Food Technology Information Center

Technology of Forage and By-Products Preservation

14. Feed Out

In this phase the silage is taken out from the silo for feeding to animals. In the course of this process, all the protective means that were applied to protect the silage from oxygen are removed; in one step the silage is exposed to air, and all the negative processes that we took pains to prevent are able to resume, especially the development of molds and yeasts, which grow approximately twice as quickly on residual sugars as on fermentation products such as lactic and acetic acids or ethanol Fig. 50. However, there is no way to feed out silage without exposing it to air, and the total time of exposure is the sum of the times of exposure in the silo (at the bunker face) and in the trough until the silage is eaten by the animals. In a bunker silo, feed-out involves lifting the plastic cover sheet from the top of the silage, and this must be done carefully; exposing only the amount of silage that is needed, and taking care that all material that is holding the sheet down be removed, and not allowed to mix with the silage. The unloading must be done in such a way that the front structure of the silage is not disrupted, since a damaged face structure will enable air to penetrate more easily. Fig. 51, Fig. 52, and Fig. 53.

It is also important that the area of the face be such that the daily removal of silage from the whole face extends to a depth of not less than 30 cm. This depth should be greater than that of air penetration, to ensure that air has not penetrated beyond the removed silage; otherwise, the silage removed for feeding will always be damaged to some degree. Therefore, the physical design of the bunker should match the daily demand for silage.

This design is calculated on the assumption that 1 m3 of silage in a bunker contains 250-300 kg of DM (or approximately three times this amount of wet silage), therefore, each 10-cm-thick slice cut from the face will supply 25-30 kg of silage DM or 75-90 kg wet silage per square meter.

It is a serious mistake to remove silage in advance of its requirement for feeding to cattle, since it begins to deteriorate immediately upon removal, and deteriorated silage, in addition to its impaired nutritional value, contains large populations of molds and yeasts, which may not be visible. The presence of molds always imposes the risk of mycotoxins, with all their negative consequences. Microbial growth rates rise exponentially with temperature up to approximately 55°C, therefore these deterioration problems are much more acute during the summer and in hot areas.

The intensity of deterioration of the silage at the feed-out stage is greatly affected by several management factors: the hygiene of the raw material (with regard to removal of weeds, soil and stubble); the technology of the ensiling procedures (chopping, compaction, sealing); and the environmental conditions (temperature and humidity). Because so many factors are involved, most of the data used to evaluate unloading losses are derived from laboratory experiments, in which it is possible to control some factors. The findings of such experiments indicate DM losses of 1.5-3.0% per day for each 8-12°C rise in the silage temperature above ambient. Only good management can minimize these losses Fig. 54.

*Sources of losses during ensiling and feed out (%)

*Zimmer 1980 (these data are probably derived from ryegrass under German conditions).
Sourceloss of net energycausative factor
Respiration1-2enzymes + bacteria
Fermentation I4bacteria
Fermentation I0-5clostridia
Effluent or wilting5-7low DM
Surface wastage0-10aerobic microorganisms
Aerobic deterioration0-10aerobic microorganisms

The lost ingredients are mainly from the digestible fractions, and what remains, in increasing percentages, comprises mainly lignin and minerals

The aerobic stability of silage is an indicator of the extent to which silage changes under exposure to air; it is also called the “bunker life” of the silage. Unstable silage becomes less palatable to livestock; this deterioration under aerobic conditions is characterized by rising temperature, intensive production of CO2 and loss of organic matter, as indicated by increasing ash content. Measurement of one or more of these phenomena can reflect the intensity of deterioration Fig. 55 and Fig. 56.

The temperature rise is measured by putting a sample of silage in a thermally insulated box and timing its temperature rise above ambient, to obtain a measure of the stability-time relationship of the silage. The equation of aerobic fermentation is: C6H12O6 + 6O2 → 6CO2 + 6H2O + 673 kcal.

Fermentation of one molecule of sugar releases six molecules of CO2, which means that fermentation of 180 g of sugar will release 264 g of CO2. Therefore the amount of sugar lost can be also calculated by collecting and weighing the released CO2 and multiplying by 180/264, i.e., 0.68. Since CO2 is 1.52 times heavier than air, it can be readily collected and measured: the CO2 sinks to the bottom of the collecting vessel where it is absorbed by an excess of a basic solution. The solution is then titrated with 1N HCl, which expels the CO2 (details of this method were published in Canadian Agricultural Engineering 1991, pp. 391-393).

The following table summarizes the results of silage deterioration:

Chemical, microbiological and CO₂ production in various silages in aerobic conditions

* log. of no. of colony forming units (CFU) per g DM., **g per kg DM, NF = not found.
Time (days)pH Yeasts*Molds*CO₂production**

It can be also seen that deteriorated silage contains more water than either good silage or the original forage. The aerobic stability of silage is a very important factor, especially in hot areas. The use of additives, which basically elicit a positive effect on the fermentation, necessitates testing for stability also. Although the additive improves fermentation, it may also render the silage more susceptible to aerobic deterioration.

Microbial genera associated with aerobic deterioration

Yeast (or fungus)MoldAerobacteria
PichiaMucor, Penicillium 
SaccharomycesRizopus, Trichoderma