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Authors: Respectively, Extension Horse Specialist; Extension Brush and Weed Specialist, both of the Department of Extension Animal Sciences and Natural Resources; and Jicarilla Apache Extension Agent, all of New Mexico State University.
Russian knapweed (Centaurea 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 stem perennial that grows to approximately 3 feet 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).
Figure 1. Russian knapweed flowers. Photo courtesy of Steve Dewey,
Utah State University, Bugwood.org.
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 feet.
Figure 2. Yellow star thistle plants. Photo courtesy of John O'Loughlin,
Grant County Noxious Weed Program Coordinator.
Figure 3. Yellow star thistle flowers. Photo courtesy of John O'Loughlin,
Grant County Noxious Weed Program Coordinator.
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 their ability to swallow is not compromised. 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 to 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.
Table 1. Herbicides Currently Labeled for Control of Russian Knapweed and Yellow Star Thistle in Pastures and Rangeland
|Application Rate Amount/Acre||
Time of Application
|Picloram||Tordon 22k||1 to 2 qt||Early flower to frost|
|Clopyralid: 2, 4-D||Curtail||1 to 2 qt||Full bloom to frost|
|Clopyralid||Reclaim||2/3 to 1 1/3 pt|
|Imazapic||Plateau||12 oz||Fall and winter|
|Aminopyralid||Milestone||5 to 7 oz|
|Chlorsulfuron||Telar XP||1 to 3 oz||Prebloom to bloom and fall rosette|
|Aminopyralid + Metsulfuron||Chaparral||2 1/2 to 3 1/3 oz||Spring or fall|
|Yellow Star Thistle|
|Metsulfuron||Escort XP||1 oz||Seedling to early bud|
|Metsulfuron + 2, 4-D: Dicamba||Cimmaron Max||Rate III 1 oz. + 4 pt|
|Dicamba: Diflufenzopyr||Overdrive||4 oz||Rosette|
|Triclopyr||Remedy||3 pt||Spring to early bud|
|2, 4-D||Esteron 99 And others||1 qt|
|Picloram||Tordon 22k||1 pt|
|Dicamba||Banvel, Clarity||1 pt|
|Picloram + 2, 4-D||Grazon P + D||2 qt|
|Aminopyralid||Milestone||3 to 5 oz|
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.
Knight, A.P. 1995. Plant Poisoning of Horses. In L.D. Lewis, Equine clinical nutrition: Feeding and care (p. 466-467). Philadelphia: Williams and Wilkins.
Burrows, G.E. and R.J. Tyrl. 2001. Asteraceae Dumort. In Toxic Plants of North America (p. 156-160). Ames, IA: Iowa State University Press.
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Printed and electronically distributed February 2011, Las Cruces, NM