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Western Washington Poisonous Plants

W estern Washington Plants Known to Be Poisonous to Horses (USDA-SCS)

Buttercup, creeping (Ranunculus repens)
Location or Season: Moist soils
Probable Toxic Dose: Very Large
Toxin: Protoaneonin
Symptoms: Inflammation and narcosis
Comments: Rarely eaten unless pasture overgrazed

Camas, death (Zigadenus venenosus)
Location or Season: Spring
Probable Toxic Dose: less than 9 lb.
Toxin: Steroidal, glycosidal, alkaloid
Symptoms: Salivation, weakness, respiratory difficulty, nausea, convulsions, coma
Comments: Deadly, easily confused with edible camas after bloom

Fern, bracken (Pteridium aquilinum)
Location or Season: Fall, when pastures overgrazed
Probable Toxic Dose: Cumulative Large quantities
Toxin: Thiaminase
Symptoms: Appetite loss, timid, stupified, incoordination
Comments:

Fiddleneck (Amsinckia sp.)
Location or Season: Overgrazed pastures
Probable Toxic Dose: Single dose 20 Ib. or cumulative
Toxin: pyrrolixidine
Symptoms: Liver damage & failure, depression, dermatitis, incoordination, death
Comments: Similar poison to Tansy Ragwort

Foxglove (Digitalis purpurea)
Location or Season: Acid soils
Probable Toxic Dose: VERY TOXIC (1/4 Ib.)
Toxin: Digitoxin & other glycosides
Symptoms: Contracted pupils, labored breathing, convulsions, death
Comments: Rarely eaten fresh, dangerous in hay

Hemlock, poison (Conium maculatum)
Location or Season: Ditches, moist disturbed areas
Probable Toxic Dose: Very toxic (5 to 10 lb.)
Toxin: Coniine and other alkaloids
Symptoms: Narcosis, paralysis, death
Comments: Hay MAY be somewhat less toxic as the poison will slowly evaporate

Hemlock, water (Cicuta douglasii)
Location or Season: Low, wet areas; in spring roots pull out of ground easily
Probable Toxic Dose: VERY TOXIC (.2 to .8 lb.)
Toxin: Circutoxin
Symptoms: Teeth grinding, muscle spasms, respiratory failure, death
Comments: Roots & stem base most toxic. The most poisonous plant known in N. America

Horsetail (Equisetum arvense)
Location or Season: Moist areas
Probable Toxic Dose: Large quantities Cumulative
Toxin: Thiaminase
Symptoms: Thiamine deficiency causes appetite loss, incoordination
Comments: Poisoning occurs when dry plants are fed in hay

Knapweed, Russian & Yellowstar thistle (Centaurca spp.)
Location or Season: Disturbed areas
Probable Toxic Dose: Cumulative 600 Ib.?
Toxin: 
Symptoms: Brain deterioration resulting in "Chewing disease"
Comments: Can eventually cause death by starvation

Larkspur (Delphinium spp.)
Location or Season: 
Probable Toxic Dose: Very toxic (1 lb.)
Toxin: Alkaloids-delphinine
Symptoms: Constipation, bloat, depression, paralysis
Comments: Deadly

Lupine (Lupinus spp.)
Location or Season:
Probable Toxic Dose: 600 to 800 lbs.
Toxin: Alkaloids-lupinine
Symptoms: Spasms, cerebral excitement & death
Comments: Can cause birth defects if eaten during pregnancy

Nightshade, black (Solanum spp.)
Location or Season: Late summer, early fall, fencerows
Probable Toxic Dose: 1 to 10 Ib.
Toxin: Alkaloid-solanine
Symptoms: Diarrhea, convulsions, incoordination, death
Comments:

Oak (Ouercus spp.)
Location or Season:
Probable Toxic Dose: Very large
Toxin: Tannins?
Symptoms: Constipation, blood in urine
Comments: Leaves can cause problems, acorn poisoning more common

Ragwort, tansy & Common groundsel (Seneclo spp.)
Location or Season:
Probable Toxic Dose:
Toxin: Alkaloid-pyrrolizidine
Symptoms: Liver Lesions, weakness, staggering, death
Comments: Liver damage is permanent. Normally avoided when fresh, eaten in hay or when wilted.

Rhododendron (Rhododendron spp.)
Location or Season: 
Probable Toxic Dose: Small
Toxin: Glycosides
Symptoms: Vomiting, vertigo, death from respiratory failure
Comments:

Yew (Taxus spp.)
Location or Season:
Probable Toxic Dose: 1 to 10 lbs.
Toxin: Alkaloid-taxine
Symptoms: Gastroenteritis, labored breathing, trembling, collapse
Comments:

Excerpted from: Pasture Management for Horses and Ponies, Gillian McCarthy; and Plants that Poison Livestock in Thurston County, Thurston County Noxious Weed Control Agency.