Mycotoxins in Feed
April 01, 1997
by Teresa Acklin
What feed manufacturers should know about mycotoxin contamination
Mycotoxins are compounds produced by molds that cause toxicity when consumed by animals. Not all molds produce mycotoxins, and molds that produce them may do so only under certain conditions.
In storage, the development of mycotoxins in grain and other feed ingredients can be controlled. Mold growth can be prevented by ensuring moisture and temperature conditions favorable to growth and toxin production do not occur.
The presence of molds does not automatically mean that toxins are present. But mycotoxins can occur naturally in the field, making it very important to monitor for their presence in grains and feed ingredients. Because mycotoxins are unevenly distributed in commodities, it is important to get a sufficiently representative sample and prepare the sample properly to assure reliable test results.
The clinical signs and the degree of toxicity exhibited by animals consuming mycotoxin contaminated feed will depend on the type and amount of mycotoxin in the feed ingredients and the class and species of animal. In general, young animals and breeding animals are more susceptible to the effects of mycotoxins.Aflatoxin
Aflatoxins are the most notorious mycotoxin because of their early discovery and the abundant amount of research information available compared with other mycotoxins. Aflatoxins are produced by the fungi Aspergillus Flavus and A. parasiticus.
These fungi are present in soil and decaying plant material. They cause heating and the decay of stored grain and may invade maize in the field. Crops and feed ingredients most susceptible to fungi and aflatoxin development include maize, peanuts (groundnut), peanut meal (groundnut cake), cottonseed and cottonseed meal.
The optimum temperature for aflatoxin production in storage is between 25° and 32°C. Kernels with a moisture content below 15% are at less risk of mold growth and aflatoxin production because optimum kernel moisture is around 18% and optimum relative humidity in the bin is 85% or higher.
The clinical signs of aflatoxins vary significantly. Short term exposure of aflatoxin for poultry should be no more than 20 parts per billion for young chicks and 50 ppb for laying hens.
A big difference exists in the effects of aflatoxin among different poultry species; ducks, for example, are 15 times more sensitive than mature laying hens, and some strains of laying hens are three times more sensitive than others.
For broilers that undergo continuous genetic improvement, the exact sensitivity to one or more aflatoxins or mycotoxins is hard to determine. Aflatoxin may increase stress susceptibility and compromise growth efficiency; chronic symptoms include liver damage (fatty liver), reduced growth, decreased feed efficiency, kidney damage, anemia, interference with the immune system, greater susceptibility to bruising, and interference with normal protein and fat metabolism.
Aflatoxicosis in swine is primarily a liver disease, although other organ systems may be involved. Decreased growth rate and lower feed efficiency are seen when aflatoxin is consumed in feed at levels between 100 and 400 ppb.
Young animals and breeding stock are the most sensitive to aflatoxin. Liver damage, bleeding disorders and death may occur when aflatoxin levels exceed 400 ppb. At these levels, sows may abort or farrow dead pigs. Beef cattle are somewhat less susceptible to aflatoxin.Deoxynivalenol and T-2 Toxins
Deoxynivalenol (DON), commonly referred to as vomitoxin, is produced by Fusarium species. Fusarium graminearum is a Fusarium species that causes root, stalk and ear rot in maize and sorghum and causes scab in wheat and barley. It produces DON in all of these grains. DON belongs to a class of mycotoxins referred to as trichothecense, which also includes the mycotoxin identified as T-2.
DON usually is not known to increase in stored shelled maize or in small grains that are contaminated in the field because Fusarium growth requires a minimum moisture content of 19% to 25%. T-2 toxins also are produced by Fusarium species and may occur under the same conditions. If moisture does exceed 19% to 25%, DON and T-2 can grow in storage.
General signs of DON and T-2 toxicity in animals include weight loss, decreased feed conversion, feed refusal, vomiting, bloody diarrhea, severe dermatitis, hemorrhaging, decreased egg production, abortion and death.
Swine appear to be most susceptible to DON. Feed containing more than 1 part per million of DON may result in a reduction of feed intake and lower weight gain. Vomiting may occur in some cases. Cattle and poultry appear to be less susceptible to DON.
T-2 toxins in poultry may produce lesions at the edges of the beaks, abnormal feathering, reduced egg production, eggs with thin shells, reduced body weight gain and mortality. Mild enteritis and loose feces may occur in calves consuming feed contaminated with T-2.Zearalenone and Ochratoxin
Zearalenone is typically produced by the fungus Fusarium graminearum, but other Fusarium species also may produce some zearalenone. Maize is the major source of this mycotoxin. Production of Zear-alenone toxin in storage is unlikely unless moisture exceeds 22%
Zearalenone is a compound that mimics the effects of the hormone estrogen. Thus, most of the effects of zearalenone are confined to the reproductive tract of swine. Sows consuming zearalenone contaminated grain will exhibit vulvar reddening and swelling. Vaginal and rectal prolapses are a frequent result of zearalenone consumption in swine.
They also will exhibit frequent irregular estrous cycles and litter size may be drastically decreased. Young boars will undergo a feminizing effect with atrophy of the testes and enlargement of the mammary gland. Zearalenone can cause embryonic death, inhibition of fetal development and decreased numbers of fetuses present in sows.
Ochratoxins are produced by several species of Aspergillus and Penicillium and can occur in cereal grains, dry beans and moldy peanuts. They develop most readily in storage of grains with moisture of greater than 22%, and the only recommended control is to keep grains cool and dry.
Ochratoxin can affect changes in the renal functions of pigs. Poultry symptoms include retarded growth, decreased feed conversion, impaired kidney function and mortality. Ochratoxin also can cause a decrease in egg shell quality and egg production.Fumonisin and Combinations Toxins
Fumonisin is produced by certain strains of Fusarium monliforme and Fusarium proliferatum and typically occurs in maize. These fungi infect maize roots, leaves, stalks and kernels. F. monliforme survives on crop residue in or on the soil surface and, under favorable conditions, can infect maize stalks directly or through wounds caused by hail or insects. The fungus is commonly seed borne, and fumonisins have been reported on visibly healthy grains.
Horses appear to be most susceptible to fumonisin. This mycotoxin, when fed to horses, causes a unique neurotoxic syndrome called leukoencephlomalacia. This disorder causes liquefaction of the horse's brain. Neurotoxic symptoms include lowered feed consumption, lameness, oral and facial paralysis, seizures and eventual death.
The toxin is believed to be a possible carcinogen and may be associated with pulmonary edema in swine. Fumonisin levels of 5 ppm for horses, 10 ppm for swine and 50 ppm for cattle and poultry have been identified as levels of concern.
The toxicity and clinical signs observed in animals when more than one mycotoxin is present in feed are complex and diverse. The effects observed during multiple mycotoxin exposure can be very different from the those observed in animals exposed to a single mycotoxin.
For example, aflatoxin produces fatty livers in poultry, which is a key presumptive diagnostic index of aflatoxicosis. But when ochratoxin is a co-contaminant of feed, ochratoxin prevents the expression of this diagnostic index.
Some mycotoxins, like the combination of aflatoxin and ochratoxin or aflatoxin and T-2, interact to produce synergistic toxicity in broiler chicks. Of the mycotoxin combinations that have been investigated in poultry and swine, the aflatoxin/ochratoxin and aflatoxin/T-2 interactions appear to be the most toxic.
This article is based on information from the U.S. Feed Grains Council as presented in the InfoNews Bulletin, July 1996.Examples of recommended maximum mycotoxin levels in feed
|Young chicks/grow pigs||20||1.0||2.0|
|Breeding cattle, mature poultry||100|||||
|Finishing beef cattle||300|||||
|*adjust downward if feed intake is reduced|
|Source: U.S. Feed Grains Council|