Mycotoxins in ruminant productionPart 1: Reassessing potential risks
Considering the prevalence of exposure and improved understanding of rumen susceptibility, it is safe to say mycotoxins have been an under-estimated risk in dairy and beef cattle production.
By Cathy Bandyk, PhD, PAS
Historically, mycotoxin risk management has not been considered a high priority for most beef and dairy producers. Beyond the recognized – and regulated – need to limit transfer of aflatoxins into milk entering the human feed chain, the prevailing assumption has often been that ruminal deactivation protects cattle from significant harmful exposure.
It is true that the rumen microbiota lessen the danger of mycotoxicosis through both degradation and deactiviation of some of these compounds. Additionally, a portion of ingested toxins may bind to feed particles while passing through the rumen (Gallo 2015), limiting damage or absorption in the lower digestive tract. However, recently developed diagnostic tools have clearly shown that cattle across a wide range of geographies and production scenarios must routinely deal with systemic exposure to multiple mycotoxins.
Part 1 of a 4 part series. Subsequent articles in this series will address options for improving mycotoxin hazard assessment and applied mitigation strategies.
As a case in point, a published and validated protocol combining feed and blood sample testing (MycoMarker®, Innovad; Lauwers, et al., 2019) is being utilized to provide direct measurement of animal exposure to 21 different mycotoxins. As part of this patented process, blood samples, consisting of single blood drops on FTA cards, are analyzed for 36 mycotoxin biomarkers. Test results from more than 1,000 cattle and sheep operations have been compiled in a database that highlights the prevalence of mycotoxin challenges across these commercial settings. (Vidal, 2023).
Notably, every blood test identified some level of mycotoxin absorption and risk, with the vast majority of animals being exposed to multiple toxins. In fact, 64% of farms exhibited co-exposure to five or more mycotoxins. It is important to note that more toxins were consistently found in blood samples than feed, supporting the use of biomarkers as more accurate indicators of true exposure (Vidal, 2023).
Reassessing ruminant susceptibility to mycotoxins
In an early review, Whitlow and Hagler (2007) noted that not all ingested mycotoxins undergo rumen degradation. This would be especially true with high levels of feed contamination, in stressed animals, and any time rumen microbial activity is reduced, i.e., low intakes, digestive upsets, major dietary changes, etc. Deoxynivalenol (DON) and nivalenol (NIV), for example, are largely metabolized into less toxic metabolites in healthy ruminants. However, this activity is reduced under conditions of low pH and acidosis (Debevere et al., 2020; Whitlow and Hager, 2007). Protozoa play a large role in ochratoxin (OTA) degradation, suggesting a correlation between defaunation and realized exposure to this toxin.
Some mycotoxins appear resistant to ruminal microbial action regardless of animal status. Fumonisins are not deactivated in the rumen (Fink-Gremmels, 2008), and when realistic levels of six mycotoxins were introduced into an in vitro rumen model (Debevere et al., 2020), enniatin B (ENN B) and roquefortine C (ROQ-C) disappeared only partially, whereas mycophenolic acid (MPA) levels were virtually unchanged over time. And in complete contrast to risk-removal via deactivation, zearalenone (ZEN) is converted in the rumen to a more potent metabolite, α-zearalenol (α-ZEL).
Even when mycotoxins are largely neutralized prior to reaching the lower digestive tract, there are potential concerns. Trichothecene toxins such as DON and T-2 can cause severe irritation and damage to the epithelium prior to being degraded (Whitlow and Hagler, 2007), negatively impacting gut immune defenses and nutrient absorption. Questions have also been raised about the inherent costs of bacterial metabolism of mycotoxins (Jeong et al., 2010). Several reviews have cited changes in ruminal microbiota populations, reduced digestion and protein synthesis, altered fermentation end products, and lowered VFA concentrations when mycotoxins are introduced to the rumen environment (Fink-Gremmels 2008; Gallo et al., 2015; Jiang et al., 2021). In the case of aflatoxin, DNA and RNA expression and enzyme activity were also shown to be altered in vitro. These changes, along with potential activation of inflammatory and immune responses, are typically associated with reduced performance and efficiency – even in the absence of clinical health symptoms.
The diets of beef and dairy cattle also bring unique potential for mycotoxin challenges. Besides the sheer volume of feed consumed, total animal exposure is the summation of toxic contributions from wet feeds, by-products, grain, and/or forage. Focusing on test results from a single feedstuff perceived to be “high risk” can significantly underestimate total mycotoxin intake. Silage, haylage, and high-moisture corn are often suspect, as their moisture level can be conducive to development of mycotoxin-producing molds. By-product feeds can be particularly concerning, as many retain any toxins occurring in the raw feed stock at concentrated levels. DDGS, for example, may contain three times the mycotoxin level as the corn grain it originated from (Wu and Munkvold, 2008). Several well-publicized surveys of grain testing results provide annual insights into trends in mycotoxin development for specific crop years and regions, and collectively confirm the wide prevalence of contamination in these commodities.
Gallo and co-authors (2015) argued in their review that the diversity of ruminant diets is responsible for a greater mycotoxin exposure than that typically encountered by monogastrics, and that the most significant contributor could be forage. While several specific, related conditions have received significant attention (fescue toxicosis, ryegrass staggers, and other ergotisms), there is limited data and research regarding the presence and impact of true mycotoxins in growing or harvested roughages. A survey of pasture grasses in the southeastern U.S. (Gott, et al., 2018), triggered by reports of mycotoxicosis symptoms in grazing cattle, involved standard testing for 15 different mycotoxins in samples from nearly 300 pastures. One or more mycotoxins were detected in 69% of the samples. The authors were able to conclude that multiple mycotoxins can occur in various pasture grasses at levels which may be of concern relative to livestock reproduction, health, and performance. Similarly, standing forage samples collected from Austrian dairies contained multiple mycotoxins and metabolites (Penagos-Tabares et al., 2021). While concentrations of individual fungal toxins and metabolites were generally low, in some tested pastures the total fungal metabolite concentration exceeded 6000 μg/kg DM.
Considering the prevalence of exposure and improved understanding of rumen susceptibility, it is safe to say mycotoxins have been an under-estimated risk in dairy and beef cattle production. They also represent an opportunity to address a potentially-limiting problem. Subsequent articles in this series will address options for improving mycotoxin hazard assessment and applied mitigation strategies.
References available upon request.
Cathy Bandyk, PhD, PAS, is Technical Development Manger with Innovad, a provider of animal nutrition and health solutions.