organisms causing reductions in weight or volume. Quality losses can occur as changes in color, smell or taste; contamination with toxins, pathogens, insects or rodent excreta; reduction in nutritional value; or loss of viability if the harvest is meant for seed.

Your major enemies to stored grain

1. Insect pests: Despite timely harvest, proper drying and shelling, and hygienic conditions, various pests still infest harvested grain. Insects are generally the most serious pests of stored grain. Storage insects are generally small in body size, rarely exceeding 2 mm in length, making them difficult to detect unless they are numerous. However, they have the capacity to multiply rapidly, so that in a very short space of time, you can easily have thousands of them attacking your grain. This rapid population growth makes them the major cause of food loss in stored grain. They are well adapted to darkness, and to movement in confined spaces and amongst stored grain. Many insect species can be found in association with stored grain, but only some are of economic importance. The most frequently encountered insect pests on stored maize include:

  • The Grain weevil (Sitophilus zeamais), which can fly and the Granary weevil (Sitophilus granarius), which cannot fly.
  • Angoumois Grain Moths (Sitotroga cerealella).
  • Grain borers: Larger grain borer (Prostephanus truncates) and the lesser grain borer (Rhyzopertha dominica).
  • Termites, which cause serious damage to the maize crop at all stages, from seedlings through to stored grain.

2. Microorganisms: Virtually all environments are surrounded by large populations of microorganisms, including bacteria, fungi and yeasts. Some enter the grain, while others contaminate or damage it from outside. Microorganisms can attack stored grain before it dries properly, when the storage environment is moist, or when it accumulates moisture. Typically, grain attacked by bacteria develops a foul rotting smell, while yeasts cause a musty, fermented smell and a slimy texture. Fungal infection is the most widespread in stored grain and appears as mold or caking on the affected ear or grain. The grain loses color, and there is loss of viability and reduction in food value. The most feared by-product of fungal attack is the production of poisonous substances called mycotoxins. Mycotoxins cause poisoning in both livestock and people.

3. Domestic rodents: This group, mainly rats and mice, causes some of the heaviest losses to stored grain. They feed on stored produce and prefer foods rich in proteins and oil, such that they may simply eat the germ of maize grain and leave the rest of the seed. A rat can eat an amount of food equivalent to about 7% of its body weight daily, (i.e. leading to losses of approximately 7 kg of grain per year). They also contaminate produce with urine, feces and other pathogens such as fleas. It is usually impossible to remove these contaminants and infested grain becomes spoiled and unfit for human consumption.

What to do to reduce post-harvest losses

The first step in managing post-harvest losses begins at planting. Some maize varieties are more susceptible to attack than others. Research has produced many improved hybrid varieties tolerant to a number of pests and diseases. In addition, some traditional varieties are resistant to some pests such as the maize weevil, and it may be important to identify those varieties. At harvest, it is important to be aware of possible sources of pest infestation. Likely sources of infestation include infestation from field to storage, insects/microorganisms remaining from previously stored grain, and cross contamination to a cleanly harvested lot. Upon identification in the field, an infested crop should can be cleaned or destroyed before harvest. The storage structure should also be cleaned thoroughly before depositing your harvest. The storage structure should be dusted with a pesticide, especially if a previous infestation was experienced. Be on the lookout for any possible hiding places for insects, while traps or baits can be set up for rodents. Other control measures are described in Table 1.

For more diagnostic information, please see the online tool ‘Maize Doctor’ (maizedoctor.cimmyt.org).

 

Stage at which losses can occur

Causes

Effect

Measures to be taken

Physiological maturity

§ Delayed harvest (increased exposure to pests, livestock and animals)

§ Varieties susceptible to diseases and pests

§ Losses in quality and quantity

§ Timely  harvest

§ Planting resistant varieties

§ Protecting crops from livestock, etc.

Harvesting

§ Poor handling

§ Poor threshing or shelling practices

§ Termites and rodents

§ Losses in quantity

§ Careful handling of produce

§ Pest control

§ Timely harvest

Mechanical damage during harvest

§ Poor handling

§ Poor threshing or shelling practices

§ Quality decreases Increased vulnerability to pests and diseases

§ Careful handling of produce

§ Threshing and shelling methods should minimize damage

Drying and storage

§ Temperatures too high during drying

§ Storage pests and fungi

§ Insufficient drying before storage

§ Moisture in storage area

§ High relative humidity

§ Losses in quality

§ Possible production of mycotoxins

§ Swelling and germination of grain

§ Avoid artificial drying

§ Control storage pests

§ Dry produce sufficiently before storage

§ Storage facility should be moisture proof and adequately aired

 Table 1.  A summary of the different stages during which post-harvest losses are likely to occur. 

References:
Derera, J., P. Denash Giga and K.V. Pixley. 2001. Resistance of maize to the maize weevil II. African Crop Science Journal. 9: 441-450.
Van den Akker, S. and D. Giga. 1992. Manual on food grain post-production technologies and practices for smallholders in Zimbabwe. FAO/UNDP/AGRITEX Project ZIM/88/018. Improving the Post-Harvest Practices of Small Farmers. Harare.

Developed with input from F. Mtambanengwe, P. Kosina, J. Jones