Agronomy - Drying and Storage

This lesson will teach the reader how to determine the best drying method to use after harvesting their crops, and how to determine the most suitable method of storage for their crops, as well as be able to recognize and control crop storage insects.

 

Lesson 1.9 - Drying and Storage

Objectives

After this lesson, participants will be able to:

1. Determine the best drying method to use after harvesting their crops.

2. Determine the most suitable method of storage for their crops.

3. Recognize and control crop storage insects.

Lesson Preparation

Before presenting the lesson, prepare the following:

1. Prepare copies of notes at the end of lesson for each of the participants.

2. Prepare samples to determine grain moisture.

3. Prepare a sample (glass bottle) of grain with insect damage.

4. Buy samples of insecticides farmers use in the area for protecting stored grains.

Concepts to be Taught

1. Drying

   Grain Moisture Guidelines For Safe Storage

   How To Determine Grain Moisture

   Methods of Drying

   Temperature Guidelines for Grain Drying

2. Storage

   Traditional Storage Methods Improved Storage

3. Insect Control in Stored Grain

   Factors Favoring Infestation

   Types of Storage Insects and their Identification

   Controlling Stored Grain Insects

   Non-chemical Controls for Storage Insects

   Chemical Controls for Storage Insects

Drying and Storage

Grain drying and storage are very good subjects, and adequate coverage is far beyond the scope of this lesson. Some of the more important principles and practices are outlined here.

Drying

Very moist grain will deteriorate and spoil during storage for two reasons: 1) Since they are alive, the seeds consume oxygen and burn up some of the food stored in the endosperm for energy. This respiration process produces heat, but is too slow to be of concern in dry grain. However, respiration and heat production are rapidly accelerated by moisture, and the moisture and heat promote rapid mold growth and spoilage in wet grain. 2) Storage insects like weevils become more active and multiply more quickly in warm, moist grain. They also produce heat and add more moisture which further increases mold growth.

NOTE: Some storage molds produce toxins called mycotoxins which are harmful to humans and livestock. Aflatoxin is one example. All cereal and legume grains are susceptible if insufficiently dried or improperly stored, especially peanuts.

Fortunately, the farmers do not have to dry their grain down to zero percent moisture, since it can tolerate about 12-30 percent depending on the type, the form in which it is stored (ears or seedheads vs. loose grain), how it is stored (bags vs. bins, etc.), and the surrounding temperature and humidity. Most loose grain has about 12-15 percent moisture at marketing or prior to processing for consumption, and crop yields are usually calculated on about a 14 percent moisture basis. In fact, there are several disadvantages to drying grain below this range. Where grain is sold by weight, overdrying will reduce the farmer's returns from a sale. It is also costly where artificial drying is used and can lead to cracking, discoloration, and poor germination.

Grain Moisture Guidelines For Safe Storage

Corn, Sorghum, MilletLoose

Threshed grain can be safely stored in silos or bins for up to a year at 25-30C and 70 percent relative humidity if grain moisture is not above 13.0-13.5 percent for corn and sorghum, and 16 percent for millet. Bagged corn and sorghum can be stored at up to 15 percent moisture, since ventilation is much better.

On the cob or seedhead

Husked corn ears can be safely placed in cribs for storage and further drying at kernel moisture contents up to 30 percent if all the ears are within 30 cm of open air. Sorghum and millet seedheads can also be safely stored and dried down from high moisture contents if kept in small stacks or hung from rafters.

Peanuts

For safe bulk storage of pods, kernel moisture content should not be above 10 percent. Pods can be safely placed in bags at kernel moisture contents up to 16 percent and will dry down adequately if loosely stacked, provided ventilation is sufficient. Otherwise, forced air will be needed.

Beans and Cowpeas

Threshed seed stored in bins or silos should not be above 13 percent moisture. Bagged seed can be safely stored at up to 15 percent moisture. Unthreshed pods can be kept at much higher moisture contents and will dry down well if ventilation is adequate.

How To Determine Grain Moisture

Grain moisture should always be calculated on a wet weight basis. In other words, 100 kg of 15 percent moisture corn contains 15 kg of water and 85 kg of dry matter. There are several ways of measuring grain easily done on the farm with very little equipment.

Salt and bottle method

This quick and easy method is accurate to within 0.5% but will only indicate the grain is above or below 15% moisture, the maximum limit for storing corn and sorghum in bags.

1. Thoroughly dry out a bottle of about 100 ml capacity and fill it three-quarters full with corn.

2. Add 5-10 teaspoons of oven-dry table salt, seal the bottle with a dry lid or cork, and shake for several minutes. If the salt sticks to the inside of the bottle, the grain has over 15% moisture.

Oven method

A grain sample of known weight should be ovendried for one or two hours at 130C if ground or 72-96 hours at 100C if in whole form. After reweighing, moisture content can be calculated as follows (cover the grain to avoid moisture reabsorption while it is cooling off):

% moisture of original sample = Wet Weight-Dry Weight/Wet Weight

Biting, Pinching, Rattling, Feeling

Most farmers will use such methods for estimating grain moisture with varying success, depending on experience. They should not be relied upon where accuracy is important as in the case of grain stored in bulk (bins or silos.)

How to Estimate the Final Weight of Grain After Drying:

Final grain weight after drying = % dry matter before drying x original weight/% dry matter after drying

Example: A farmer has 2000 kg of shelled corn at 20% moisture. How much will this amount of corn weigh after it has been dried down to 14% moisture?

Solution: To obtain the percent dry matter needed for the formula, subtract grain moisture content from 100 percent then use the formula.

Final grain weight after drying = 80%/86% x 2000 kg = 1860 kg after drying to 14%

Methods of Drying

Traditional sun-drying is the most common method used by small farmers and consists of spreading the grain out in a shallow layer on the ground for sun exposure. Depending on the weather, the thickness of the grain layer, and the amount of stirring, the results range from poor to good. The disadvantages are poor air circulation, contamination with dust and stones, and moisture absorption from the ground. The small farmer's experience can provide grain storage recommendation on general improvements for this system.

Enclose solar drying reduces sun-drying time, requires no fuel, and can be used on other crops like cassava, copra, fruits, and vegetables. However, grain can be damaged or have its germination impaired by the extremely high temperature (65-80C) that can build up under the plastic or glass sheet. Solar drying may not dry down grain rapidly enough when operated under very cloudy conditions (see appropriate technology section for information or plans for solar driers.)

Fuel-Heated and/or Forced-Air Drying

For large quantities of grain, fuel-heated and/or forced-air drying is used. For the individual small farmer such drying may not be feasible. However, the procedure can be justified on a cooperative basis and can offer several advantages:

1. Farmers can harvest their crops earlier at a higher moisture content to avoid losses caused by natural field drying. Earlier harvesting also permits earlier land preparation and planting of the next crop.

2. The grain may end up at a lower, safer moisture content for storage and keep in better condition. Its market value may also be higher.

On the other hand, construction and fuel costs may outweigh these advantages, so a thorough analysis of the factors should be conducted before deciding to buy or build such dryers.

Temperature Guidelines for Grain Drying

Excessively high drying temperatures can cause cracking, breaking, and discoloration of the kernels and also lower germination and protein quality. Peanuts may become bitter if dried at temperatures above 32-35C, and overdrying increases splitting and skin slippage during shelling. Beans are also best dried at low temperatures.

STORAGE

Storage losses of grain due to molds, insects, and rodents are estimated to be about 30 percent worldwide. Small farmers are especially vulnerable to such losses since their traditional storage methods are often inadequate to protect the crop. In many cases, farmers may be forced to sell much of their grain shortly after harvest at a low price rather than risk spoilage. A few months later, they may end up buying it back at a much higher price. By improving their storage facilities, farmers can ensure more food for their families, more stable prices, and better quality seed for planting. Crop improvement programs should place a higher priority on providing ample safe storage for the expected production increases.

Traditional Storage Methods

If a farmer's production is small, it is often stored in the family dwelling. Corn ears and seedheads are commonly hung from rafters in the cooking area, the smoke acting as an insect deterrent. Clay pots, closely-woven baskets, and gourds are also frequently used. While such methods may work well with small amounts of grain, they are no well suited to large quantities.

Improved Storage

The small farmers of the area may use some kind of improved storage. Through discussion, participants can design and/or put together some guidelines for other types of improved storage. The major points are summarized briefly here.

Storing in sacks made of burlap, local grasses or cotton does not afford much protection against rodents, insects or moisture. However, sacks are easy to label and move around, and grain can be stored at about 2 percent higher moisture than is needed for airtight storage (i.e. about 15 percent versus 13 percent.) For sacks storage:

1. The walls and the roof of the storage building should be waterproof.

2. Sacks should be stacked on platforms (pallets) raised off the floor or on a plastic sheet. They should no lean directly against walls.

3. The sacks should be stacked in a way that favors good ventilation.

4. The building should be insect- and rodent-proof.

5. The sacks should be sprayed or fumigated for insects, but only when grain will not be consumed directly by humans or animals (seed grains.)

Silos and bins made from sheet metal, mud bricks, cement blocks or cement with metal staves can be built with capacities ranging from 500-4500 kg of dried, threshed grain. Some of them can be made virtually airtight. However, whenever grain is stored in such large quantities, more care must be taken to ensure that it is well dried. Unless well insulated, the containers should be shaded to prevent large temperature variations which cause moisture migration, condensation, and spoilage of grain at the top and bottom.

Airtight storage in sealed gourds, underground pits, plastic bags, drums, and bins provides excellent insect control and also prevents the grain from reabsorbing moisture from humid outside air. The air present in the container when it is sealed is soon used up by grain respiration and any insects already present. For successful airtight storage:

1. The grain should not be above 12-13 percent moisture.

2. The containers must be made airtight by using metal, plastic, cement (with vapor barrier) or a waterproofing material like tar, oil-base paint or pitch.

3. Containers should be filled to the top to exclude as much air as possible before sealing.

4. Airtight storage should not be used where the containers must be frequently opened, since the added air will make the system ineffective for controlling insects.

5. Containers, especially metal ones, should be shaded to prevent condensation and moisture migration.

Insect Control in Stored Grain

Weevils and grain beetles feed on grain in both the adult and larval stages. In addition, the larvae of several types of moths attack the seeds. Aside from the actual losses due to feeding, storage insects promote mold and spoilage of grain by adding moisture and increasing temperature. A heavy infestation can increase grain moisture content by 5-10 percent within several months. Even if the grain does not spoil, it may be rendered unmarketable by the presence of insects or the physical damage caused by their feeding.

Grain can become infested both in the field and during storage. Some storage insects like the corn weevil, rice weevil, and angoumis grain moth which attack cereal grains and the bruchid beetles that attack pulses, have wings and can infest grain in the field. These and other types can also begin attacking grain during storage. The adults lay eggs on or in the grain, and the developing larvae hollow out the kernels.

Factors Favoring Infestation

Temperature: This is the most important factor. As temperature increases from 10C to 26C, storage insect activity increases, and life cycles are reduced from about eight weeks to three weeks. At optimum temperatures, 50 insects could theoretically multiply to 302 million in just four months! Activity and breeding shows considerably below 10C and above 35C, and death occurs below about 5C or above 59C.

Moisture: Storage insects prefer under-dried grain, but can still cause serious problems in grain as dry as 12-13 percent. Grain moisture content has to be 9 percent or below before activity ceases, and this degree of dryness is difficult to achieve and maintain.

Storage practices: Storing new grain next to old grain or using storage facilities or sacks that have not been disinfected are sure ways of inviting infestations.

Types of Storage Insects and their Identification

It is useful to be able to precisely identify the types of insects attacking a farmer's grain for three reasons:

1. Not all insects found in grain are serious pests. On the other hand, lack of visible grain damage does not necessarily indicate that the insects are harmless or minor pests, since it may take some weeks for damage to become apparent.

2. Although control measures are fairly similar for most storage insects, there are some differences.

3. Some storage insects are known as secondary and tertiary pests since they feed mainly on grain which is cracked or already damaged by primary pests. The presence of these non-primary pests often indicates that more serious pests are at work.

Checking for Infestations

Early recognition of an infestation is very important for reducing potential grain losses. Stored grain should be closely checked every several weeks for sign of an insect buildup. Exit holes in the kernels, cobweb-like accumulations on sacks and corn ears, and the presence of adult insects are sure signs. When sampling grain, the farmer should examine kernels from various sections of the container or sack, since infestations often develop and spread from very localized areas or "hot spots" where temperature and moisture may be very high.

Controlling Stored Grain Insects

A brief summary on non-chemical and chemical controls for insects is given in this lesson. Specific or most used chemicals will need to be discussed with the participants and additional information may be required from other sources.

Non-chemical Controls for Storage Insects

1. Unhusked: Storing corn in the form of unhusked ears is somewhat effective.

2. Sunning the Grain: Beetles and weevils will leave grain if it is placed in the hot sun in a shallow layer. However, this usually will not kill all the eggs and larvae inside the kernels.

3. Smoking the grain: By building a smoky fire under a platform or corn crib, many of the insects can be killed by both the smoke and heat.

4. Mixing materials with the grain: Effectiveness varies with the substance used, but control can be quite good in some cases.

5. Sand, burned cow dung, wood ashes, and lime give varying results. Sand helps exclude air by filling in the spaces. It also scratches the insects' shells which can lead to dehydration and death if the grain is already very dry (9-10 percent moisture.) The other materials may have some insecticidal properties. It was discovered by CIAT that adding wood ash to bean seed at the rate of one part to three reduced bruchid weevil infestations by about 80 percent if applied before the insects appeared. Slaked lime (calcium hydroxide) or burned lime (calcium oxide) added at 4-8 parts per 100 is also fairly effective (both types are caustic).

6. Plants: In some areas, certain plants are known to have insecticidal properties and are mixed with the grain.

7. Vegetable Oil: The oils of peanuts, sesame, coconut, cottonseed, and mustard seed have given excellent protection from bruchid infestation in beans and cowpeas when added at the rate of 0.5-1.0 percent (5-10 ml per kg of seed.) Protection lasts for up to six months and does not affect the physical appearance of the grain since it is absorbed.

8. Airtight storage: see storage methods.

Chemical Controls for Storage Insects

Grain that will be stored only a few weeks or even up to two to three months may not warrant the use of insecticides. However, the best time to treat grain is when it is first put into storage, before an infestation becomes serious.

CAUTION!: Some insecticides like Malathion, Lindane, Actellic, and Pyrethrins can actually be mixed with food grain without harmful effects or residues if used correctly. Many other insecticides would make the grain very toxic and unsuitable for consumption. Many farmers are not aware of these differences and in fact may refer to all insecticides by one name such as "DDT."

Where to obtain recommendations: Check with your local extension agent for insecticides available and used in the area. Other farmers can provide information on what insecticides they have been using.

Return to the first lesson in this series

Notes to Lesson 1.9

Some Important Grain Drying Principles

1. Warm, dry, moving air encourages more rapid drying to a lower moisture content than cool, damp, still air. In fact, if the air becomes too damp, grain may actually begin to absorb moisture and become wetter.

2. Air flow through the grain and air moisture content (relative humidity) have the biggest influence on drying. The lower the air's relative humidity the greater its ability to pick up moisture from the grain and carry it away.

3. Warm air has a much higher moisture-holding capacity than cool air. This means that warm air is more effective at picking up moisture from wet grain than cool air when the relative humidity is low.

4. Supplemental heat from either sunlight or fuel can be very effective at improving the drying ability of cool air if it is very damp (high relative humidity). For each 0.55C rise in temperature, the relative humidity of the heated air is reduced by about 2 percent.

5. The rate of drying slows down as grain moisture content falls, since the remaining moisture is given up less readily. Unless the air is very hot and dry, a point is eventually reached beyond which no further drying occurs. This is known as the equilibrium moisture content.

The maximum safe drying temperature depends on the crop and its use:

Table I - Maximum Safe Drying

Crop and Use	Temperature
Livestock feed	75˚C
Cereal grains for human food except rice	60˚C
Milling for flour	60˚C
Brewery uses	45˚C
For planting seeds	45˚C
Rice for food	45˚C
Beans for food	35˚C
Peanuts	35˚C

Principles of Safe Storage

1. Grain must be adequately dried before being put into storage, although corn stored on the ear and other crops stored in the form of seedheads or pods can often be stored and dried at the same time using cribs or loose stacks.

2. Undamaged, winnowed grain has a much longer storage life. Uncleaned grain reduces air movement, and the dirt and chaff hold moisture and encourage molds and insects. Damaged grain deteriorates two to five times more rapidly than undamaged grain.

3. Grain should be kept as cool as possible and protected from fluctuations in outside temperatures that encourage condensation and moisture buildup inside the container.

4. Drain should be protected from storage insects and rodents.

5. Containers and buildings must be waterproof and free from groundwater.

6. New grain should be stored separately from older grain.

7. The old grain should be used first.

8. The grain should be checked every two or three weeks for signs of heating and insects.

Pre-storage Guidelines

1. Be sure the grain is well dried and cleaned.

2. Clean out and repair the storage facility. This includes the sweeping out old grain and debris and patching all the holes and cracks where insects might hide or moisture might enter.

3. Spray or dust the facility with an approved insecticide.

4. Disinfect used grain sacks before filling by boiling, sprayed with an approved insecticide or placing them on a hot tin roof.

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