What is mold?
Mold is a term used to describe various fungi that grow in animal feed and degrade the quality of that feed. Mold growth is estimated to reduce feed value by at least 10%. Mold germinates at temperatures as low as 45°F, with most rapid growth at 75° to 90°F. They require humidity for germination, therefore, an upper limit of 14.5% moisture is recommended for stored corn or milo. Other grains should maintain moisture levels less than 13%.
What are some signs of mold in feed?

• Dustiness
• Caking of feed
• Poor feed flow out of bins
• Feed refusal by animals for no apparent reason
• Moldy, mildewy smell of feed grains
• Darkening of feed and grain

What factors lead to mold growth and mycotoxin production in field crops?
Stress, such as water stress, high temperature stress, insect damage, and subsequent reduced vigor predispose crop plants to infestation, colonization and contamination by molds and mycotoxins.
What effects do mold and mycotoxins have on feed value and performance?

• Destruction of nutrients
• Reduced palatability

What are Mycotoxins?
A mycotoxin is a highly toxic by-product of mold growth in feed and grain.  Myco means fungus and toxin means poison.  They represent a broad spectrum of acute and chronic diseases in livestock.  Mycotoxins can remain as a residue in meat and milk, posing a possible threat to human health.  Mycotoxins are poisons generated from the secondary metabolic processes which occur naturally in a variety of molds.  The amount and type of mycotoxin or secondary metabolite varies with environmental conditions such as temperature and humidity.  Aspergillus, the mold which produces aflatoxins, grows in warm, dry conditions.  Fusarium, which produces zearalenone and the "T" toxins, grows in cool, wet conditions.
Effects of Mycotoxins
Mycotoxins can have a very wide range of effects on animals depending on quantitative and qualitative aspects of their production in animal feedstuffs. The diseases in animals caused by mycotoxins are referred to as mycotoxicoses..
Mycotoxins act within the animal to:

• Modify nutrient quality, absorption and metabolism
• Alter endocrine and neuroendocrine functions
• Depress the immune response

While obvious clinical mycotoxicoses can result in lesions, abortion and even death, even moderate amounts frequently result in:

• Lower feed consumption
• Lower feed conversion
• Reduced disease resistance
• Increased reproductive problems

All levels of mycotoxins should be considered problematic, since even small doses of those commonly found in feeds can negatively impact animal productivity.
When and/or where is mold and mycotoxin contamination most likely to occur?
Mycotoxins often occur in the field before harvest. Post-harvest contamination can occur if crop drying is delayed and if, during storage, water activity is allowed to exceed critical limits for mold growth.
Common Mycotoxins
Aflatoxin:
Aflatoxin has been the most heavily researched of the mycotoxins. Aflatoxin is produced from certain strains of the molds Aspergillus flavus and Aspergillus parasiticus.  Aflatoxins are most abundant in hot, humid geographic areas.  The effects in animals of ingesting excessive amounts of the toxin range from chronic health and performance problems to death.  Aflatoxins' pricipal target within an animal is the liver.  The damage to the liver results in abnormal blood clotting, development of jaundice, hemorrahaging, and reduction of immune response.
Aflatoxin levels greater than 20 ppb constitute contaminated corn by the FDA. This is the maximum level that grain can contain when fed to dairy cattle. The aflatoxin level acceptable in milk is 0.5 ppb.
According to the FDA, there are set maximum allowable levels of aflatoxin in food and feed.  The regulatory levels for aflatoxin issued by the FDA are as follows:

What can be done to prevent growth of aflatoxin?

• Variety resistant to insects
• Field practices that lessen likelihood of damage to crops
• Maintain post-harvest conditions unfavorable to fungi growth
• Sampling and testing for aflatoxin

Vomitoxin (DON):
Deoxynivalenol (DON) is most commonly produced by the pink mold Fusarium graminearum.  Vomitoxin can cause livestock, especially swine, to reject corn. 
Diets that are highly contaminated will result in severe vomiting by animals. Vomitoxin levels greater than 6.0 ppm are not recommended for cattle. Dietary swine limits for vomitoxin are less than 1.0 ppm.
What conditions promote production of vomitoxin?

• Rain and warm weather from flowering time to harvest promote infection in corn and small grain.
• Low temperatures following infection may increase production.
• Vomitoxin will not develop in storage if grain is free of vomitoxin at harvest.
• Amount of vomitoxin present in grain will not decrease in storage unless grain is stored in a crib.
• Vomitoxin (DON) should be considered a "marker" for spoiled feed;  if it is present, conditions exist for mold growth and production of other unidentified toxins.

Zearalenone:
Zearalenones are a group of estrogenic metabolites produced from Fusarium gramineaum.  Corn is the major crop effected by Zearalenone.  When Zearalenone-contaminated feed or grain is eaten by livestock, it can cause a wide variety of reproductive problems.  Some animal species exhibit constant heat under the influence of Zearalenone, while others show total absence of heat.  The FDA has issued no advisory levels for Zearalenone recommending only that the levels of concern for DON be observed.  Swine are the most susceptible to the effects of Zearlenone.  The swine industry has voiced concern levels for Zearalenone at 250-500 ppb.  It is suggested that Zearalenone not exceed 300 ppb in the total dairy cattle diet.
What conditions promote production of Zearalenone?

• Moderate prevalence of fusarium ear rot in the field before harvest
• Storage of infected ear corn in cribs at higher moisture levels
• Natural occurrence favored by high humidity and low temperatures
• Period of several weeks of low or fluctuating temperatures
• High temperatures promote mold growth
• Low temperatures promote zearalenone

T-2 Toxin:
T-2 toxin is another Fusarium-produced toxin. T-2 toxin is associated with gastroenteritis, intestinal hemorrhages, and death in cattle. The upper limit for T-2 is 500 ppb. Low levels in animal diets have shown decreased weight gains and poor performance.
Ochratoxin:
Ochratoxins are produced by at least seven species of Aspergillus and six species of Penicillium. In monogastric animals, Ochratoxin impairs normal kidney functions. With laying hens, it is about 10 times more toxic than aflatoxin.
Ergot:
Ergot (Claviceps purpurea) is another fungi that attack cereal grains.  The damage of Ergot is two fold.  It decreases the yield of infected crops by replacing healthy kernels and robbing the host plant of needed nutrients.  It also contains toxic alkaloids that can have adverse helath effects on humands and animals.
Slaframine:
Slaframine is a toxic alkaloid which can be the cause of slobbers syndrome. This is associated with black patch disease in red clover. Other symptoms in cattle include bloat, diarrhea, and frequent urination.
Patulin:
Sheep fed toxic levels of feed containing Patulin have shown nasal discharge and loss of appetite, rumination, and body weight. Little is known of Patulin effect on swine, cattle, or poultry.
Citrinin:
Citrinin is primarily a metabolite of Penicillium citrinum.  Citrinin is probably the most widely produced Penicillium toxin.  Citrinin is a kidney toxin, which has been associated with mycotoxicoses in swine, horses, dogs and poultry.
Gossypol:
Gossypol is a natural-occurring pigment in cottonseed, Gossypol toxicity is due to a deficiency of fat-soluble vitamins within a ration.
Symptoms of Gossypol

• Muscular degeneration
• Sterility
• Liver necrosis
• Reduced hemoglobin levels
• Reproductive performance

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References:
Whitlow, L.W., Applied Aspects of Mycotoxins, Dept. of Animal Science, CSU.
Whitlow, L.W., Mycotoxin Contamination of Silages, North Carolina State University, pp. 220-227.
Dairyland Laboratories, Inc., Arcadia, WI.
Kemin Industries, Des Moines, IA.
Christiansen, C.M., C.J. Mirocha, R.A. Meronuck, Molds and Mycotoxins in Feeds, University of Minnesota.
Mycotoxins: Economic Health Risks, Council for Agriculture Science and Technology, November, 1989.
Allenstein, L.C., DVM, Mycotoxins Probably Affected Breeding in this Texas Herd, Hoards Dairyman, August 10, 1995, p. 519.
Hoffman, Patrick, Feed Molds & Mycotoxins, University of Wisconsin-Madison, Department of Dairy Science, 3-89.
Feed Management, Molds & Mycotoxins, July, 1987.
Gossypol Intake May Affect Vitamin Status of Dairy Cattle, Feedstuffs, A.G. Lane, R.l. Stuart, July 9, 1990, p. 13.
Further information can be found at Dairylandlabs.com