Frank Mickan is the Pasture and Fodder Conservation Specialist with the Department Environment and Primary Industries (DEPI) at Elinbank in Victoria.
“Animal and human health problems associated with silage are not very common but when they do occur, they are not quickly forgotten!” says Frank Mickan, Pasture and Fodder Conservation Specialist, DEPI, Ellinbank. Animal health problems can appear as decreased intake and resultant decreased production, abortions, scouring and even death! However despite silage being the obvious and guilty party in many cases, it is also the scapegoat for many not so obvious causes, bit like snakebite as a cause for dead animals with no obvious reason. That’s the bad news but the good news is that most of these need not occur at all.
Proper silage harvesting and storage will minimise any such problems.
- Harvest quickly24 – 36 hours ideally)
- Ensure that dirt, manure and dead animals are not incorporated
- Compact the stack or bale very tightly
- Seal the stack or bale airtight with plastic or alternative as soon as possible after harvesting is completed
- Regularly check and repair any hole immediately
- Feed out at least 150 mm of silage face each day (300 mm every 2nd Day)
- Ensure baled silage is eaten by day 3 once opened.
Hazards to human and animal health can be caused by undesirable micro-organisms such as Enterobacteria, Listeria, Clostridia, moulds/fungi, and by mycotoxins.
Surveys have revealed a huge variation in the preservation qualities of silages, with many being unstable and badly preserved. Rapid lactic acid production in anaerobic conditions is the desired type of fermentation. It is when this environment does not occur, or is later changed, that most problems occur.
Undesirable micro-organisms can enter the silage storage by soil and livestock waste during harvest or increase in numbers by an environment suited to them in the silage storage.
Enterobacteria.Also known as coliform organisms, the most important micro-organism in this group is Escherichia coli (E. coli) species and can cause diarrhoea and death. E. coli and other related enterobacteria (Bacillus and Clostridia species) become a greater risk when effluent is applied to paddocks closed for silage. Effluent sprayed onto paddocks closed for silage must be well washed in by rain before harvesting.
A slow fermentation favours the growth of enterobacteria and so these guys can compete more strongly for the water-soluble carbohydrates (plant sugars and starches) with the more desirable lactic acid bacteria in the early phase of fermentation. The result is that less lactic acid is produced and the desired rapid decrease in pH ie. Increase in acidity, does not occur.
Leguminous crops such as balansa clover and lucerne have a higher buffering ability than grass type silages and pH decrease can be slow. That is, they are less inclined to undergo a satisfactory fermentation once ensiled. However wilting leguminous crops quickly to 33 – 40% DM for chopped silage and 40 –55% baled silage will reduce this problem.
Listeria monocytogenes is found in soil, faeces and rotting vegetation and can reproduce at low temperatures, as well as in heating silage. It is more commonly found in the out layers of baled silage although it may occur in the layer just below the plastic sheet in chopped stack silage.
Slow air entry via holes, poorly wrapped or insufficiently wrapped bales and plastic degradation can lead to this problem. As the silage breaks down the silage pH rises, ie. becomes less acidic, and upon air entry, water, carbon dioxide and heat are generated. The moisture, rising pH and slow air ingress favours the Listeria blighters.
If possible avoid feeding the obviously mouldy silage to pregnant animals. Do not feed it to sheep as they are particularly vulnerable to Listeriosis. Prevention is the most effective cure by following the proper harvesting and storage requirements mentioned above.
The direct effects of clostridial fermentations on animal and human health are less evident than those on silage composition quality. These anaerobic (without air) bacteria are responsible for the secondary fermentation of glucose and the more desirable lactic acid to a less desirable butyric acid. Clostridia bacteria are also the cause of excessive protein breakdown. Clostridai bacteria could grow in wet silages which have a pH above 4.6, which can occur in pit silage where air has gained entry. The silage is not pleasant to smell. The bales are often shrunken, effluent in the bottom and sagged heavily.
Costridia affected silages are less palatable, have a lower digestibility silage and results in reduced intakes and possibly signs of acetonaemia in high yielding cows in early lactation. The extensive protein degradation of plant proteins to ammonia may also have detrimental effects on animals although scientists debate this issue.
Clostridium botulinium is the Clostridia bacteria of most concern to health. The risk of botulism is increased substantially if the crop is contaminated with animal remains. Botulism has been reported from silage to which poultry manure was applied. Horses are most susceptible.
Clostridial spores from silage can infect milk and cause problems in cheese making.
Fungal spores pose health problems in two ways. Firstly as airborne spores affecting mucosal surfaces of the lungs and respiratory passages, and secondly as metabolic disorders from ingestion of fungal mycotoxins. Humans and horses are more at risk of respiratory disease than ruminants but the disease caused by silage is rare due the open areas associated with its feeding. Respiratory problems, such as farmer’s lung, are more common in mouldy hay, especially in enclosed areas such as hay sheds. Figure 2 shows an extremely mouldy bale of silage.
Mycotoxins are products of fungal metabolism and can be found in silage, hay and any feed that has deteriorated during storage. To produce toxins fungi need a temperature above freezing, a moisture content above 20%(ie. less than 80% DM), and air (oxygen).
For the scientifically inclined reader, Fusarium, Aspergillus and Penicillium are the most prolific producers of mycotoxins in silages. Mycotoxins of concern are deoxynivalenol (DON), aflatoxin, T-2 toxin, zearalone, moniliformin, ochhratoxin, roquefortin C and patulin. They are most likely to be found in silages that have undergone aerobic spoilage.
Mycotoxins affect animals through three mechanisms:
a) alteration of nutrient content, absorption, and metabolism
b) changes in endocrine and neuroendocrine functions
c) suppression of the immune system.
Signs of mycotoxicosis in animals are reduced feed intake, decreased animal performance, poor fertility, and increased incidence and severity of disease. However mycotoxin presence may not always be the blame for poor animal performance. Even if they were, the multiplicity of potential toxins renders it sometimes impossible to diagnose accurately the cause of the disease. But mycotoxins can be a serious hazard to both animals and humans and so their presence should be minimised by proper harvesting and storage.
Considering the increased importance of silage as a feed for livestock more studies are needed to establish epidemiologically and mechanistically the risks to animal and human health from silages contaminated with pathogenic bacteria and mycotoxins. Research is also needed to understand more completely the relationships between the physical and chemical composition of silage and metabolic disorders in animals. Farmers will then be in a position to avoid the predisposing conditions by changing crop and ensiling management.