Mycotoxins are highly toxic secondary metabolic products of molds on almost all agricultural commodities worldwide. They occur under natural conditions in feed as well as in food. Several studies proved that economic losses occur at all levels of food and feed production, including crop and animal production, processing and distribution. According to the FAO (Food and agriculture organization) 25% of the world’s crop harvests are contaminated with mycotoxins. There are currently more than 400 mycotoxins known. There are six major classes of mycotoxins that frequently occur: aflatoxins, trichothecenes, fumonisins, zearalenone, ochratoxin and ergot alkaloids. They are formed by different kinds of fungi and each fungi species can produce more than one mycotoxin. The major classes of these mycotoxin-producing fungi are listed in the table below.

Mycotoxins differ in their structure, which explains the great variation of symptoms (see table: "Selected mycotoxins and their basic symptoms"). The main toxic effects are carcinogenicity, genotoxicity, nephrotoxicity, hepatotoxicity, oestrogenicity, reproductive disorders, immunosuppression or dermal effects. There are several factors which influence symptoms:

Moreover, mycotoxins can occur even though negative analytical findings. Firstly, mycotoxins often occur in so-called “hot-spots” and therefore they might stay undetected if sampling procedures were not carried out in a correct way. Secondly, masked mycotoxins could appear in the feed. Masked mycotoxins are chemically modified mycotoxins by specific biochemical reactions in which mycotoxins can be bound to certain feed ingredients including e.g. glycosides, glucuronides, fatty acid esters and proteins. Consequently, these masked mycotoxins are not detectable with conventional analytical methods, i.e. in regular testing samples will show up as having no contamination whilst containing mycotoxins in a bound form. However, in the course of the digestive process, masked mycotoxins are released again and affect the animal.

Each plant can be affected by more than one fungus and each of them can produce more than one mycotoxin. Consequently, there is a great probability that many mycotoxins are present in one feed, thus increasing the odds of interaction between mycotoxins and the occurrence of synergistic effects, which are of great concern in livestock health and productivity. Synergistic effects can already occur at low levels when the combined effects of two mycotoxins are much greater than the individual effects of each toxin alone. In contrast additive effects that occur when the combined effects of two mycotoxins are equal to the sum of the effects of each toxin given alone.

Mycotoxins are often invisible, tasteless, chemically stable and resistant to temperature and storage. They resist the normal feed manufacturing processes.

Mycotoxin producing fungi can be divided into two groups:

However in special cases like under unusually hot or dry conditions Aspergillus and Penicillium species can also affect crops during the growing season. On the other hand field fungi can continue growing and produce mycotoxins during transport and storage.