Milkweed Poisoning of Horses

Guide B-709
Revised by Jason L. Turner and Greg Alpers
College of Agricultural, Consumer and Environmental Sciences, 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)

Introduction

There are a wide variety of milkweed plants responsible for poisoning and death of horses. These plants grow in a range of soil and moisture conditions, from roadsides and ditches to pastures and rangelands. The growing season varies with the particular species, but growth (and the greatest incidence of poisoning) occurs primarily in warm weather from March to September.

Description Of Plant

Milkweed (Asclepias spp.) is an erect-stemmed, herbaceous perennial plant that grows from 3 to 4 feet tall (Figure 1). The more common species can be differentiated by their leaf shape and arrangement as well as the color of the flowers. The plants are characterized by the thick, milky sap that seeps out when the plant stem is broken. Milkweed seed pods contain many seeds, and each seed has a tuft of silky white hairs. For more information on identifying milkweed varieties, consult your local county Extension agent (https://aces.nmsu.edu/county/) or see NMSU Extension Circular 678, Poisonous Plants of New Mexico Rangelands (https://pubs.nmsu.edu/_circulars/CR678/).

photo of Horsetail milk weed

Figure 1. Horsetail milkweed (Asclepias subverticillata) is one of the more toxic species of milkweed plants.

Toxic Principles

The toxic nature of milkweed is due to one of two agents: cardiac glycosides (cardenolides) or an unidentified neurotoxin. The cardiac glycosides are found in the majority of milkweed species, while the neurotoxin is specific to the whorled-leaf types such as horsetail milkweed. Of the two, the neurotoxin is the most lethal.

The lethal dose of milkweed varies among species and, to a lesser extent, plant parts, but consuming green plant material in an amount equivalent to 0.005 to 2.0% of the horse’s body weight is considered to be a toxic dose. For a 1,000-pound horse, this toxic dose could be between 0.5 and 20 pounds of plant matter. While the fresh, green plant material is the most toxic, dried plants present in pastures or hay retain their toxicity.

Symptoms

Signs of milkweed poisoning due to cardiac glycosides include depression and reluctance to stand, irregular heartbeat, colic, dilated pupils, muscular weakness or tremors and uncoordinated gait, and labored breathing, with death following within 24 hours. Unlike the neurotoxicity described below, poisoning due to cardiac glycosides rarely results in convulsions, so death usually occurs without signs of struggle.

Signs of whorled milkweed poisoning affecting the nervous system include severe colic, dilated pupils, muscle tremors and falling down, incoordination, violent convulsions, and respiratory failure, normally leading to death within 24 hours of ingesting the toxin.

If you believe that your horse may have been exposed to milkweed toxins, remove the suspect feed immediately and call your veterinarian. He or she may be able to administer a course of treatment that can help minimize the absorption of the toxic compounds by the digestive system. Supportive therapy may aid horses in combating the neurotoxicity symptoms.

Management: Prevention And Control Measures

Milkweed is very unpalatable, and horses do not readily consume it. The risk of poisoning increases when pastures or rangelands are overgrazed and horses have no other suitable forage to eat. Therefore, horse owners should closely monitor grazing conditions on their pasture or range to prevent overuse, and develop a weed management plan for areas such as field edges, fence lines, irrigation ditches, and roadsides where milkweed infestation is more common. In addition to sound grazing management, herbicides can be an important component of a comprehensive weed management plan. Herbicides that are currently labeled for control of milkweed are listed in Table 1.

The greatest incidence of milkweed poisoning in horses occurs when milkweed species have been baled along with the normal hay crop. Although horses may be able to sort broadleaf and course-stemmed milkweed plants from the hay, the whorled-leaf plants commonly found in hay fields have finer stems and leaves, making them more difficult to find and sort out. While horse owners should always be diligent in inspecting hay and grain as it is fed to their horses, they should be extra cautious when feeding hay that is harvested from high-risk areas (such as field edges or roadsides) where milkweed infestation is likely to occur.

Table 1. Herbicides and Application Rates for Control of Milkweed on Rangeland and Pastures¹
Common name	Trade name	Application rate (amount/acre)	Time of application
Picloram	Tordon 22K*	1 qt	Seedling to bloom
Picloram + 2,4-D	Trooper P+D*	1 to 2 qt
Dicamba	Banvel, Clarity	1 pt
Dicamba + 2,4-D	Weedmaster	1 to 2 qt
Metsulfuron + 2,4-D + dicamba	Cimarron Max	Label Rate II 1/2 oz + 2 pt
Picloram + fluroxypyr	Surmount*	1.5 pts
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* (https://pubs.nmsu.edu/_circulars/CR597/).

Conclusion

Although milkweed is unpalatable to horses, the potential for death due to ingestion of the plant is high. Therefore, horse owners should take appropriate measures to rid their pastures of milkweed and ensure that the hay they provide their horses is safe and free of milkweed. As is the case with many plant poisonings, preventing consumption of the plant is the only guaranteed means of avoiding toxicity and potential death.

References

Allison, C.A., J.L. Turner, and J.C. Wenzel. 2016. Poisonous plants of New Mexico rangelands [Circular 678]. Las Cruces: New Mexico State University Cooperative Extension Service.

Burrows, G.E., and R.J. Tyrl. 2001. Asclepiadaceae R.Br. In Toxic plants of North America (pp. 125–135). Ames: Iowa State University Press.

Hart, C.R., T. Garland, A.C. Barr, B.B. Carpenter, and J.C. Reagor. 2003. Toxic plants of Texas (pp. 34–39). College Station: Texas Cooperative Extension Service.

Knight, A.P. 1995. Plant poisoning of horses. In L.D. Lewis, Equine clinical nutrition: Feeding and care (pp. 486–489). Philadelphia: Williams and Wilkins.

Knight, A.P. 2003. Asclepias species. In A guide to poisonous house and garden plants (pp. 38–40). Jackson, WY: Teton NewMedia.

USDA–ARS. 2018. Milkweed (Asclepias spp.). Retrieved September 12, 2022, from https://www.ars.usda.gov/pacific-west-area/logan-ut/poisonous-plant-research/docs/milkweed-asclepias-spp/

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-710: Russian Knapweed and Yellow Star-thistle Poisoning of Horses
https://pubs.nmsu.edu/_b/B710/

B-712: Oleander Poisoning of Horses
https://pubs.nmsu.edu/_b/B712/

B-713: Locoweed Poisoning of Horses
https://pubs.nmsu.edu/_b/B713/

Original authors: Jason L. Turner, Extension Horse Specialist; Keith Duncan, Extension Brush and Weed Specialist; and Kyle Tator, Valencia County Extension Agent. Subsequently revised by Jason L. Turner and Kert Young, Extension Brush and Weed Specialist.

Jason L. Turner is Associate 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.

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Revised January 2023 Las Cruces, NM