Mycotoxins are secondary fungal metabolites which occur in a wide variety of feed ingredients. Although mycotoxins have been described frequently in various livestock feeds, these toxic metabolites are not typically the top concern for dairy producers, as other more recognizable challenges dominate day-to-day activities on-farm. However, mycotoxins can contribute to poor health and performance, and may be an underlying piece of the puzzle when troubleshooting issues in herds.
Mycotoxicoses (the effects caused by mycotoxins) frequently present with non-specific signs – meaning the outcomes observed in animals could be attributed to a variety of causes, often complicating diagnosis (Figure 1). Therefore, utilization of a differential diagnosis (process to distinguish two or more conditions which share similar signs) is essential to determine the cause of the problem to guide appropriate corrective actions and implementation of preventive measures. This article will cover some of the common challenges in herds which have been associated with mycotoxins but can also be attributed to a variety of other causes.
Mycotoxin basics
More than 400 mycotoxins have been identified, but few are well understood, especially when it comes to their potential negative effects in cattle. Some of the most studied mycotoxins are grouped into six major categories including aflatoxins, trichothecenes such as deoxynivalenol (DON aka “vomitoxin”) and T-2 toxin, fumonisins (FUM), zearalenone (ZEN), ochratoxins and ergot alkaloids. Additional mycotoxins occur but are not routinely screened for, and more research is needed to better understand their occurrence and toxic effects.
Crops are subject to mycotoxin contamination both during growth in the field as well as in storage. Each year’s crop provides its own unique challenges when it comes to mycotoxins, and contamination patterns vary by region as weather conditions heavily influence the production of mycotoxins.
Mycotoxins in dairy cattle
Dairy rations are composed of a wide variety of ingredients which may provide multiple sources of contamination and potentially increase mycotoxin exposure. Combinations of mycotoxins can result in more extreme outcomes than would be expected following exposure to a single toxin. Additionally, other factors related to the animal (age, sex, production stage, health status) and environment (thermal stress, stocking density, hygiene, etc.) influence how animals respond to mycotoxins. While many of these combinations and relationships are not fully understood, it is common to see animal health and performance affected in the field at toxin concentrations below the tolerance levels reported in controlled scientific research.
Ruminants are often thought to be less sensitive to mycotoxins due to natural detoxification in the rumen. However, the extent of protection depends on the type and concentration of mycotoxin present and may be limited by increased passage rate in high-producing cows as well as low rumen pH. Therefore, toxins can persist and cause negative effects in the cow. High concentrations of mycotoxins are generally needed for the expression of classical signs of mycotoxicosis, but low to moderate levels can cause problems less easily attributed to mycotoxins.
Potential challenges in dairy herds
Aflatoxin residues in milk
Aflatoxin residues in milk are a well-recognized issue exclusively attributed to mycotoxins, specifically aflatoxin B1 which is carried over from the feed into milk as aflatoxin M1 (AfM1). Aflatoxins are carcinogenic (potentially cancer-causing), so most countries have strict regulatory limits for allowable levels in milk, as this is a human health concern. In the U.S., AfM1 is restricted to 0.5 parts per billion (ppb) in milk. Feeds destined for lactating dairy cows also have strict regulations, limited to 20 ppb total aflatoxins.
Decreased milk production
Decreased milk production can be linked to a variety of causes related to nutrition, management or disease. When it comes to mycotoxins, lower milk yields may be related to depressed feed intake or feed refusal, altered rumen function due to changes in microbial populations (many mycotoxins have antimicrobial properties), decreased microbial protein synthesis, decreased nutrient absorption in the intestinal tract or impaired metabolism which ultimately leads to reduced availability of the precursors needed for milk synthesis.
Reproductive effects
Reproductive effects arise in many ways and are also attributed to a variety of diseases, environmental factors such as heat stress and are influenced by management practices. Zearalenone is the mycotoxin most frequently associated with reproduction issues, as it disrupts hormonal balances. Reported effects in ruminants include vaginitis, early embryonic death, abortions, infertility and premature mammary gland development in virgin heifers. Ergot alkaloids also disrupt endocrine function and have been associated with poor conception rates and reduced birthweight. It’s important to keep in mind that male reproduction can also be negatively impacted by mycotoxins.
Gastrointestinal effects
Gastrointestinal effects associated with mycotoxins can include altered rumen function, diarrhea, leaky gut and intestinal hemorrhages, among a variety of other challenges on-farm. These issues can also be linked to a variety of potential causes ranging from pathogens such as Escherichia coli and salmonella to nutrient imbalances, among others. The intestinal tract has two key roles: to absorb nutrients and to act as a barrier to prevent harmful substances from entering the bloodstream. Various mycotoxins are reported to disrupt both functions, leading to reduced nutrient uptake and increased passage of toxins and pathogens into circulation.
Elevated somatic cell count (SCC) and mastitis
Elevated SCC and mastitis are frequently caused by intramammary infections, but many factors influence udder health, and numerous reports from the field have tied these issues back to consumption of mycotoxin-contaminated diets. More research is needed, but mycotoxins may affect udder health by disrupting immune function through various mechanisms, including making the cow’s immune response less effective, which can increase the severity and duration of mastitis or other infections.
Altered milk composition and technological properties
Altered milk composition and technological properties can be caused in many ways. Various mycotoxins have been reported to modify milk composition. Depressed butterfat is a common complaint in the field, but there is relatively limited scientific research on the exact mechanism. Mycotoxin-induced fluctuations in feed intake, as well as damage to the gastrointestinal tract which can disrupt nutrient absorption, may contribute to variations in milk composition by limiting or changing the nutrients available for milk synthesis. Altered cheesemaking properties, including negative effects on curd quality and firmness, have also been documented following known mycotoxin exposure.
Other potential issues
Other potential issues have been associated with mycotoxin consumption in dairy cows, including laminitis, increased incidence of metabolic disorders, neurological signs, skin lesions, rough hair coats – and death, when levels are elevated. This list is not all-inclusive, as other complications have been associated with exposure to mycotoxins in cattle, beyond what has been described in this article. Again, a variety of factors could lead to these challenges in addition to mycotoxins, but it is worthwhile to keep mycotoxins on the list of potential causes to investigate when troubleshooting issues on-farm, as they can be linked to a wide range of outcomes.
Mycotoxins are often only considered once other potential causes have been ruled out or addressed, but the problem persists despite corrective actions. Mycotoxins should be considered when working through a differential diagnosis to troubleshoot challenges on-farm. Proactive screening of feeds can help identify potential mycotoxin challenges early and help guide development of a comprehensive mycotoxin risk management strategy.
