is more likely to occur in early spring before grasses become available, or in late summer when grasses have been eaten and weeds remain. Here is a
list of some of the more prevalent poisonous plants that we see around the area.
species grow in Colorado and contain alkaloids that cause liver damage
. The plant is biennial and has a
rosette form (leaves radiate out from central area) the first year, and bloom from May to late summer in year two.
Second Year Plant
Irregular Leaf Tip Margins
The rosette displays dentate or irregular leaf tip margins, and is easy to pull up, with a purple hued single tap root. The second year plant flowers
are yellow appearing like pencil erasers early, then with rays like a daisy. The defining character is the presence of parallel bracts, the green outer
supports of each flower head, that don’t overlap.
Horses will eat all parts of the plant in early spring, especially if it is located near more palatable plants, and the toxic effects are cumulative.
The liver inflammation can be irreversible with scarring and may be fatal. See also the case, The Lessons of Dusty
, for more details.
Like Senecio spp
, Hound’s Tongue contains Pyrollizidine Alkaloids that are cumulative in effect and cause severe liver inflammation and fibrosis
(scarring) and eventual irreversible liver failure after several months. The plant is abundant in many areas.
Biennial, forming a rosette the first year of based leaves up to 18 inches long, densely hairy and tongue shaped. In the second year a 2-4 foot erect
flowering stem is produced. Flowers are reddish purple in color and are produced from the terminal leaf axils. The fruits are pyramidal, separating into
4 parts which are hooked and adhere to animal hair and clothing.
First Year Rosette
Flowers and Seeds (2nd Year)
Water Hemlock is the most lethal wildflower in Colorado, to horses, livestock, and people. It has a hollow stem, white flowers, and a
tuberous root, growing up to four feet high, and looks like the wild carrot. It grows along stream beds in wet, marshy conditions. Horses may die
within two hours of eating Water Hemlock
Call your local weed specialist (Frank Ratliff, Archuleta County Weed and Pest Control, 264-6773) for current info on weed sprays and cautions.
Locoweeds are poisonous at all times, even when dried. Some horses become very depressed and sleepy after eating significant amounts of Locoweed, but the
plant is most often associated with sudden changes in temperament, aggressiveness, wobbliness, falling over unexpectedly, and violent reaction to
routine management practices such as putting a halter on, or picking up a foot. Weight loss and reproductive failure are also likely. Locoweed prefers
open prairie, and foothills, often in well drained soils of decomposing granite. The toxin, Swainsonine, inhibits the enzymes which are essential for
normal carbohydrate and glycoprotein metabolism in cells. As a result, these carbohydrates accumulate in the cells of the brain and most other organs
with the result that normal cell function is impaired. Depending on the duration of locoweed consumption, the affected cells can be permanently damaged.
Swainsonine is secreted in the milk of lactating animals and will therefore affect the young animal suckling its mother.
: Perennial herbaceous plants with long tap root. Leaves are grouped basally 8-12 inches long, odd-pinnate compound and covered with
silvery hairs. The flowers are borne on a leafless stalk in a raceme. The flower of white locoweed is white, pea-like, with purple-tipped, pointed keel.
Purple locoweed flowers immediately after white locoweed and has fewer leaves and fewer flower heads. There can be a mix of the two. The seed pods are
erect, stalkless, with a short beak that splits open to release numerous smooth brown seeds. Seeds may remain viable in the soil for 50 years or more.
Most poisoning occurs when horsetail is incorporated into hay fed to livestock. Signs include central nervous system depression, weakness and
incoordination of the hind limbs
, paralysis from muscle degeneration, and difficulty in seeing from a toxic enzyme thiaminase which depletes an
essential B vitamin. Animals continue to eat relatively well.
: Moist, sandy soils in fields, along roadsides and banks of rivers, dams, etc.
generally preferring cooler, moist climates.
: Horses suspected of horsetail poisoning should be immediately taken off the hay or pasture containing the Equisetum
and fed a nutritious
diet. Treatment with thiamine hydrochloride (1-2 mg/kg subcutaneously for several days) is beneficial in restoring thiamine levels to normal. Feeding
grain as part of the diet affords a protective effect against the thiaminase.
Lupine causes restriction of fetal movements and eventual birth defects
so all pregnant animals should be prevented from eating lupine. The plant has
palmate leaves with 6-9 narrow leaflets. The flowers are white to blue/purple, pea-like, produced at the end of branches. Seeds are produced in pea-like
Milkweeds grow in the dry soils of roadsides, waste area,
and meadows and contain various toxic cardiac glycosides that have effects on the heart and
resinoids that have direct effects on the respiratory, digestive and nervous systems causing breathing difficulties, colic and diarrhea, muscle tremors,
seizures and head pressing. Milkweeds are most toxic during rapid growth, and retain their toxicity when dried in hay.
: Whorled milkweeds are erect perennial herbs which have narrow linear leaves arranged in whorls and contain a milky sap or
latex. The flowers are produced in terminal or axillary umbels consisting of two, 5-parted whorls of petals, the inner one being modified into a
characteristic horn-like projection. The color of the flowers varies amongst species from greenish-white to yellow-red. The characteristic follicle or
pod contains many seeds each with a tuft of silky white hairs that aids in its wind born dispersion.
Animals grazing the green vetch develop a severe granulomatous disease affecting many organs including the skin. Initially animals have welts on
the skin, with hair loss, thickening of the skin, itching and rubbing of the affected areas, and peeling of the skin around the nose and eyes.
The seeds of hairy vetch when eaten in quantity by cattle and horses cause nervous signs and sudden death. The seeds may contain cyanide.
: An annual with stems 4-6 feet in length, with hairy stems and leaves. The leaves have 10-20 leaflets up to 1 inch in length which
are narrow and lance-shaped, 20-60 per spike, all on one side of the flower stalk. Tendrils at the end of the leaves are well developed.
Flowers are purple to red. Pods are about 1 inch in length containing several hard seeds.
: Lymphocytosis and hyperproteinemia are common features of hairy vetch poisoning. Microscopically, the skin, heart, liver and other organs
have cellular infiltrates of lymphocytes, monocytes, and multinucleated giant cells typical of an immune-mediated granulomatous reaction. Mortality
is usually high.
Seeds are present in some pasture mixes. Clover likes moist areas in fields, pastures, roadsides and waste places. A fungus growing in the clover may
play a part in causing disease in the liver, jaundice, and weight loss and photosensitization. The white areas on the legs and face become raised,
reddened, painful and eventually dry and slough-off .
The plant grows 1-2 feet in height and leaves are alternate, trifoliate, and hairless; leaflets are broadly elliptical with toothed
margins. Alsike clover flowers are usually pink and white in color but can be darker depending on growing conditions. The flower stem originates from the
same point off the main stalk as separate leaflet stems in contrast to white clover where the flower stem develops at the end of a low stalk and purple clover
where the flower is just above leaflets on the same stem.
Remove all clover from the horse's diet, keep the horse indoors out of the sun. Do not use pasture mixes containing alsike clover seed.
Photo courtesy BCMAL
Yellow Sweet Clover
A common weed in many pastures that was seeded by helicopter in this area to stabilize bare slopes after road construction. The plant is a legume
with high protein levels similar to other clovers and alfalfa and is very palatable. Certain molds convert a chemical in the plant to dicoumarin
(like rat poison) that causes hemorrhaging due to failure of the clotting mechanisms in blood. Hay containing moldy yellow sweet clover is also
Both the green and dried plant is toxic. Hay contaminated with the plant is often the source of the problem. Horses develop diarrhea and lameness with
signs ranging from stiffness and swollen lower legs to laminitis and severe lameness. Pregnant mares may abort. Affected horses should be
taken off of the hay or pasture that contains the hoary alyssum and treated with oral and intravenous fluids if diarrhea and shock have developed.
Recovery is usually uneventful if the horse is treated early in the course of poisoning.
: An erect branching annual growing 2-3 feet in height. It has alternate, narrow, lanceolate leaves that are densely covered with
white hairs that give the leaves and stems a gray-green appearance. The flowers are produced at the ends of the branches, and are white with 4
deeply divided petals. The seed pods are basically round and slightly flattened containing 6 brown seeds.
Photo by Leo Michels, Source
Often grows near larkspur which is mostly toxic to cattle. Can cause muscle weakness, staggering gait, and eventual inability to stand due to muscle paralysis. In cattle, death due to bloat, respiratory paralysis, or cardiac arrhythmias. Monkshood can be differentiated from larkspur if the flowers are not present by the fact that the stems of monkshood are not hollow like those of larkspur. The flowers are usually deep blue-purple, but occasionally white or yellow, and are produced on simple racemes.
Pictures in the above section unless noted were taken from an excellent Guide to Poisonous Plants by Dr. Tony Knight
- Clovers eaten in excess at one time—several hours on pasture with clovers
- Alfalfa—eaten in several pound amounts when a horse is not accustomed to it.
- Mold on hays, especially hay containing alfalfa or clover
- Cool season grass pastures (Brome, orchard grass) in late spring and early summer, in the afternoon.
- In the fall after a hard freeze, any grass pasture at this altitude. Rye grass, crested wheat, bluegrasses and meadow brome are sweeter
in the afternoon unless temps are over 80 degrees.
- High risk weeds. The following have high sugar/starch/fructans content, often more than grass, and are therefore sweeter and palatable:
Red stem Filaree
Yellow Sweet Clover
Horses prone to founder should not be grazed in the afternoon during spring and summer, and not at all after freezing nights in the fall. Remember that horses will eat more weeds after grasses are low in the fall. But highly palatable clovers, dandelion, filaree, and medic are popular with horses all season long.
Pictures of Red Clover, White Clover and Yellow Sweet Clover: Photo © Al Schneider, www.swcoloradowildflowers.com
Pictures of Black Medic, Mallow, Dandelion and Red stem Filaree are from Judy Sedhook’s collection at www.coopext.colostate.edu