Silage can be contaminated with a wide range of mycotoxins, originating from infection of the crop or growth of moulds in silage during storage or feeding. Prevention of silage contamination should be considered at the field, silo and feeding levels.
Pre-harvest or field moulds develop during plant growth and maturation as humidity is high (>70%) and the temperature fluctuates between day and night. Insect damage can also increase contamination as it predisposes the crop to mycotoxin infection.
The risk of mycotoxin contamination significantly increases with late harvest when the DM of the crop rises above 30-35%. This is, probably due to longer time in the field and also because dry silage is more difficult to compact so more predisposed to the presence of oxygen and heating leading to mould growth and the proliferation of mycotoxins.
Poor compaction, poor sealing, and low feed-out rate promote aerobic conditions and favour the growth of toxic moulds. When storage conditions are not optimum, the inoculum of pre-harvest toxic moulds will find favourable conditions for their proliferation, leading to a substantial accumulation of mycotoxins in the final diet.
Animals may reduce their feed intake and efficiency due to reduced cellulose digestion, volatile fatty acid production, and rumen motility when consuming mycotoxin-contaminated silage for extended periods. Dramatic drops in milk production, weight loss, altered endocrine and exocrine systems, and fertility problems (e.g., irregular oestrous cycles, embryonic mortalities, pregnant cows showing oestrus, and decreased conception rates) have all been reported as a result of consumption of mycotoxin-contaminated silage.
Further, mycotoxins can lead to a variety of diseases such as displaced abomasum, ketosis, retained placenta, metritis, mastitis, fatty livers, and respiratory problems caused by interstitial changes in the lung. Other diseases such as generalised deterioration typical of protein deficiency, malnutrition, diarrhoea, irritability, and abnormal behaviour are also common. In most of these cases, affected animals may not easily respond to veterinary treatment.
The immune status of animals can be affected by mycotoxin contamination as well. The following modes of action have been described:
Decreasing serum concentrations of IgM, IgG, and IgA
Affecting immune cells and modifying immune responses as a consequence of tissue damage elsewhere
Inhibiting DNA, RNA, and protein synthesis and impairing the synthesis of cytokines that regulate the communication network of the immune system.
Mycotoxins in silage also affect human health as they can be found in milk and accumulate in animal muscle and other organs, thus causing adverse effects in humans when consuming milk and meat products. Unfortunately, most mycotoxins are heat-tolerant when such foods are prepared and processed under conventional temperatures (80–121°C). Therefore, little or no reduction in feed or food-borne mycotoxins occurs as a result of cooking, boiling, and pasteurisation.
Preventing the occurrence of mycotoxins produced by toxigenic fungi in forage crops starts in the field. The following are basic inhibitory conditions for fungal infection and proliferation:
Seed hybrids: If mycotoxins or diseases have been present in previous years, selecting seed hybrids that are resistant to them can reduce the risk and/or the severity of the infection. Some diseases can also be seed-borne, so it is important to be selective with the seed hybrids chosen for upcoming years.
Crop rotation and tillage: Due to the cycle of fungi and spores wintering in the soil and on crop residues, increased tillage and crop rotation are recommended to help control crop residues and potential mycotoxin contamination. Removal, burning, or burial of crop residues aids in the reduction of Fusarium inoculum, which could affect the subsequent crop.
Irrigation: This process can be very useful in avoiding plant stress in particular plant growth stages, but excess irrigation may favour infection with some mycotoxins such as Fusarium.
Application of fungicides: This should be done with an adequate fungicide dose since sub-lethal dosage can result in increased mycotoxin contents in crops showing no or low fungal infection.
Plant nutrition: Well-nourished plants have more effective defences. A proactive fertiliser programme, accompanied by the measures mentioned above can help reduce infection.
Long narrow silos are preferred to the short wide ones because they allow a rapid rate of feedback. Also, a smooth face of the silo is preferred to a rough one because of the smaller surface area that allows less penetration of air.
Silos should be filled rapidly immediately after harvest to preserve anaerobic conditions. When oxygen gets access to ensiled material, due to delayed filling, yeasts and acetic acid bacteria start to oxidise the preservative acids and residual water-soluble carbohydrates. The aerobic deterioration results in a rise in temperature and pH, thus allowing fungi to proliferate.
Compaction is also important for the dispersal of oxygen into a silage mass. This is closely related to the dry matter content of the ensiled material, so ensiling forage crops at optimal dry matter content (350-450g per kg-1 for grass: and 300-350 g per kg-1 for whole crop maize).
Sufficient sealing also prevents the build-up of aerobic microorganisms and ensures subsequent aerobic stability of silage. The sealing should remain intact during the whole ensiling period. It was observed, for example, that P. roqueforti spores stored under airtight conditions for 3 months had a strongly reduced germination ability. Thus, leaving silos sealed for at least 3 months before starting to feed the silage to livestock should have a beneficial effect on the prevention of fungal growth and mycotoxin production.
Inoculation with the heterofermentative lactic acid bacteria species Lactobacillus buchneri has proved to increase aerobic stability due to its anti-yeast and antifungal properties. With other inoculants, such as propionic acid bacteria no improvement in aerobic stability of silage was observed due to the acidic environment arising during the fermentation phase, which is not favourable for the growth of propionic acid bacteria. In another study with corn ensiled with mycotoxins at 0.006 mg per kg fresh weight, application of lactic acid has achieved a modest detoxification while hydrochloric acid was more effective. For instance, 50 and 100% of the mycotoxins remained after 21 days following treatment with 1.0 and 0.1 M lactic acid, respectively, whereas the corresponding figures were 4 and 40% with 1.0 and 0.1 M hydrochloric acid, respectively.
Increasing dietary levels of nutrients such as protein, energy, and antioxidants may be advisable. Animals exposed to aflatoxin show marginal responses to increased protein. Acidic diets seem to exacerbate the effects of mycotoxins, and therefore adequate dietary fibre and buffers are recommended. Because mycotoxins reduce feed consumption, feeding management to encourage intake can be helpful. Dry cows, springing heifers, and calves should receive the cleanest feed possible. Transition rations can reduce stress in fresh cows and reduce the effects of mycotoxins. Strategic use of mould inhibitors can be beneficial.
References are available upon request.