Plants of Canada and the northern United States that are poisonous to livestock, or that have tainted animal products
Research Scientist and Research Institute Director (retired).
Honorary Research Associate (1987 - 2022), Agriculture and Agri-Food Canada,
Ottawa, Ontario K1A 0C6, Canada
Awarded the George Lawson Medal by the Canadian Botanical Association in 2006.
Awarded the Faculty of Macdonald, McGill University, Most Distinguished Alumni Award on October 18, 2014.
Read his biography "The Real Weed Man" available in print and ebook.
Information is presented about the plants, of Canada and the northern United States, that are reported to have poisoned livestock, or to have tainted animal products. Some general information is also included on poisonings by blue green algae and mycotoxins, and on plants that can cause mechanical injuries. The number of publications examined is very large, listing all of them and the information that they contain is beyond the scope of this website. More detailed information can be obtained by consulting the texts of Frankton and Mulligan (1987), Fyles (1920), Johnston et al. (1965), Keeler et al. (1978), Kinghorn (1977), Kingsbury (1964), Muenscher (1975), and Mulligan and Munro (1990), by contacting your appropriate educational agency or veterinarian, and by entering either the common or the scientific name of a specific plant into any of the search engines on your computer. Plants that are poisonous to livestock, or that taint animal products, often have disagreeable odors, tastes, or textures. As a general rule, livestock usually avoid eating these plants, either in the field or in confinement, unless other sources of food are very scarce or are absent. However, livestock will often eat some of these when they are present as impurities in hay or in other types of dried feed.
1. A list of the plants poisonous to livestock, or that have tainted animal products.
2. Accounts of plants poisonous to livestock, or that have tainted animal products.
3. Poisonings of livestock by blue green algae.
4. Plants causing mechanical injuries to livestock.
5. Poisoning of livestock by plant mycotoxins.
6. Selected references.
1. List of plants poisonous to livestock, or that have tainted animal products
The names of all the plants in this list are arranged alphabetically, with the appropriate family name following the English common name of each plant. Throughout this text, plants poisonous to livestock are shown in boldface, whereas plants tainting animal products are in bold-italics.
prostrate pigweed (amarante fausse-blite); Amaranthus blitoides S. Wats. (=A. graecizans L.)
Widespread, prostrate, annual weed spreading by seeds.
redroot pigweed # (amarante à racine rouge); Amaranthus retroflexus L.
Widespread native annual weed, spreading by seeds.
smooth pigweed (amarante hybride); Amaranthus hybridus L.
A native riverbank annual of eastern North America that is gradually spreading westward. Poisonings and deaths of pigs and cattle have been reported for all.
Arrow-grass family (Juncaginaceae)
seaside arrow-grass (troscart maritime); Triglochin maritimum L.
Widespread native herb. Sickness and deaths have occurred in cattle and sheep.
Barberry family (Berberidaceae)
May-apple (podophylle pelté); Podophyllum peltatum L.
Native in eastern part of our area. Poisonings of cattle and poisonings and deaths of pigs have been reported.
Beech family (Fagaceae)
black oak (chêne noir); Quercus velutina Lam.
red oak (chêne rouge); Quercus rubra L.
Native trees in east. Poisonings and deaths of cattle, sheep, and horses have occurred when animals grazed trees that fell into pastures, and in years when the crops of acorns were very large.
Borage family (Boraginaceae)
fiddleneck (amsinckie intermédiaire); Amsinckia intermedia Fisch. & C.A.Mey. [=A. menziesii (Lem.) Nels. & Macbr.]
Native in prairie areas. Poisonings and deaths of cattle, horses, and pigs have been reported.
hound's tongue (cynoglosse officinale); Cynoglossum officinale L.
A biennial introduced weed, primarily eastern. Deaths of horses feeding on hay containing hound's-tongue, and cattle poisoned after grazing on waste land containing this weed. Its seeds can become tangled in the wool of sheep.
Buckwheat family (Polygonaceae)
garden-sorrel (grande oseille); Rumex acetosa L.
Cultivated as a salad plant, and sporadically naturalized. Sheep were poisoned from grazing a field of garden-sorrel.
Tartary buckwheat # (sarrasin de Tartarie); Fagopyrum tataricum (L.) Gaertn.
Sporadic introduced annual weed; sporadic, only locally common. Heavy consumption and exposure to sunlight has caused poisoning of livestock, especially pigs.
veined dock (rumex veiné); Rumex venosus Pursh
Native prairie herb. Sickness and deaths of cattle have occurred.
Composite family (Compositae [Asteraceae] )
absinth # (armoise absinthe); Artemisia absinthium L.
Throughout, especially common in prairies. When eaten by cattle, causes a taint in dairy products.
black-eyed Susan # (rudbeckie hérissée); Rudbeckia serotina Nutt.
Native perennial, spreading by seeds. Throughout our area, but often common locally. Ingestion of large quantities has caused severe poisoning.
broom snakeweed (gutierrézie faux-sarothra); Gutierrezia sarothrae (Pursh) Britton & Rusby
Native perennial in mid-west. Cattle poisonings have occurred during winter grazing, especially when other vegetation is scarce.
chicory # (chicorée sauvage); Cichorium intybus L.
Widespread, but rare in the west. If eaten in large quantities by cows, the herbage imparts a bitter flavor to milk.
cocklebur (lampourde glouteron); Xanthium strumarium L.
Widespread annual weed; both native and introduced populations exist here. Poisonings of cattle, sheep, horses, and swine. Other species of cocklebur are also poisonous. Milk from cows grazing on leaves has an undesirable flavor. The burs often become entangled in the manes of farm animals and contaminate the wool of sheep.
Colorado rubberweed (hyménoxys de Richardson); Hymenoxys richardsonii (Hook.) Cockerell
Native herb in southern Saskatchewan, southern Alberta, and southward in the United States. Poisonings and deaths of sheep; rarely also goats and cattle.
common groundsel (séneçon vulgaire); Senecio vulgaris L.
Introduced annual weed; widespread. Sickness and deaths of horses fed hay that was about 25 percent common groundsel.
common ragweed # (petite herbe à poux); Ambrosia artemisiifolia L.
Native herb; throughout, but most common in the east. Dairy products from cows that have grazed on this plant have an objectionable odor and taste.
common yarrow # (achillée millefeuille); Achillea lanulosa Nutt. (= Achillea millefolium L.)
Widespread native and introduced herb. Dairy products from cows grazing on this plant have an undesirable flavor.
cut-leaved coneflower (rudbeckie laciniée); Rudbeckia laciniata L.
A native, naturalized and cultivated herb. Poisonings and deaths of pigs, sheep, and horses have occurred.
false ragweed # (ive à feuilles de lampourde); Iva xanthifolia Nutt.
Abundant in prairie areas, less common elsewhere. Milk from cows grazing on the leaves has an undesirable flavor.
naked-flowered sneezeweed (hélénie nudiflore); Helenium flexuosum Raf.
Native herb in east. Poisonings and deaths of cattle, horses, sheep, and mules.
ox-eye daisy # (marguerite blanche); Leucanthemum vulgare Lam. (= Chrysanthemum leucanthemum L.)
Widespread introduced herb; rare in prairie areas. When eaten by dairy cattle, milk has an disagreeable taste.
prickly lettuce # (laitue scariola); Lactuca serriola L. (= Lactuca scariola L.)
Introduced annual or winter annual. One case of cattle poisoning has been reported.
Russian knapweed # (centaurée de Russie); Acroptilon repens (L.) DC. (= Centaurea repens L.)
Introduced perennial, forming dense patches; spreading by seeds and horizontal creeping roots. Widespread on the prairies, but rare elsewhere. The deaths of sheep and horses has been reported.
sneezeweed (hélénie autumnale); Helenium autumnale L.
Native herb sporadic throughout area. Poisonings and deaths of cattle, horses, and sheep.
stinking rabbitbush (bigelovie puante); Chrysothamnus nauseosus (Pall. ex Pursh) Britt.
Native in western one-third of our area. Livestock poisonings have been reported.
tansy ragwort # (séneçon jacobée ); Senecio jacobaea L.
Introduced biennial or short-lived perennial weed; in eastern and western maritime areas. Many cases of poisonings when present in hay eaten by cattle, sheep, and horses. Animals usually avoid grazing plants growing in fields.
velvety goldenrod (verge d'or veloutée); Solidago mollis Bartl.
Native prairie herb. Occasional deaths of livestock.
white snakeroot (eupatoire rugueuse); Ageratina altissima (L.) King & H. Rob.
Native herb in eastern half of our area. Poisonings and deaths of cattle, horses, sheep, and goats.
yellow star-thistle (centaurée du solstice); Centaurea solstitialis L.
Weed in central part of our area. Poisonings and deaths of horses has been reported.
Crowfoot family (Ranunculaceae)
larkspures (pied-d'alouettes); Delphinium species
Native species of larkspurs. Documented poisonings and deaths of cattle and suspected poisonings of horses and sheep.
tall buttercup # (renoncule âcre); Ranunculus acris L.
An abundant introduced perennial weed, especially in moist locations. Contains an acrid bitter juice that causes a severe pain and a serious inflammation when this plant is grazed. Considered harmless in hay as the juice is volatile and is dispelled when the hay is cured.
Dogbane family (Apocynaceae)
hemp dogbane (apocyn chanvrin); Apocynum cannabinum L.
spreading dogbane (apocyn à feuilles d'androsème); Apocynum androsaemifolium L.
Both widespread native herbs. Toxic to all types of livestock, but eaten only when other forage is scarce.
Fern family (Polypodiaceae)
bracken # ( fougère-aigle ); Pteridium aquilinum (L.) Kuhn
Native perennial herb with extensive underground rhizomes; throughout, except the mid-west. Sickness and deaths have occurred in cattle, horses, sheep, and pigs.
sensitive fern (onoclée sensible); Onoclea sensibilis L.
Native perennial herb. Sickness and deaths of horses is reported to have occurred after eating hay containing this plant.
Figwort family (Scrophulariaceae)
foxglove (digitale pourpre); Digitalis purpurea L.
Sporadic garden escape; frequently cultivated. Pigs, cattle, turkeys, and other animals have been poisoned, although animals normally avoid eating foxglove.
Fumitory family (Fumariaceae)
Dutchman's-breeches (dicentre à capuchon); Dicentra cucullaria (L.) Bernh.
squirrel-corn (dicentre du Canada); Dicentra canadensis (Goldie) Walp.
western bleedingheart (dicentre à belles fleurs); Dicentra formosa (Haw.) Walp.
Native herbs in extreme west. Poisonings and deaths of cattle reported for all.
Goosefoot family (Chenopodiaceae)
five-hooked bassia (bassia à feuilles d'hysope); Bassia hyssopifolia (Pall.) Ktz.
greasewood (sarcobate vermiculé); Sarcobatus vermiculatus (Hook.) Torr.
Native in western one-third of our area. Poisonings and deaths of sheep by both plants have occurred.
kochia # (kochia à balais); Kochia scoparia (L.) Schrad.
Introduced annual, spreading by seeds. Occurs widely, but most common in mid-west. Photosensitization of cattle has resulted in poisonings and deaths.
lamb's-quarters # (chénopode blanc); Chenopodium album L.
Introduced annual, spreading by seeds. Widespread in disturbed habitats. Poisonings and deaths of cattle, horses and pigs have occurred.
poison suckleya (suckleya); Suckleya suckleyana (Torr.) Rydb.
Native on central prairies. Poisonings and deaths of cattle have occurred.
Russian thistle # (soude roulante); Salsola kali L. subsp. ruthenica (Iljin) Soó
Widespread introduced weed; most common in prairie area. Has caused poisoning to sheep.
Grass family (Gramineae [Poaceae])
Johnson grass (sorgo d'Alep); Sorghum halepense (L.) Pers.
Rhizomatous perennial weed, introduced as a forage plant. Locally common in southern Ontario and southward. Can produce cyanogenic compounds in leaves and stems, causing prussic-acid poisoning in grazing animals.
tall manna grass (glycérie géante); Glyceria grandis S.Wats.
Native and naturalized throughout. Deaths of cattle have been reported.
Heath family (Ericaceae)
azaleas (azaleés); Rhododendron species
Native and ornamental shrubs. Losses of livestock have occurred.
bog-laurel (kalmia à feuilles d'andromède); Kalmia polifolia Wang.
Widespread native shrub. Poisonings and deaths of livestock have been reported.
false azalea (menziézie ferrugineuse); Menziesia ferrruginea Sm.
Native shrub in west. Poisoning and death of sheep.
rhododendrons (rhododendrons); Rhododendron species
Native and ornamental shrubs. Losses of livestock have occurred.
sheep-laurel (kalmia à feuilles étroites); Kalmia angustifolia L.
Native shrub in the east. Poisonings and deaths of livestock have been reported.
Horse-chestnut family (Hippocastanaceae)
horse-chestnut (marronnier); Aesculus hippocastanum L.
Outdoor ornamental tree. Leaves and fruits have caused illness in cattle, horses, and pigs.
Horsetail family (Equisetaceae)
field horsetail # (prèle des champs); Equisetum arvense L.
marsh horsetail (prèle des marais); Equisetum palustre L.
Both are native herbs with extensive underground rhizomes. Sickness and deaths has occurred in horses and sheep; very rarely in cattle.
Iris family (Iridaceae)
blue flag (iris versicolore); Iris versicolor L.
Widespread native herb. Poisonings and deaths of calves have occurred.
Lily family (Liliaceae)
death camas (zigadène vénéneux); Zigadenus venenosus S. Watson.
Native on western prairie.
white camas (zigadène élégant); Zigadenus elegans Pursh
Both have caused sickness and deaths of sheep, cattle, and horses.
false hellebore (vérâtre vert); Veratrum viride Ait.
Native herb, absent on prairie. Sickness and deaths of sheep, cattle, and chickens.
onion (oignon); Allium cepa L.
A food plant. Poisonings and deaths of cattle, and horses have occurred after they were fed large quantities of onions.
Maple family (Aceraceae)
red maple (érable rouge); Acer rubrum L.
Native and cultivated trees. Acute hemolytic anemia reported in horses that ate wilted leaves from cut trees.
Mezereum family (Thymelaeaceae)
daphnes (daphnés); Daphne species
Ornamental shrubs. Cattle, sheep, goats, pigs, deer, and horses have been poisoned after eating fallen or cut branches.
Milkweed family (Asclepiadaceae)
common milkweed # (asclépiade commune); Asclepias syriaca L.
eastern whorled milkweed (asclépiade verticillée); Asclepias verticillata L.
showy milkweed (belle asclépiade); Asclepias speciosa Torr.
Native herbs. Poisonings and deaths of sheep and cattle reported. Livestock usually avoid eating milkweeds.
Mint family (Labiatae [Lamiaceae])
ground-ivy # (lierre terrestre); Glechoma hederacea L.
Widespread but most common in east. A perennial introduced weed; most common in lawns, gardens, and vacant land. Horses were poisoned when large quantities of ground-ivy were eaten.
Mustard family (Cruciferae [Brassicaceae])
garlic mustard (alliaire officinale); Alliaria petiolata (M. Bieb.) Cavara & Grande
Biennial or winter annual, probably introduced as a medicinal and salad plant. Locally common in east, rare in west. Cows eating garlic mustard produce milk with a disagreeable flavor.
green tansy mustard (moutarde-tanaisie verte); Descurainia pinnata (Walt.) Britt.
Native herb, mainly in the western part of our area. Poisonings and deaths occurred after cattle foraged almost exclusively on this plant.
hare's-ear mustard # (vélar d'Orient); Conringia orientalis (L.) Dumort.
Introduced annual weed; widespread but most common in prairie region. Seeds have caused poisonings when fed in grain.
hoary alyssum # (bertéroa blanc); Berteroa incana (L.) DC.
An introduced annual or winter annual. Contamination in hay by more than 30% can cause lameness and in extreme cases the death of horses.
stinkweed # (tabouret des champs); Thlaspi arvense L.
A widespread introduced annual or winter annual weed. Poisonings and deaths occurred after cattle were fed hay with 25 percent or more of stinkweed. It can result in an off-flavor of dairy products.
wild mustard # (moutarde des champs); Sinapis arvensis L.
A widespread introduced annual weed, reproducing by seeds. Poisonings and deaths of cattle, chickens, and swine have occurred after eating very large quantities of plants or seed screened from grain.
Nightshade family (Solanaceae)
black nightshade # (morelle noire); Solanum nigrum L.
Widespread introduced herb. Poisonings and deaths of cattle, sheep, pigs, goats, chickens, and ducks have occurred.
buffalobur (morelle rostrée); Solanum rostratum Dunal
Annual weed, native further south; widely distributed but most common in prairie region. Has caused poisoning to livestock.
climbing nightshade # (morelle douce-amère); Solanum dulcamara L.
Widespread introduced woody vine. Mild poisonings of sheep and cattle have been reported.
horse-nettle (morelle de Caroline); Solanum carolinense L.
Perennial weed, native of Gulf States. Has caused poisonings of cattle, sheep, and deer.
jimsonweed (stramoine commune); Datura stramonium L.
Widespread introduced herb. Poisonings and deaths of cattle, horses, pigs, chickens, and mules have been reported.
potato (pomme de terre); Solanum tuberosum L.
Food plant. Farm animals have been poisoned by eating large quantities of spoiled potatoes or potato tops.
tobacco (tabac); Nicotiana tabacum L.
Cultivated herb. Deformities have occurred in offspring of sows fed tobacco leaves.
Parsley family (Umbeliferae)
giant hogweed # (berce du Caucase); Heraclium mantegazzianum Somm. & Lev.
Tall introduced perennial herb, spreading by seeds; local throughout, except in drier regions. Causes a severe contact photo-activated dermatitis in livestock; probably toxic when eaten.
poison-hemlock # (ciguë maculée); Conium maculatum L.
Introduced biennial herb, with a very disagreeable odor. Fairly rare in our area. Has caused poisonings and death of all classes of livestock, and deformities in calves.
northern water-hemlock (cicutaire du Nord); Cicuta virosa L.
Native in wet locations in north.
spotted water-hemlock # (carrotte à Moreau); Cicuta maculata L.
Widespread in wet locations.
western water-hemlock (cicutaire pourpe); Cicuta douglassii (DC.) Coult. & Rose
All are perennials, spreading by seeds. All are very poisonous. Death of all types of livestock has occurred after they have consumed leaves or roots.
wild carrot # (carotte sauvage); Daucus carota L.
Introduced annual, biennial, or short-lived perennial weed. Locally common, especially in east. Will taint milk. Cows will avoid eating this plant, unless nothing else is available to them.
Pea family (Leguminosae [Fabaceae])
black locust (robinier faux-acacia); Robinia psuedoacacia L.
Naturalized shrub or tree; sometimes grown as an ornamental. Has caused poisonings and deaths of horses, cattle, and chickens.
golden bean (thermopsis rhombifolié); Thermopsis rhombifolia (Nutt.ex Pursh) ex Richards.
Native herb in west. Toxic to cattle.
hairy vetch (vesc velue); Vicia villosa Roth
Forage crop and occasional weedy herb. Has caused poisonings and deaths of cattle.
locoweeds (oxytropes); Oxytropis species
Native herbs in west. Has caused poisonings and deaths of cattle.
lupines (lupins); Lupinus species
Native herbs of west. Although usually considered a valuable range forage plant, poisonings and deaths of cattle, sheep, horses, and pigs have occurred.
milk-vetches (astragales); Astragalus species
Native western herbs. Poisonings and deaths of chickens, horses, cattle, and sheep have occurred.
Pine family (Pinaceae)
ponderosa pine (pin ponderosa); Pinus ponderosa Dougl.
West coast native tree. Abortion and birth of weak offspring in cattle and goats.
Pink family (Caryophyllaceae)
purple cockle (nielle); Agrostemma githago L.
Introduced annual weed; formerly common, but now rare. Eating purple cockle seeds has resulted in poisonings and deaths of chickens.
Pokeweed family (Phytolaccaceae)
pokeweed (phytolaque d'Amérique); Phytolacca americana L.
Native herb in the eastern part of our area. Turkeys and pigs have been poisoned.
Primrose family (Primulaceae)
scarlet pimpernel (mouron rouge); Anagallis arvensis L.
Sporadic introduced herb. Poisonings and sometimes deaths of dogs, mules, poultry, rabbits, wild birds, calves, and sheep have occurred after they consumed large quantities of vegetation or seeds.
Rose family (Rosaceae)
black cherry (cerisier tardif); Prunus serotina Ehrh.
Native tree in the eastern part of our area; sometimes cultivated.
red chokecherry (cerisier de Virginie); Prunus virginiana L.
Widespread native shrub or small tree. Both have caused poisonings and deaths of livestock.
St. John's-wort family (Guttiferae)
St. John's-wort # (millepertuis commun); Hypericum perforatum L.
Introduced perennial weed, spreading by seeds. Absent from the mid-west but common elsewhere. It contains a toxic substance that adversely affects white-haired animals when they are exposed to strong sunlight after they have eaten this plant. Sickness and even deaths has occurred in cattle, sheep, horses, and rabbits.
Spurge family (Euphorbiaceae)
cypress spurge # (euphorbe cyprès); Euphorbia cyparissias L.
Perennial herb, with underground rootstocks, often forming large patches; sometimes grown as an outdoor ornamental, frequently becomes an aggressive weed. Occurs sporadically throughout our area, but mainly in the east.
leafy spurge # (euphorbe ésule); Euphorbia esula L.
Introduced perennial weed spreading mainly by persistent vertical and horizontal underground rootstocks. Occurs throughout, but mainly in the mid-west.
Both have caused poisonings and deaths of cattle, horses, and sheep, especially when animals eat contaminated hay. Grazing animals usually avoid grazing plants growing in the field.
Yew family (Taxaceae)
yews (ifs); Taxus species
Native and ornamental shrubs; widely distributed. Cattle, sheep, goats, pigs, deer, and horses have been poisoned, especially after eaten fallen or cut branches.
3. Poisonings of livestock by blue green algae (Division Cyanophyta)
Occasionally extensive losses of cattle, horses, swine, and domestic fowl have occurred after they have ingested water containing high concentrations of blue green algae.
4. Plants causing mechanical injuries to livestock
Some plants can cause mechanical injuries to livestock. This is particularly true of members of the grass family (Gramineae). Seeds and fruiting inflorescences of some of our grasses have various kinds of sharp barbs or awns that can become lodged in the nose, eyes, or throat of an animal, and cause it to become disorientated or make it extremely painful for it to eat. The following are some of the worst offenders: downy brome # (brome des toits), Bromus tectorum L.; foxtail barley # (orge queue-d'écureuil), Hordeum jubatum L.; yellow foxtail (setaire glauque), Setaria pumila (Poir.) Roem. & Schult.; and porcupine grass (stipe à balai), Stipa spartea Trin.
5. Poisonings of livestock by mycotoxins
Certain fungi can produce toxic metabolic products (mycotoxins), under some environmental conditions, in many types of plant material eaten by livestock. These mycotoxins have caused the deaths of large numbers of livestock. The economic losses are difficult to determine, but many experts believe that it is considerable.
6. Selected references
Frankton, C., and Mulligan G.A. 1987. Weeds of Canada. Revised. N.C. Press, Toronto, Ont. 217 pp.
Fyles, F. 1920. Principal poisonous plants of Canada.. Agric. Can.. Exp. Farms Bull. 39 (2nd Ser.). 120 pp.
Johnston, A., Smoliak, S., and Avery, R.J. 1965. Veterinarian's handbook of poisonous and injurious plants of the Prairie Provinces. Agric. Can. Res. Stn., Lethbridge, Alta. 54 pp.
Keeler, R.F., Van Kampen, K.R., and James, L.F. (eds.). 1978. Effects of poisonous plants on livestock. Academic Press, New York, N.Y. 600 pp.
Kinghorn, A.D. 1977. Toxic plants. Columbia Univ. Press, New York. 195 pp.
Kingsbury, J.M. 1964. Poisonous plants of the United States and Canada. Prentice-Hall, Englewood Cliffs, N.J. 626 pp.
Muenscher, W.C. 1975. Poisonous plants of the United States. Revised Ed., Collier Books, New York, N.Y. 277 pp.
Mulligan, G.A., and Munro, D.B. 1990. Poisonous plants of Canada. Agric. Canada Res. Br. Tech. Bull. 1842/E. 96 pp.
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