Previous Page Table of Contents Next Page


The silo is no substitute for the production and harvesting of high-quality forages

by M.E. McCullough

1. Factors influencing silage fermentation

The main trends in silage making today are a return to the basic principles of silage fermentation and the proper supplementation of the silage to ensure maximum economic utilization. To the casual observer of livestock agriculture, these may appear to be so elementary that one might wonder why they have not been the primary aims of silage making all along.

M.E. McCullough is Professor and Head of the Department of Animal Science (Feed Evaluation) at the University of Georgia College of Agriculture, Experiment Station, Georgia 30212, United States.

Following World War II, silage production in the United States and other affluent countries was engulfed by the “mechanization” revolution of the fifties and sixties. What started out to be a substitution of human labour by mechanized methods of production, harvesting and storage soon became a race to see who could develop the most complicated and expensive machinery. The silage itself frequently became less important than the equipment used to handle it. Thus, the dry matter content of the crop to be ensiled was considered more in terms of silo structure and unloading equipment than in terms of the preservation of the nutrients ensiled. Unfortunately, these trends caused many developing countries either to conclude that silage was too expensive for them or to try to emulate the silage production systems in the United States with an obvious waste of valuable development funds.

A trench silo dug in firm soil is as useful as a specially engineered and highly automated upright. However, the silage resulting from the methods of preparation illustrated on the right will be of very poor quality: the material being ensiled is too coarse and stemmy, and compaction will be insufficient.

During the 1950s and 1960s silage production in some of the developed countries was engulfed by the mechanization revolution, resulting in the construction of expensive silos like those below.

The current energy crisis and world food shortage with the resulting emphasis on animal production from forages has created the proper climate for reevaluating silage production. Thus, this article will consider the basic principles of silage fermentation as a means for preserving forage nutrients.

What is the aim of silage making?

Barnett (1954) defined the aim of silage making: “to achieve within the ensiled mass a sufficient concentration of lactic acid, produced as a result of the presence of micro-organisms within the cut crop, to inhibit other forms of bacterial activity and thus preserve the material until such time as it is required.” This basic concept includes the factors necessary for lactic acid production such as exclusion of air, availability of adequate carbohydrates, adequate moisture content and the initiation of an early and rapid fermentation. Under proper conditions, silage fermentation will result in the preservation of over 90 percent of the harvested energy and protein and the concentration of energy within the silage.

2. Relationship between silage dry matter content and effluent production

* 1 gallon equals 3.78 liters

Role of the silo in silage making

Despite all the advertising and promotion designed to sell various makes and types of silos, the silo has only three functions in the ensiling process:

If the silo meets the first two requirements, the cost of its construction is of little significance. Thus, a trench silo dug in firm soil is as useful as a specially engineered and highly automated upright. The role of the silo in mechanization of silage handling should not be confused with its role in silage fermentation.

Factors influencing silage fermentation

The major factors influencing silage fermentation are illustrated in Figure 1. The successful manipulation of the factors contributed by the crop and the technology of ensiling have significant effects on the losses sustained in the silo and on the feeding value of the resulting silage.

The crop

Since a major influence on the production of lactic acid is exerted by the crop ensiled, it is important to know the ensiling properties of the latter. The buffer capacity of the crop to be ensiled, which is largely determined by its protein and mineral content, must be exceeded by the lactic acid that is formed during fermentation. It is therefore important that the forage ensiled should be brought down to a pH level of 4.0 as quickly as possible to prevent further fermentation and to create a stable condition within the silo. Thus, legumes and high nitrate grasses are more difficult to ensile than cereals such as maize and sorghum. At the same time, forages with high protein and mineral contents are also usually low in carbohydrates. This is particularly relevant when highmoisture (above 75 percent) forages are ensiled, since the moisture dilutes both the carbohydrates available for acid production and the acid formed.

For these reasons, silages should be made from crops with at least 25 percent dry matter. Conversely, silages with insufficient moisture have an adverse influence on fermentation with the result that the optimum range in dry matter is between 25 and 35 percent. Forages should never be permitted to reach optimum dry-matter levels to the detriment of their nutritive value. It is preferable to correct for moisture by adding high dry-matter roughages than to harvest the forage at a late stage of maturity. A major problem in ensiling high moisture forage is the effluent, or “run-off” produced in the process. The data in Figure 2, compiled by Murdock (1954), show that seepage was greatly reduced at more than 25 percent dry matter and almost eliminated at 30 percent dry matter. The large variations at each level of moisture were due to differences in crops and amount of material ensiled. Large silos with heavy compaction produce more effluent than small silos. Proper disposal of inffluent should be considered in planning for silo construction and location.

3. Influence of covers on spoilage in trench silos*

* Data are given for three experiments in each treatment.

4. The mode of action of silage additives


This refers primarily to the system of harvesting and storage used to eliminate air from the fresh forage ensiled and to the protection of the ensiled forage from air and water. Although a high level of technology is required in all climates, it is far more important in warm, humid climates. An important consideration in such climates is the prevention of mould growth both within the silo and in the silage prior to feeding. The primary effect of mould is a lowered rate of milk production or weight gain.

To ensure a rapid exclusion of air, forage should be chopped to an average length of 1.3 cm or less, ensiled as rapidly as possible, packed as ensiled and handled to give the maximum density possible under existing circumstances. The exact methods for doing this can only be decided in terms of conditions existing on a given farm at a given point in time. For this reason, it is not profitable to try to set down rules for any and all conditions. Since silage cannot always be easily compacted, every effort should be made to do a fast and thorough job within the restrictions of the farm situation.

A major failure in silage technology, particularly in warm, humid areas, arises from lack of protection of the silage mass after ensiling. Owen and Miles (1956) in Mississippi demonstrated the need for and effectiveness of silo covers. Their data in Figure 3 is of particular interest, since it illustrates losses under warm, humid conditions. The use of straw and earth was of little value, since it permitted rain water to permeate the silo. The felt and sawdust cover was also found to be inadequate. A heavy plastic, covered with sawdust, reduced spoilage by 88 percent, and was a very economical use of both labour and money in the silage-making process.

Silage additives

Silage additives are usually included as a part of the technology of silage making. However, due to the current economic value of additives and the wide range of their use, an understanding of the mode of action or classification of additives is essential. The four basic forms are illustrated in Figure 4. Traditionally, silage additives have been considered as useful tools for silage-making in cool, wet climates where legumes are part of the silage mixture. In recent years, new emphasis has been placed on their use in warm, humid areas and with crops other than legumes. In part, this change in emphasis comes from the realization that forages grown under high temperatures must be harvested at earlier stages of maturity if they are to be of sufficient quality to result in acceptable levels of animal performance. Research in Australia by Minson and McLeod (1970) showed that dry-matter digestibility was negatively correlated with mean temperature (r = -0.76) and rate of evaporation (r = -0.64). Thus, to obtain highly digestible forages in warm climates, forages must be harvested while they are high in moisture.

Table 1. Digestibility of forage sorghum silages

Silage additiveDry matterCrude proteinCrude fibreEither extractNFE
Lactic acid bacteria59b46b58b68a63b
Formic acid60b49b59b70a63b
Urea + molasses58b47b57b64a61b
Urea + molasses + lactic acid bacteria59b47b58b62a62b

Values having the same letters are not significantly different (P<.01).

Substitutes for fermentation: Since the object of silage fermentation is the production of sufficient lactic acid to lower the pH to 4.0, an alternative would be the addition of sufficient acid at ensiling to achieve the same objective. Perhaps the best known system was developed in Finland by A.I. Virtanen. His process, called the A.I.V. system, used a mixture of sulfuric and hydrochloric acids. The system has declined in popularity since the farmer must handle strong acids, and the inorganic acids require careful mineral supplementation at feeding time. In northern Europe and several other areas of the world, formic acid has largely replaced the A.I.V. system. The change to formic acid did not remove the problem of handling acids, but it does have the attraction of being a natural acid with nutritional value. In recent years, salts of formic acid have been tried, since they can be handled in a dry form. The idea of reducing the pH by adding organic acids has proved to be a most useful method of controlling silage fermentation and will undoubtedly continue to be an alternative additive. The current energy crisis with its influence on the chemical industry may result in these additives becoming too expensive for farm use.

Enhanced fermentation: The second class of additives are those which aid the compaction of the mass, increase the rate of fermentation, and/or in some way improve the efficiency of the normal fermentation process. Included in this classification are cellulases, lactic-acid bacteria, antioxidants, fermentation residues, and miscellaneous enzymes. Although this area has frequently been indiscriminately exploited by commercial enterprises, there are mixtures available with definite biological merit. A well researched example of this type of additive is a product called SiloGain (NuLabs, Portland, Oregon) which consists of cultured strains of lactic acid producing bacteria originally cultured from high quality silage. Recent research at this laboratory showed that, added to wheat silage, this product reduced energy losses in the silo from 16 percent, in the control silage, to 7 percent with the additive. In maize silage, the reduction was from 25 percent in the control to 12 percent with the additive. These reductions of half the energy loss during ensiling are evidence that the normal process of fermentation can be improved. The fact that many natural silage fermentations result in excellent silage must not be equated with efficient fermentations. Since the first 96 hours in the silo are critical in determining the efficiency and success of the ensiling process, research and development of highly reliable and cheap additives that enhance this area of silage making should do much to improve the use of silage.

Added nutrients: The best known additives in silage making are sources of carbohydrates and nitrogen. Since the added nutrients contribute to both the ensiling process and the nutritive value of the silage, the major question in their use is one of cost. Whenever and wherever feeds such as citrus pulp, maize, distillers' grains, molasses and similar products are available at a usable cost, they could be valuable additions to the silage making process. Animal wastes may also be usefully ensiled in combination with forages.

Table 2. Relationships between silage quality, silage intake and average daily gain of heifers

Forage NumberDry matter digestibilityBody weightDigestible dry-matter intakeAverage daily gains

Values having the same letters are not significantly different (P<.01).

Biological acid formation: This is largely a new area of research and is mentioned here only to explain its possible future use. There is growing evidence that certain mixtures of products are capable of producing lactic acid through fermentation within the silo without significantly affecting the nutrients in the original forage. In short, it is a fermentation within a fermentation. Although not ready for use in farm practice, the process does bear watching for future development.

Table 3. Influence of grain supplements on animal performance (N = silage without additive. DG = silage with distillers' dried grains)

ItemHigh acetic1High propionic2Normal3
Daily 4% milk35.538.131.935.632.238.4
Percent fat5.
Daily total solids4.314.753.934.484.004.70
Means4% milk% fat
No additive
100 lb distillers'
High acetic
High propionic
Preliminary production:
No additive
100 lb distillers'

1 High acetic - Grain ration had 16% crude protein, 72% TDN and 11% crude fibre.
2 High propionic - Grain ration had 16% crude protein, 78% TDN and 2% crude fibre.
3 Normal - Grain ration had 16% crude protein, 76% TDN and 5% crude fibre.

Additives in action

Since many of the developing nations as well as many established livestock areas are in warm, humid climates, it might be useful to illustrate the classes of additives when used with forages typical of such areas. Forage sorghum is widely adapted to warm, humid climates. The data in Table 1 illustrate the comparative effects of additives with three modes of action when used with high moisture, low buffer, low soluble carbohydrate forage — in this case, forage sorghum. Comparing the control silage with any of the treated silages clearly indicates the destructive nature of the ensiling process on the feeding value of such forages. At the same time, the comparisons show that using a fermentation enhancer (lactic acid bacteria) to establish an immediate fermentation, or lowering the pH with formic acid or adding carbohydrates and nitrogen results in significant improvements in silage feeding value. The method of choice would be determined by the availability of materials and the relative costs of the alternative additives.

Feeding value of silage

The data in Table 2 illustrate the high correlation between the dry matter digestibility of silages, the intake of silage and the average daily gains of dairy heifers. The multiple correlation between intake, digestibility and average daily gains was +0.89. These data show that only the best available crops should be used for silage and they should be harvested and ensiled to conserve them at their optimum stage of maturity in terms of dry-matter digestibility. Since the ensiling process at its best can only preserve what was ensiled, the silo is no substitute for the production and harvesting of high quality forages.

The data in Table 3 illustrate the combined effects of using a silage additive to enhance the silage-making process and the feeding of a supplement to maximize silage utilization. The data from this laboratory are the results of ensiling maize silage with or without dried distillers' grains and the use of low- or high-fibre grain rations for milking cows. In all rations, the use of an additive significantly increased milk production due to improved silage fermentation. Within silages, using a grain ration with sufficient digestible crude fibre to ensure adequate acetic acid formation in the rumen increased both milk and butterfat production. The two factors were apparently additive in their effect on milk production since the cows receiving the high acetic type ration produced more milk than the cows on the control silage receiving the same grain ration. Current trends in silage production involve the application of known principles of silage fermentation to conserve a maximum of the nutrients ensiled. When these are combined with modern ration formulation techniques, maximum animal performance can be obtained with the resulting silage.


Barnett, A.J.G. 1954. Silage Fermentation. New York. Academic Press Inc.

Minson, D.J. and McLeod, M.N. 1970. The digestibility of temperate and tropical grasses. Proceedings, 11th International Grassland Congress.

Murdock, J.C. 1954. Seepage from silos. Agriculture 61:224.

Owen, J.R. and Miles, J.T. 1956. Relative efficiency of three different covers for a trench silo. Mississippi State College Information Sheet 524.

Previous Page Top of Page Next Page