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.
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:
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.
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).