Toxic Parts:
terpenes, sesquiterpene lactones
Flower Color:
  • flower color
meadows, mountains, rangelands, haybales

Time of Greatest Risk


Geographical Distribution

Yellow star thistle distribution - United States

Related Species

Yellow Star Thistle

Centaurea solstitialis

St. Barnaby's Thistle, Golden Star Thistle, Yellow Cockspur, Chewing Disease
8/ 10
Yellow star thistle (Centaurea solstitialis) is an annual weed found predominately in the western United States. Yellow star thistle is a grayish-green colored plant which has multiple rigid stems extending in all directions from its base. It produces bright, star-like yellow flowers that are shielded by long, spiny bracts. Yellow star thistle has a deep taproot, which allows it to thrive during dry, hot summers and drought periods. Yellow star thistle begins to experience significant early growth in late May to early June and flowers early to mid-July. It can often go unnoticed until the plant begins to flower, but once the bright yellow, dandelion like flowers bloom, the plant is easily detected.

Toxic components
Yellow star thistle contains eight sesquiterpene lactones that consist of: solstitialin A, repin, subluteolide, acroptilin, janerin, cynaropicrin, lactones solstitiolide and episolstitiolide. These toxins have a cumulative effect on horses, meaning they accumulate in the body over time. Upon ingestion of 85-100% of their body weight results in the appearance of clinical signs of equine parkinsonism. Ingestion of these plants causes irreversible neurological damage to the horse, and if left untreated, they will usually die from starvation, inhalation pneumonia, or dehydration. Yellow star thistle causes the same clinical signs as Russian knapweed (Centaurea repens), but is considered more toxic.


  • Anxiousness
  • Impaired Ability To Eat Or Drink
  • Sudden Onset Of Involuntary Facial Movements
  • Continuous Chewing Movements
  • Jaw Held Open Or Tightly Closed
  • Dehydration
  • Severe Weight Loss
  • Depression
  • Submerging Head Into Deep Water To Drink
  • Yawning
  • Violent Head Tossing
  • Drowsiness


MECHANICAL: Mow at early flowering.

BIOLOGICAL: Three seed head weevils, two seed head flies, yellow thistle bud weevil. Graze after bolting prior to spine formation. Several years needed to deplete seed reservoir.


ChemicalTrade NameApplication Rate (Amount/Acre)Time of Application
MetsulfuronEscort XP1 ozSeedling to early bud
Metsulfuron + 2,4-D DicambaCimmaron MaxRate IIISeedling to early bud
Dicamba DiflufenzopyrOverdrive1 oz + 4 ptSeedling to early bud
TriclopyrRemedy4 ozRosette
2, 4-DEsteron 99 And others3 ptSpring to early bud
ImazapyrArsenal1 qtSpring to early bud
PicloramTordon 22k1 ptSpring to early bud
DicambaBanvel, Clarity1 ptSpring to early bud
ClopyralidReclaim2/3 ptSpring to early bud
Picloram + 2,4-DGrazon P + D2 qtSpring to early bud
AminopyralidMilestone3 to 5 ozSpring to early bud