Farming

Seed Storage

Once shelled, the raw seed may be stored until delivery to the seed company factory. Raw seed may be stored in bags or bulk but no seed should be stored unless it is at a moisture content low enough to maintain seed quality, which is less than 13%. However, the length of time that seed may be safely stored even at low moisture content depends on the air temperature and relative humidity. The lower the air temperature and relative humidity, the longer the seed can be stored with minimal deterioration. As a rule, if the relative humidity of the air is above 60%, seed deterioration is likely to be rapid; such conditions will increase the moisture content of the seed and foster the development of diseases and storage pests. Ideally, the seed should be stored in a shed to protect it from rain and heat, while providing security.

Protecting seed from storage pests

Seed should be fumigated and protected against storage pests. Fumigation is usually done with phosphine gas, either released from aluminum phosphide tablets or injected from hydrogen phosphine gas canisters. Fumigation should be supplemented with insecticidal grain protectants applied directly to the seed or sprayed onto the bag exteriors and building interior, to prevent re-infection.

Seed to be fumigated should be enclosed in an air-tight plastic sheet that will prevent the escape of the phosphine gas from the seed stack. Fumigation sheets should be polythene or polyester scrim of 250 to 350 g/m2 mass. If the sheets are UV protected, they should last for 3 to 4 years, provided they are cared for properly. Particular care should be taken to avoid punctures and tears. Phosphine gas is slightly heavier than air and will disperse downwards from the point of application, but will eventually spread to all parts of a contained area. Where phosphine tablets are used, these should be distributed at various places in the bag stack or bulk bins, according to the recommended dosage, to ensure rapid and even gas dispersion throughout the seed.

The ideal conditions for phosphine fumigation are an air temperature of 21°C, relative humidity of 60%, and grain moisture of 12%. Under such conditions, the seed should be exposed to the gas for at least 10 days, but the longer the better. Time of exposure to fumigation is better than dosage. The higher the air temperature, the faster the gas will be released from the phosphine tablets. If the air temperature is less than 15°C, it is better not to fumigate. Phosphine gas is HIGHLY TOXIC. Operators must observe safety precautions, such as placing warning signs around the fumigated area, wearing suitable protective clothing and respirators, and prohibiting smoking. Phosphine gas will corrode electrical equipment. A mixture of phosphine gas and the air is combustible and will explode if ignited.

Electrical equipment should be well-grounded to prevent sparking or static discharges. Protection from rodents. Rodents can cause tremendous damage to seeds. Losses may be reduced by storing seed in rodentproof stores, ensuring bags in seed stacks are tightly bonded to prevent rodents from penetrating into the center of the stack and controlling rodent populations with baits and predators.

Physical space for seed storage

Maize seed may be stored temporarily on the cob, either in loose stacks or in cribs, provided the grain moisture is less than 13%. Cribbed or binned ears of high moisture content are prone to attacks by storage pests. Only at about 13% moisture content will the kernels and the cob be in moisture equilibrium (Table 7). At higher seed moisture contents, the cobs contain significantly higher moisture than the seed, which will promote disease development and insect growth. A key factor in cribbing maize on the cob is the width of the crib –  the narrower the crib the better (2 to 3 m maximum), so as to allow sufficient natural ventilation through the cobs.

Storing seed in bags

Seed may be stored in bags for lengthy periods, provided the seed has a moisture content of less than 13%, the storage conditions are favorable for maintaining seed viability (i.e., cool and dry), the seed is protected from storage pests and the bags enable gas exchange and are stacked in an orderly manner. Normally, the raw seed is stored in 25 or 50 kg jute or polypropylene bags prior to processing or packing. Where bags are to be stored in stacks, use the following guidelines.

25 kg bags

Stack in 3 m x 3 m stacks, with 32 bags per layer (i.e., 0.8 t) and up to 20 layers high (total of 16 t). This stack will normally take 8 bags width-wise times 4 bags length-wise.

50 kg bags

Stack in 7 m x 7 m stacks, with 130 bags per layer (i.e., 6.5 t) and up to 32 layers high (total of 208 t). This stack will normally take 13 bags width-wise times 10 bags length-wise.

Bags should be laid so that they are bonded together, with alternate layers laid cross-wise, to strengthen the stack. Sides should be inclined inwards with height to avoid collapse. When starting a stack, the outer ring of bags is laid first, with a gap of about 2.5 cm between bags on the bottom layer. With each successive layer, the gap between bags is reduced, so that the width of the stack reduces with height. Lanes of at least 1m should be left around the stacks to allow for inspection, insecticide spraying, or covering with fumigation sheets. Stacks that have been laid well will have a density of about 650 kg per m.

As a rule of thumb, 1 t of seed may be stored per 1 m2 of floor space, allowing for alleyways.

Bulk storage of seed

The advantage of bulk storage of seed is that it requires less space since the density of seed in bulk is about 0.75 t/m3. Furthermore, with well-constructed silos and conveyors seed losses are reduced, less labor is required for handling the seed and the cost of storage bags is eliminated. Bulk stored seed is also easier to fumigate. The disadvantage is the high capital cost, although this may be offset over time with the saving in bags. Silos should always be filled and emptied from the center to avoid excessive pressure building up on one side of the silo. The angle of repose of seed of most field crops is about 25°.

Seed drying

On most farms, the seed is left to dry in the field before harvesting and shelling. This is usually possible for summer-grown crops that mature in the dry winter period. However, it might be necessary to harvest the seed when it is above the safe storage moisture content and dry the seed artificially. Early harvesting and artificial drying of seed have the advantages of minimizing disease and insect infestation in the field, avoiding field losses from birds, rodents, and theft, and enabling earlier processing and sale of seed. The disadvantages are that seed is more vulnerable to damage at high moisture content, poorly controlled drying may adversely affect seed viability and artificial drying is costly. Maize may be artificially dried pre- or post-shelling.

The most common artificial drying method is to force heated air through the cobs, to evaporate and remove the excess moisture from the seed. The cobs may be harvested at a grain moisture content of 25 to 30% and transferred rapidly to a drier, where an air of moderate temperature (not greater than 35°C, cf. Table 8) is used to dry the seed to 13% moisture. The density of maize cobs is about 480 kg/m3, which is equivalent to a seed density (on cobs) of about 360 kg/m3. The batch-type of drier is most suitable for seed since continuous flow or re-circulatory driers involve a significant amount of seed movement, which might cause seed during drying and the crop is moved only during filling or emptying the drying bin. However, the drying pattern in batch driers is uneven, with the unevenness increasing with depth. The seed at the bottom of the batch, closest to the incoming air, will be drier than the seed at the top of the batch.

Evenness of drying may be improved by ensuring that the crop is clean, having a uniform depth of seed within the drier, and minimizing the depth of the seed. The maximum depth in a batch drier should be 1 to 2 m for raw seed, and 2 to 4 m for maize cobs. Some driers are built to enable the reverse flow of air to improve the evenness of drying. The rate of drying depends on the air temperature and the rate of airflow through the seed. The maximum air temperature for seed drying is less than that for grain drying. The maximum air temperature also depends on the seed moisture content, with lower drying air temperatures required at higher moisture contents to avoid “cooking” the seed.

The rate and pressure of the airflow required to depend on whether cobs or seeds are being dried, the depth of the crop, and the required rate of drying. For example, raw seed maize requires about three times the air pressure of that for cobs, because of the greater resistance offered by the seed. The smaller the seed and the higher the bulk density, the greater the resistance to airflow through the mass of the seed. Increasing the airflow will increase the rate of drying, but will require more fuel to heat the greater volume of air.

The initial moisture content of the crop also influences the drying rate, because the rate of moisture extraction is not a linear function of moisture content. Less energy is required to remove moisture from wet grain than dry grain. In general, the drying rate for seed should not exceed 0.5% moisture removal per hour.

Diverse air-heating systems are available, using a variety of energy sources (e.g., coal, diesel, cobs). The choice of system depends on the required air temperature increase, the volume of air to be heated, the availability of fuel, and the cost of the system. Solar drying may be possible given that the air temperatures used are usually less than 35°C. Similarly, there are a number of fan options for creating airflow. The design of a seed drying system is complicated and requires expert advice.

When designing a drier, the following issues need to be considered.

  • The kind of crop(s) and the type(s) of product (raw seed or cobs) to be dried.
  • The quantity of product to be dried over a specific time.
  • The size and number of bins.
  • The filling and emptying of bins.
  • The location of the facility (accessibility, altitude, ambient air temperatures and relative humidity during drying).
  • The fuel sources available.
  • The type of heat exchanger required.
  • The fan requirements.

Cleaning seed

Seed leaving the sheller often contains foreign material, including bits of cob, husk, broken kernels, stones, dirt, weed seeds, and insects. This debris must be removed to improve seed appearance and to promote storability and plant ability. Cleaning may be done manually or using seed cleaning machines, which use a combination of screens and air movement to remove unwanted material from the seed.

Seed grading

One of the unique aspects of seeds on a maize ear is the dramatic differences in size and shape due largely to position on the ear. Large round seeds are often found at the base of the ear and small rounds at the tip. About 75% of the seed in between this round type is flattened as a result of their tightly packed position. As part of processing, the seeds are graded into various classes. These classes are determined by bypassing the seed through two types of screens that separate the seeds based on their length (rectangular-shaped slots) and width (round-shaped slots). The graded seeds are classified as large, medium, or small based on length, and round, thick or flat based on thickness. It is possible to obtain extra-small classes of all grades of seed.

Due to their relative sizes, the different grades have different weights per thousand kernels (TKW) and contain different numbers of kernels per specific weight (kernels per kg). The average weight of a thousand kernels and the average number of kernels per kg of various classes. This is important because it affects field seeding rates.

Seed treating

Maize seed should be treated with a fungicide or a combination of fungicide and insecticide to protect the seed and developing seedlings from diseases and to give short-term protection against storage insects. Fungicides are particularly helpful when sowing in conditions where soils are clayey, crusted, and/or cold and wet at sowing. Seed treatment chemicals may also help offset vulnerability to disease in seeds that have been chipped or cracked during harvest or conditioning operations. The fungicides and insecticides are commonly mixed into a slurry and applied to the seed using a seed treater device. Colored dye is often added to the slurry to impart a distinctive color and thereby clearly identify treated seed. Uniform coverage of the seed with the correct dosage of chemical(s) is critical in the treatment operation. Different fungicides and insecticides are available for seed treatment. The locally registered chemicals can be used.

Seed bagging

Bagging is the final step in the conditioning process. A seed package accomplishes several essential functions.

  • Serves as a convenient unit for handling, transport, and storage.
  • Protects seed against contamination and mechanical damage.
  • Provides a suitable environment for storage.
  • Provides a barrier against seed loss and the escape of pesticides.
  • Serves as a sales promoter.

Seed packaging material may consist of cloth, jute, plastic, paper, metal, or various combinations of products. Plastic, paper, or plastic/paper combinations are the materials of choice for packaging maize seeds. Clear plastic bags are preferred in some areas because farmers can see the seed. However, the use of plastic bags may be risky if the seed is exposed to the sun and warm temperatures, resulting in accelerated respiration and likely deterioration. Seed may be packed by hand or with semi-automatic or automatic equipment. After filling the bag with the required amount of seed, the bag is sewn or heat-sealed and a tag is attached. The tag commonly includes information about the variety, seed lot number, physical and genetic purity, germination test results, and the seed treatment chemicals used. Maize seed package size varies depending on farmers’ requirements, normally ranging from 1 to 25 kg.

Seed quality testing

After harvest, seeds should be inspected at different stages of seed processing. Tests may be conducted with samples to evaluate physical and phytosanitary quality, genetic purity, and vigor. Since moisture content has such a large influence on the life and quality of a seed, the initial quality control test is often simply an evaluation of moisture content. This is typically followed by physical purity analysis, which examines the amount and type of damaged or off-type kernels, along with the quantity of inert matter and weed seeds present in the sample. Genetic purity analysis is important to guarantee that the seed is uniformly true to type. The conventional approach has been through post control grow-out tests. These tests involve planting out a sample of the seed lot in the greenhouse or in an off-season field to evaluate plant development in comparison to the varietal description. The disadvantages of this system include the time required and the fact that the genetic material may be evaluated in an environment where it is not well adapted, making it hard to identify off-types and true-to-type plants. Today sophisticated molecular techniques available provide a precise determination of the genetic make-up of the seed. The most common seed quality test is the germination test, which measures seed viability under ideal conditions. For a maize seed lot, 4  replications of 100 seeds each are sown either in sand or on a paper substrate and placed under adequate moisture conditions at either 25°C (with 12 hours light/day) or 20° and 30°C alternating.

  • The number of normal and abnormal seedlings and ungerminated seeds are determined at 4 and 7 days after initiation. Vigor tests evaluate the ability of a seed to emerge in a timely, uniform and adequate fashion under a field conditions. To reduce the possibility of spreading harmful pathogens via seeds, four types of seed health test have been developed.
  • A physical examination of the internal and external seed parts, including use of a microscope to spot pathogens.
  • Plating seeds on agar followed by identification of emergent organisms.
  • Germinating seeds and growing the seedlings in conditions known to encourage the development of diagnostic symptoms.
  • Washing seed samples with distilled water followed by centrifugation and observations under a microscope.

Safety

Worker safety is an integral part of seed production.

Mechanical safety | Machines should be fitted with emergency shutdown switches. Dangerous moving parts should be encased in protective cages.

Electrical safety | Operators need to be protected against electric shock and precautions followed when maintaining electric components.

Dust and waste control | Seed processing generates dust, which should be removed by air-extraction ducts. Cleaning plants also produce unwanted residues, which can be a fire hazard or become an

environmental waste problem.

Hazardous chemical safety | Appropriate safety procedures must be followed during fumigation and seed treatment.

Vehicle safety | Movement of seed around the plant with tractors or forklifts may cause accidents.

Seed storage safety | Care must be taken with seed stack construction, and silos should not be entered without due precautions.

Major reasons for seed deterioration in storage.

  • Low quality seed is placed in storage.
  • Seed with high moisture content is stored.
  • The first-in, first-out rule not followed, so that seed remains for too long in storage.
  •  The warehouse is not ideal; e.g., poorly ventilated, prone to heating, exposed to moisture. penetration and/or insecure.
  • The environment is too humid or too hot for seed storage.

President

The divine scriptures are God’s beacons to the world. Surely God offered His trust to the heavens and the earth, and the hills, but they shrank from bearing it and were afraid of it. And man undertook it.
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