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CLOSE THIS BOOKTraditional Field Crops (Peace Corps, 1981, 283 p.)
Harvesting, drying, and storage
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VIEW THE DOCUMENTFrom maturity to harvest
VIEW THE DOCUMENTHarvesting and threshing
VIEW THE DOCUMENTDrying and storage
VIEW THE DOCUMENTLessons of the ''Green Revolution''

Traditional Field Crops (Peace Corps, 1981, 283 p.)

Harvesting, drying, and storage

For the farmer, the challenges of agriculture do not end when a successful crop reaches maturity in the field. Losses between maturity and consumption or sale are frequently serious, especially for the small farmer, and are also a major contributing factor to the world food problem. This chapter focuses on specific practices that will keep these losses at a minimum.

From maturity to harvest

Maize, Sorghum And Millet

When these cereal crops reach physiologic maturity, the grain is still too wet and soft for damage-free threshing (separation from the stalk or ear) or for mold-free storage. Most small farmers let the crop dry naturally in the field for several weeks prior to harvest, unless immediate land preparation is needed for the next crop. During this "dry-down" period, the crop is vulnerable to losses caused by several factors:

· Rodents: Losses are especially high where lodging or stalk breakage is severe.

· Lodging and stalk breakage: These may occur during the drydown period and are encouraged by overly-high plant densities, low soil potassium levels, high winds, and stalk rots. They promote rodent damage and grain rots, especially when the ears or seedheads touch the soil.

· Grain rots: Wet weather during dry-down may provoke fungal grain rots (head molds, ear rots) or accelerate those that may have begun during grain filling. Some small farmers bend the maize ears downward near maturity to prevent water from entering through the tips.

· Weevils and other storage insects: Some storage insects like the rice weevil (Sitophilus oryzee) and the Angoumis grain moth (Sitotroga sp.) can fly to the fields and infest crops from the soft dough stage onward. Maize varieties with long, tight husks have good resistance, but high-yielding varieties tend to be inadequate in this respect.

· Birds: Most species prefer younger, softer grain but can still cause problems after maturity. Bird-resistant sorghum varieties lose their repellent ability by the time maturity is reached.

· Theft: Farmers should be encour encouraged to harvest their crops as soon after maturity as is practical to prevent losses from theft.

Beans and Cowpeas

Losses between maturity and harvest of beans and cowpeas are caused by:

· Pod shattering: Spillage of seeds from drying pods that split can be a problem, but losses are not usually serious unless harvest is delayed.

· Bruchid weevils: (See section on storage.) These are not only serious storage pests of pulse crops but also can fly to the fields to infest beans and cowpeas by laying eggs in cracks or cuts in the pods.

· Seed deterioration: This can be a serious problem in beans and cowpeas and can occur soon after maturity if rainfall continues. Studies by IITA have found that cowpea seed quality and germination decline rapidly when harvest is delayed. In tests under wet conditions, seed germination fell to 50 percent or lower within three weeks after maturity, and preharvest fungicide sprays were of little benefit in preventing this.

· Delayed maturity: Literature from CIAT mentions that bean plants may put out new growth and flowers during maturation under high rainfall. The new foliage can interfere with the proper drying of the maturing pods and may lead to rotting.


Peanuts pose a special problem since the nuts do not mature simultaneously. Those that ripen first may become detached from the pegs before the rest mature. The timing of the harvest is critical and will be covered in the next section.

Harvesting and threshing

Nearly all small farmers in the developing countries harvest their cereal crops and beans by hand and thresh them later. In the case of peanuts, harvesting involves lifting the plants and attached pods from the ground, then allowing them to cure (dry) in the field for a period of from several days to four to six weeks before threshing.

Threshing consists of separating the seeds from the seedheads, cobs or pods by beating, trampling or other means. With peanuts, threshing separates the pods from the pegs that hold them to the plant and does not include actual shelling. (With maize, the term "shelling" is usually used in place of "threshing".)

With cereal crops and beans, the small farmer has several options as to when to thresh the crop. If the matured crop has stood in the field for some time during dry weather, the seeds may be low enough in moisture content to be threshed without damage right after harvest. However, the farmer may still prefer to delay threshing for two reasons:

· The grain may still be too high in moisture content to escape spoilage if stored as loose seed. Grain stored in unthreshed form on the cob, on the seedhead or in the pod can be safely stored at a much higher moisture content since there is much more air space for ventilation and further drying.

· Maize stored as unhusked ears and pulses stored in their pods are more resistant to storage insects.

Winnowing follows threshing and consists of separating chaff and other light trash from the grain using wind, fan-driven air or screens Winnowing may need to be repeated several times before consumption or marketing and is usually supplemented by manual removal of stones, clods, and other heavy trash.

Guidelines for Harvesting and Shelling Maize

Determining Maturity

In the 0-1000 m zone in the tropics, most maize varieties reach physiologic maturity within 90-130 days after seeding emergence or 50-58 days after 75 percent of the plants have produced silks. As maturity nears, the lower leaves begin to yellow and die off. In healthy, wellnourished plants, this should not occur until the ears are nearly mature. Ideally, most of the leaves should still be green when the husks begin to turn brown. Unfortunately, such high-yielding plants are not often seen in small farmer fields because of stress factors like low fertility, insects, diseases, and inadequate weeding. More typically, most of the leaves are dead by the time the plant matures.

The "black layer" method: When a maize kernel reaches physiologic maturity (maximum dry weight), the outside layer of cells at its base where it connects with the cob will die and turn black, thus preventing any further cob-to-kernel nutrient transfer. This "black layer" provides an indication of maturity. The layer can be seen by detaching kernels from the cob and examining their bases. Newly-matured kernels may have to be slit lengthwise with a pocketknife to expose the black layer. However, with older kernels, the layer can be readily seen by scraping the base with the fingernail.

Keep in mind that physiologic maturity is not reached until all the kernel's milky starch has solidified. This process begins at the tip of the kernel and moves downward toward the base. The kernels at the ear tip are the first to mature, followed by those in the middle and finally the ones at the lower end (the difference is no more than a few days).

With healthy plants, kernel moisture at physiologic maturity will vary from about 28-36 percent. This is usually too high for damagefree threshing or for mold-free storage except in the form of husked ears placed in very narrow cribs. The black layer may form much earlier in the maize plant's growth cycle if growing conditions are adverse. Such kernels will be small and shrunken and have much higher moisture contents when the black layer forms. The drydown rate of maize: When maize plants are left standing in the field after maturity, the kernels lose about 0.25 0.5 percent moisture per day, but this can range from 0.1 - 1.0 percent depending on weather conditions and whether the ears are pointing downwards to prevent water entry.

Methods of Harvesting Maize:

· By Hand: The ears are removed by hand from the plants with or without husking. Husked ears require a smaller storage area and are more resistant to insects, but may rot more easily if stored at a high moisture content.

· Mechanical: Tractor-drawn pickers and picker-shellers can handle one to two rows at once, but self-propelled combines are available which can harvest up to six to eight rows. By changing the front attachment (the "head"), combines can also harvest other cereal crops (if not overly tall) and bush beans, but cannot be used on peanuts. Well-adjusted pickers and combines should have losses of less than 2 percent and 4 percent respectively unless lodging is severe.

When to Begin Harvesting

Harvest should begin as soon as is practical after maturity, but this depends on the farmer's harvest method and storage and drying facilities.

Hand harvesting: Since husked ears can be safely stored in narrow cribs (see storage section) at up to 30-32 percent kernel moisture, harvest can be started at or soon after maturity if desired. Most small farmers prefer to let the maize dry down further in the field first.

Mechanical harvesting

· Pickers If narrow cribs (see storage section) are used for storage, mechanical picking can be started once kernel moisture is down to 30-32 percent.

· Picker-shellers and combines: In this case, adequate drying facilities and kernel damage from shelling are the main concerns. In the tropics, shelled maize above 14 percent moisture will not store more than a week to a few months without spoilage. Rapid drying is essential and usually requires forced air and heated dryers when large volumes are involved. Kernel damage from mechanical shelling may become serious above 28-30 percent or below 15-18 percent moisture.

Methods of Shelling Maize

If done too roughly or at too high a moisture content, shelling can cause kernel damage such as tip loss, cracking, stress cracks, and pulverization. Studies have shown that damaged kernels spoil two to five times more rapidly during storage than undamaged ones. Hilysine varieties and other floury types are more susceptible to damage. Shelling methods and guidelines for small farmers include these:

Traditional methods

· By hand: This method is very tedious and labor-intensive, but causes little damage to the kernels. It is more thorough than other methods and also allows for separation of damaged and insect-infested grain. This method is best suited to small amounts.

· Beating: Dry ears are placed in bags and beaten with sticks. This is quicker but less thorough than hand shelling and may cause damage.

Wooden, hand-held maize sheller

Improved methods

· Wooden hand-held maize sheller: The model shown in the drawing was developed by the Tropical Products Institute and has an output of roughly 80 kg/hour. (Plans are available from ICE.) Other types of hand-held shellers are available commercially. Cobs must be husked first.

· Hand-cranked or pedaloperated shellers Small, hand cranked models have outputs of about 50-130 kg/hour. The Ransomes Cobmaster twin-feed pedal-operated sheller has an hourly output of 750-900 kg. For details write Ransomes Ltd., Ipswich 1P3 9QG, England. Maize at too high or too low a moisture content is likely to be damaged, but this can be checked visually. Ears must be husked first.

· Motor-driven shellers have outputs of about 1000-5000 kg/hour. The comments above also apply to this type.

Winnowing Methods

Reliance on wind is the traditional method, but hand-cranked or pedal-driven fans can be constructed easily. The larger models of the hand-cranked or pedaloperated shellers usually are equipped with blowers.

Guidelines for Harvesting and Threshing Sorghum And Millet

Determining Maturity

When grown under favorable conditions and good management, grain sorghum reaches physiologic maturity while the stalks and most of the leaves are still green. Like maize, sorghum kernels also develop a "black layer" at their base when physiologic maturity is reached. The layer can be checked by pinching off some kernels from the bracts that hold them to the head and examining their bases. If present, the black layer can be seen without splitting the kernel. Sorghum flowers and pollinates from the tip of the seedhead downward, a progression which takes from four to seven days. The kernels mature in the same direction, with those at the bottom lagging about a week behind those at the top. Kernel moisture content is about 30 percent at physiologic maturity.

Methods to Harvest Sorghum

· By Hand: The seedheads are cut off using a knife or sickle.

· Mechanical: Tractor driven or self-propelled combine harvesters can harvest and thresh short (dwarf) and medium varieties.

When to Harvest Sorghum

In most sorghum-growing regions in developing countries, maturity often coincides with the start of the dry season, and the crop may be left standing in the field to dry for a number of weeks before harvest. Crop losses during this period can be heavy. If dry conditions prevail, the crop can be harvested at or shortly after maturity and stored on the head with little danger of spoilage. Sorghum can be harvested and threshed with a combine once kernel moisture reaches 25 percent. However, loose grain that is this "wet" must be dried down to around 14 percent within a few days to avoid spoilage. If large amounts of grain are involved, some form of forced air or heated drying would probably be needed.

Methods of Threshing Sorghum

· Traditional methods: These include pounding, beating, and animal trampling and are very tedious except for small quantities. Kernel damage is possible unless care is taken.

· Mechanical methods: Tractoror motor-driven stationary threshers come in many models with outputs of 600-3000 kg/ hour. All but the simplest models will also clean the threshed grain by the use of shaking screens and/or blower fans.

Plans for a four-person pedalpowered grain thresher/mill for sorghum, millet, and wheat designed by VITA can be obtained from ICE. As of 1979, however, this thresher/mill had not been adequately field tested and is not suited to local village construction.

NOTE: Millet is harvested and threshed much like sorghum.

Guidelines for Harvesting and Threshing Peanuts

Peanuts reach maturity when the veins on the inside of the pods turn dark. However, since the plants produce flowers over a period of from 30-45 days, the nuts do not mature simultaneously. Unfortunately, harvesting cannot be delayed until all the nuts have ripened, because heavy losses may occur for two reasons:

· By the time the last pods ripen, many of those which matured earlier will have become detached from the plants due to peg rotting. This pod "shedding" can be especially serious when Cercospora leaf spot causes premature leaf loss or when lifting occurs in dry, hard soils.

· In Spanish-Valencia varieties, the early-maturing kernels may sprout if kept too long in the ground. The Virginia types have a lengthy seed dormancy period which prevents this.

Likewise, if harvesting occurs too early, an undesirably high proportion of the kernels will be immature, shrunken, low in weight, and inferior in flavor. The choice of harvesting date can easily make a 400-500 kg/ha difference on a high yielding crop.

How to determine "peak maturity": The farmer should aim for a harvest date that will recover the largest number of mature kernels before excessive pod shedding or sprouting has occured. This is often referred to as "peak maturity", and there are no easy rules for determining it. The pattern of flowering, pod setting, and kernel maturation varies from year to year due to differences in weather and leaf spot incidence. The first 40-60 flowers to bloom are generally the ones that end up as mature kernels at peak maturity. Flowering starts about 30-45 days after plant emergence in warm areas and begins very slowly. In fact, most of these 40-60 flowers usually bloom near the end of the flowering period, although there maybe several "bursts" of flowering.

Peak maturity cannot be determined by looking at the aboveground portion of the plants. The best method is to carefully dig up a few plants every several days beginning near the end of the growing period and examine the pods. With experience, the farmer can learn to estimate quite accurately how many young pods will ripen before the matured pods begin to shed or sprout.

Minimizing crop losses: Pod shedding can be reduced by keeping the plants green and healthy until maturity. This often requires controlling Cercospora leaf spot with fungicide sprays or dusts. This also increases yields by prolonging the growing season by as much as two to three weeks. Some farmers, however, may object to having leafy green foliage at harvest time, since it may slow down the rate of field curing when the harvested bushes are placed in stacks. In this case, farmers may purposely stop their fungicide applications late in the season to promote defoliations. This also has the effect of making maturity more uniform, although yields are reduced. Such a practice may be justified in some regions, especially where field curing weather is not always dry. On the other hand, farmers can use leafy plants for lifestock feed after harvest.

(NOTE: In the U. S., extension service advise against feeding peanut hay to dairy or beef animals if it has received fungicide applications, except in the case of copper or copper-sulfur products.)

Peanut Harvesting

Whether traditional or modern methods are used, the harvesting process basically consists of four steps:

· The taproots are cut and the plants are pulled (lifted) from the ground with the attached pods.

· Under traditional methods, the plants are cured (dried) in the field for up to 4-6 weeks before threshing. With modern methods, the plants are cured in the field for 214 days, depending on whether artificial drying is available afterwards.

· The pods are threshed from the plants.

· The threshed pods are placed in bags for storage and possible further drying. In dry areas, the pods are often stored in outdoor piles.

Note that shelling the nuts from the pods is not normally a part of the harvesting process, since the kernels dry and store better in the pod. Shelling damage can be high unless kernel moisture is at or below 10 percent.

Methods of "Lifting" the Crop:

· By hand: The plants are pulled from the ground manually after loosening the soil with hand tools. It takes about 30 hours to pull and stack a hectare with this method.

· Animal-drawn methods: Special animal-drawn lifters are available and consist of a sharpened, horizontal blade that is run under the plants right below the nuts to cut the taproots, loosen the soil, and partially lift the plants. One hectare can be lifted and stacked in about 15 hours. A carefully operated weeding sweep (see Chapter 5) about 30-40 cm wide can be used, but the blade should be adjusted to slice rather than push through the soil to minimize pod losses. Some farmers use moldboard or lister plows on ridge-planted peanuts.

· Tractor-drawn methods: Tractors can be equipped with front mounted cutter bars and rear-mounted pullers that lift the plants. Two to four row setups are common, and some of the pullers will combine two or more rows into one windrow for curing. Peanut inverters are available that flip the bushes over to expose the nuts to the sun.

Some General Guidelines for Lifting

· Lifting the crop when the soil is too wet can weaken the pegs. It may cause excessive amounts of soil to adhere to the pods which can also slow down curing.

· Lifting losses can be high in very hard, dry soils.

· If cutter blades are used, they should be kept sharp and be set at a slight forward pitch to aid in lifting the plants and loosening the soil.

Methods for Curing and Threshing Peanuts

The method and length of curing prior to threshing varies considerably with weather conditions and the availability of equipment and artificial methods of drying. The most common methods are:

· The "stackpole" method: This is often used by mechanized and unmechanized farmers alike where curing weather can be wet and no means of artificial drying are available.

Poles are placed firmly in the ground, and two slats are nailed at right angles to each other about 50 cm above ground on each pole. After being allowed to wilt, the plants are stacked around the pole with the pods facing inward. The slats hold the bottom layer off the ground and also improve air curculation. The stack is built in a cone shape and the top covered with a few vines to help shed water. In some cases, the plants are kept in the stacks until kernel moisture is down to 8-10 percent. This may take up to four to six weeks in cool, wet weather.

If harvest takes place at the start of the dry season, the plants may be stacked right on the ground.

· Row or windrow curing: If artificial methods of drying are available or effective sun drying is possible, the plants may be cured in the field in rows or windrows for two to five days before threshing. Where post-threshing drying is less efficient, the curing period lasts about 7-14 days so the pods will be drier at threshing time.

Windrows can be made by hand or through careful operation of a side-delivery rake (tractordrawn). The main advantage of windrows is that they save time when self-propelled modern threshers are used.

The plants can be placed upside down to expose the nuts to the sun. This will reduce damage in wet weather, hut can lower quality under hot, sunny conditions.

Windrows that are overly compact and dense increase curing time and spoilage under wet conditions. After a heavy rain, it may be necessary to gently turn the windrow to prevent mold. This should be done before it dries out to minimize pod shedding. Avoid placing windrows over depressions in the field.

Methods of Threshing

·Traditional: Peanuts can be manually threshed by stripping the pods by hand or by striking the base of the plants (above the pods) against the edge of a barrel or wooden box.

· Improved: A hand-cranked thresher with an output of 200 kg/hr is being marketed in Senegal.

Stationary motor-driven threshers are available. Tractor-drawn or self-propelLed threshers are used in modern farming and pick up the plants right from the windrows.

Threshing Guidelines

· Peanuts can be threshed any time after the plants are lifted as long as adequate natural or artificial drying methods are available (in the case of high-moisture nuts). Further drying will be needed after threshing for peanuts above 10 percent moisture intended for bulk storage and for peanuts above 16 percent intended for storage in loosely stacked bags under good ventilation. Peanut moisture content at lifting may be over 35 percent.

· Tips on mechanized threshing: llull damage and splitting is lowest for peanuts threshed at 25-35 percent moisture. Letting the lifted plants dry down longer in the field reduces post-threshing drying requirements but increases the weather risk. Unless the vines are dry enough to be easily torn apart, rough threshing action may be needed which will increase kernel damage.

Shelling Peanuts

Peanuts are not usually shelled until shortly before consumption or oil extraction. The shelling percentage is about 68 percent (1000 kg of unshelled peanuts yields about 680 kg of shelled kernels), and the process is most easily accomplished when kernel moisture is below 10 percent. Hand shelling is very tedious and the output is only about 10-20 kg/day Various models of hand-cranked or pedal-operated shellers are commercially available with outputs about 15-90 kg/hour.

Plans developed by VITA for a beltdriven peanut huller made from scrap motor vehicle parts are available from ICE; some simple welding and cement work is needed. Power can be supplied by a water wheel, small motor or animal.

Guidelines for Harvesting and Threshing Beans And Cowpeas

Determining Maturity

The pods begin to turn yellow during the final stages of growth and become brown and rather brittle once maturity is reached. Determinate bush varieties and some indeterminate types have fairly even pod maturity, and the plants have usually lost most of their leave< by the time the pods have ripened. Most indeterminate vining types mature much less uniformly, and a good number of pods may ripen while most of the leaves are still green. Seed moisture content is around 30-40 paecent at physiologic maturity.

When to Harvest

Indeterminate varieties with an uneven maturity are usually harvested in several pickings, while determinate bush types are harvested all at once when most of the pods are dry.

Method of Harvesting

The following methods apply to bush or semi-vine varieties with uniform maturity:

· By hand: The mature plants are pulled from the ground and placed in piles for drying. Pulling is best done in the early morning when the pods are moist to prevent shattering.

· Mechanized: Two basic methods are used. The plants are cut or "glided" out of the ground using a tractor with frontmounted horizontal blades with blunt cutting edges or rotating disks operated slightly below the soil surface. Several rows are combined into one windrow using a side-delivery rake which can be rear-mounted behind the cutters. The windrows are dried for 5-10 days before threshing with tractor-drawn or selfpropelled threshers.

Direct harvesting is popular in the U. S. and Canada using grain combines with modifications.

Threshing Methods for Beans

Beans can be threshed manually by beating the plants or bagged pods with sticks once they are dry enough. Whatever the method used, bean seed can be easily injured if threshed too roughly or when too dry. Injured seed, when planted, will produce weak, stunted plants and other abnormalities (see Chapter 6 on bean diseases).

Winnowing beans: Refer to maize.

Drying and storage

Grain drying and storage are very broad subjects, and adequate coverage is far beyond the scope of this manual. Some of the more important principles and practices are outlined here. More detailed information can be found in the references listed in the bibliography.


Very moist grain will deteriorate and spoil during storage for two reasons:

· 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.

· 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

Maize, Sorghum, Millet

· Loose: 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 maize and sorghum, and 16 percent for millet. Bagged maize and sorghum can be stored at up to 15 percent moisture, since ventilation is much better.

· On the cob or seedhead: Husked maize 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.


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 maize contains 15 kg of water and 85 kg of dry matter. There are several ways of measuring grain moisture, some of which can be 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 maize and sorghum in bags.

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

· 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 reabsorbtion while it is cooling off):

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

Biting, pinching, rattling, feel-in": 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 / % dry matter after drying ] x original weight

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

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

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

Some Important Grain Drying Principles

· 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.

· 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.

· 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.

· 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.

· 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.

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 PC/ICE Small Farm Grain Storage Manual recommends general improvements for this system.

Enclosed 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 bibliography for references containing 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:

· 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.

· 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.

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

Crop and Use

Maximum Safe Drying Temperature

Livestock feed


Cereal grains for human food except rice


Milling for flour


Brewery uses


For planting seed


Rice for food


Beans for food





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.

Principles of Safe Storage

· Grain must be adequately dried before being put into storage, although maize 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.

· 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.

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

· Grain should be protected from storage insects and rodents.

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

· New grain should be stored separately from older grain.

· The old grain should be used first.

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

Traditional Storage Methods

If a farmer's production is small, it is often stored in the family dwelling. Maize 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 not well suited to large quantities.

Improved Storage

The PC/ICE Small Farm Grain Storage manual contains design details and guidelines for many 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 sack storage:

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

· Sacks should be stacked on platforms (pallets) raised off the floor or on a plastic sheet. They should not lean directly against walls.

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

· The building should be insectand rodent-proof.

· 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:

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

· 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.

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

· 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.

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

Crib storage: See drying methods.

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 maize 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 snows 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:

· 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.

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

· 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.

The Small Farm Grain

Storage manual has a very complete identification guide to cereal grain pests, while the Insect Pests guide mentioned in the bibliography has pictures of both cereal and pulse storage insects.

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 signs of an insect buildup. Exit holes in the kernels, cobweb-like accumulations on sacks and maize 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

Early recognition of an infestation is very important for reducing potential grain losses. Stored grain should be closely checked every several weeks for signs of an insect buildup. Exit holes in the kernels, cobweb-like accumulations on sacks and maize 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.

The Small Farm Grain Storage manual contains a detailed section on non-chemical and chemical controls for storage insects. A brief summary is given here plus some additional information from other sources.

Pre-storage Guidelines

· Be sure the grain is well dried and cleaned.

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

· Spray or dust the facility with an approved insecticide (more on this further along).

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

Non-Chemical Controls for Storage Insects

· Unhusked: Storing maize in the form of unhusked ears is somewhat effective.

· 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.

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

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

· 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 hyroxide) or burned lime (calcium oxide added at 4-8 parts per 100 is also farily effective (both types are caustic).

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

· 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.

· 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: The Small Farm Grain Storage manual gives recommendations for treating both grain and storage areas. However, storage insects vary in their susceptibility to different insecticides and resistance to Lindane and Malathion has become a problem in many areas. Actellic (pirimiphosmethyl) is a newer product that has proven very effective. Two other sources of information on storage insect control are:

African Rural Storage Center
PMB 5320
Ibadan, Nigeria

Tropical Stored Products Institute
London Road
Slough SL3 7HL
Bucks, England


The Small Farm Grain Storage manual contains a very complete section on rodent control.

Lessons of the ''Green Revolution''

The "Green Revolution" of the 1960's and 1970's was really the first organized attempt to develop yield improving practices for staple food crops in the developing world. Most of the efforts of the Green Revolution were directed towards a number of the cereals, namely wheat, rice, and maize.

A major impetus was the development of short-strawed varieties of wheat, rice and maize that would respond well to high rates of fertilizer, especially nitrogen, without lodging.

The term "revolution" is really a misnomer; nearly two decades of plant breeding and local adaptive research were required before the new wheat and rice varieties were ready for widespread introduction in India and Pakistan. The true origins actually go back to breeding programs for wheat and maize in Mexico in the 1940's and to similar work with rice in the Philippines.

Supported by a "package" of complementary improved practices involving factors like fertilizer use, pest control, and plant spacing, the new varieties were adopted in many developing regions. By 1972-1973, some 33 million hectares in Africa and Asia were were being sown to the high yielding wheat and rice varieties. Average yields were increased about 100 percent for rice compared with traditional varieties.

Despite these increases, the efficacy of the Green Revolution in overcoming hunger and rural poverty in the developing world is a hotly debated issue worthy of a manual in itself. There is no doubt that the Revolution has been the major factor behind the gains in food production in many developing countries during the past 15-20 years and has also laid a solid basis for additional agricultural research in the region. It was conducted in a spirit of humanitarian and largely apolitical international cooperation that should be commended.

On the other hand, it has not proved to be the hoped-for panacea for several reasons:

· The high yielding variety (HYV) "packages" it developed required relatively high levels of inputs (fertilizers, pesticides, and, in some cases, irrigation pumps) and investment. At least initially, the smaller farmers were often bypassed due to deficiencies in the infrustructure that made it difficult for them to obtain both the credit and the inputs. Unless special provisions were made to provide small farmers credit, lending institutions naturally favored the larger ones. This situation has improved considerably over the past decade in many areas, but is still a serious problem.

· The high costs of these inputs, some of which are very petroleum-dependent (i.e. nitrogen fertilizer and pumping fuel), raises doubts about their continued practicality, estpecially in view of the current energy crisis. Fertilizer rates are often well above the threshold of diminishing returns in the case of nitrogen and phosphorus; the latter is a non-renewable resource with limited world reserves. Fortunately, there is a growing awareness of the need for an appropriate technology in harmony with both the environment and economics.

· An important lesson learned is that increased production does not automatically improve rural wellbeing. In some parts of India, for example, the HYV package actually had a negative effect on income distribution, rural employment, and dietary habits. A disturbing number of small landholders and tenant farmers were squeezed off the land by the new production economics, and urban industrialization was insufficient to provide them employment. Cereal cropping was favored over grain legumes, sometimes resulting in actual declines in pulse production and consumption. With a bad case of Western economic ethnocentricity, many "experts" argued that this was the necessary price to pay for modernizing agriculture along "big is better" lines.

Fortunately, there is a growing realization that the small farmer must be included in agricultural development which should be tied into integrated rural development so that nutrition, health, education and general rural welfare are also considered. In fact, as the small farm family's income and production increases, receptivity to these other programs is usually enhanced.

The Green Revolution is far from over. Rather, its goals are being redefined and extended to other food crops. Future progress will largely depend on how the developing world handles two key issues:

· The conservation of natural resources and the total environment.

· Choosing appropriate scales of production: The Western bias is that "big is better", yet evidence strongly suggests that small, intensively cultivated units are the most efficient. This brings up the issue of agrarian reform, as well as the ultimate goals of agricultural development. The conventional approach of trying to integrate the small farmer into a modern agribusiness system usually fails (as it did in the U.S.). Others feel the goal should be to enable the marginal small farm family to achieve self-sufficiency with a small surplus left over for education and general welfare.

Agricultural extension workers will have a central role in this effort to extend the benefits of the Green Revolution. By spreading the knowledge gained in trials conducted by the major research institutes to increase production of traditional crops, agriclutural extension workers will help ensure that the Green Revolution truly serves to improve the lives of small farmers and their families in the developing world.