Food Technology Information Center

Technology of Forage and By-Products Preservation

12. Compaction

Compaction is necessary to expel the air from among the forage particles. Loosely packed forage contains a large amount of oxygen that enables aerobic respiration processes to continue and aerobic microorganisms to remain active for a long time, which increases losses. In the feed-out phase, especially in bunker silos, this loose forage will enable air to penetrate easily from the face, deep into the silage, and to cause much damage. In such a case the quality of the silage that is fed will always be impaired Fig. 26, Fig. 27. The quality of compaction is affected by the DM content, the chopping length, and the character of the forage H4>12. Compaction Fig. 28.

In a tower silo compaction results from the weight of the forage itself – every layer compresses those beneath it – but in a bunker silo compaction is done by driving a heavy tractor to and fro along the bunker. It is important to emphasize that the compacting effect of the tractor is mainly on the upper layer of the forage – 30-50 cm (10-20 inches) – therefore compaction is applied to each load of forage separately. The width of the bunker must be at least twice that of the tractor, otherwise an uncompacted strip of silage will be left along the middle. Fig. 30 Compaction must be done with a wheeled tractor, which is much more effective for this than a tracked one. It is almost impossible to improve a layer that was not compacted well at the correct time, and the forage in such a layer will remain loose, with all the negative consequences; the same applies to air pockets, therefore, the rate of supply of forage to the bunker has to match the practicable rate of compaction. However, there is no logical reason to continue to run the tractor on the top layer after the forage has been well compacted. Fig. 31 and Fig. 32

The technique of filling a bunker is from the back to the front. The forage is pushed to the back, with the surface maintained at a slope that the tractor can climb. The slope established will depend on the width of the bunker and the rate at which the crop is arriving; the key requirement is that it must be shallow enough for the tractor to run up and down with complete safety. When the rear part reaches the correct height it is time to start to seal it, so as to enable the anaerobic fermentation to begin. Covering is continued as each part reaches the correct height.

The ensiling process can be divided into three major phases:

  1. The aerobic phase.
  2. The fermentation phase.
  3. The anaerobic phase.

1. The aerobic phase

Once the material is in the silo, the populations of aerobic and aerobic-facultative microorganisms (bacteria, yeasts and molds) increase greatly. Respiration and proteolysis continue after the crop is cut, and the chopping and crushing of the forage damage the plant cell wall, which promotes the release of WSCs and enhances enzyme activities, and consequently the temperature increases. Excessive temperature (above 42-44°C) can result in a Maillard or a Browning reaction. In these reactions, sugars, and free amino acids groups are formed into polymers that will be measured as ADF (acid detergent fiber) and ADIN (acid detergent insoluble nitrogen). In all silages, excessive heating will significantly reduce the digestibility of protein, fiber and other nutrient compounds, and in drier silages this process can even result in a fire in the silo. The aerobic phase is characterized by many negative effects, including losses, therefore it is very crucial to minimize this phase, and rapid filling, good compaction and effective sealing all work towards this end.

2. The fermentation phase

Once the aerobic conditions are established in the silo, several processes begin. The intact plant cells start to break down and to release juice. In wet forages this process occurs within a few hours, whereas in dried forages (and at low temperature) it can extend over a few days or more. The juice so released provides sugars for the LAB to ferment and also releases numerous enzymes similar to those released by the chopping process. In forages with low levels of WSCs the enzymes that degrade polysaccharides are beneficial and provide additional sugars for fermentation, but the proteolysis enzymes that are also released break down proteins to soluble NPN compounds. In this phase effluent is produced if the forage is too wet.

3. The anaerobic phase

With the onset of anaerobic conditions, anaerobic microorganisms begin to multiply rapidly. The microorganisms having greatest relevance to silage preservation are LAB, enterobacteria, yeasts and clostridia. Which microorganism will dominate the silage will be determined by the epiphytic microorganisms that were brought from the field with the forage, and by the conditions within the silage. The fermentation process will continue until the pH drops to a level that stops the microorganism activities. From this point the silage will be stable as long as it remains in a sealed condition.

The composition of the fermented WSCs, the volatile fatty acids (VFAs) in the silage has a very important effect on silage intake by the animals, and on the silage stability. The composition of the VFAs also reflects the pattern of the fermentation process. In some cases ordinary analysis indicates that the silage is good, but the cattle react differently, and reduce their intake. In most cases the reason is the presence of excessive undesirable VFAs.

The table shows the importance of VFAs and alcohol in silage palatability:

VFAVFA content (g/kg DM)
 Silage ASilage B

Silage A had intake characteristics of 45 kg/cow per day.

Silage B had intake characteristics of 60 kg/cow per day.

The two silages had almost identical standard chemical analyses.