Silage preparation for biogas plants

During the ensiling process, freely available carbohydrates are digested by lactic acid bacteria in an anaerobic climate. This process results in various short-chain organic acids and invariably leads to a loss of substrate. To keep this loss to a minimum, it is necessary to lower the pH as quickly as possible. This ensures that enzymatic reactions and further energy loss caused by microorganisms that impair fermentation are effectively prevented.

Critical phase: contact with ambient air when silo is opened

Considerable loss occurs often after the silo is opened, as the silage is then exposed to the oxygen in the ambient air. Microorganisms that prevent proper fermentation such as moulds and that are found in every silo begin to grow. The generation of heat in the silage material is a clear indicator for spoilage and associated loss of energy. Energy that has been lost during storage or preparation in the form of heat is of course not available in the subsequent fermentation process for the production of biogas.

If untreated silage becomes hot, it is being digested by yeast. Silage containing high concentrations of acetic acid prevent yeast from growing.

Preservation with acetic acid

The storage stability of silage is determined by its fermentation acid pattern. The protective effect of acetic acid, in particular against yeast growth, is well researched. For this reason, selected heterofermentative lactic acid bacteria are used in ensiling agents. They simply increase the acetic acid content of the silage!

Taking into account the purpose of the silage

While an increased acetic acid concentration is beneficial for silage for biogas production, it might not be appropriate for forage silage for dairy cows, as they might not like the taste. The farmer must thus decide prior to the harvest whether the crop is to be used for biogas production or as an animal feed!

High acetic acid concentration accelerates the startup of the methane production

During the methanogenic phase of the biogas production process, acetic acid has the added advantage of being directly convertible to methane. This is another reason why a high acetic acid content is desirable in biogas silage.

Scientific studies prove positive effect of homofermentative/heterofermentative ensiling agents

The biogas potential of various substrates was examined in batch tests carried out at the University of Rostock. Lab tests showed that acetic acid has a significant effect on the methane production: silage treated with the ensiling agent produced higher methane yields.

Study on the effect of ensiling agents on the methane yield - PDF file

The crop must be suitable for the purpose

Good silage can only be produced from a crop that is actually suitable for ensiling, which in term is mainly determined by the dry substance content and the availability of easily fermentable carbohydrates in the substrate.

Time of harvest

Crops harvested at the ideal harvesting time show good compaction properties, a low tendency to heat generation and thus a reduced susceptibility to mould growth and fusarium toxins.

Density

The optimum density for minimum energy loss depends on the crop species and the dry substance content of the harvested material. It can be achieved by adjusting the shredder chip length, layer thickness and roller pressure.

Anaerobic conditions

By compacting the material and covering it quickly, it is possible to maintain anaerobic ensiling conditions.

Two-foil system for effective coverage

To cover the silage, we recommend using a two-foil system with a 0.04 mm lower foil and a 0.15 mm cover foil. It is advisable to cover the substrate during prolonged ensiling breaks (> 6 h) in order to reduce the gas loss.

Guideline values for crops used for energy silage preparation

Depending on the harvested plant, certain guideline values must be adhered to during harvesting and storage.

The linked documents contain the main parameters that determine the suitability of the crop such as storage density, layer thickness, roller pressure in the silo, etc. It also shows how the silage is covered with the two-foil system, and how the silo can be opened:

Maize silage - key properties - PDF file
Forage rye - key properties - PDF file
Cereal whole crop silage - key properties PDF file

Cereal instead of maize? There are many good arguments for this in certain regions

Compared with maize, the production of cereal can offer a range of advantages, especially as regards the crop's tolerance to cold, its low humus consumption, the use of winter moisture and the ideal harvesting time. There are however certain disadvantages, such as the often low yield per hectare, which can however be compensated for by increasing the yield of other crops in the rotation cycle.

Choice of cereal depends on purpose, local climate and soil

As a rule, all cereals are suitable for whole crop silage. The decision for the production of a specific cereal and variety is thus made by considering local climate and soil conditions and the purpose for which the crop is grown. On land with sufficient water supply from June to August, good experiences have been made with two-crop systems such as winter forage rye (first crop) and maize, Sudan grass or millet as secondary crop.

Forage rye

Winter forage rye is the ideal first crop for two-crop system with maize, Sudan grass or millet as the secondary crop.

Forage rye - key properties - PDF file

Cereal whole crop silage

Thanks to its specific properties, rye is used as a sample crop to demonstrate the versatile use of whole crop silage:

Cereal whole crop silage - key properties - PDF file