3 misconceptions about mycotoxins in ruminants

Although the existence of mycotoxins is recognized, still many misconceptions circulate in the industry leading to wrong conclusions. In this article, we focus on three misconceptions related to mycotoxins in dairy cattle.

Misconception #1

“Silage free of visible mold is free of mycotoxins”

Dairy farmers often look at the silage to evaluate whether they are exposed to a mycotoxin risk. However, there is little relationship between mold and mycotoxins in silage.

Fungi producing mycotoxins can be classified in two categories, field fungi and storage fungi (Table 1). Field fungi are molds infecting and growing on crops in the field. These fungi mainly belong the class of the Fusarium species and produce zearalenone (ZEA), deoxynivalenol (DON), T-2 toxin (T2) and fumonisin (FUM). These toxins are stable so concentrations of Fusarium toxins present in silage reflect the contamination levels at the time of harvesting. This is the main class of toxins found in silage, so even though no visible fungi are present, the silage can be heavily contaminated.

CategoryFungal speciesMycotoxin
Aspergillus flavus
Aspergillus parasiticus
Aflatoxin B1, B2, G1 and G2
Storage fungiAspergillus ochraceus
Penicillium verrucosum
Ochratoxin A
Penicillium roquefortiMycophenolic acid
Rocquefortine C
Fusarium graminearum Fusarium culmorumDeoxynivalenol
Field fungiFusarium graminearum Fusarium culmorum Fusarium sporotrichioidesZearalenone
Fusarium sporotrichioides Fusarium poaeT-2 Toxin
Fusarium verticillioides Fusarium proliferatumFumonisin B1, B2 and B3
Table 1: Overview of the main mycotoxins and fungi occurring in cattle feed

The second group of fungi is called storage fungi. These fungi grow and produce mycotoxins during storage. This type of molds can be recognized as mold pockets in silage and appear in different colors depending on the specific fungal species. A common mold in silage is Penicillium roqueforti (figure 1) because it is acid-tolerant and it can grow at low oxygen concentration. Besides mycophenolic acid this mold produces roquefortines which might cause symptoms such as reproductive disorders, mastitis, lack of appetite and paralysis. Overall, these molds should be avoided as they reduce the nutritional quality of silage.

Figure 1: Penicillium roqueforti in silage has a blue-green to whitish-grey color

Misconception #2

“Little to no mycotoxins are present in silage or TMR”

Recent research by Ghent University in Belgium evaluated 257 samples of whole crop maize for silage at harvest across Flanders over the course of 3 years. For a country with a moderate climate it could have been thought that mycotoxin contamination would be minimal. After testing for 22 mycotoxins, it appeared that 47% of samples contained 5 or more mycotoxins, 99.2% was contaminated with nivalenol (NIV), 85.6% by DON and 49.8% by ZEA (Table 1). Contamination exceeded the EU guidelines in 2.8% and 7.8% for respectively DON and ZEA.

Positive samples %Average concentration (µg/kg DS)Maximum concentration (µg/kg DS)Samples above EU guideline (%)b
Nivalenol (NIV)99.2748.76776.3
Deoxynivalenol (DON)85.2396.45322.42.3
Zearalenone (ZEA)49.8159.72791.67.8
Enniatin B (ENN B)36.3149.51984.9
Fumonisina (FUM)28.6131.86293.5
Roquefortine C (ROQ-C)1.70.430.4
Table 2: Selection of mycotoxins found in harvested silage maize in Flanders in 2016, 2017 and 2018 (University Ghent, Belgium)
a Fumonisin = sum of Fumonisin B1, Fumonisin B2 and Fumonisin B3
b ‘-‘ means that there is no EU guideline for this mycotoxin, EU guideline is 2000 ppb for DON and 500 ppb for ZEA

Looking at silage level, the mycotoxin pattern was similar to the occurrence in the harvested maize. Typical storage mycotoxins such as aflatoxin and ochratoxin A were not found in Belgian silages, but Roquefortine C, another storage mycotoxin, was present in 6.8% of samples with an average concentration of 24.4 µg/kg DS and a maximum level of 1065 µg/ kg DS. This research clearly indicates that even in moderate climates mycotoxins are present at serious levels imposing challenges for dairy farmers relying on own-grown roughage quality.
Misconception

Misconception #3

Ruminants are not sensitive to mycotoxins

It is often stated that ruminants are not sensitive to mycotoxins as the rumen microbiota is able to neutralize or detoxify toxins. Recent research has shown that the natural detoxification process in the rumen is in many cases not sufficient to protect ruminants against the toxic effects of mycotoxins.

The toxic effects of mycotoxins in ruminants depend on different factors including the natural detoxification rate, rumen pH, microbial activity, type of mycotoxins, lactation stage, the absorption rate in the intestines and the specific toxicity. Table 3 gives a general summary of the risk of DON and ZEA in lactating cows considering that the feed contains a substantial concentration of these mycotoxins and that the rumen transit time is approximately 10 hours.

MycotoxinHealth condition rumen1Rumen detoxification after 10 hours*Cytotoxic effect GIT**Systemic effect***
DONaNormal± 50%MediumMedium
SARA± 5%HighMedium
ZEAbNormal0%LowHigh
SARA0%LowHigh
Table 3: Simulated risk level of DON and ZEA in lactating dairy cows under different rumen health conditions (based PhD research Ghent University, Debevere, 2020)

a Rumen detoxification of DON refers to its degradation into DOM-1.
b ZEA is not detoxified in the rumen but part is metabolized into α- and ß-zearalenol. α-zearalenol is 10 times more estrogenic than the original mycotoxin .
1 A normal rumen pH is assumed to be 6.8, under subacute rumen acidosis (SARA) conditions the rumen pH drops below 5.8

* Calculated based on detoxification rates at 6 and 24 hours after ingestion
** Taking into account rumen detoxification assuming that average transit time of feed in the rumen is 10 hours.
*** Taking into account reported absorption rate in the gastrointestinal tract.

We can conclude that mycotoxins are omnipresent. Trends such as climate change, no tillage and reduction of fungicides will likely increase the mycotoxin load. Dairy cows are not able to fully detoxify mycotoxins. On top of that, symptoms are hard to connect to daily observations and problems, which are non-specific.

It is time to take mycotoxin risks more serious. Nowadays reliable, fast and relatively cheap methods for mycotoxin detection are available. When elevated levels are found in total mixed rations or silage, it is advised to use a toxin binder with proven efficacy. This toxin binder should neutralize toxins in the gastro-intestinal tract before they can cause harm to the animals. For high-producing dairy cattle it is recommended to apply a maintenance dosage of a toxin binder.

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Kevin Vanneste
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