Russian Knapweed and Yellow Star-Thistle Poisoning of Horses

Guide B-710
Revised by Jason L. Turner and Greg Alpers
College of Agricultural, Consumer and Environmental Science, New Mexico State University

Authors: Respectively, Professor/Extension Horse Specialist, Department of Extension Animal Sciences and Natural Resources (ASNR); and Interim Brush and Weed Specialist, ASNR, New Mexico State University. (Print Friendly PDF)


Russian knapweed (Rhaponticum repens, formerly Centaurea repens or Acroptilon repens) and yellow star-thistle (Centaurea solstitialis) are unusual among poisonous plants in that they are toxic to horses—causing “chewing disease”—but cattle and sheep consume the plants without any apparent signs of toxicity. Since these two noxious weeds are aggressive invaders of pasture, range, and vacant lands in New Mexico, occasional poisoning of horses has been reported.

Description of Plants

Russian knapweed is a woody-stemmed perennial that grows to approximately 3 ft tall. It is characterized by gray hairs (knap) that cover its leaves and stems. The terminal branches of the stem give rise to purple thistle-like flowers (Figure 1).

Fig. 1: Photograph of Russian knapweed flowers.

Figure 1. Russian knapweed flowers. (Photo courtesy of Steve Dewey, Utah State University,

Although it is not as prevalent as Russian knapweed, yellow star-thistle (Figure 2) is found in certain areas of New Mexico. It is an annual weed with multiple branching stems that yield characteristic star-like yellow flowers (Figure 3) protected by long, spiny bracts. It also grows to a height of approximately 3 ft.

Yellow star-thistle plants.

Figure 2. Yellow star-thistle plants
. (Photo courtesy of John O’Loughlin, Grant County Noxious Weed Program Coordinator.)

Yellow star-thistle flowers.

Figure 3. Yellow star-thistle flowers. (Photo courtesy of John O’Loughlin, Grant County Noxious Weed Program Coordinator.)

Toxic Principles

The exact chemical compound responsible for toxicity in Russian knapweed and yellow star-thistle has not been defined; however, a sesquiterpene lactone, repin, is believed to be the key neurotoxin present. The toxic effects of Russian knapweed and yellow star-thistle are cumulative, meaning that poisoning normally results when levels of the toxin build up in the body over time due to horses routinely grazing these plants.

Horses must consume relatively large quantities of the green or dried plants before the toxic threshold is reached. It has been suggested that a horse must consume 60% of its body weight in green Russian knapweed plant material before toxicity symptoms appear. For yellow star-thistle, toxicity symptoms may arise after horses have ingested 85 to 100% of their body weight in green plant material. Once these thresholds are reached, disease symptoms have a rapid onset.


The clinical signs of poisoning observed in horses that have consumed large quantities of these plants result from accumulation of the toxin in the brain, resulting in necrosis, or death, of neural tissue. Initial symptoms of the disease include impaired ability to eat or drink, as well as anxious or confused behavior. In the following couple of days, the horse will begin showing the classic symptoms of hypertonicity (sustained contraction) of the muscles of the muzzle, lips, and tongue. The mouth may be held open or closed, with the tongue hanging out in a curled manner to form a “V” shape. This is accompanied by constant chewing-like motions of the mouth, which can injure the tongue and other mouthparts. During this stage of chewing disease, horses are unable to eat pasture or hay, but they are still able to swallow. Muscle paralysis means that they are unable to drink water in a normal fashion, and horses may learn to submerge their muzzles deeply so that water will flow into the esophagus, allowing it to be swallowed. Other abnormal behaviors observed include yawning, violent head tossing, drowsiness, and other locomotor impairments.

If left untreated, horses normally die of starvation, dehydration, or inhalation pneumonia. Due to the irreversible neurological damage that occurs, euthanasia of afflicted animals is recommended.

Management: Prevention and Control Measures

Generally, these plants are not highly palatable to horses, so toxicity stems from horses being forced to eat Russian knapweed or yellow star-thistle because no suitable forage is available. Horse owners should monitor grazing conditions on their pastures or rangeland, and if they notice horses consuming these toxic plants, immediately remove the horses from the infected area and provide alternative forage. As part of a weed management program, herbicides such as those listed in Table 1 can be used to control Russian knapweed and yellow star-thistle.

Since clinical symptoms result from irreversible damage to brain tissue, the outlook for recovery of horses showing signs of poisoning is poor. If a horse survives, the owner can expect permanent impairment of the horse’s nervous system. Therefore, preventing consumption is the only certain means of preventing clinical symptoms and death.


Table 1. Herbicides Currently Labeled for Control of Russian Knapweed and Yellow Star-Thistle in Pastures and Rangeland1

Common Name

Trade Name

Application Rate


Time of Application

Russian Knapweed


Tordon 22K*

1 to 2 qt

Early flower to frost

Clopyralid + 2,4-D


1 to 2 qt

Full bloom to frost



2/3 to 1 1/3 pt



12 oz

Fall and winter



5 to 7 oz


Telar XP

1 to 3 oz

Prebloom to bloom and fall rosette

Aminopyralid + metsulfuron


2 1/2 to 3 1/3 oz

Spring or fall

Aminopyralid + florpyrauxifen


16 oz


Aminopyralid + 2,4-D


2.1 pt


Yellow Star-Thistle


Escort XP

1 oz

Seedling to early bud

Metsulfuron + 2,4-D + dicamba

Cimarron MAX

Rate III: 1 oz + 4 pt

Dicamba + diflufenzopyr


4 oz




3 pt

Spring to early bud


Esteron 99 and others

1 qt



1 pt


Tordon 22k*

1 pt


Banvel, Clarity

1 pt



2/3 pt

Picloram + 2,4-D

Grazon P+D*

2 qt



3 to 5 oz

Aminopyralid + florpyrauxifen


16 oz

Aminopyralid + 2,4-D


2.1 pt

Picloram + fluroxypyr


1.5 pt

1Always follow the herbicide label, which supersedes this table. Some herbicides may injure non-target plant species and have use restrictions.

*A pesticide applicator license may be required to purchase the product. Be sure to use adjuvants described on herbicide labels. Further control recommendations can be found in NMSU Extension Circular 597, Chemical Weed and Brush Control for New Mexico Rangelands (


Burrows, G.E., and R.J. Tyrl. 2001. Asteraceae Dumort. In Toxic plants of North America (pp. 156–160). Ames: Iowa State University Press.

Knight, A.P. 1995. Plant poisoning of horses. In L.D. Lewis (Ed.), Equine clinical nutrition: Feeding and care (pp. 466–467). Philadelphia: Williams and Wilkins.

USDA. 2017. Field guide for managing Russian knapweed in the Southwest [TP-R3-16-13]. USDA Forest Service, Southwest Region. Retrieved September 12, 2022, from

USDA. 2017. Field guide for managing yellow starthistle in the Southwest [TP-R3-16-07]. USDA Forest Service, Southwest Region. Retrieved September 12, 2022, from

Young, K., and C. Spackman. 2021. Chemical weed and brush control for New Mexico rangelands [Circular 597]. Las Cruces: New Mexico State University Cooperative Extension Service.

For further reading

B-709: Milkweed Poisoning of Horses

B-712: Oleander Poisoning of Horses

B-713: Locoweed Poisoning of Horses

Original authors: Jason L. Turner, Extension Horse Specialist; Keith Duncan, Extension Weed and Brush Specialist; Jesse LeFevre, Jicarilla Apache Extension Agent.

Photo of Jason L. Turner.

Jason L. Turner is a Professor and Extension Horse Specialist. Jason was active in 4-H and FFA while growing up in Northeastern Oklahoma. His M.S. and Ph.D. studies concentrated on equine reproduction, health, and management. His Extension programs focus on proper care and management of the horse for youth and adults.

The pesticide recommendations in this publication are provided only as a guide. The authors and New Mexico State University assume no liability resulting from their use. Please be aware that pesticide labels and registration can change at any time; by law, it is the applicator’s responsibility to use pesticides ONLY according to the directions on the current label. Use pesticides selectively and carefully and follow recommended procedures for the safe storage and disposal of surplus pesticides and containers.

Brand names appearing in publications are for product identification purposes only. No endorsement is intended, nor is criticism implied of similar products not mentioned. Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.

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Revised January 2023 Las Cruces, NM