The menace of mycotoxins in the silage
As can be seen in
Table 1, laboratory analyses of silages confirm the presence of mycotoxins in silage samples. Analyses were performed using standard procedures. Aflatoxins, ZON, DON and total FUM were analyzed by HPLC (High Pressure Liquid Chromatography). For the purpose of data analysis, non-detect levels were based on the quantification limits of the test method for each toxin: Aflatoxin B1 <0.5 μg/kg; ZON <10 μg/kg; DON <150 μg/kg; and FUM <25 μg/kg.
The occurrence of AfB1 and FUM was less frequent than that of others such as ZON and DON. Only 2 samples out of 191 (1.1%) were found positive for AfB1. Scudamore and Livesey (1998) consider that surveillance for aflatoxin in silage and forages has rarely been reported, despite the acknowledged hazardous effects this mycotoxin implies. Aflatoxin degradation in the rumen is generally weak, less than 10% with dosages from 1 to 10 μg/ ml (Yiannikouris and Jouany, 2002). In lactating animals, AfM1 and other metabolites are excreted in the milk (Gratz and Täubel, 2007). This carcinogenic mycotoxin was proven to be related to increased lameness (subclinical mastitis) and impaired fertility (cystic ovaries) (Özsoy
et al., 2004). Out of the 43 samples tested for fumonisins, 11.6% presented a positive result. The maximum contamination level found was 989 ppb. FUM has shown to reduce milk production in dairy cattle (Diaz
et al. 2000). ZON and/ or DON contamination was found in many samples tested for these mycotoxins. From the 191 samples tested approximately 20% and 40% were positive for ZON and DON, respectively. Levels as high as 26 728 μg/kg (ZON) and 1256 μg/ kg (DON) were found for these mycotoxins. Several case reports have related ZON to an estrogenic response in ruminants and included abortions as a symptom.
Minimizing mycotoxin contamination in the field
Ensiling has become an important process in the conservation of harvested crops. This process is based on anaerobic storage in order to promote the growth of desirable microorganisms (lactic acid bacteria which lead to a deep acidification) and to prevent contamination with undesirable microorganisms (especially
Clostridium spp. and
Listeria spp. bacteria, moulds and yeasts; Kalac and Woodford, 1982). According to Richter and Bauer (1997), the most frequently occurring mould in corn silage is
Penicillium roqueforti (
Figure 1), whereas in grass silages
Monascus ruber (
Figure 2) and
Aspergillus fumigatus (
Figure 3) are the most common ones. The last two moulds were classified by Pelhate (1977) as tolerant in their tolerance to oxygen, whereas
Penicillium roqueforti is considered as microphilic or indifferent to oxygen presence.
Since more than 90 % of the mycotoxins in feed are already produced on the field, the first step to avoid mycotoxins in silages should be done at the site of crop production. Several environmental factors play a role in the growth of moulds on the field: temperature, composition of the gas atmosphere, substrate properties including moisture content and water activity (aw), pH and chemical composition, as well as biotic factors (insects, rodents, and other microorganisms) (Ramakrishna
et al. 1993, Ominski
et al. 1994).
The use of resistant plants against
Fusarium spp. is recommended, as well as a good crop rotation. Rain and high thermal amplitude are supporting risk factors, therefore the weather forecast or the weather conditions should be known as they are valuable sources of information concerning risk management.
The level of field mycotoxins is known to increase with plant maturation. As can be assessed in
Figure 4, the contamination level was negligible, from a practical point of view, for the grains with 63.3% of dry matter (DM).
At this maturity stage, 90 % of the starch yield potential is already reached. However, 3 weeks later, the mycotoxin load increased substantially, reaching over 800 μg/kg, 400 μg/kg and approximately 50 μg/kg DM of DON, Acetyl- DON and ZON respectively. This practical example is in accordance with other authors (Jones
et al., 1981; Warfield and Gilchrist, 1999), supporting the need for
adequate planning of harvesting activities.
Avoiding mycotoxin contamination in the ensiling process
While fusariotoxins are mainly produced on the field,
Aspergillus and
Penicillium fungi will most likely develop after harvest leading to the production of aflatoxins and ochratoxins, especially under poor storage conditions. However, as most of the toxic compounds present on agricultural commodities will remain stable after harvest under aerobic conditions (Scudamore and Livesey, 1998), crop management should not be discarded as an important factor.
Hygiene (clean crop, clean silo) should be maximized; dirt can considerably increase the number of undesirable microorganisms, namely
Clostridia and
Listeria, and fungi due to the ubiquitous existence of
Fusarium spores in the soil (Schrödter, 2004).
According to Scudamore and Livesey (1998), field-derived fungi will, in time, be replaced by storage fungi, particularly with inadequate drying or if the moisture content is not maintained below about 15%. In the case of silages, the moisture content is 3 to 5 times higher than this value and therefore water activity (aw) is much higher than needed by fungi (0.65 according to the same authors), which will increase the contamination risk. The most common silage materials are grass, corn, whole crop cereals and different industrial byproducts. Authors like Richter
et al. (2005) gave provisional orientation values for contaminations with moulds in corn and grass silages (
Table 2).
Most of these fungi are known producers of mycotoxins which will persist throughout the ensiling process. In spite of the fact that some moulds can grow even under anaerobic conditions/low amount of oxygen, the creation of
anaerobic conditions in the silage can considerably reduce the growth of fungi and subsequently mycotoxin formation. Two aspects are essential to control the oxygen entrance into the silage: compaction and coverage. Compaction eliminates the oxygen inside the material and coverage maintains the silage anaerobically preserved. When the silage is well compacted, the oxygen entrance and penetration will be limited to the layer in contact with the air in the feed out phase (Losand, 2003) (
Figure 5) and the aerobic stability will be improved (Kleinmans, 1996).
Crop particle length is closely related to compaction. The "rule of thumb" at this stage is: the drier the material to be ensiled, the smaller the crop particle should be. The
coverage of the ensiled matter should be done immediately with plastic sheets (polyethylene). It is very important to use exclusive adequate sheets especially for this purpose. A low quality sheet will permit air penetration and enable mould growth and the further production of mycotoxins, also leading to losses of dry matter and energy content. Once the silo is air tight, respiration stops and fermentation can be initiated. Although not useful in preventive situations due to its highly questionable efficacy, storage length is considered by some authors to have an impact on the mycotoxin content. Richter (2006) reported a decrease in ergot alkaloids produced by
Claviceps purpurea during storage. Zearalenone and some of the trichothecenes appear not to be affected by anaerobic and acid conditions in silage (Lepom
et al., 1988).
a. the field, should continue in
b. the silage production process and finalize with
c. the correct management of the open silo.