Russian Knapweed and Yellow Star-Thistle Poisoning of Horsesaces.nmsu.edu/pubs/_b/B710.pdf · The...
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1 Respectively, Professor/Extension Horse Specialist, Department of Extension Animal Sciences and Natural Resources; Extension Brush and Weed Specialist, Department of Extension Animal Sciences and Natural Resources; and Extension Agent, Jicarilla Apache Nation Extension Office, New Mexico State University. COLLEGE OF AGRICULTURAL, CONSUMER AND ENVIRONMENTAL SCIENCES aces.nmsu.edu/pubs • Cooperative Extension Service • Guide B-710 Russian Knapweed and Yellow Star-Thistle Poisoning of Horses Revised by Jason Turner, Kert Young, and Jesse LeFevre 1 All About Discovery! TM New Mexico State University aces.nmsu.edu The College of Agricultural, Consumer and Environmental Sciences is an engine for economic and community development in New Mexico, improving the lives of New Mexicans through academic, research, and extension programs. INTRODUCTION Russian knapweed (Rhaponticum repens, formerly Centaurea repens or Acro- ptilon repens) and yellow star-thistle (Centaurea solstitialis) are unusual among poisonous plants in that they are toxic to horses—causing “chewing dis- ease”—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 approxi- mately 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). 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 ap- proximately 3 ft. Figure 1. Russian knapweed flowers. (Photo courtesy of Steve Dewey, Utah State University, Bugwood.org.)
Transcript of Russian Knapweed and Yellow Star-Thistle Poisoning of Horsesaces.nmsu.edu/pubs/_b/B710.pdf · The...
1Respectively, Professor/Extension Horse Specialist, Department of Extension Animal Sciences and Natural Resources; Extension Brush and Weed Specialist, Department of Extension Animal Sciences and Natural Resources; and Extension Agent, Jicarilla Apache Nation Extension Office, New Mexico State University.
COLLEGE OF AGRICULTURAL, CONSUMER AND ENVIRONMENTAL SCIENCES
aces.nmsu.edu/pubs • Cooperative Extension Service • Guide B-710
Russian Knapweed and Yellow Star-Thistle Poisoning of Horses
Revised by Jason Turner, Kert Young, and Jesse LeFevre1
All About Discovery!TM
New Mexico State Universityaces.nmsu.edu
The College of
Agricultural,
Consumer and
Environmental
Sciences is an
engine for economic
and community
development in New
Mexico, improving
the lives of New
Mexicans through
academic, research,
and extension
programs.
INTRODUCTIONRussian knapweed (Rhaponticum repens, formerly Centaurea repens or Acro-ptilon repens) and yellow star-thistle (Centaurea solstitialis) are unusual among poisonous plants in that they are toxic to horses—causing “chewing dis-ease”—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 PLANTSRussian knapweed is a woody-stemmed perennial that grows to approxi-mately 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).
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 ap-proximately 3 ft.
Figure 1. Russian knapweed flowers. (Photo courtesy of Steve Dewey, Utah State University, Bugwood.org.)
Guide B-710 • Page 2
TOXIC PRINCIPLESThe exact chemical compound responsible for toxic-ity 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 ef-fects of Russian knapweed and yellow star-thistle are cu-mulative, 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 con-sume 60% of its body weight in green Russian knap-weed 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.
SYMPTOMSThe 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 accom-panied 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. Mus-cle 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 esopha-gus, allowing it to be swallowed. Other abnormal be-haviors 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 irre-versible neurological damage that occurs, euthanasia of afflicted animals is recommended.
MANAGEMENT: PREVENTION AND CONTROL MEASURESGenerally, 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 moni-tor 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
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.)
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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 dam-age 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 consump-tion is the only certain means of preventing clinical symptoms and death.
REFERENCESBurrows, 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. 2015. Field guide for managing Russian knap-weed in the Southwest [TP-R3-16-13]. USDA Forest Service, Southwest Region. Retrieved July 12, 2017, from https://www.fs.usda.gov/Internet/ FSE_DOCUMENTS/stelprdb5410125.pdf
USDA. 2015. Field guide for managing yellow starthis-tle in the Southwest [TP-R3-16-07]. USDA Forest Service, Southwest Region. Retrieved July 12, 2017, from https://www.fs.usda.gov/Internet/ FSE_DOCUMENTS/stelprdb5410133.pdf
Young, K. 2017. Chemical weed and brush control for New Mexico rangelands [Circular 597]. Las Cruces: New Mexico State University Cooperative Extension Service.
Table 1. Herbicides Currently Labeled for Control of Russian Knapweed and Yellow Star-Thistle in Pastures and RangelandCommon Name Trade Name Application Rate
(amount/acre)Time of Application
Russian Knapweed
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 Cimarron 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
Imazapyr Arsenal 1 pt
Picloram Tordon 22k 1 pt
Dicamba Banvel, Clarity 1 pt
Clopyralid Reclaim 2/3 pt
Picloram + 2,4-D Grazon P+D 2 qt
Aminopyralid Milestone 3 to 5 oz
Original authors: Jason L. Turner, Extension Horse Specialist; Keith Duncan, Extension Weed and Brush Specialist; Jesse LeFevre, Jicarilla Apache Extension Agent.
Jason L. Turner is a Professor and Extension Horse Specialist at NMSU. He was active in 4-H and FFA while growing up in Northeastern Oklahoma. His M.S. and Ph.D. studies concentrat-ed on equine reproduction, health, and management. His Extension programs focus on proper care and management of the horse for youth and adults.
Guide B-710 • Page 4
Contents of publications may be freely reproduced for educational purposes. All other rights reserved. For permission to use publications for other purposes, contact [email protected] or the authors listed on the publication. New Mexico State University is an equal opportunity/affirmative action employer and educator. NMSU and the U.S. Department of Agriculture cooperating.
Revised October 2017 Las Cruces, NM
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.
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 direc-tions on the current label. Use pesticides selectively and carefully and follow recommended procedures for the safe storage and disposal of surplus pesticides and containers.
