Introduction
There is no evidence to support the misconception that silage feeding has significantly greater animal health risks than feeding other forms of conserved forage. Reports of animal health problems associated with silage feeding are not common.
Animal health issues are only covered briefly in this publication. Producers who are concerned about health risks associated with the feeding of silage should seek veterinary advice.
The potential health risks most likely to be associated with feeding silage to livestock are caused by listeria (listeriosis), moulds and Clostridium botulinum (botulism). The risks of health problems caused by listeria and moulds can be almost eliminated by good silage-making practices, particularly effective compaction and sealing.
Poor silage-making practices may increase animal health risks. However, the main issue is that poor practices will always result in significant economic penalties from increased DM and quality losses.
Listeria: Listeriosis is an infection caused by the bacteria Listeria monocytogenes. Listeria can cause abortions (usually in late pregnancy), brain damage (‘circling disease’) in sheep, or even death.
Listeriosis is more common in animals with weakened immune systems – particularly new-born and pregnant stock. Sheep are inherently more susceptible than cattle. Listeria require aerobic conditions to grow and multiply, but are able to survive under anaerobic conditions.
They are intolerant of acidic conditions and, under anaerobic conditions, activity is severely restricted below a pH of about 5.5. Therefore, listeriosis is generally only associated with poor quality silages – inadequate air exclusion, poor sealing and limited fermentation (high pH).
European studies have found the incidence of listeriosis is marginally more common with baled silages, where adequate compaction and air exclusion are more difficult to achieve, there is a greater surface to volume ratio and the fermentation is limited.
If listeria are present they are usually in the surface spoilage layer. If this layer is removed prior to feeding, the risk of listerosis is reduced. The most effective strategy to avoid listeriosis is effective sealing.
Listeria
Moulds: Some moulds are capable of producing toxins, which if eaten, can be fatal to livestock. Inhaled mould spores are also capable of causing allergic reactions in humans – asthma and farmer’s lung.
Moulds require aerobic conditions for growth. In well-made silages – rapid filling and compaction of the silo, good air exclusion and adequate sealing – any mould growth is limited and confined to the surface of the silo or bale. If mould is observed, and potential animal health risk is a concern, take the following precautions:
- Remove the mouldy material prior to feeding, if possible.
- Feed sufficient silage to allow livestock to avoid eating the mould. Because it is unpalatable, stock will generally not eat mouldy silage, if given a choice.
- Avoid feeding the silage to very hungry livestock and to pregnant animals. Feeding mouldy silage is more likely to lead to animal health problems when it is used for drought feeding.
Most authorities consider the risk to livestock from mouldy silage to be minimal and no greater than the risks associated with feeding mouldy hay. Reports of livestock deaths from either source are not common. There is no evidence to suggest that colour of the mould is any indication of toxicity.
Botulism: The disease caused by the bacteria Clostridium botulinum. When the carcases of dead animals are ensiled, these bacteria multiply and produce a toxin. Although the incidence is very low, eating contaminated silage or hay causes death very quickly.
The most common sources are probably rats, snakes and other small animals picked up during harvest. As a precaution, remains of dead livestock should be removed prior to sowing a silage crop or locking up pasture. Vermin that burrow into and nest in silos and bales, and then die may also be a source of contamination.
Clostridium botulinium
Chop length
The chop length of the ensiled forage can affect the rate and extent of silage fermentation, the extent of losses during storage and animal production. Reducing the length of chop causes more physical damage to plant cells, releasing Water Soluble Carbohydrates WSCs more rapidly for the silage microorganisms.
This allows the fermentation to develop more rapidly and the LAB to ferment more WSC to lactic acid. The pH will decline more rapidly, with a reduced loss of DM and energy, and less degradation of the protein fraction.
For forages with low levels of WSC, such as legumes or tropical grasses, a finer chop length will assist in the production of more acid, which will, in turn, assist successful preservation.
As well as making WSCs more available, short chopping increases bacterial activity in wilted silages by releasing moisture from the cells. This increases the amount of WSC fermented to lactic acid. The effect of chop length on the silage fermentation, as indicated by rate of pH decline.
Reducing the chop length makes the silage easier to compact and reduces the amount of trapped oxygen in the silo. As a result, losses due to aerobic respiration and the risk of mould growth are lower
The advantage of a finer chop length is greater for silages that are difficult to compact, e.g. heavily wilted forage and grasses compared to legumes. However, finer chopped, low DM silages produce more effluent, at the same DM content, due to the release of moisture from damaged cells
