Danger in the Pasture
Many horse owners don't give a great deal of thought about
what is growing in their horse's pasture, but multiple problems
can arise due to poor pasture management. One of these problems
is plant poisoning. There are several species of plants that are
quite toxic to horses, and to monitor the horse's pasture
regularly in order to prevent their presence is very important.
Most plants that are toxic to horses are not very palatable.
This should not, however, excuse the need to check the horse's
pasture often and rid it of these plants. In many cases, despite
having a taste that is unappealing to horses, these plants are
ingested by them and can have very serious effects on the horse.
Such situations as overcrowding of a pasture or scarce forage in
the pasture can lead the horse to eat anything green that can be
found. Other environmental changes such as moving the horse to a
different pasture or introducing new members to a herd can cause
changes in eating habits, which may lead to ingestion of toxic
Symptoms of Plant Poisoning:
Poisoning by a toxic plant can produce a number of different
symptoms depending on the plant eaten, the amount ingested, the
period of time over which it has been eaten, and even varies from
individual horse to individual horse. It is often very difficult
to diagnose poisoning in a horse primarily because many of the
symptoms can mimic those of other conditions.
Plant poisoning is often extremely serious and even fatal. For
this reason it is important to contact a veterinarian as soon as a
horse begins exhibiting any abnormal behavior thought to be
caused by the ingestion of a toxic plant.
Some symptoms include, but are not limited to, lack of control
of body functions due to the attack by the toxin on the nervous
system, disorientation, unprovoked frenzy, tremors, muscle spasms,
and difficulty swallowing.
In addition, the horse may show signs of colic (abdominal pain),
respiratory distress, excessive salivation, or may even suddenly
collapse without warning. These symptoms are general, and,
again, they vary from plant to plant and horse to horse. Below
is a list of common toxic plants that a horse may get a hold of,
and it is followed by a detailed description of a few.
A List of Some Poisonous Plants:
Wild Cherry Tree
Red Maple Tree
Yellow Star Thistle
Black Walnut Tree
Nerium oleander is a shrub commonly grown for ornamental
purposes in California and the Southwest. It is uncommon that a
horse will eat this shrub due to its inpalatability, but
poisoning cases by this plant have been reported when clippings
from these shrubs have been put in a place accessible to the
It does not take a large amount of this plant to produce
toxic effects on the horse. As few as thirty leaves have been
known to be fatal if eaten. Poisoning by the oleander usually
causes death within twelve to twenty four hours after ingestion.
Symptoms of oleander poisoning include abdominal pain,
profuse sweating, bloody diarrhea, abnormal heartbeat, and
labored breathing. If poisoning by this plant is suspected, the
veterinarian may try administering atropine, which is a drug used
to relax smooth muscles in organs such as the stomach and
intestines and increase the heart rate. Further treatment is
symptomatic, which means that the veterinarian will treat the
symptoms as they arise.
Locoweed is a member of the legume family of plants. There
are several types of locoweed that are toxic to horses. It is
thought that it is an alkaloid present in these plants that
causes the poisoning. An alkaloid is a bitter tasting organic
base. Selenium is also believed to be a toxic substance in
Symptoms of locoweed poisoning include excessive salivation,
aimless wandering, excitability, irritability, and disturbed
vision. Horses may also experience chronic weight loss,
weakness, and convulsions. These symptoms appear after a
significant amount of the toxin has accumulated in the horse's
Treatment of this poisoning involves the administration of
sedatives and laxatives. It is also advisable to keep the horse
quiet during this period. The most affective treatment is to
remove the horse from the source of the toxicity as early in the
course of the disease as possible.
Tobacco plants contain nicotine which is toxic to horses.
Nicotine poisoning in horses is usually as a result of the
ingestion of wild tobacco plants as opposed to domesticated
species grown as crops. Nicotine affects the nervous system of
the horse, which results in symptoms such as tremors,
incoordination, and finally paralysis. Pulse and respiratory
rates become dramatically weaker and slower. Death usually
occurs within about three hours of ingestion due to respiratory
A successful treatment for this type of poisoning has not yet
been discovered. It has, however, been found that horses with
mild cases of poisoning tend to recover well.
Yellow Star Thistle Poisoning
The yellow star thistle is a common plant in the western
United States, particularly in northern California. This plant
usually becomes a problem when pastures dry out in the late
summer and early fall leaving the thistle as one of the few
available forages left. The aspect of the yellow star thistle
that makes it a particular problem is that, although it is fairly
unpalatable, some horse tend to develop a liking for it, and many
even begin to seek it out when looking for food.
The poisoning caused by this plant is specifically known as
chewing disease. The reason it is referred to as chewing disease
is because the toxins in the plant damage the part of the brain
that is involved in the control of picking up and chewing feed.
As a result the horse loses control of the facial and lip
muscles. The lips and jaw will take on a rigid appearance, and
grasping and chewing feed becomes impossible. It usually takes
one to three months of eating the plant before symptoms begin to
appear. If left alone, the horse will eventually starve to
The horse can be assisted in eating and drinking for a while,
but the effects tend to be irreversible. Maintaining a horse in
this condition is difficult and hard on the horse. The best
solution to this poisoning is euthanasia in order to prevent a
long and painful death by starvation.
Sorghum grasses such as milo, sundangrass, and Johnsongrass
are common in much of the eastern United States as well as the
southwest. Some sorghum grasses contain toxic levels of the
chemical, hydrocyanic acid. Hydrocyanic acid is the precursor to
cyanide. New growth grasses contain the highest levels of this
chemical, and Johnsongrass is the most toxic of the three
mentioned above. Toxicity of these grasses declines as they
Symptoms of cyanide poisoning can be acute or chronic. An
acute poisoning (a sudden and severe condition) will result in
flaring nostrils due to sudden respiratory distress, loss of
control of urination and defecation, staggering gate, collapse,
and respiratory arrest. Death due to acute sorghum poisoning can
occur in minutes after ingestion.
Equine sorghum-cystitis ataxia is the condition associated
with the chronic ingestion of low levels of this toxin over a
long period of time. Symptoms of this condition include
incoordination of the hind limbs and paralysis of the bladder.
It is possible to treat the acute condition if it is caught
in time. Treatment involves administering antidotes containing
sodium thiosulfate and sodium nitrate. The chronic form may be
improved by removing the toxin from the diet, but complete
recovery does not occur. Many horses affected will eventually
fall victim to kidney failure.
The bracken fern becomes a problem in the fall when it is one
of the only plants in the pasture that is still green. During
other times of the year horses will normally avoid the plant.
Poisoning is due to an accumulation of the toxin over thirty to
sixty days. Only after the toxin has accumulated for this amount
of time do horses begin to show symptoms. The fern appears to
cause a deficiency of thiamin in the horse. Thiamin is a
B-complex vitamin which is important in energy metabolism.
Therefor it is believed that the toxic substance in this plant is
a thiaminase, which is an enzyme that denatures or inactivates
the vitamin, thiamin.
Symptoms of bracken poisoning are due to the deficiency of
thiamin and include emaciation (severe weight loss), loss of
coordination, muscle tremors, rapid heart rate, and eventual
paralysis. Fortunately this condition can effectively be treated
with infection of thiamin over a period of seven to fourteen days
to replenish that which was lost, and it is possible for the
horse to completely recover. However, if the condition is left
untreated, it will usually result in death.
Sagebrush is actually a category of plants that includes as
many as 200 species. It is found primarily in the western United
States. Sagebrush contains the toxin, monoterpeniod oil, which
is most potent in the fall and winter while are other forages are
The symptoms of sagebrush poisoning are somewhat similar to
locoweed poisoning. They include incoordination in the front
limbs, excitability, endless circling, and overreaction to
normal situations. The effects of this poisoning are not
normally permanent, and complete recovery usually occurs after
about two to four weeks of being on a nutritious diet.
How Can We Prevent Poisoning by Plants in the Pasture?
There are several preventative measures that can be taken to
reduce the chance that your horse does not fall victim to these
types of poisonings. Most involve sound pasture management. If
a horse is to be kept at pasture, and pasture is intended to be
the primary source of roughage (feeds that contain mostly bulky,
course plants with high fiber and low digestible nutrient
content) for the horse it is important to keep several things in
mind. As a general rule there should be at least one acre of
good pasture provided for each horse. This will minimize
overgrazing, which in turn will detour horses from eating
harmful, unpalatable plants.
If adequate pasture is not available, the horse's diet must
be supplemented with hay or other forms of roughage such as
cubes. However, it must be noted that roughages such as hays
have a higher nutrient concentration, because they lack the water
content that fresh pasture contains. This means that horses can
get by with eating a lower volume of hay than pasture. Horses by
nature graze as much as 70% of the time. If they consume all the
hay that has been provided, they will instinctively search for
other sources of food. When the amount of quality pasture is
low, and toxic plants are present, it is very likely that horses
will start to graze on these out of boredom. For this reason it
is extremely important that the pasture is monitored for toxic
plant species when horses are on a more concentrated diet.
Horse owners need to be able to recognize the signs of boredom.
In the case that plants that are potentially toxic are found
in the pasture, measures must be taken to get rid of them. These
plants can be removed by burning them, digging them up or
applying weed killer. The latter must be used with caution to
prevent any poison to the horse by the herbicide.
In addition, do not turn hungry horses out into strange
pasture. It is likely that they will grab the first thing that
they see. It is important to be sure that lawn clippings that
might contain plant cuttings are not dumped into the pasture by
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Encyclopedia for Horsemen. Ed. M. M. Vale D.V.M. Equine
Research Inc., Grand Prairie, Texas. 1977
Giffin, James M. and Tom Gore. Horse Owner's Veterinary
Handbook. Howell Book House, New York, New York. 1989
Hayes, Captain M. Horace. Veterinary Notes for Horse Owners.
Prentic Hall Press, New York, New York. 1987
Moore, Jack. Poisonous Plants: A Survival Guide. Equus, volume
212. Ed. Ami Shinitzky. June 1995