Mycotoxins are metabolites produced by fungi that contaminate forages. What effect do they have on cows?

What are mycotoxins?

Mycotoxins are secondary metabolites produced by fungi. These metabolites contaminate forages in three stages. Mycotoxins in forages can occur in cultivation (under field conditions), during transportation and forage storage. In general, mycotoxins are chemical compounds with pathological effects on humans and animals. These compounds can affect several organs or systems, depending on the amount ingested and the time of consumption of the contaminated feed.

Mycotoxins in forages are presented worldwide. They have an enormous impact on animal production and human health.

Factors favoring mycotoxins in forages

Mycotoxins in forages mainly affect crops such as corn, barley, wheat, and sorghum. Sometimes they can also contaminate silage, pastures, and oilseed crops. For mycotoxins to appear, some favorable conditions are required for the fungus to release these toxic metabolites.

1. Physical factors: humidity and the amount of water determine the appearance of mycotoxins in forages. In general, mycotoxins in forages develop with relative humidity between 80 and 90%.
2. Temperature: for fungi to produce mycotoxins, ideal temperature conditions between 20-25°C are required. However, some mycotoxins in forages can grow between 15-30°C.
3. pH: fungal colonies grow well in an acidic environment between 4 and 8.
4. Oxygen: mycotoxin-producing fungi need oxygen (aerobic medium) to survive.

On the other hand, the toxicity of mycotoxins in forages depends on some conditions that can change according to situations, such as:

1. Mycotoxin: the type of metabolite involved, its bioavailability, and concentration in the forage.
2. Synergism: toxicity in animals increases if more than one mycotoxin is involved.
3. Amount of feed: toxicity depends on how much forage contaminated with toxic metabolites is consumed by the animal.
4. Individual: the weight of the individual, the physiological state, and age are significant factors.

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Types of important mycotoxins

Mycotoxins in forages are produced mainly by fungi of the genera Aspergillus, Penicillium, Fusarium, and Claviceps. Approximately 300 mycotoxins have been discovered, but veterinary medicine has focused on those that cause clinical signs in animals and humans. The most important mycotoxins are aflatoxins, zearalenone, ochratoxins, trichothecenes. Other mycotoxins of smaller impact are fumonisins, patulin, and ergot alkaloids.

Table 1 Types of mycotoxins and the fungus that produce them, with contamination levels.

Aflatoxins 

Aflatoxins take their name from the group of fungus that produce them: Aspergillus. These metabolites are the most studied group of mycotoxins since their discovery, in 1966.

The most significant Aflatoxin in cows is AFB1 since it is metabolized in the animal’s liver. AFB1 is eliminated in milk as AFM1. In goats, was discovered that Silicoglycidol reduces the excretion of aflatoxin M1 in goat milk.

This AFM1 has a severe health impact, by affecting DNA in humans. It is suspected to be carcinogenic and may remain in dairy products. Constant monitoring is carried out in milk industries to measure the quantity of aflatoxins.

Table 2 Types of aflatoxins and the fluorescence that characterizes them.

Thanks to ultraviolet (UV) light, it is possible to identify the presence of aflatoxins in forages. Aflatoxins, under this type of light at 365nm, emit colors (Table 2). This is a quick method to identify them in the forage.

Ochratoxin

It is a forage mycotoxin produced by fungus of the genera Aspergillus and Penicillium. There are five types of Ochratoxins: A, B, C, α (alpha), and β (beta). The most important is Ochratoxin A (OTA).

OTA is produced by the fungus Aspergillus ochraceus and A. nigri. It can cause lesions in the kidneys and liver. In addition, this mycotoxin is carcinogenic and mutagenic, affecting the immune system. These lesions are generated by the effect of oxidative stress that blocks protein synthesis, damages DNA, and alters metabolism.

Trichothecenes

Trichothecenes are forage mycotoxins produced by fungi of the genus Fusarium. Among the most important types of trichothecenes, there are four: T-2 toxin, DAS, DON or vomitoxin, and Nivalenol. The first two are the most toxic to cattle.

Zearalenone

Zearalenone is a forage mycotoxin produced mainly by Fusarium graminearum. This toxic metabolite has potent estrogenic effects that affect cows and other farm animals.

Sporodesmin

Sporodesmin is a mycoxotin produced by Pithomices chartarum. At the macro level, colonies of this fungus can occasionally be observed as black powder or black spots on forage leaves. Commonly affected forages are Ray Grass, white and red clover, Lotus, and rye.

What are the effects of mycotoxins on cows?

The diversity of mycotoxins in forages is considerable, so the number of effects and symptoms is also varied. For this reason, the effects of mycotoxins in cow forage can be divided according to the affected systems.

1. Liver: In acute exposure, mycotoxins in the liver can cause fatty degeneration, hemorrhage, and parenchymal necrosis. Affected cows may have yellow mucous membranes (jaundice), hemolytic anemia, and suffer photosensitization. In chronic exposure, cows may suffer liver fibrosis and cirrhosis.
2. Kidneys: there may be damage to the renal tubules leading to tubular toxic nephrosis.
3. Vascular system: mycotoxins in cow fodder can cause diffuse hemorrhages, hematomas, weakness, anemia, and susceptibility to infections.
4. Tegument: toxic alkaloids produced by the fungus Claviceps purpurea, present in fescue, can produce gangrene in the extremities. Other mycotoxins can produce ulcers and oral necrosis.
5. Reproductive and endocrine system: some mycotoxins have hormonal effects such as hyperestrogenism. They can decrease fertility and libido in males. In cows, it can generate agalactia and premature calving.
6. Immune system: aflatoxins can decrease the response of the immune system, predisposing cows to infections.
7. Teratogenesis: mycotoxins in forages such as aflatoxin and ochratoxin are teratogenic.

Prevention of mycotoxins in forages

Multiple topics can be used for the prevention of mycotoxins in cow fodder.

1. Improve agricultural practices: to prevent contamination of growing crops, it is recommended to use high-quality, fungus-free seeds at planting. It is also useful to avoid physical damage to crops because fungi grow there.
2. Use of antifungals: Propionic acid has been used to inhibit mycotoxigenic fungi. This organic acid is allowed as a feed additive, and it works by changing the pH of the forage and prevents fungal growth. However, it is most effective if used preventively and as a supplement. Other effective antifungals are those with cimenol ring together with citric acid, as they have a bactericidal and fungicidal effect on the feed. Thanks to this, they prevent food spoilage and thus reduce infectious agents such as fungi, avoiding the appearance of mycotoxins.
3. Correct storage: the storage of fodder is a critical moment for the development of mycotoxins. Silage should be kept in anaerobic and low humidity conditions so that fungi do not proliferate.
4. Physical control: grains contaminated with mold can be selected for discarding, all containers should be kept dry, and each stage of forage production should be carefully managed.
5. Mycotoxin binders and microbiological control: there are several types of mycotoxin binders, such as glucomannans, enzymes and natural silicates. These substances bind to mycotoxins and prevent their toxic effect on the animal’s organism. Some species of bacteria and fungi that compete with and degrade mycotoxins have also been used, but the process is slow and not fully effective. Recently, the yeast Saccharomyces cerevisiae has been used with good results.

CONCLUSIONS

Mycotoxins in forages are toxic metabolites produced by several species of fungi. Some environmental conditions interfere in their occurrence. The most important mycotoxins in cows are aflatoxins, zearalenone, trichothecenes, and ochratoxins. These toxic metabolites in forages have health effects on cows and humans. Prevention measures for mycotoxins focus on controlling conditions that may favor their development.