H ay, Molds, a nd Feeding
Problems in Monta na
All hay contains molds, but not all molds are directly dangerous to livestock.
Molds in hay can affect animals by infecting them, producing
mycotoxins (or changing natural plant compounds to toxic
compounds), producing allergic reactions, and, ultimately,
reducing the palatability and nutritional value of the hay.
Mold problems in hay can result from infection of the forage
before cutting. A common example is ergot, which is caused
by a fungus that infects the floral organs of grasses at or near
the time of flowering. Ergot is typically a problem when cool,
wet springs favor infection by the ergot fungus. The small
black ergot sclerotia (i.e., black, hard bodies varying from the
size of the grass plant’s seed up to ½ inch, depending on the
grass) can be found in the heads of all grasses. When ingested
by animals, it will cause ergotism, which is characterized by
skin necrosis (i.e., death of cells), rough hair coats, necrosis
of the ears or tail, lameness or loss of extremities, poor milk
production, and poor weight gain. When these symptoms are
visible, hay should be examined for the presence of the small
black ergot sclerotia that resemble rodent feces.
Fescue toxicosis, caused by alkaloids produced by fungal
endophytes (i.e., fungi organisms that live between plant
cells) of forage grasses, can also cause problems similar to
ergot poisoning. In Montana, grasses infected with endophytic
fungi, particularly tall fescue, have been identified, but a
case of fescue toxicosis has never been observed. This is
most likely because the problem is typically associated with
pure stands of tall fescue and in Montana, mixed species of
grasses are more common in pastures and hayfields.
Slobber syndrome, another issue related to moldy hay, is
associated with Rhizoctonia infection of red clover and a few
other legume forages. The Rhizoctonia fungus produces the
mycotoxin slaframine which, upon ingestion, causes excessive
salivation. This problem is not common in Montana, but is much
more common in the Midwest and adjoining portions of Canada.
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Once hay is cut, molds, including yeasts, will grow until
moisture is less than 14-15%. The most common fungi
associated with moldy hay are members of the fungal genera
and include Alternaria, Aspergillus, Cladosporium, Fusarium,
Mucor, Penicillium, and Rhizopus. As these fungi grow, they
produce heat and moisture as they break down plant tissues,
which, ultimately, can result in spontaneous combustion
of haystacks under certain conditions. The temperature,
moisture content, and duration of moisture content above
15% will determine the extent of mold growth. Depending on
which molds predominate in a hay source, there may or may
not be mycotoxins present. Analysis of moldy hay samples,
particularly those showing heat damage, will show lower
levels of total digestible nutrients (TDN) and lower levels of
vitamins A, D3, E, K, and thiamine availability, creating an
immediate loss of nutritional value in moldy hay. A second
effect is that mold spores can cause allergic reactions and
symptoms resembling human asthma in both horses and
cattle. Production of mycotoxins by molds that can cause
direct toxicity or suppress the immune system are members of
the genera Alternaria, Aspergillus, Fusarium, and Penicillium.
While Mucor species do not produce mycotoxins, they, along
with some species of Aspergillus, can cause mycotic abortion
when they infect cattle.
Every year, MSU receives samples of moldy hay with
questions regarding feed safety, colic, abortion, poor
performance, or even death. Frequently, these samples have
been submitted to a lab and a mold spore count is reported.
Labs will discuss livestock feeding risks based on mold spore
counts. While this might be useful in addressing potential for
allergic reactions, it is the mold species present that is critical
in determining mycotoxin risk - not the total mold count.
PHOTO BY KELLY GORHAM, MSU
by Barry Jacobsen
Associate Director, Montana Agricultural Experiment Station and Emeritus Professor of Plant Pathology, Montana State University
To determine mycotoxin risk, it is important to determine what
mold species are present. If Aspergillus or Fusarium species are
present, there are laboratory tests that can determine levels of
some common and specific mycotoxins such as aflatoxin, T2,
DON, and Zeralenone. Unfortunately, there are few laboratories
that can do specialized determination of mycotoxins produced
by Alternaria and Penicillium species. It is likely that multiple
mycotoxins from a single species or from multiple species
can be present and their effects can be additive or even
synergistic. Representative samples can be sent to the Schutter
Diagnostic Lab at Montana State University for determination
of fungal species present. If mycotoxigenic fungi are present,
diagnosticians can suggest laboratories where actual mycotoxin
content can be determined.
Determining the risk of mycotoxin poisoning from moldy hay
can be very difficult and often can only be implied following a
veterinary pathologists’ report or observation of the symptoms
expressed on the animal(s) in question. Diagnosis can be further
complicated by the fact that the suspect hay has typically
already been consumed, making sample availability minimal.
PHOTO BY BARRY JACOBSEN, MSU
Symptoms associated with feeding moldy hay include, though
are not limited to, feed refusal, allergy symptoms similar to
asthma (e.g., heaves or recurrent airway obstruction in horses
or interstitial pneumonia in cattle, also known as “Farmers
Lung” in humans), reduced feed intake, poor weight gain, colic
in horses, diarrhea, lower fertility, abortions, lethargy, poor
hair coats, increased water consumption, estrogenic effects
such as unexpectedly swollen nipples and vulva, poor kidney
or liver function, lameness, and vaccine failure, among other
serious problems. It is important to remember that allergies
and health problems associated with mold spores also affect
people, not just livestock. When working around moldy hay,
wear a mask capable of excluding 5 to 10 micron particles.
Determining whether moldy hay is safe to feed depends on
several factors, including:
• Species: Ruminant animals are generally less sensitive
since many mycotoxins are broken down in the rumen.
Horses, which are cecal digesters, are considered to be the
highest risk.
• Reproductive state: Pregnant animals and young animals
are at higher risk.
• Nutritional status: Animals in poor condition are more
likely to be affected.
• The dose: The dose makes the poison. Consider what
proportion of the ration moldy hay comprises and what the
level of mycotoxin present is. If feeding moldy hay, consider
spreading the hay out on the ground so that animals avoid
the moldy portions while consuming good hay.
• Total digestible nutrient (TDN) content of hay: The
higher the TDN, the lower the chance of nutritional
problems associated with moldy hay.
Other livestock problems that can mimic mycotoxin
intoxication are the presence of toxic plants incorporated into
the hay or the presence of toxic seeds. One common example
is the presence of dodder seeds in hay. Dodder is a parasitic
plant that will appear in field as yellowish or golden-colored
string-like vines that produce numerous tiny seeds from midsummer to fall. Ingestion of these seeds will cause colic in
horses. It is always a good idea to survey fields for poisonous
plants or dodder before cutting.
If you have questions about mycotoxins or are concerned
about moldy hay, contact Barry Jacobsen (bjacobsen@
montana.edu), the Schutter Diagnostic Laboratory (http://
diagnostics.montana.edu/) or your local MSU county or
reservation Extension office. 
Where hay or straw has been flooded or kept above 90%
moisture, the Stachybotrys species can grow and produce
mycotoxins in straw or be associated with inhalation of the
black mold spores they produce. This is one of the key fungal
species responsible for “building sickness.” These mycotoxins
affect humans and most warm-blooded animals. Strachybotyrs
symptoms include dermatitis, fever, various chest and upper
airway symptoms, inflammatory disorders of the mouth,
rhinitis, conjunctivitis, and neurological disorders. Generally
symptoms start within two to three days of exposure. Without
new exposure, symptoms may last for three weeks or more.
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