Introduction
The anaerobic fermentation phase commences once anaerobic conditions are achieved within the silo. During this phase, acids are produced, lowering the silage pH and preventing further microbial activity, and so preserving the silage.
The silage will not deteriorate until exposed to oxygen. A slow fermentation increases Dry Matter and energy losses, and reduces the palatability of the silage.
The silage quality and fermentation products are determined by the forage characteristics and which micro-organisms dominate.
After the fermentation phase commences there is a short period, about one day, when breakdown of cell walls and the release of fermentation substrates by plant enzymes continue.
Bacteria then begin to multiply rapidly, increasing to a population of about 1 billion (109) per gram of fresh forage. These silage bacteria ferment Water Soluble Carbohydrate (WSC), converting them to acids and other products.
Ideally, Lactic Acid Bacteria (LAB) dominate the fermentation, but enterobacteria and clostridia may be dominant in some silages. Aerobic yeasts can also be present. This phase may be dominated, in the early stages, by enterobacteria. These bacteria ferment WSCs, producing mainly acetic acid, with lesser quantities of lactic acid.
Close up view of Enterobacteria
Ethanol, 2,3-butanediol and carbon dioxide are also produced and DM and energy is lost. In well-fermented silages, as lactic acid is produced the pH drops, enterobacteria cease growing, and LAB quickly begin to dominate the fermentation.
If the decline in pH is slow, enterobacteria may continue to dominate the fermentation, and produce an acetate silage. LAB ferment WSCs to lactic acid, with only very small quantities of other compounds being produced.
Fermentation dominated by ( LAB is preferred because lactic acid production is the most efficient chemical pathway. The decline in pH is rapid and there are only very small fermentation losses of DM and energy.
The proportion of lactic acid to other compounds produced will depend on the relative activity of homofermentative and heterofermentative LAB . Providing sufficient WSC are available, fermentation will continue until a pH of about 4 is achieved.
Lower pH values have been observed in silages produced in Australia from forages with high levels of WSC and low buffering capacity, such as maize and forage sorghums. In drier silages, the fermentation is inhibited and the ultimate pH achieved is higher, and can exceed pH 5 in heavily wilted silage.
Clostridial silages result if insufficient lactic acid is produced or it is produced too slowly. Clostridia require moist conditions to thrive and are not usually a problem in silages wilted to >30% DM content.
If the population of clostridia increases, a secondary fermentation can occur. Clostridia ferment WSC, lactic acid, and protein to produce butyric, propionic and acetic acid, and ammonia-N (NH3-N) plus a number of other intermediate compounds.
As the secondary fermentation proceeds, the pH rises. Final pH will be higher than for a lactic acid fermentation and depends on the final products of the fermentation. This is because the acids produced are weaker than lactic acid, and the ammonia-N has a buffering effect against these acids.
