The term mycotoxins refers to a large, diverse group of chemical substances, all produced by different fungi but with very different toxic effects. The actual cause of their occurrence is not fully understood but mycotoxin problems are more pronounced under less-than-ideal production and storage conditions of grains and feed.
Unlike monogastric animals, ruminant nutrition has to ensure a good intake of roughage to keep the rumen in good condition. The complexity of these diets which include concentrates and fibre sources and the fact that fibre sources are a potential source of mycotoxins, increases the risk of exposure to more than one mycotoxin. The interaction between mycotoxins often leads to synergistic effects, when the negative effect of one mycotoxin is amplified by the presence of another. This is especially true in the case of fusariotoxins. Fusarium graminearum and Fusarium culmorum are known to produce several different fusariotoxins under similar conditions, for example, zearalenone (ZEN) and deoxynivalenol (DON), which are also known to interact synergistically in the animal.
High-yielding cows have higher mycotoxin uptake
The assumption of an efficient inactivation of mycotoxins in the rumen relies on the fact that feedstuffs are retained in this ‘compartment’ long enough to allow proper activity of rumen microorganisms. But this is becoming more difficult to guarantee as animals are fed increasing quantities of feed in order to increase milk yields. With large amounts of feed comes the risk of increased mycotoxin exposure, higher passage rates and less time available for proper feed digestion. Complete mycotoxin degradation in the rumen is not possible.
Various mycotoxins are able to modify the rumen microflora as they exert antimicrobial, antiprotozoal and antifungal activity. In practical terms, this means that mycotoxins that are not degraded by the rumen microflora will escape the rumen and reach the site of absorption intact. These mycotoxins will then be absorbed by the organism just as in the case of monogastrics and the animal will be encumbered with a high mycotoxin load. Reduced ruminal motility, decreased dry matter intake, acid detergent fibre and starch digestion are some other negative impacts reported due to the ingestion of mycotoxin-contaminated feed.
Scientific studies demonstrate that only a few mycotoxins are subject to enzymatic cleavage by rumen micro-organisms. Only up to 42% of aflatoxins are rumen degradable; thus up to 58-100% may still be absorbed by the animal. Nonetheless, other scientists did not find any degradation of this mycotoxin in the rumen.
Mycotoxins impair dairy fertility
Since fertility and milk yield are directly related parameters, all factors disrupting fertility have a negative economic impact on herds. ZEN is an estrogenic metabolite that has been reported to occur in silage (Figure 1), corn, wheat, barley, oats, sorghum, hay and soybean worldwide. Chemically, ZEN shows a similar configuration to the female hormone estradiol, enabling it to connect to specific cell receptors. Thus, ZEN causes estrogenic effects as well as abnormal estrous cycles which ultimately impair fertility. Besides ZEN, other mycotoxins have been shown to cause fertility problems in dairy animals. Reduced fertility in dairy cattle is also caused by ergot alkaloids. Low conception rates, cystic ovaries and uterine infection were observed in dairy animals consuming aflatoxin-contaminated diets.
Mycotoxins impact milk production and lead to toxic residues in milk
In the case of aflatoxins, the most worrying effect is their carry-over into milk as aflatoxin M1 (AfM1). Aflatoxins are considered carcinogenic by the Institute of Applied Research on Cancer. Although the maximum amount of aflatoxin B1 on dairy feed is well regulated, if the carry-over rates ranging from 1.8 to 6.2% as found in the literature are taken into account, the maximum amount of contamination allowed in dairy rations to meet those requirements would be 1.78 ppb which is well below the 5 ppb allowed by the European Commission Regulation.
|Figure 1. Biomin worldwide mycotoxin survey. Analysis results for silage samples tested in 2012 and 2013. Fusarium toxins, such as zearalenone (ZEN), deoxynivalenol (DON) and fumonisins (FUM) were the most prevalent mycotoxins.|
Mycotoxins increase the incidence of metabolic problems in dairy animals
Figure 2 provides an overall picture of the effects of mycotoxins in dairy cattle. The most common and difficult challenges to identify occur when rations contain low levels of mycotoxins and the health effects are subclinical. In dairy cattle sub-clinical effects of mycotoxins are often overlooked due to the fact that a single herd is made up of individual cows in different stages of production and/or age, impairing the observation of a reduction in milk production which indicates the subclinical effects of mycotoxins.