Toxic and Poisonous Weeds

June 11 2015

Horses will usually avoid eating poisonous plants (they don't taste very good) as long as there is an abundant supply of good quality hay or pasture available. However, faced with no pasture or hay, a horse might decide to sample one of the poisonous weeds still left standing in the field.

The best medicine for dealing with poisonous plants is PREVENTION. Ensure that horses on pasture have adequate hay and/or pasture so that they won't have to resort to eating poisonous weeds. Avoid overgrazing, if no supplemental hay is provided.

Learn to recognize poisonous weeds and control them by pulling or by use of commercially registered herbicides. Finally, because you can't control the conditions it is grown in, examine your hay for unwanted plants.

If you are looking for a specific plant, select a letter to display all entries in the index which begin with that letter.

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(Echinochloa crusgalli)

Annual growing to 1.5 metres tall; stems usually flattened in cross-section at the base; smooth leaves often purplish at the base; flowers covered with short, stiff hairs and often a straight or twisted bristle.

Considered one of the world’s worst weeds, it reduces crop yields and causes forage crops to fail by removing up to 80% of the available soil nitrogen. The high levels of nitrates it accumulates can poison livestock.


(Echium vulgare)

Cariboo, Central Kootenay, Columbia-Shuswap, Okanagan-Similkameen,Thompson-Nicola

Biennial to short-lived taprooted perennial growing to 1 metre high; stems covered in stiff hairs with swollen reddish to black bases where attached to stem.

Invades rangelands, pastures, roadsides and idle areas particularly on coarse, sandy to gravelly soils. Blueweed can infest pasture and rangeland, causing potential impact to livestock.

Contains pyrrolizidine alkaloids, which can be toxic to horses and cattle when ingested.


(Cirsium vulgare)

Bulkley-Nechako, Cariboo, Columbia-Shuswap, Fraser-Fort George, Kitimat-Stikine,North Okanagan, Okanagan-Similkameen,Peace River, Thompson-Nicola

Taprooted biennial with spiny winged stems growing 0.3 to 1.5 metres tall; leaves end in long, sharp spines; upper surface with short prickles, undersurface cottony; flowerhead bracts tipped with prickles; purple flowers 4 to 7.5 cm across.

Often confused with Canada thistle (Cirsium arvense) which has creeping roots, much smaller flowers and weak prickles, note spines on the leaves.

Toxicity: None known. However, bull thistle spines can irritate the mouths of grazing livestock and significantly reduce weight gain.


(Arctium spp.)

Bulkley-Nechako, Cariboo, Columbia-Shuswap, Fraser-Fort George, Kitimat-Stikine,North Okanagan, Okanagan-Similkameen,Peace River, Thompson-Nicola

Biennial weeds common in farmyards,fencelines, roadsides, streambanks and idle areas well known for their rounded flower heads with hooked spines that easily attach to clothing and animals.

Common Burdock (Arctium minus) grows 1 to 3 metres in height; lower leaf stalks are hollow; flower heads are less than 2.5 cm across and scattered along the stems.

Great Burdock, sometimes called Giant Burdock (Arctium lappa) is similar in appearance but lower leaf stalks are solid and flower heads are over 2.5 cm across and are arranged in a flat-topped cluster rather than along the stem.

These are not toxic but bear mentioning as the spiny burrs from these plants have been known to cause corneal ulcers in equines. Common burdock is considered toxic due to potential diuretic effects, and there are reports of allergic reactions when the hooked bristles of burs lodge under the surface of the skin.


(Cirsium arvense)

Creeping rooted perennial growing erect to 1.2 metres; stalkless dark green leaves with irregular spiny lobes; flowerheads spineless and small compared to other thistles; flowers variable in colour from rose-purple to pink to white. Only thistle with male and female flowers on separate plants.

Canada thistle reduces forage consumption in pastures and rangeland because horses typically will not graze near infestations. Livestock do not eat thistles and will not graze between thistle plants on more desirable forage.

While not particularly dangerous to horses, this plant can cause nitrate poisoning in cattle.


(Malva neglecta)

Annual to short-lived perennial with prostrate to semi-erect stems, 10 to 60 cm long; longstalked rounded leaves with heart-shaped base and 5 to 7 broad shallow-toothed lobes; white to pale lilac flowers; seeds in round, flat button-like disk of 12 to 15 smooth nutlets.

Has been implicated as the cause of poisoning cases in horses, sheep and cattle. The incidence is rare as the plants are usually unpalatable. It should be considered a risk because it is a common weed of horse pastures. The toxin in this plant is unknown. Young animals seem to be most at risk.

Signs of mallow poisoning:
• Profuse sweating
• Rapid breathing
• Incoordination—'staggers'
• Muscle tremors Symptoms worsen when the animal is forced to move or is otherwise stressed. Most animals recover when rested and when access to mallow is restricted.

Veterinary attention should be sought if mallow poisoning is suspected in a horse. Treatment and supportive care may help with recovery.


(Tanacetum vulgare)

Central Kootenay, Columbia-Shuswap,North Okanagan

Aromatic perennial growing to 1.8 metres tall; deeply divided dark green leaves; yellow "button" flowers in cluster at top of plant; no ray flowers.

Often confused with Tansy Ragwort (Senecio jacobaea) that has ray flowers. It isn’t likely that your horse would eat this as it is very strong. In fact, some old fashioned remedies called for making a tea out of this plant.

However, it can cause abortion, colic, cardiac and respiratory suppression, so make sure your horse doesn’t have access to it.Common tansy is considered undesirable forage and may be toxic to livestock.

It can be toxic to humans if large quantities are consumed.


CURLED DOCK (Curly Dock)
(Rumex crispus)

Perennial in the Buckwheat Family growing 0.8 to 1.5 metres tall; deep penetrating yellow taproot; dark green leaves waved and crisp along the margins; plant turns a rusty-red colour when mature.

A similar plant, Broadleaved Dock (Rumex obtusifolius) is distinquished by its large leaves which are rounded or heart-shaped at the base. Seeds and vegetation are toxic to poultry and can cause dermatitis and gastric problems in cattle. It is an alternate host to many crop diseases.

The leaves contain oxalates. Mildly irritating. In large quantities can be an issue. Unless there’s absolutely nothing else to eat (and in that case, I’d put malnutrition higher on the problem list than plant toxicity), horses are unlilkely to have any issue with the stuff.

Toxin involved: Soluble Oxalates

Potential for Toxicity: Moderate.

Symptoms: Oxalates bind to calcium and magnesium in the blood leading to muscle tremors, weakness, depression and recumbency.

Treatment: Intravenous Ca, Mg, glucose, electrolytes. Oral limewater to decrease further oxalation.


(Ranunculus repens)

Perennial with slender, fibrous roots; dark green leaves, often with white spots, are divided into 3 leaflets and are hairy and deeply obed; hairy stems root at the nodes; bright yellow waxy flowers divided into 5 petals; seeds are smooth with a hooked apical beak.

A common weed of grain, forage crops, and turf that depletes the soil of potassium and other nutrients. Creeping buttercup contains an acrid juice that causes oral and gastrointestinal inflammations in livestock. It often grows by and can dominate streams, swamps, ponds, and forest openings.

Sometimes confused with Tall Buttercup(Ranunculus acris) which has a more upright growth habit and more deeply cut leaf lobes.

Toxicity rating: Low to Moderate

Toxins involved: Ranunculan. The juice of the entire plant contains the toxins.

Animals affected: All animals, primarily cattle and sheep. When eaten, the buttercup irritates the mouth and intestinal tract.

Symptoms: Blisters and ulcers in mouth, in an infested pasture, milk cows can eat enough to taint their milk.

Treatment: Recovery is uneventful when animals removed from source.

Tall Buttercup: The toxin in this plant makes horses (and humans!) lips swell on contact, so it is unlikely (but not impossible) that horses would eat it. If ingested it could contribute to gastrointestinal upset, or colic.


(Linaria dalmatica)

Creeping rooted perennial to 1.2 metres tall; pale green waxy leaves clasp the stem and are heart-shaped with a pointed tip; bright yellow “snapdragon-like” flowers with orange spot on the lower lip (2.5 to 4 cm long).

Likely introduced to North America as an ornamental. Toadflax, which is toxic to animals, competes with native grasses and wildflowers and reduces forage for cattle and wildlife.

Contains a glucoside, a quinoline alkaloid, and peganine, which make it toxic to livestock. Poisoning is rare as livestock will generally not eat it.


(Cuscuta spp.)

Annual parasitic plant having no leaves or green parts. Also called strangleweed for the thread-like yellow to orange twining stems that coil around and attach to host plants with wartlike suckers. A particular concern in vegetable and forage crops and ornamentals.

Dodder lives by extracting food from a host plant through penetrating suckers. It attacks many vegetables, legumes in particular, forage crops, especially alfalfa, as well as native plants.

High levels of dodder in fodder are toxic to cattle and horses. Poisoning can occur if horses and cattle are fed contaminated hay for several weeks. Problems are usually only experienced when dodder makes up about 50% of the contents of the hay.

Affected animals typically suffer from abdominal pain and diarrhoea, and can also experience weight loss. On some occasions liver damage may occur, and can be associated with haemorrhages throughout the body and secondary brain damage. The brain damage makes the affected animal’s behaviour erratic and unpredictable. It will be inclined to stagger about and wander aimlessly before it eventually lies down, becomes comatose or convulsive, and dies.


(Convolvulus arvensis)

Creeping-rooted perennial from Europe often called morning-glory; vine-like stems trail on the ground or twine counter-clockwise round supports; arrow-shaped leaves with sharp-pointed or rounded basal lobes; funnelshaped white to light pink flowers 2.5 cm across; two small bracts 2.5 cm below flower.

Roots can penetrate to over 5 metres in soil; seeds can remain viable for up to 50 years.

Often confused with Hedge Bindweed (Convolvulus sepium) which has larger leaves and flowers (up to 5 cm across).

Field bindweed is not palatable, but may be consumed in severely overgrazed pastures. Hay contaminated with large amounts of field bindweed can cause colic in horses. Invasive plants should never be considered as forage.

Field bindweed will accumulate nitrates. It also contains various tropane alkaloids including pseudotropine, tropine, tropinone, and cuscohygrine. Pseudotropine, the predominant alkaloid, is capable of affecting smooth muscle activity.

Other nortropane alkaloids (calystegins) present in Bindweed (Calystegia spp., some Solanum spp., and Ipomoea spp.) are potent glycosidase inhibitors.


(Hordeum jubatum)

Perennial growing in tufts to 0.6 metres tall; flowerheads, 5 to 13 cm long, break into 7-bristled clusters with 3 spikelets at each joint; green or reddish bristles (awns), 7.5 cm long, are cream-coloured when mature.

Often found on the edges of alkaline sloughs and salt marshes. Upward pointing barbs on the bristles can cause mechanical injury to grazing animals.


GIANT HOGWEED (a.k.a. Giant Cow Parsnip)
(Heracleum mantegazzianum)

Large, hairy perennial herb in the Parsley family with stout taproot of fleshy, fibrous roots; hollow green stems with purple spots grows to 5 metres tall; dark green, coarsely toothed leaves divided into 3 large segments; lower leaves can exceed 2.5 metres in length; small white flowers are produced in large flat top umbrella-like terminal clusters up to 0.8 metres across.

Warning: small hairs on stems and leaves contain a poisonous sap that can cause severe irritation, blistering and dermatitis.

The greatest concern from Giant Hogweed is human health. The blister like pustules on stems and stalks exude a clear watery sap that sensitizes skin to ultraviolet radiation.

Affected areas are subject to severe burns that usually result in blistering and painful dermatitis. Blisters often result in purplish to blackened scars.

Giant Hogweed’s tenacious and invasive nature allows it to readily occupy and crowd out native vegetation. In riparian areas it forms a dense canopy, out-competing native species and causing streambank erosion.


(Senecio vulgaris)

Annual or biennial growing to 60 cm; yellow disk flowers but no ray flowers; black tipped flower head bracts; seeds windborne.

Common Groundsel contains pyrrolizidine alkaloids, which can cause irreversible liver damage and possibly death.

Toxins involved: Pyrrolizidine alkaloids (same as found in Tansy Ragwort)

Animals affected: Horses are most sensitive, followed by cattle, hogs and chickens.

Symptoms: Lethargy, liver lesions, weakness, staggering, death. Liver damage is permanent.

Large amounts of Groundsel can kill an animal in a few weeks or less.

Clinical Signs: 15 mg/kg BW over 2 weeks induces irreversible liver disease. May also cause photo-sensitization, weight loss, and jaundice.

Treatment: Once liver damage is done, treatment is unsuccessful. Humane euthanasia recommended.


(Berteroa incana)

Annual to short-lived perennial in the Mustard family growing erect to 0.7 metres tall; the whole plant covered with star-shaped hairs; upper leaves are elliptic and clasp the stem; white flowers with deeply notched petals; oval seedpods are 5 to 8 mm long, somewhat flattened and held close to the stem.

Consumption of mustard plants by mares has caused a condition called Congenital Hypothyroid and Dysmaturity Syndrome in foals.

Signs of this condition include:
• Abnormally long pregnancy
• Foals commonly born with facial and lower jaw deformities
• Deformities of the limbs

This syndrome occurs most often in mares that are bred late and fed hay that is contaminated with mustard. Or when pastured in early spring in fields that contain mustard plants, such as Blue Mustard, Tumble Mustard, Flixweed, Shepherd’s-purse and Hoary Alyssum.

The syndrome appears to be caused by ingestion of certain mustards during late pregnancy. The chemical—glucosinolates, are broken down into compounds that are goitrogenic or act on the thyroid gland. Make sure that hay is free of mustards and keep mares that are late in pregnancy off weedy pastures that contain plants in the Mustard family.


(Equisetum arvense)

Leafless fertile stems light brown in colour are about 0.3 metres tall with a spore-bearing cone (up to 4 cm) on top; fertile stems to 0.6 metres tall, produce whorls of green, four-angled leaflike branches.

Over 15 species of Equisetum grow in British Columbia. Giant Horsetail (Equisetum telmateia) is similar but taller and more robust throughout.

Toxicity rating: Moderate for most animals, high toxicity in horses.

Toxins involved: Thiaminase. All parts of the plant are toxic, both fresh and dried. Hay containing this weed may be more poisonous than fresh plants in the field.

Animals affected: Horses, other animals can be affected.

• Weight loss
• Weakness
• Gait abnormalities
• Abnormal heart rate
• Inability to rise, death Horses suffer from Vitamin B1 deficiency, causing degeneration of peripheral nerves.


(Cynoglossum officinale)

Biennial taprooted weed growing 0.5 to 1.2 metres tall; soft, hairy rosette leaves (7 to 30 cm long) produced in first year resemble the shape of a dog’s tongue; stem leaves are shorter and stalkless; dull reddish-purple flowers bear 4 rounded triangular nutlets (seeds) that are covered with hooked prickles.

Easily spread by seed attachment to clothing and animals. Well adapted to forested areas, roadsides and meadows. Contains toxic alkaloids that can cause liver damage in grazing animals.

Toxicity rating: Moderate

Toxins involved: Pyrrolizidine alkaloids. All parts of the plant are toxic; most poisonous in the rosette stage.

Animals affected: Horses and cattle

• Weight loss
• Jaundice
• Depression
• Diarrhea
• Photosensitivity of non-pigmented skin

Horses and cattle are most susceptible while sheep seem to be tolerant. The alkaloids have a cumulative effect on the liver and can induce fatal poisoning once 5–10% of an animal’s body weight has been consumed.


(Kochia scoparia)

Peace River

Much branched erect annual growing from 0.3 to 2 metres in height; main stem often tinged with red; stalkless narrow leaves with entire margins often turn purple in autumn; inconspicuous flowers usually surrounded by cluster of long hairs.

Livestock will eat kochia, which sometimes contains high nitrate levels and sulphate toxicity. Can cause Nitrate Poisoning in cattle.

Kochia is very common, especially around corrals. It can shift it’s response to herbicides rapidly, so changing your spray every year can help.

Kochia weed is capable of accumulating significant levels of nitrate as a young, rapidly growing plant. It may also accumulate oxalates and sulfates and has been responsible for causing liver disease and photosensitization in some years due to an as yet undefined toxin.

The variability in the reported toxicity of Kochia weed is poorly understood and is probably related to the growing conditions of the plant.


(Chenopodium album)

One of many similar "goosefoot" species this heavily branched annual varies from 0.3 to over 2 metres in height; ridged green stems are often striped with purple; leaves are somewhat triangular in shape, greyish-green underneath and covered with mealy particles; the greenish inconspicuous flowers are crowded in leaf axils or at end of stems.

One of the most common agricultural weeds. There are several plants that are considered "Nitrate-Accumulating" plants.

Animals that eat 0.05% of their body weight can be poisoned. Cows seem to be the most susceptible and poisoning usually occurs due to infested hay and when there is a high consumption rate. Fertilization, soil and drought conditions are factors in how high the nitrates are in plants.

Some plants to watch out for include:
• Corn
• Pigweeds
• Kochia
• Russian Thistle
• Lambsquarter
• Sorghum/Sudan
• Nightshades
• Oat Hay


(Euphorbia esula)

Perennial with persistent vertical and horizontal creeping roots; grows to 0.8 metres tall; leaves spirally arranged on the stem; inconspicuous greenish-yellow flowers inserted above 2 leaflike yellow-green bracts.

All parts of the plant contain a white milky latex that can irritate skin of livestock and humans.

Toxicity rating: Moderate

Toxins involved: White, latex sap that has co-carcinogenic factors that can increase the cancer-causing properties of other substances. All parts of the plant are toxic.

Animals affected: Any animal consuming spurge exclusively, or that come into contact with the sap. Humans that come in contact with the sap can experience severe skin irritations as well as temporary blindness (seldom permanent) if sap gets in their eyes.

• Gastrointestinal irritation
• Dermal and ocular irritation
• Weakness
• Prolonged exposure to skin (legs and head primarily) will cause irritation, redness, swelling and salivation and head shaking if the oral mucosa is affected
• Blistering and open sores are possible from exposure to the sap


(Asclepias speciosa)

Perennial from 0.6 to 1.5 metres tall with thick creeping rootstocks; short-stalked oval leaves covered with fine hairs; pinkish fragrant flowers are arranged in ball-like clusters at top of stem and in leaf axils; all plant parts contain a sticky, white juice (latex); flowers produce grayish pods which split open in fall to release hundreds of tufted airborne seeds.

Toxicity rating: Moderate

Toxins involved: Cardenolides and resinoids.

Leaves and other above ground parts of the plant are poisonous. Milkweed may cause losses at any time, but it is most dangerous during the active growing season.

Animals affected: Sheep and cattle, occasionally horses. Most livestock losses are a result of hungry animals being concentrated around milkweed infested corrals, bed grounds and driveways. Poisoning may occur if animals are fed hay containing large amounts of Milkweed.

• Loss of appetite
• Depression
• Diarrhoea, maybe blood stained
• Bloating, swelling under jaw
• Slow irregular heart-beat, feeble pulse
• Breathing difficulties, bluish gums
• PM shows internal haemorrhages

• See Vet as soon as possible
• Try activated charcoal drench
• Electrolyte replacement


(Solanum species)

There are several plants in the nightshade family that are considered toxic, including:
• Black Henbane
• Bitter Nightshade
• Hairy Nightshade
• Cutleaf Nightshade
• Jimsonweed

They contain a complex of glycoalkaloids and are highly toxic to cattle, sheep, horses, swine and poultry. People have been poisoned after eating the berries of nightshade plants.

Climbing Nightshade (Solanum dulcamara), also known as bittersweet: perennial with long branches trailing or climbing on other plants; lance-shaped leaves lobed at the base; bluishpurple flowers; immature green berries turning bright red.

Hairy Nightshade (Solanum sarrachoides): annual to 0.6 metres tall; white flowers; dense covering of short hairs on leaves and stems; yellowish-brown berries at maturity; calyx cupped 2/3 around berry. Black Nightshade(Solanum nigrum): annual to 0.9 metres tall; white flowers; smooth leaves; black to dark purple berries at maturity; short calyx does not cup the berry. Young plants and immature berries are particularly poisonous.

Poisonous Principle: Tropane alkaloids, notably apropine, scopolamine and hyosycamine.

• Constipation and infrequent urination
• Some initial excitation and later depression, muscle trembling and weakness, leading to recumbency
• Dilated pupils, dry mouth and nose
• Increased heart rate but a weak pulse
• Loss of rumen sounds, laboured respiration
• In humans, nausea, fainting, loss of coordination, respiration and heart difficulties, drop in temperature and blindness
• Some cases may progress further to either coma or terminal convulsions—within 24 hours
• May block the effects of acetylcholine on smooth muscle tissue Many cases recover

• Try activated charcoal


(Tribulus terrestris)


Annual, branching from the base and spreading along ground to form dense mats; leaves are hairy and grow in pairs on opposite sides of stem; leaf divided into 4 to 8 pairs of oval leaflets; yellow flowers originate in leaf axils; fruit with 5 sections that form tough, sharp sometimes curving spines.

Sharp spines easily penetrate leather, and skin and can flatten rubber tires. The hard, spiny burrs of Puncturevine can damage wool and injure livestock and other animals. It can be toxic to livestock, especially sheep. The spiny burrs can penetrate human skin and have been known to puncture bicycle tires.

Puncturevine’s burrs can puncture the feet of livestock, causing suffering, infection, and lameness—especially in horses.


(Amaranthus retroflexus)

Course annual growing to 1 metre in height; leaves are long-stalked with prominent whitish veins; taproot is often pinkish to red; inconspicuous green flowers borne in spike-like clusters in leaf axils and at stem ends; stiff, spine-like bracts give the head a bristly appearance.

This weed is a host to several pests of vegetable and ornamental crops. It can cause allergic reactions.


(Acroptilon repens)

North Okanagan

Creeping rooted perennial with erect stems to 1 metre tall; young stems covered in soft, gray hairs; rounded flower heads produced singly at the ends of branches; flowers light pink to purple; small pearly bracts with papery margins that are slightly hairy at the tip. (Also see Yellow Star Thistle)

Knapweed or Yellow-Star Thistle intoxication is a disease of neglect, as horses must be forced to eat them. Poisoning occurs when horses are kept on a small pasture with limited feed. As other forages are exhausted, some horses develop a taste for Knapweed and may continue eating it even when supplemented with other forage.

At doses of 50 percent to 200 percent of their body weight over 30 to 90 days, horses develop dysfunction of facial, mouth and throat nerves and muscles. This poisoning has been called "the chewing disease" (horses chew but can’t swallow). Next comes facial-muscle hypertonicity that causes "smiling," tongue lolling, protruding tongue and head tossing. Trying to drink, some horses submerge their muzzle in the water.

These early neurologic signs become worse and lead to lethargy, loss of interest in food, dehydration, malnutrition, difficulty breathing, incoordination, muscle tremors and severe depression. Lesions include those of dehydration and starvation, in addition to damage to specific parts of the brain (the substancia nigra and globus pallidus—thus the name negropallidal encephalomalacia.

Horses are uniquely susceptible to this disease. The long exposures and lack of a smaller animal model have made it difficult to positively identify the toxin. As there is no treatment and the disease is irreversible, it is best to avoid exposure to the plants for prolonged periods.


(Cytisus scoparius)

Upright, taprooted evergreen shrub in the Legume Family growing from 1 to 3 metres in height; stems are ridged, woody, green to brownish-green and prominently 5-angled; upper leaves are simple and unstalked, lower leaves are stalked and comprised of 3 oval leaflets; pea-like flowers are bright yellow (sometimes with red markings in the middle); seedpods are flat, brown to black in colour with fine hairs on the margins.

Escaped garden ornamental now naturalized throughout south coastal BC and introduced at a few Interior locales.

Poisoning from this shrub is usually of a mild type. Large amounts are required to cause symptoms in animals.

Alkaloids have been identified as being the toxic principle. Cytisin, sparteine, and isosparteine are found in the twigs, leaves, and seeds in small amounts. A glycoside, scoparin, has also been isolated. Horses are most susceptible to poisoning by Scotch Broom.

Symptoms: The alkaloid portions cause depression of the nervous system, and the glycoside causes a diuretic effect. Symptoms include incoordination and occasional excitement. Ingestion of large amounts of this shrub can cause coma and death.

Treatment: Nonspecific. Treat symptoms.


(Rumex acetosella)

Perennial with slender rootstocks growing 15 to 60 cm tall; lower leaves arrow-shaped with 1 or 2 outward turning lobes at the base. Tiny flowers are borne at the end of flowering stalks with male and female flowers on separate plants; male flowers are orange-yellow; female flowers are redorange.

Seeds are said to poison horses and sheep. Leaves, when eaten in large quantities, are poisonous.


(Capsella bursa-pastoris)

Annual to winter annual in the Mustard family growing erect from 7 to 90 cm in height; white four-parted flowers; seeds develop in a triangular, flattened pod (purse), notched at the top.

Consumption of mustard plants by mares has caused a condition called Congenital Hypothyroid Dysmaturity Syndrome in foals.

Signs of this condition include:
• Abnormally long pregnancy
• Foals commonly born with facial and lower jaw deformities
• Deformities of the limbs

This syndrome occurs most often in mares that are bred late and fed hay that is contaminated with mustard. Or when pastured in early spring in fields that contain mustard plants, such as Blue Mustard, Tumble Mustard, Flixweed, Shepherd’s-purse and Hoary Alyssum.

The syndrome appears to be caused by ingestion of certain mustards during late pregnancy. The chemical – glucosinolates, are broken down into compounds that are goitrogenic or act on the thyroid gland. Make sure that hay is free of mustards and keep mares that are late in pregnancy off weedy pastures that contain plants in the Mustard family.


(Hypericum perforatum)

Perennial from underground runners, growing 0.3 to 1 metre in height; transparent dots are visible over the surface of the oblong leaves when held to light; bright yellow flowers with 5 petals; plants turn a rusty red colour at maturity.

Toxicity rating: Low to moderate
Toxins involved: Hypericin. All parts of the plant are toxic.
Animals affected: Most animals are susceptible
Symptoms: Sunburn, skin slough, eye irritation, inflammation of non-pigmented skin.

Hypericin is a pigment that when absorbed by the body and activated by sunlight can result in a condition where white or light-skinned animals become seriously sunburned under normal exposure to sunlight. Animals must consume the plants for 4 to 5 days before clinical signs are noted.

Also known as: Goat Weed or Klamath Weed, St. Johns Wort can cause SERIOUS photosensitivity in horses who eat it. This condition can become so serious, a horse’s entire skin might slough away from it’s body, which of course is very painful and fatal.


(Senecio jacobaea)

Biennial to short-lived perennial growing 0.3 to 1.2 metres tall; leaves deeply cut into irregular segments give plant a "ragged" appearance; yellow flowers in a flat-topped cluster.

Contains a toxic alkaloid which reacts with enzyme in livestock to create cumulative liver damage.

Toxicity rating: High
Toxins involved: Pyrrolizidine alkaloids.

Generally unpalatable to livestock, it is only eaten if no other food source. Toxic when fresh or in dry hay.

Animals affected: All animals, especially cattle and horses. Symptoms:
• Lethargy
• Diarrhea
• Weakness
• Crustiness around eyes/nose, red and watery eyes, irreversible liver damage
• Cattle may develop a piglike odor

Animals eating 5% or more of their diet of pre-bloom tansy for a period of 20 days can expect to die within 6 months.


(Cicuta douglasii)

Highly toxic native plant in the Parsley family. Hollow, jointed stems grow 1 to 2 metres tall; oval leaflets with saw-toothed margins; leaflet veins tend to end at the base of the notch on leaf edge; enlarged taproot with horizontal hollow chambers.

These contain an extremely poisonous oil (cicutoxin). CAUTION: Clean knife blades well if used for cross-sectioning roots.

Found throughout British Columbia in sloughs, wet meadows, stream banks and other wet areas.

Toxicity rating: High
Toxins involved: Cicutoxin.

The root contains the highest concentration of poison, but the whole plant can be considered toxic. This is one of the most toxic plants in the United States. Animals have been poisoned by drinking water that had been contaminated with trampled water hemlock roots, humans are poisoned when this plant is mistaken for water-parsnip.

Animals affected: All animals, including humans.
• Nervousness
• Breathing difficulties
• Tremors
• Collapse
• Sudden death As little as 8 ounces can kill a horse.

Signs will develop within minutes of ingestion, death can occur in 30 minutes. If the animal survives 4 to 6 hours, they may recover but could suffer permanent damage to the heart.


(Centaurea solstitialis)

Annual taprooted heavily branched weed growing from 0.6 to 1 metre tall; stems are winged and covered with fine hair; yellow flowers are borne on ends of branches and armed with sharp thorns up to 2 cm long.

Not currently known from British Columbia but close to our borders with Washington and Idaho.

Russian Knapweed and Yellow Starthistle can cause a neurological disease called nigropallidal encephalomalacia, better known as "Chewing Disease".

The effects of the toxins are cumulative and horses will not eat these plants unless it is the only forage available. Chewing Disease can eventually cause death by starvation. Unfortunately, horses LIKE it.

See Russian Knapweed for description of "Chewing Disease"


(Linaria vulgaris))

Creeping rooted perennial to 0.6 metres tall; the stalkless leaves are narrow and pointed at both ends; bright yellow "snapdragonlike" flowers with an orange spot on the lower lip are 2 to 3.5 cm long. Flowers similar to but smaller than Dalmatian Toadflax (Linaria dalmatica); likely introduced as an ornamental.

Can cause gastrointestinal upset in horses. It looks like a Snapdragon with it’s pretty yellow flowers.