Silage making is a widespread practice to preserve forage. However, forage silage as a source of mycotoxins requires attention, considering that on a dry matter (DM) basis, the silage is the main dietary component for ruminants. What can a farmer do to prevent mycotoxins in silage?
Luckily, rumen microflora can degrade and inactivate mycotoxins, which make ruminants one of the least susceptible animal species. However, several mycotoxins resist rumen degradation, and rumen detoxification capacity might be saturable and can vary with changes in diet or presence of metabolic diseases. Another point of concern is that some mycotoxins have a high carry-over rate from feed to milk. Fusarium, Penicillium and Aspergillus species are the primary toxin producing moulds, although other moulds should be considered as mycotoxins producers. Several mycotoxins, including the most studied aflatoxins, deoxynivalenol, nivalenol and zearalenone can be found in silages (Table 1).
Multi-mycotoxin contamination in silages is the most common type of contamination. At farm level, animals are more frequently exposed to a moderate amount of different mycotoxins over a long period of time, rather than to high amounts of a single mycotoxin. This aspect is of particular concern because of the detrimental additive and/or synergic effects of mycotoxins on animal health. In maize silage, a high incidence of co-occurrence of deoxynivalenol and zearalenone is observed globally, while the incidences of other mycotoxins widely differ as a result of regional and environmental condition differences.
The main critical points of good ensiling practices
– proper assessment of the size of the silo to match silo size to feeding rates
– evaluation of the right moisture content and stage of maturity of the crop
– assessment of the best chop length and particle size allowing a good compaction, density, rapid pH drop and fermentation
– rapid harvest, filling and sealing choosing the right covering material to reduce the risk of air penetration
– use of microbial inoculants. The use of heterolactic LAB inoculants, because of the higher production of antifungal compounds such
as acetic acid, may improve the silage aerobic stability.
– maintain a clean-cut face in the feed out phase.
Fungi can colonise and produce mycotoxins during either pre-harvest or post-harvest stages. Pre-harvest events are dictated by environmental and agronomic factors (moisture, relative humidity, temperature, maturity stage, mechanical damage, pesticide and fungicide treatments, plant variety, stress, insects) and are controllable only to a limited extent. Whereas post-harvest events are largely controlled by the farmer. Post-harvest, favourable ensiling conditions decrease the growth of moulds and mycotoxin contamination.
However, some moulds can survive and, furthermore, toxins existing prior to ensiling can survive in the silage even if proper silage fermentation occurs (Table 2). An effective silage management and control programme must cover all the production phases, from the field through the ensiling process, until the management of the open silo.
When a silo is opened, oxygen becomes available to the front of the mass and the activity of yeasts and moulds could reduce its aerobic stability, thus favouring potentially toxigenic fungi development. Correct silage face management practices should consider:
1) maintenance of a smooth surface (with no cracks) perpendicular to the ground;
2) advance in the silo of 1-2 m/week in winter and twice that rate in summer. DM loss from maize silage exposed to air for just 1 to 2 days was measured to be as high as 6%. This means that, for example, 1000 tonnes of bunk maize silage at 33% DM with a 6% DM loss will result in a loss of 20 tonnes of silage DM;
3) mouldy parts should be discarded. An inclusion of 25% of spoiled silage in steer ration may cause a significant decrease of 7% DM intake and DM digestibility. If possible, in cases of severe contamination, silage could be mixed with less contaminated silage from a different silo. This option must be carefully evaluated, because it implies a reduction in the removal rate, which can predispose higher oxidation of the open front of the silo and
4) controlling spoilage at the feed bunk, by removing uneaten feed, cleaning the bunk daily, and keeping water out.
In hot weather, feeding multiple times per day, limiting wet ingredients in the ration, mixing TMR for one feeding and eventually adding mould inhibitors. Moreover, effective dietary strategies to promote rumen health, coupled with the administration of effective and broad-spectrum mycotoxin detoxifiers, are essential to minimise the negative impact of mycotoxins.
Mycotoxin analysis should become part of the routine evaluation of silages. Regular sampling and testing of silage allows picking up any variations in mycotoxin contamination. From a farming perspective, the most critical point is the sampling procedure of silage. To know the quality of the silage that is fed to animals, samples must be collected from the front of the silo, and the procedure should be repeated at different times. For instance, 12-15 sub-samples need to be collected from the front of the silage to form a pool of 500-1000 g as the final sample. The depth of sampling would be proportional to the thickness of the front removal for feeding animals. It must be underlined that each sample only represents the portion of silage from which it was taken as mycotoxin distribution may change within a silo and metre by metre when the feed out phase proceeds. Testing TMR is another critical point to evaluate the total mycotoxin burden with the diet. Samples from the top, middle and bottom from the entire length of the TMR windrow must be collected immediately after the distribution (within 20 min).
In conclusion, given the potential for multi-mycotoxin contamination, it is important to obtain information about the type and distribution of mycotoxins in silages. It is not possible to rectify spoiled silage at the end of the fermentation phase. Therefore, good silage-making practices must be applied. Moreover, producers should test silage samples prior to feedout for “in field decision-making” with regard to the rapid diagnosis of the quality of given forage and identification of possible risks for animals.
References are available on request.