Silage factors
Composition. Silages with high levels of fermentable carbohydrates, including Water Soluble Carbohydrates (WSCs), remaining after the fermentation (e.g. wilted silages), tend to be less stable.
Fermentation quality. Silages which have a poorer fermentation quality and higher levels of volatile fatty acids (acetic, propionic and butyric) tend to be more stable. Silages can be more susceptible to aerobic spoilage where homofermentative LAB have dominated.
Porosity. Silage stability declines as air infiltration increases. The susceptibility of a silage to air infiltration is influenced by the physical characteristics of the silage, silage density (kg/m3)and DM content.
Dry Matter (DM) content. Wilted silages can be more susceptible to aerobic spoilage due to higher levels of residual WSC, and greater difficulty in achieving adequate compaction.
There is, however, some evidence that susceptibility to aerobic spoilage is less once DM exceeds 50%.
- Population of aerobic spoilage organisms. An extended aerobic stage at the commencement of the ensiling process, or air entry during storage, allows aerobic organisms to proliferate. They remain dormant during the anaerobic storage phase, until the silage is opened.
Feed-out factors
Ambient temperature. Silages tend to be more susceptible to aerobic spoilage during warmer weather.
Feed-out rate. Slow feedout of silage or bales from the feeding face increases aerobic spoilage. This is one of the most important factors influencing aerobic spoilage.
Management of the feeding face. Excessive disturbance of the face during removal of silage increases air penetration, increasing the spoilage rate.
Mixing prior to feeding. Mechanical processing of silage in a feedout wagon, mixer wagon or bale chopper increases aeration and can increase aerobic spoilage.
Silage micro-organisms. Lactic acid bacteria (LAB) Bacteria belonging to this group convert Water Soluble Carbohydrate (WSC) to lactic acid and other fermentation products. LAB are classified as either homofermentative or heterofermentative
Domination of the fermentation by homofermentative LAB leads to a more efficient utilisation of available WSC and a more rapid decline in pH with less loss of DM or energy. In forages with low WSC content, achieving a successful fermentation may be dependent on homofermentative LAB dominating the fermentation.
The population of LAB is low on growing crops and pastures, and is concentrated on dead and damaged plant tissue. Studies with chopped silage show that the population increases rapidly between mowing and delivery to the silage pit or bunker.
Damage to the plant tissue releases nutrients and minerals and is suggested as a possible reason for this rapid increase in bacteria numbers. Some studies have shown that LAB numbers also increase rapidly during the wilting phase, although this is not always the case.
Commercial bacterial inoculants usually contain cultures of homofermentative LAB bacteria to improve the rate and efficiency of fermentation.
However, recent information indicates that production of some acetic acid may improve aerobic stability of the silage upon opening. This would be an advantage in warm Australian conditions, particularly for maize silage, which is inherently unstable. This suggests that some heterofermentative LAB may be desirable during fermentation.
Further studies are required to confirm this.
Clostridia virus
Clostridia
Clostridia are classified as either saccharolytic or proteolytic according to whether the main substrate they ferment is WSCs and lactic acid or protein, respectively, although some species possess both saccharolytic and proteolytic activity .
Clostridia require a neutral pH (about 7.0) and moist conditions for optimal growth. As pH falls during an effective lactic acid fermentation, clostridia become less able to compete, until their growth is completely inhibited. Wilting to a DM content greater than 30% severely restricts clostridial growth.
Enterobacteria
These bacteria prefer a neutral pH (about 7.0) and warm conditions for optimal growth. The warm Australian conditions are ideal for enterobacteria to flourish early in the fermentation phase. In low WSC forages (e.g. tropical grasses), where pH drops slowly, these bacteria can dominate the fermentation.
If they dominate, an acetate silage will be produced with a pH of about 5.0. Below this level the growth rate of enterobacteria s inhibited.
Levels of enterobacteria are low on crops and pasture, and decline during wilting.
However, numbers can increase rapidly during the first few days of the fermentation and compete with LAB for available WSC. In most silages, they are only likely to be significant during the early stages of fermentation, before pH starts to decline significantly.
The main products of their fermentation process are acetic acid, lactic acid and CO2, and increased ammonia-N levels due to the degradation of protein. Although some acetic acid production may improve aerobic stability, DM and energy losses can be significant if the fermentation is prolonged and enterobacteria are dominant. The resulting silage is also less palatable to stock.
