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close this bookUtilization and Construction of Pit Silos (Peace Corps, 1976, 41 p.)
View the document(introduction...)
View the documentEnsilage hay and pasture crops
View the documentHarvesting silage crops
View the documentHandy silage preservative guide
View the documentCharacteristics of high quality hay
View the documentStorage of forage
View the documentCorn or sorghum silage vs. grass silage
View the documentPit silos
View the documentProject ensilage - Interim report
View the documentProject ensilage - Termination report

Pit silos

The pit silo is shaped like the conventional tower silo, but is inverted into the ground. It resembles a well or cistern. The walls of a pit silo may or may not be lined. Where the water table is low enough that the silo will not become filled with water, such as in semi-arid areas, the pit silo is very satisfactory.

In comparison with tower silos, pit silos have the following advantages:

1. they are never damaged by storm or fire,
2. they require less reinforcing
3. they minimize silage loss because of not having doors,
4. less expensive and practical for the African farmer.

Requisites of a good pit silo

1 That its size be inkeeping with the number and kind of animals to be fed daily, the length of the feeding period and the amount of forage available for ensiling.

2 That the sidewalls are straight and smooth in order to prevent the formation of air pockets.

3 That it be of adequate depth thus making for better packing and less surface area exposed: factors which will help to keep spoilage losses to a minimum.

4 mat it be conveniently located and accessible from standpoint of both filling and feeding.

5 That it be located in a well drained area.

6 That the soil structure be such that the sidewalls will not collapse. (Where there is deep sand, it is not recommended that a pit silo be dug)

Advantages of silage

1 It retains a higher proportion of the nutrients of plants. Thus grass silage preserves 85% or more of the feed value of the crop.

2 It is feasible to produce a top quality feed during times of inclement weather when it would be impossible to cure properly hay or fodder.

3 It is the cheapest form in which the whole plant can be processed and stored.

4 It practically aleviates the danger of fire loss to feed.

5 It is the most satisfactory and economical way in which to preserve a number of by-products of feeds. For example: peanut vines, millet and sorghum that is planted too late, etc..

6 It is a better source of protein and of certain vitamins, especially carotene, and perhaps some of the unknown factors, than dried forage.

7 It is a very palatable feed and slightly laxative in nature.

8 It makes for less waste, the entire plant being eaten; an important consideration with coarse stemmy forages.

9 It makes possible the production of the maximum quality of feed per acre or hectare of land.

10 Cattle, sheep, and goats like it.

Kinds of silage

A great variety of crops can be and are made into silage. A rule of thumb is that crops and forages that are palatable and nutritious to animals as pasture as green feed, or as dry forage also make palatable and nutritious silage. Likewise crops and forages that are unpalatable and unnutritious as pasture, as green feed, or as dry forage also make unpalatable and unnutritious silage.

The varieties of grasses and legumes that can be used depends on those that are available in each locality. Some of the more common forages found on African savannas which make good silage are hyparrhenta sp., andropogon gayanus, roettebbia exaltata, peanut greens, millet, sorghum, bean vines, and many others.

Characteristics of good silage

In order to make good quality silage, farmers need to know what constitutes silage quality. They need to be acquainted with those recognizable characteristics of silage which indicate high palatability and nutrient content. The easily recognized characteristics of silage of high feeding value are:

1 Odor - it has a "clean" rather pleasing acid odor, in contrast to the foul or objectionable odor of poor silage.

2 Taste - the taste is pleasing; but not bitter or sharp.

3 Absence of mould and rot - there is no visible mold, and it is not musty or slimy.

4 Uniformity - it is uniform in moisture and color. Generally green or brownish silage is good. Tobacco brown or dark brown silage indicates excessive heat, and black silage is rotten and should not be fed.

5 Animals acceptance - animals like and thrive on good silage.

The ensiling process

The ensiling process is principally governed by the interaction of three factors: 1, the bacteria on the plant material, 2, the composition of the plant material placed in the pit silo, 3, the amount of air entrapped or allowed to enter the stored forage.

The ensiling process refers to the changes which take place when green forage is stored in a pit silo in the absence of air. An understanding of these changes is likely to lead to the production of more high quality silage.

The entire ensiling process requires two to three weeks, during which time the following aerobic bacteria (with air) and anaerobic bacteria (without air) predominate:

Aerobic bacteria - the living plant cells of forage continue to respire, or breath; consuming the oxygen of the silage - entrapped air, producing carbon dioxide (CO2) and water (H2O), and releasing energy or heat. Simultaneously, aerobic yeasts and molds thrive and multiply. During this period the temperature may rise to about 100 degrees fahrenheit. The bacteria on the plant material when harvested are largely aerobes. These bacteria, along with the facultative bacteria increase in number and their activity aids in the exhaustion of oxygen (O2) From the silage mass.

Anaerobic bacteria - when the available oxygen of the entrapped air has been completely consumed by the respiration of the plant and aerobic bacteria, a four to five hour transition period takes place whereby anaerobic bacteria conditions prevail. Chiefly acid forming and proteolytic bacteria are formed. The lactic acid bacteria increase in number. Despite the fact that the lactic acid bacteria may originally be present in such small numbers that they are difficult to isolate, they are always adequate in numbers to produce good silage, under suitable conditions. Even a very few lactic acid bacteria, given proper conditions, may increase to several hundred million per gram of silage in three or four days.

Simultaneously, the molds and yeasts die, but continue to function as enzyme systems which produce alcohol and other end products.

The combined anaerobic activity produces the following changes:

1 The carbohydrates and sugars (especially the sugars) are broken down into lactic acid (the acid in sour milk), some acetic acid (the acid in vinegar), and certain other acids and alcohols. The sugars that are broken down are largely sucrose and monosacharides, glucose and fructose.

2 Some of the proteins are broken down into ammonia, amino acids, amines, and amides.

3 The acidity finally reaches a point where the bacteria themselves are killed, and the silage-making process is completed. Silage in a good pit silo will remain unchanged for a long period of time during the dry season months in African savannas.

The acid development "pickles" the plant material by reducing the ph to 4.0 or below. The low ph inhibits further bacterial growth and enzyme action and preserves the silage. Also it inhibits proteolytic and putrefactive bacteria from growing, which cause rotting and putrefaction.

The presence of small amounts of ethyl and other alcohols is desirable, because they combine with the acids to form compounds which impart to silage the characteristic aroma

Dangerous silage gases

Gases formed during fermentation may become hazardous when making and feeding all types of silages unless precautions are observed. The gases are heavier than airand may accumulate near the surface of the silage in pit silos. Pit silos are always dangerous, even after filling.

It has been long known about the suffocating effect of carbon dioxide (CO2) gas formed in silage. It is the most common and most dangerous of gases from silage, because it is invisible.

It has also been recognized that nitrogen dioxide gas is formed by high-nitrate silages and can cause a sometimes fatal disease called nitrogen dioxide pneumonia in man and livestock. Some plants such as legumes, oats) barley, wheat, corn, sorghums, many pasture grasses and certain weeds appear to accumulate especially high concentrations of nitrates during droughts and when grown on high-nitrate soils. When these plants are made into silage, poisonous nitrogen dioxide gas forms until a week or ten days after filling the pit silo.

Carbon dioxide gas may be detected by lowering a lighted lantern into the level of the silage. If the flame goes out, the oxygen content of the atmosphere in the pit silo is dangerously low. Nitrogen dioxide gas can be detected by 1: its yellow or yellowish brown color or 2: by means of starch-iodide paper (obtained from drugstores or chemical supply houses), which turns blue in the presence of nitrogenous compounds.

Precautions against silage gases

Before entering a pit silo, swing a piece of canvas, a tree branch, a burlap bag, or something to agitate the air and dilute gases that may be present with oxygen from the air. Adequate ventilation is essential.

Notice: A victim of silo gas should be moved into fresh air as soon as possible, and artificial respiration should be applied, and taken to a physician immediately.

Selecting a site for a pit silo

When selecting the site for a pit silo, the following must be considered:

1: depth of loose sand,
2: depth of water table,
3: availability of forage
4: convence of feeding the silage,
5: flood plains during rainy season,
6: shade.

When selecting a site where the sand is deep, (more than two feet), there is a tendency of the side walls to be unstable. When the pit silo is empty, the side walls generally cave-in. Therefore it is recommended that pit silos be dug where there is less than two feet of sand.

Before choosing a site, one must always determine the depth of the water table. In many areas of African savannas, the water table is near the surface, especially near rivers. One good method of determining the depth of the water table, is to look down an open well near the proposed site, or ask the villagers how deep the water is in their well. Always remember, the water table usually descends at the onset of the dry season, and ascends during the rainy season. Do not put silage in a pit silo that contains water.

Select the site of pit silos where forages are readily available. When forages are readily available, less time is consumed transporting cut forages to the pit silo.

Select a site that will be convenient to feed silage to livestock. Preferably near where livestock are usually tethered.

The site location should never be in a flood plain. Usually the water table is near the surface, and during the rainy season, the sidewalls are more likely to cave-in.

It is preferred that the pit silo be located between two large trees or on the east side of a large tree. It is important that the shade cover the hole during the heat of the day. If no shade is available, the silage will be more apt to dry out after the silo is opened. Shade is also necessary for the workers while filling

Proper size of pit silo

The diameter and depth of the pit silo should depend on the number and kind of animals to be fed from it and the length of the feeding period. The silo should be of such that two to four inches of silage will be removed from the entire exposed surface daily to prevent spoilage.

The size of the pit silo required may be computed from determining the length of the feeding period and kinds of livestock to be fed. Knowing the number of animals of each kind to be fed, the entire amount of silage which will be consumed daily, and the length of the feeding period, the total tonnage of silage needed can be estimated. By referring to table 1, one can determine the proper size and dimensions for a pit silo that will suffice the needs of individual farmers.

The amounts of silage commonly fed per head daily to the various classes of livestock are estimated as follows:

2 - 3 year old oxen

25-30 pounds

3 - 8 year old oxen

30-50 pounds


2-3 pounds per 100 lbs. live wt.


2-3 pounds per 100 lbs. live wt.


Mr. Keita owns two oxen, ten sheep, and ten goats. be wants to store enough silage to feed all his livestock during the dry season. How large must he make his silo.

Step 1 Determine the length of dry season or the number of days he must feed his livestock silage. In this case we sill say five months or 150 days.

Step 2 His two oxen will eat an estimated 60 pounds of silage each day, with the sheep and goats eating an estimated total of 60 pounds of silage each day, making a grand total of 120 pounds of silaged needed each day.

150 days X 120 pounds of sillage = 18,000 pounds or 9 tons

Mr. Keita needs an estimated nine tons of silage to meet his livestock needs for the coming dry season. Refer to table 1, a pit silo with a diameter of 12 feet and a depth of 5 feet of settled silage will suffice his needs.

Table 1

Depth of settled silage

Total quantity of settled silage, from the top to the depth indicated, in silos having a diameter of:

10 feet (tons)

12 feet (tons)

14 feet (tons)

16 feet (tons)












































































From USDA circular number 603, by J.B. Shepherd and J.E. Woodward: with all decimals rounded off to the nearest whole number.

Digging a pit silo

Remembering that a pit silo can be used for several years, the following observations should be considered when digging the pit: 1; shape of pit, 2; having pit repaired for filling at proper time, 3; keeping the sidewalls straight, 4; keep top edge of pit free from piled soil.

There are basically three shapes one can make a pit silo, round, square or rectangular. The shape of the pit depends entirely upon the wishes of the farmer. From the standpoint of experience, round pit silos are recommended over square or rectangular shapes. Round silos have the following advantages: 1, silage will tend to settle more uniformly which is desirable for high quality silage, 2, corners are eliminated, which have a tendency to form air pockets causing spoilage, 3, with round silos increase volume can be gotten from a given site.

It is recommended that the pit be readied for filling prior to the time when the forages are at their best for harvesting. Pit silos should be prepared for filling immediately following the rainy season in African savannas.

When making a new pit silo, it is important that the sidewalls be kept straight and smooth. Often there is a tendency for the sidewalls to slope inwards as the pit gets deeper, thus having a larger diameter at the top compared to the bottom. Having the sidewalls smooth and free of holes, roots or stones, reduces the possibilities of air being entrapped which may cause some spoilage and the silage will also settle more uniformly.

If using a pit silo from previous years, all rotten and moldy silage should be removed. Sidewalls should be straightened if portions have fallen in during the preceding rainy season.

While digging the pit silo, it is recommended that the soil from the pit be piled a minimum of one to two feet from the edge of the pit, thus eliminating the possibility of soil being pushed into the silage by workers while moving about in filling the pit silo.

Filling a pit silo

Filling a pit silo properly will pay dividends during the African savanna dry season when feed for livestock is difficult to obtain. When filling a pit silo in African savannas, the following points should be considered: 1, cutting forages to be chopped, 2, tools, 3, transporting forages to pit silo, 4, chopping the silage, 5, tramping chopped silage, 6, amount of time required to fill a pit silo, 7, workers.

Cutting - The kinds of forages to be cut will depend upon the availability of local grasses, legumes, crops and crop residues. Nevertheless, a large volume of forages must be cut and placed near the edge of the pit silo. Cutting the forages in the late afternoon and placing at the edge of the pit for chopping the following morning has been found to be a satisfactory system for these reasons: 1, cutting the forages in late afternoon for chopping the following morning allows the plants to wilt somewhat before putting into the pit silo. Water evaporated from the plant juices before putting into the silo decreases the amount of nutrients that will escape from the silage in the plant juices and water that normally drain from the silage mass. Wilted forage increases the amount of sugar per pound of forage, because part of the water is removed, thus improving the quality of chopped silage. 2, Workers usually do not like to cut forage during early morning hours when plants are heavily covered with dew. Cutting forages in late morning hours or late afternoon when forages are dry improves work efficiency.

Cutting forages and chopping without wilting is also an approved practice when making silage. The system used depends upon the wishes of the farmer

When cutting forages, if the grasses or legumes tend to be stemmy, cut plants higher from the ground using only the succulent and leafy part of the plant, eliminating the coarse, woody stem of more mature plants. For high quality silage, cut only leafy succulent forages.

Tools - Tools used to cut forages depend on what is available locally, local made knives and machettes have been found to be satisfactory for cutting and chopping forages. Factory made machettes, military machettes and scythes are also very satisfactory for cutting forages, when available.

It is recommended that a file be kept available for workers to sharpen knives and machettes when necessary. It has been found that work efficiency is greatly improved when tools are kept sharp.

Transporting - Transporting cut forages to the pit silo depends upon what the farmer has available and how far forages are located from the, pit silo. Ox drawn carts have proved to be very satisfactory in transporting cut forages to pits. Where ox drown carts are not available, carrying bundles of cut forages on the workers head has been found to be satisfactory. Attaching bundles of cut forage on the backs of horses and/or donkeys has been done successfully for long distance transporting. However the method used, transporting cut forages to pit silos must be economical and practical for the farmer. Transporting cut forages long distances could prove to be both uneconomical and unpractical

Chopping - Chopping cut forages into two to four inch lengths improves the quality of silage; makes forages more palatable, especially coarse, mature forages causes the silage to pack more tightly excluding the air, and, increases the volume of forages that can be put into a given pit silo.

Where forage chopping machinery is not available, manual methods can be successfully accomplished by placing logs at the edge of the pit silo. With workers sitting by the log, placing forages on the log towards the pit silo, using machettes or locally made knives, silage can be chopped into two to four inch lengths falling directly into the pit silo. This method has been found to be practical and well accepted by local farmers who are making silage for the first time.

Tramping - While filling a pit silo with chopped silage, it is very important that the silage be periodically tramped. Tramping the chopped silage facilitates packing and excludes air pockets that may cause spoilage. It is most important that the edges be well tramped where there is more of a tendency for air pockets to form. Remember, one cannot over tramp silage in a pit silo. Before entering a pit silo, precautions must be taken that no dangerous silage gases are present.

Time - Filling a pit silo as quickly as possible will further insure high quality silage. It is recommended that a minimum of one to two feet of chopped silage be added daily until the pit silo is completely filled. The time required to fill a pit silo depend upon the following: 1, size of pit, 2, number of workers, 3, availability of forages, 4, method of transporting forages, 5, local factors such as holidays, sickness, deaths, etc..

Workers - The number of workers participating will greatly influence the time necessary to completely fill a pit silo. Farmers should avoid when possible, attempting to fill a pit silo alone. It is highly recommended that farmers work cooperatively in filling pit silos. The more workers available, the more quickly the silo can be filled, thus improving the over-all quality of the silage.

Sealing pit silo's

When the pit silo has been completely filled with chopped forages and has been well tramped, it is necessary to seal the silage mass so that it may be properly preserved. On African savannas where pit silos have been successfully used, the following has been found to be very satisfactory, acceptable, and practical.

Cover the entire silage mass with a layer of long grass, bannana leaves, palm tree leaves or dry grass, anything that will prevent soil from being mixed with the chopped silage. Thence the entire surface of the silage mass is covered with 18-24 inches of soil.

When the pit silo has been properly sealed, two to four days afterwards, the silage mass will have settled approximately one foot or more, depending upon how well the chopped silage was tramped before sealing. The weight of the soil cover aids in forcing out any entrapped air pockets, thus further eliminating possibilities of spoilage.

Opening pit silos

Keeping in mind the number of animals that will be fed daily, it is necessary, after the silo is opened, that two to four inches of silage or more be removed from the exposed surface daily. Therefore the amount of silage a farmer exposes when opening a pit silo, will depend a great deal upon how much silage he will need each day. By removing the daily minimum of two to four inches of silage, reduces the time silage near the surface is exposed to the air, thus curtailing loses caused by spoilage.

When only a small number of animals will be fed daily from a pit silo, it is suggested that the pit silo be divided into three vertical sections. Thus only a portion of the pit silo will be opened at a time. When only a portion of the silage is exposed, the minimum of two to four inches of silage will more likely be removed from the exposed surface when a small number of animals are to be fed.

When removing silage from the pit silo, care should be taken not to cause loose soil to fall on exposed silage. The soil should be placed at least 2; inches from the edge of the pit, when opening each section Soil mixed with the silage reduces its palatability.

After soil has been removed from the silage, one will find between four to six inches of spoiled silage on both the top and sides of the silage mass. This small amount of spoiled silage is normal and must be thrown away and not fed to livestock. This thin layer of spoiled silage forms a seal following the ensiling process.


In many areas of African savannas, heavy termite infestations exists. When introducing silage to farmers one of the first questions they ask is "will termites eat the silage stored in pit silos?"

Thus far, experience and studies have indicated that termites will not eat well preserved silage. It is suspected that due to the low ph or well preserved silage that perhaps the acidity may discourage termites from damaging silage.

Pit silos have been filled where termites were visible on the sidewalls while filling, but no detremental effects have been observed while the silage was being removed during the dry season.

Termite infestation has been occasionally observed in the thin layer of silage that normally spoils next to the sidewalls, but no infestation or damage has been noted in the usable silage.

Written by: James E. Diamond PCV - Chad, 1971

Soil piled away from edge of pit

Soil too close to edge of pit

Sideview of an opened pit silo