Harvesting
1.1 Green maize
Maize for the green market is ready for harvest when the grain hardens or when the silky flowering at the top of the maize cob turns black.
1.2 Dried Maize harvesting
Maize should be harvested at physiological maturity. Maize can be left in the field beyond physiological maturity to allow for further drying. This can be done through stooking for about 2 to 4 weeks. Maize is physiologically mature when:
1. Most leaves have dried up
2. Cob husks are no longer green
3. Stalks turn yellow or brown
4. Cobs begin to droop on the stalk (figure 1)
5. Kernels show a black layer between the seed and point of attachment to the cob (figure 1).
6. Grains are hard and not milky
7. Cobs are no longer good for roasting
Fig. 1: Drooping cobs (left) signifying readiness for harvesting; and checking for a black layer in the grain (right) [Source: Kenya Maize Hand Book, 2010]
1.2.1 Preparations for the harvest
Before the new harvest it is important that farmers are prepared for postharvest activities. They must ensure that equipment needed for the harvest and post-harvest activities are available and in good condition; they decide where activities such as drying and threshing will take place; that there will be sufficient storage space for the crop; grain stores and sacks have been thoroughly cleaned before the new harvest so that residues of last season’s crop are removed from all cracks and crevices; the new harvest should never be mixed with grain from the previous season as this will encourage the movement of pests from the old to the new harvest. The old harvest can be stored in a separate place for consumption.
In addition, good hygiene should be ensured to prevent postharvest losses, the new harvest should never be placed on, or with, grain from the previous season as this will encourage the movement of pests from the old to the new.
1.2.2 Methods of harvesting
Harvesting of maize is done by hands for small-scale maize farming in Kenya. Maize stalks are cut using sickle and machete (panga) and stooked on farm (Fig. 2), or carried to the storage area.
Fig. 2: Sickle (left) and machete (middle) used for manual harvesting of maize; and stooks of maize in the field (right)
Stooking has the following enables the maize to dry in the farm and thus allowing farmers to have enough time to prepare the stores. It is also easier to pick cobs from one spot and dehulling the maize on the farm.
1.2.3 Precautions during harvesting
During harvesting, care should be taken to ensure that produce is not affected in quality or quantity:
- • The crop should be harvested at the right maturity stage depending on the intended use.
- • Delayed harvesting is not encouraged as the crop may get spoiled, thus reducing the quality and quantity available for utilisation.
- • Harvesting is best done when weather is dry because wet weather enhances rotting of produce.
- • Avoid contact of the harvested produce with soil to reduce contamination.
1.2.4 How to transport harvested maize
After harvest, maize may be transported from the farm to the homestead by any of the following methods: Head loads, Bicycle and motorbikes (boda boda), Oxen carts, Pick-up trucks and, tractors and lorries. Farmers are encouraged to use methods which do not cause damage (bruising, cracking or breakage) or loss through spillage.
1.2.5 Drying of maize on cobs
After harvesting, the maize must be dried before threshing and storage. Proper drying of maize enables grains to be stored for long and reduces conditions favourable for pest and mould infestation by lowering the moisture content. Drying should be done by spreading the cobs on mats, tarpaulins (figure 3) or in cribs (figure 4). When drying maize at the homestead, it should not be placed in direct contact with the soil and should be kept away from farm animals, or else the grain may be damaged or eaten. Maize cobs may be dried either with or without the husk cover.
Dryingmaize cobs on mats or tarpaulin
- Find a large plastic sheet or several small plastic sheets or plastic sacks that can be laid out so that they overlap to form a large covered area see figure 3(a).
- Build a flattened mound of hard-packed earth on which to place the tarpaulin. If instead you use level ground, dig a shallow trench around the area on which the tarpaulin will be placed to direct any rain water away from the drying floor.
- Make sure there are no sharp objects on the ground that will tear the tarpaulin.
- Place the tarpaulin on the place you have prepared.
- To make the process of drying quicker, cobs should be placed in a single layer and turned at intervals of every hour. If they are placed in a deeper layer on the tarpaulin then drying will be slower.
- Protect the grain from rain and night dew by covering with a tarpaulin prior to any rainfall, see figure 3 (b). The recommended moisture content for dry maize is 13%.
Fig. 3(a) and (b): Sun drying of maize cobs on mats or tarpaulin
(Source: APHLIS, 2012)
Dryingmaize cobs in cribs
Cribs are special shelters recommended for drying grain. They are long and narrow, with wooden slats or chicken wire sides that allow free ventilation, and a roof that protects against rain. To protect from rodents, the stands of the crib should be at least 1 m above ground level, beyond the maximum distance that rodents can jump. The stands are supplied with rodent guards that will keep rodents out (figure 4). The cribs are built across the prevailing wind to promote drying.
The maximum width of a crib depends on the prevailing climatic conditions, i.e.:
- 0.6 m in humid areas where maize is harvested at high moisture content (30-35%)
- 1.0 m in drier zones with a single rainy season where maize is harvested at about 25% moisture content 3. 1.5 m in very dry places.
Farmers should clean the cribs adequately prior to each harvest, and ensure none of the timbers are infested by storage insects, especially larger grain borer (LGB). Timbers infested with LGB should be replaced, otherwise the pests will move directly into the freshly harvested grain.
Fig. 4: Maize drying cribs
(Source: APHLIS, 2012)
1.2.6 Knowing when the grain is dry enough Biting grain method
Farmers need to judge when grain drying is complete, that is when grains reach a moisture content of 13.5%. Experienced farmers test the moisture content of dry grain by biting or pinching with fingers (figure 5). If the grain is dried sufficiently for storage it will be hard and does not break easily into many parts.
Fig. 5. Checking when the grains are dry for threshing, using the teeth or pinching with fingers (Source: APHLIS, 2012)
The ‘salt method’ of checking grain moisture content
The ‘salt method’ of checking grain moisture content is quick and easy but only indicates that grain is above or below 15% moisture content (Table 5). Dry salt will absorb moisture from grain. It should be noted that the ideal or recommended grain moisture content should be 13% or below. The salt must first be dried by spreading on plastic sheet in the hot sun and leaving it for at least 3–4 hours until it is hard. It should be turned at intervals during this time. It can also be dried in an oven. The dry salt should be placed in a sealed container until it is ready for use. Table 5: The ‘salt method’ of checking grain moisture content
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• | Shake the bottle vigorously for 1 minute to mix the salt and grain. |
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• | Allow the grain to settle for about 15 minutes. |
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• | If after 15 minutes the salt sticks to the side of the bottle then the moisture content of the grain is above about 15% and so is not safe for storage. |
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• | If the salt does not stick to the bottle then the moisture content is below 15% and so is safe for storage. |
Source: Soniia (1998) as described in KALRO-KCEP (2016); APHLIS (2012)
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• |
Shake the bottle vigorously for 1 minute to mix the salt and grain. |
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• |
Allow the grain to settle for about 15 minutes. |
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• |
If after 15 minutes the salt sticks to the side of the bottle then the moisture content of the grain is above about 15% and so is not safe for storage. |
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• |
If the salt does not stick to the bottle then the moisture content is below 15% and so is safe for storage. |
Source: Soniia (1998) as described in KALRO-KCEP (2016); APHLIS (2012)
Use of moisture meter
Moisture meters are quick, portable, simple to use and fairly accurate. The procedure of testing moisture content using the Dickey-John multi-grain moisture tester is as below:
- About 150 g of the grain is scooped by hand and poured into the moisture meter, which is then closed.
- The device is given a few seconds to detect moisture and temperature.
- The digital electronic machine will display the readings on the screen once the detection is over.
Moisture meters may be used routinely, for example, when grain is delivered to a grain store. They are expensive for individual farmers, and would normally be used by farmers’ groups and warehouses that handle a lot of grain.
Fig. 5: Dickey-John multi-grain moisture tester
(Source: http://www.ruralking.com/dickey-john-mini-gac-grain-moisture-tester-mini…. html)
5.3 Sorting/Grading
Maize cobs need to be selected and sorted before storage or threshing. Cobs infected with diseases, moulds, attacked by pests and other visible damages should be removed.
5.4 Mycotoxins and Aflatoxin Control
The most important mycotoxins in maize are produced by Aspergillus (which produces aflatoxin) and Fusarium species (which produces fumonisin) (figure 6). Once these mycotoxins are produced on the grain, they cannot be destroyed by cooking or heating. The only way to prevent moulds from growing and spreading is to dry maize grain quickly at harvest to moisture levels of less than 13%, keep them in clean bags, dry conditions and off the ground during storage. Farmers should check their maize regularly, to ensure that moisture levels do not increase and allow fungus and mould to grow on it.
Fig. 6: Healthy maize cob (left), Fusarium infected cob (middle) and Aspergillus infected cob (right)
(Source: IITA; www: researchgate.net)
5.5 Threshing/Shelling
Threshing or shelling is the process of separating the grain from the cobs. It is important to minimise damage to grain during this process as damaged grain is much more prone to attack by insects and fungi. The grain should be neither too moist (soft) nor too dry (brittle) at the time of threshing. Threshing is best done when grain is around 14-15% moisture content.
To shell maize, the cobs must first of all be dehusked. It is also important to select cobs that are insect or mould damaged since the grain from these cobs would reduce the quality of the other grain if they were mixed. Maize cobs can be shelled using bare hands (Figure 7), this is slow and tedious when large amounts are done at one time. It causes blisters on hands. A hand Sheller may also be used, with a rate of 7-12 kg/hour (figure 7).
Fig. 7: Hand shelling of maize (left) and a hand Sheller (right)
[Source: https://www.oneacrefund.org; IITA]
For large-scale production a range of different models of hand-cranked, pedal operated and motor-driven sellers are available powered by electricity or diesel. These give outputs of about 50-130 kg/hour (figure 8).
Fig. 8: Tractor shelling (a shelling machine connected to the tractor engine); and hand cranked shellers
5.6 Drying after threshing
After threshing, the maize kernels are dried in the sun either on plastic tarpaulins (figure 9), concrete slabs, or in commercial dryers if available. Maize should not be dried directly on the ground so as to avoid contaminating it with dirt, insects, or other pollutants. Maize laid on the ground could pick fungi spores that can lead to mycotoxin contamination.
Drying on tarpaulins
The maximum depth of the maize on the tarpaulin should be up to your ankle bone; any higher means the grain at the bottom of the pile will not dry quickly. Heavy-duty polythene sheets or sheets made from cut-open nylon sacks can also be used. If you turn over the grain several times a day, it will dry more quickly.
Fig. 9: Sun drying of maize on a tarpaulin (Source: www.oneacrefund.org)
Drying on concrete slabs
Maize grain can also be dried on a layer of concrete. Concrete heats up quickly and speeds up drying. Concrete slabs are easy and cost-effective to build. They are usually 5m×5m or 10m×10m and can be enlarged depending on requirements. Remember to sweep the concrete clean before drying the maize on it. In clear, sunny weather, a 5m×5m slab can dry 1,000 kg of maize in a day and a 10m×10m slab can dry up to 4,000 kg of grain.
5.7 Cleaning the grain after threshing
Cleaning grain after threshing can substantially improve its quality, and hence grade and price. Cleaning involves removal of foreign matter such as stones, plant material from harvesting such as husks, broken grain and dust produced during threshing. Insect damaged and mouldy grains are removed by hand picking. Cleaning is often done manually by winnowing. This involves tossing the grain into the wind which carries off light impurities, while the heavier grains falls onto a mat (figure 10). Heavier impurities are separated by sieving. The mesh sizes of sieves for maize are 4.5 mm (figure 11).
Fig. 10 (a): Winnowing (left) Fig. 10 (b): Sieving the grain (right) after threshing
(Source: APHLIS, 2012)
Mixing insecticidal dust with grain is a simple process that involves treating one or two bags of shelled.
5.8 Mixing insecticidal dust to shelledgrain
grain at a time (figures 12a, 12b and 12c). The recommended insecticidal dust in this manual is Actellic dust. The process is as follows:
- You will need a shovel and enough insecticidal dust to treat your grain. Read the instructions on the packet of insecticide very carefully so that you add the correct amount of insecticide to the grain.
- Make a heap of the grain on a clean concrete floor or tarpaulin (not on bare earth)
- Open the packet of insecticide and sprinkle the correct amount of powder all over the heap of grain, making sure the wind does not blow it away.
- Using a clean shovel, gently mix the powder into the heap as well as you can (see figure 11a).
Fig. 11 (a): Mixing insecticidal dust with shelled grain (Source: APHLIS, 2012; Kenya Maize Handbook, 2010)
- Shovel the heap to another part of the clean plastic sheet/ tarpaulin/concrete floor
- Then shovel it back again.
- Then shovel it back again for a third time (see Figure 11b).
Fig. 11(b): Mixing insecticidal dust with shelled grain (Source: APHLIS, 2012; Kenya Maize Handbook, 2010)
When you have finished, you should not be able to see any patches of insecticidal powder, and all the grains should be coated in a thin layer of powder (figure 11(c)). Using your shovel carefully load the grain back into a storage structure or sacks.
Fig. 11(c): Mixing insecticidal dust with shelled grain (Source: APHLIS, 2012; Kenya Maize Handbook, 2010)