by: Stephen B. Blezinger
Ph.D., PAS

Part 1

Over recent years, producers have become increasingly familiar with the term “mycotoxins.” Unfortunately many cattle owners, while they have heard the term and know that consuming enough of these materials can be detrimental to the animal, still do not have clear understanding of what mycotoxins really are, where they come from and the problems they can cause.

Mycotoxins are chemical agents (toxins) produced by a wide variety of naturally existing fungi. The word mycotoxin comes from “myco” which refers to fungi and the metabolites the fungus may produce that are toxic to other organisms. Not all mycotoxins are bad though. Penicillin is derived from a fungus and is toxic to certain bacteria and therefore could be considered a mycotoxin. There are about 100 known types of fungi that grow on standing crops or stored feeds and produce toxic substances. It is believed that there are still others that have not been identified. Approximately 20 of these have been associated with naturally-occurring diseases. Toxigenic (toxin producing) fungi spores are present almost everywhere, and they can germinate, grow, and interject their toxins into a variety of substrates (in this case, plant materials) when moisture, temperature, and aeration conditions are favorable for production and growth. Optimal conditions for toxin production by different fungi may be quite variable.

Cattle may be exposed to mycotoxins in a variety of ways:

• Growing pasture (fungal contamination of grasses)

• Silage

• Hay and straw (including bedding)

• Contaminated grains

• Seeds manufactured with contaminated grain

Economic losses associated with mycotoxicosis may include:

• Reduced milk production

• Poor fertility

• Decrease immune response

• Reduced longevity

• Reduced growth

• Reduced feed efficiency

Most cattle feeding programs generally include both forages and some grain or feed from time to time, which can increase the risk of mycotoxin exposure. It has previously been thought that ruminants were not as affected by mycotoxins (compared to monogastrics – pigs, chickens) due to metabolism (breakdown or deactivation) of these toxins in the rumen. However, although some detoxification may occur, the breakdown process can also result in the production of new and more toxic metabolites, making the problems more severe. Mycotoxin metabolism in the rumen is difficult to predict and is affected by the type of forage and feed fed to the cows. In general, the metabolism of toxins is more efficient at a more neutral rumen pH (forage based diets) as compared to cattle on a high grain diet that results in a lower rumen pH. Cattle affected by any level of acidosis will be more severely affected (e.g. those exposed to high grain diets, or lacking fiber in the diet).

Using a variety of feeds may increase the probability of multiple mycotoxin contamination but decreases the risk of high mycotoxin concentrations because any one feed ingredient is diluted by the presence of other feedstuffs overall. This can be problematic, since the presence of multiple, low levels of various toxins may result in sub-clinical mycotoxicoses which can be much more difficult to identify and diagnose.


The greatest exposure to toxins from molds for cows comes from the forage they consume (grass, hays, silages). Most cattle are maintained on pasture-based production systems around the world – climate permitting. Grazing systems are not safe from mycotoxin contamination - fresh grasses can be contaminated with several mycotoxins. These typically include fungal endophytes that produce mycotoxins, which protect the plant in some way, such as ergovaline and lolitrem B, as well as Fusarium mycotoxins, such as zearalenone or deoxynivalenol (DON).

One of the more commonly recognized mycotoxicosis problem is that seen with Lolitrem infected ryegrass – which causes ‘ryegrass staggers' (loss of co-ordination) in cattle.

Cattle are typically fed preserved forages (as silage or haylage) during times of the year when the grass growth is insufficient (winter, times of drought). Ensiled forages are more likely to harbor molds and associated mycotoxins than dry forages, especially in poorly made silages where the fermentation and anaerobic conditions have not been strictly controlled. Any silage, haylage, baleage or even dried hays showing signs of mold growth should be avoided.

Cheap Feedstuffs

Other by-products (especially those that are moist or poorly kept) may contain fungal contamination and include fruit pulp, wet distiller's grains, brewers grains or brewery wastes. As these materials are often handled in a wet form or in facilities where storage and management of materials is very basic, molds have ample opportunity to grow and mycotoxins are often produced during storage and transport. In some regions, the use of these less expensive by-products has been limited for dairy diets, due to the high risk of mycotoxicosis, as well as the potential for contamination of milk. These same ingredients may readily be used in beef cattle programs so care should be taken.

Specific Mycotoxins in Cattle Production

To effectively recognize mycotoxicosis, herds have to be carefully inspected for symptoms. Symptoms are often very general and can greatly vary according to the mycotoxins present, making proper diagnosis difficult. Careful recognition of symptoms and blood analyses in combination with adequate feed analyses, provide the most accurate diagnosis of mycotoxicosis. In many cases, accurate diagnosis will require the help of both a nutritionist and a vet.


Ochratoxins are produced during storage of feedstuffs by different species of fungi and are found in both temperate and tropical regions. Ochratoxin A is the most prevalent of this group of toxins. The primary effect of ochratoxin A is damage to the kidneys. Fortunately, in a correctly-functioning rumen, this toxin is known to be rapidly degraded and can, therefore, be assumed to be a lesser threat to cattle compared with other mycotoxins.

Clinical signs of ochratoxin toxicity include:

• Pulmonary edema (fluid accumulation in air spaces, predominantly in the lungs)

• Increased mortality at high inclusion levels (e.g. 3ppm or .0003%)


Aflatoxins are of greatest concern in warm and humid climatic conditions. Although aflatoxins are not considered to be a major problem in cold or more temperate regions, caution must be exercised in colder climates when using feedstuffs imported from warmer, more humid areas. Aflatoxins are of the most reactive mycotoxins and can damage DNA, leading to cell death or its transformation into a tumor (cancer). Due to the transfer of aflatoxin into the milk (1.7% of dietary aflatoxin B1 is transferred as carcinogenic aflatoxin M1 in milk) aflatoxin is regulated with upper legal dietary limits established for both beef and dairy cattle.

Clinical signs of aflatoxin toxicity are extensive and include:

1. Reduced weight gain

2. Reduced milk production

3. Decreased feed intake

4.Decreased feed conversion efficiency

5. Reduce fertility

6. Mortalities at very high exposure levels gain

7. Liver lesions with congestion and bleeding

8. Fatty acid accumulation in the liver, kidney and heart.


Trichothecenes include T-2 toxin, diaceptoxyscripenol (DAS), deoxynivalenol (DON), HT-2 toxin. Trichothecenes are common ‘field' toxins found on grain. They are produced on crops and enter the feed via contaminated ingredients. They can contaminate ruminant diets via concentrate or forages, although they can be partially metabolized in the rumen. As seen for ochratoxin, their breakdown can be inhibited by acidic rumen conditions (low pH). Research has shown differences in susceptibility between breeds, species and management systems, for example, beef cattle and sheep are more tolerant to DON consumption than dairy cattle.

Clinical signs of trichothecenes toxicity include:

• Reduced feed intake

• Reduced weight gain

• Reduced milk production

• Diarrhea

• Emesis

• Reproductive failure

• Mortality

• Gastro-enteritis and lesions

• Intestinal hemorrhage


Zearalenone often occurs in combination with DON in naturally contaminated cereals or in forages. This toxin mimics the activity of hormones (as an estrogen analog), which causes the majority of the reproductive-related symptoms seen, especially in pregnant animals. Zearalenone is partially metabolized in the rumen to α-zearalenol and, to a lesser extent, to β-zearalenol. These breakdown compounds have shown no toxic effects on rumen bacteria, however, α-zearalenol is about four times more estrogenic than the parent mycotoxin and so this rumen-mediated transformation actually causes greater toxicity. The rate of zearalenone transfer into the milk is low and is currently thought to present no real risk to consumers of dairy products.

Clinical signs of zearalenone toxicity include:

1. Abortions

2. Decreased embryo survival

3. Infertility and mammary gland enlargement of virgin heifers

4. Edema and degradation of the genitalia in pre-pubertal females

5. Vaginitis

6. Vaginal secretions

7. Reduced male trait expression in young bulls

8. Infertility of young bulls


Mycotoxins can cause serious problems when consumed at critical levels. Care needs to be taken not to feed moldy forages or feeds and grains. In the next part of this series we'll continue this discussion to help provide a clearer and more extensive picture of how to manage potential mycotoxin contaminations.

Copyright 2014 – Dr. Stephen B. Blezinger

Steve Blezinger, Ph.D., PAS is a management and nutritional consultant with an office in Sulphur Springs, TX. He can be reached by phone at (903) 352-3475 or by e-mail For more information please visit us on Facebook at Reveille Livestock Concepts.

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