Poisonous Plants
and Equine
AG/Weed Control/2006-03
10/01/2006
Clark E. Israelsen
Scott S. McKendrick
Clell V. Bagley
Introduction (never a good idea). A surprising number of
ornamental trees and shrubs that are used
Poisonous plants can be found in almost all for landscaping are toxic to horses. Green
plant communities. Like most other species lawn clippings, often contaminated with
of livestock, equine/horses will not usually cuttings from ornamental plants, are very
choose to consume weeds or poisonous tempting to horses, especially those that
plants when quality forages are available. have been on dry or limited feed. Horses
They do have discriminating tastes and can easily ingest toxic poisonous plants
exhibit a preference for quality legumes as they anxiously gulp down green grass
and/or grasses over less desirable poisonous clippings. Horse owners should talk to kind
plants. However, horses deprived of green and helpful neighbors and ask them not to
vegetation when confined to stalls and fed feed any clippings to horses because of the
dry hay, or horses restricted to marginal risks involved.
pastures, may sample a large number of
green plants and accidentally or intentionally It is also important to recognize that
ingest poisonous plants into their system poisonous plants are usually more
when allowed to graze. If large enough concentrated in their toxins during certain
amounts of toxic plants are ingested, serious times of the growing season. Sometimes
health problems and even death can result. poisonous plants ingested in small quantities
are not a problem, while at other times, even
In most cases, horses consume poisonous small quantities become lethal. Occasionally
plants only when their pasture is overgrazed a horse will develop an appetite for certain
and quality forage is not available. This poisonous plants and must be watched
is because most poisonous plants are less closely when grazing at home or on the
palatable than quality forages. It is also trail. Mineral deficiencies in the diet may
possible for poisonous plants to be harvested also cause the horse to seek out poisonous
and introduced to the horse simply as a plants. A well balanced and adequately
matter of consuming its regular feeding available ration will usually alleviate this
of cured hay. Another scenario may arise problem.
when lawn clippings are fed to horses
Prevention Suggestions:
It is important for horse owners to carefully
survey the local plant population within
their area and learn to recognize as many
poisonous plants native to their geographic
region as possible. This fact sheet will
identify common poisonous plants found in
Utah horse pastures, but the list is by no
means complete.
Persistent efforts to eliminate or control
poisonous plants in the home pasture are
essential. If there are only a few plants,
they can be dug out by hand and disposed Cache Valley pasture full of goatsrue
of before they spread to a larger area. If
the infested area is too large, troublesome feeding harvested hays or when riding on
plants can be sprayed with an appropriate the trail. Each county has a local Extension
herbicide. Agent who can help identify plants and
control methods unique to their area. The
Perhaps the best defense against weed USDA Poisonous Plant Research Lab is also
species is cultivating a healthy stand of on the campus of Utah State University
grasses or legumes that can compete with in Logan. Researchers at that lab are a
them. This can be accomplished with proper valuable resource to horse and livestock
fertilization, irrigation, controlled grazing and owners and veterinarians.
timely mowing. Too often, small pastures
become overstocked, becoming holding pens
instead of productive pastures. Mowing
Symptoms:
pastures before weeds and toxic plants head Symptoms of equine poisoning are varied
out and go to seed will significantly minimize and diverse. Some cases may consist of
spreading. Care must also be taken when slight illness and the horse’s inability to
perform to its fullest potential for a few
days. More serious symptoms may include
slobbering, tremors, uncoordination, erratic
behavior, convulsions or even sudden
death. A common symptom, especially for
light-skinned animals, is photosensitization
- which consists of blisters, swelling and
lesions (like severe sunburn) on the white or
light colored areas of the skin. Unfortunately
there are other illness and conditions that
may cause similar symptoms. As such, if
poisoning is suspected call a veterinarian
immediately. Because horses cannot vomit,
other methods must be utilized to help rid
the horse’s digestive system of troubling
plants.
Common Utah Plants Yellow Starthistle and
Poisonous to Equine: Russian Knapweed
These invasive weeds are
Poison Hemlock becoming more prevalent in Utah
A relatively common range or pastures and rangelands. Horses
pasture plant often found along will not eat these plants unless
roadsides, fence lines, creek good quality, palatable feed is
beds and in pastures. This plant gone. These plants are also
kills a number of cattle, sheep toxic when dried in hay. Usually,
and horses each year and is also toxic to horses must eat the plant over an
humans. This plant is not very palatable extended period of time before
and is unlikely to be ingested when other symptoms appear. The most common
forage is available. However, since poison symptom is the horse’s inability to chew or
hemlock is one of the first plants to grow in swallow, although it may attempt to do so.
the spring, horses An infected horse may chew feed only to spit
may eat this plant it out when it cannot swallow. Owners may
in the absence think the horse has something caught in its
of other green throat. The horse may also chew without
feeds. Early in having anything
the spring, before in its mouth
the plant flowers, and move its
the leaves lips and tongue
are especially in a peculiar
poisonous. fashion.
Common Drinking is
symptoms difficult and the
of poisoning horse will often
include nervous lap the water
trembling, like a dog, and
uncoordination and dilated pupils. The dehydration frequently occurs. Since the
horse may be disoriented and appear to be ingested toxins cause damage to the brain,
unaware of his surroundings. Humans are an infected horse may yawn frequently, roll
poisoned when they confuse poison hemlock its tongue, toss its head, hang the head low
roots with wild parsnips. The toxic potential and/or push
of this plant diminishes significantly as the against solid
plant dries. However, the seeds (both green objects. The
and mature) remain toxic. As such, small prognosis
quantities of poison hemlock accidentally for recovery
baled into properly cured hay prior to seed once clinical
production, are not considered a high risk. signs appear
The best way to prevent exposure to poison is usually
hemlock is to make every effort to eliminate poor, usually
it from the field. It is also wise to provide resulting in
adequate quality forage to horses. euthanasia.
Houndstongue Locoweed
This biennial weed, often found Locoweed is a range plant. Some
in pastures and along roadsides, years there is very little growth
forms a rosette the first year and other years there can be
and produces a flowering stalk significant growth. Horses usually
the second year. The leaves avoid Locoweed, but after they
are rough and resemble a dog’s tongue. have sampled it a few times, they can
This plant is toxic, containing pyrrolizidine become accustomed to it. Symptoms do not
alkaloids that cause liver damage. This may appear until horses have grazed this plant
result in jaundice, possible photosensitization for a period of time and the most obvious
(sunburn) and/or death. Houndstongue is signs may not appear until after the horse
not very palatable so horses generally do has stopped eating it. Common symptoms
not eat this plant include aimless wandering, altered gait,
in pastures, unless tremors, weakness and erratic behavior.
there is a limited Horses poisoned with locoweed may also
amount of quality have impaired vision and actually bump into
feed. However, things or try to jump imaginary objects.
houndstongue This cumulative effect of alkaloid poisoning
is invading an may be irreversible, though mares fed good
increasing number of quality hay may regain their reproductive
alfalfa and grass hay function. Some neurological damage
fields and when baled may be permanent and horse owners
into hay, horses are should be warned that recurrent, aberrant
less discriminating
and may ingest
toxic amounts of the
plant. Clinical signs
may not be apparent
for several months after ingestion and the
effects are cumulative, even if the periods of
ingestion are months apart. The syndrome
of liver failure progresses rapidly over a few
days to a week. Clinical signs include weight
loss, weakness, sleepiness, uncoordination, behavior may cause the horse to become
and with a yellowish discoloration to the unpredictable and dangerous, especially
mucous membranes. The horse may have when stressed, loaded onto trailers or
shown no signs, but upon being ridden may forced into working conditions. Horses
seem to tire quickly and the clinical condition are very sensitive to the toxin. Sheep
may deteriorate rapidly to death. Once can be managed and can graze locoweed
affected, there is no effective treatment. intermittently, but horses should be kept
The best prevention is to properly maintain away from ranges which contain it, especially
pastures and hay fields to encourage the during years of abundant growth. Locoweed
production of quality forages and to be sure seeds will survive in the soil for years,
that hay fed contains no houndstongue. waiting for the right growing conditions.
Field Horsetail seasons of the year. Yews contain a cardiac
depressant called taxine. Symptoms include
Field horsetail is a small trembling, uncoordination, collapse, a
herbaceous perennial that prefers slowed heart rate and cardiac failure. These
moist, cool areas such as ditches, symptoms may not appear for several hours
meadows or road sides. Like most of they may occur
poisonous plants, horsetail is most within minutes after
likely to be ingested when horses ingesting the plant.
have very little quality vegetation to eat. Some dead horses
Young horses are more readily affected than have even been found
mature horses. Horsetail contains several with yew leaves or
compounds, but twigs still in their
thiaminase is the mouths. Because this
only one of clinical toxin acts so quickly,
importance. there is seldom time
Thiaminase is to initiate treatment
an enzyme that and thus far, no
breaks down known antidote is
thiamine, a available. Most yew
necessary vitamin poisonings are the result of homeowners
for metabolism carelessly dumping lawn and shrub clippings
and central into pens or pastures with curious horses.
nervous system As stated earlier, horse owners should talk to
function. Horses kind and helpful neighbors and ask them not
need to ingest to feed any clippings to horses because of
field horsetail the risks involved.
for several
days, or even
weeks, before Milkweed
adverse clinical signs of muscle weakness, This herbaceous perennial is
uncoordination, paresis, convulsions and common along roadsides, ditch
potential death appear. Therapeutic doses banks, in pastures and even in
of thiamine, administered under veterinary cultivated fields. Colonies spread
supervision, followed by subsequent vitamin by underground rootstalks that
administration may prove successful are not disturbed during tillage practices.
in overcoming the effects of poisoning. The milky latex sap that exudes when these
Obviously, poisoned horses should be plants are cut or grazed is
removed from horsetail infested areas and not palatable to livestock.
provided access to clean, good quality feed The western whorled
and water. milkweed and labriform
milkweed are the most
Yews toxic of the milkweed
Yews are common small species. This plant is
ornamental evergreen shrubs easily controlled with
or trees that are toxic during all proper management.
Deathcamas stressed, making it highly toxic to horses.
Symptoms include nervousness, convulsions,
This native perennial is one of respiratory failure and sudden death. Cured
the earliest plants to emerge in hay containing arrowgrass has also been
the spring and grows on foothills reported toxic to young stock. Supportive
and in meadows. As such, most medical treatment may prove valuable, but
livestock poisonings come from death is common.
early spring grazing when other plants are
still dormant. The Water Hemlock
underground scaly bulbs
are often mistaken for This plant frequently grows in
wild onions and can moist areas such as wet meadows
cause severe illness and pastures, or on the banks of
in humans as well. ponds and streams. It resembles
All parts of this plant the taller poison hemlock plant,
contain a poisonous though the leaves are different, and also
alkaloid at all stages has hollow stems with occasional purple
of growth. Pastures streaking near the lower portion of the plant.
containing deathcamas Water hemlock can be easily identified by the
should not be grazed cross-sectional chambers in tuberous roots.
in early spring and The knife used for cutting must be cleaned
should be sprayed with thoroughly or it can carry the toxicity. All
herbicides early in the parts of the water
season when other hemlock plant
plants are still dormant. contain a toxin
After other forages become available, horses called cicutoxin.
will usually not consume this toxic weed. Young leaves,
Symptoms of poisoning include excessive tubers and green
salivation, rapid breathing, weakness, seed are especially
staggering and convulsions. Severely toxic. Some
poisoned animals usually die, while those consider this to be
less affected may recover. one of the most
poisonous plants in
Arrowgrass the U.S. Poisoning
most often occurs
Arrowgrass
in the spring when
is a native
young, vegetative
perennial that
growth is eaten
starts growth
before most
in early
other forages
spring and
are available. Cicutoxin acts quickly as a
reproduces from seed
direct stimulant to the nervous system.
and rhizomes. This plant
Clinical signs such as muscle twitching,
contains hydrocyanic
teeth grinding, convulsive seizures and
(prussic) acid, especially
excessive salivation may appear as early as
when drought or frost
15 minutes after ingestion. Death, resulting
from respiratory distress, may occur as early
as 45 minutes after ingestion. The best
Goatsrue
way to prevent water hemlock poisoning This tap rooted, perennial legume
is to consistently destroy the plants and to was intentionally introduced
provide adequate quality forage early in the from the Middle East as potential
spring. livestock forage, but was found to
be unpalatable and highly toxic.
Sorghum and Sudangrass Unfortunately, it was allowed to escape and
has spread primarily by irrigation water to
These productive grasses pastures, fence lines, roadways and marshy
are excellent livestock feeds areas. Most goatsrue plants can be found
when grown, harvested and in Cache County, Utah, where an intense
cured correctly. Under certain eradication program is underway. Unless
conditions however, especially horses are near starving, they will not eat
when stressed with drought or frost, these this plant.
plants can accumulate cyanogenic glycosides
and nitrates. Nitrates can create problems
for ruminants, but the cyanogenic glycosides
can affect horses. Typical signs include
loss of nerve function to the hind legs and
bladder. It is most apparent as horses
demonstrate an inability or reluctance to
back up. If the horse loses the ability to
urinate normally, there will be constant urine
dribbling, urine scalding of the hind legs and
potential bladder infection. There are no
adequate treatments available after clinical
signs appear. It is safest to avoid pasturing
horses on Sorghum or Sudangrass.
Additional Poisonous Plant Resources:
“Small Pasture Management Guide for Utah,”
Utah State University Extension AG 508.
“Weeds of the West,” Western Society of
Weed Science, P.O. Box 963, Newark, CA.
The USDA ARS Poisonous Plant Research
Laboratory, UMC 6300, Logan, UT 84322.
http://www.ars.usda.gov/main/site_main.
htm?modecode=54-28-20-00
“Plants Poisonous to Livestock and Horses,”
Oregon State University, http://extension.
oregonstate.edu/linn/content1/poisonplants.
php.
http://www.horsequest.info/
Design by: Peak Media, Logan, Utah
Photos courtesy of: Peak Media, Cache County Weed
Department, Richard Old, William & Wilma Follette @ USDA-
NRCS PLANTS Database / USDA NRCS, Dr. Charles Hart and
Bruce Barnes.
Utah State University is committed to providing an environment free from harassment and other forms
of illegal discrimination based on race, color, religion, sex, national origin, age (40 and older), disability,
and veteran’s status. USU’s policy also prohibits discrimination on the basis of sexual orientation in
employment and academic related practices and decisions.
Utah State University employees and students cannot, because of race, color, religion, sex, national origin,
age, disability, or veteran’s status, refuse to hire; discharge; promote; demote; terminate; discriminate
in compensation; or discriminate regarding terms, privileges, or conditions of employment, against any
person otherwise qualified. Employees and students also cannot discriminate in the classroom, residence
halls, or in on/off campus, USU-sponsored events and activities.
This publication is issued in furtherance of Cooperative Extension work. Acts of May 8 and June 30, 1914,
in cooperation with the U.S. Department of Agriculture, Noelle Cockett, Vice President for Extension and
Agriculture, Utah State University.
