By Nettie Liburt, MS, PhD Equine Specialist
Horses will usually avoid toxic plants, but if they are hungry, bored or curious enough, they might be willing to try something new. Good pasture management, including regular mowing and smart planting, along with common sense can eliminate many potential hazards. Plenty of good forage will help deter a horse from sampling toxic plants, chewing fences or picking up vices. It will also help maintain normal gut function.
Below is a brief (i.e. not all-inclusive) list of some of the common plants that have the potential to harm horses, along with symptoms of toxicity. The information was summarized from the Horse Owner’s Field Guide to Toxic Plants by Sandra Burger.
Alsike Clover: Photosensitivity, slobbers
Black Walnut: If used for bedding, can cause symptoms of laminitis, leg swelling, depression
Boxwood: Often used for landscaping. One pound of leaves can kill a horse. GI upset, blood in stool, respiratory distress
Horsechestnut: Often used as a decorative planting. Toxins affect the nervous system, may cause twitching or incoordination
Hydrangea: Contains cyanide. Labored breathing, weakness, gastroenteritis, bloody diarrhea
Japanese Yew, related to American Yew (Ground Hemlock): Some reactions are immediate, other delayed. Confusion, diarrhea, decrease in circulation and death
Mistletoe: Sudden death with major poisoning, colic in less severe cases
Nightshade varieties: Weakness, twitching, congestion in heart, lungs and spleen
Oak Tree: Anorexia, excessive thirst/urination, bloody diarrhea, kidney & liver damage
Poison Hemlock: Pupil dilation, trembling, cold extremities, paralysis, death
Pokeweed: Blood in stool, muscle weakness, salivation
Red Clover: If allowed to become moldy, a fungal toxin is produced (evidenced by brown spots seen on the leaves) causing salivation, stiffness or diarrhea.
Red Maple Tree: Anemia, causing depression and pale mucous membranes
Rhododendron: Excessive salivation, depression, colic, kidney and/or liver damage
St. John’s Wort: Photosensitivity, increased temperature and heart rate
A lot of the symptoms described above could be characterized as “non-specific.” Any time a horse seems weak, depressed, has blood in the stool or shows any sign of abnormal behavior or attitude, a veterinarian should be consulted immediately. For further reading, some excellent sources of information are listed below. There are some websites that have links and pictures, all are guaranteed to teach you something you didn’t know. The book, Horse Owner’s Field Guide to Toxic Plants, comes highly recommended, complete with summaries and pictures. More info below:
• Horse Owner’s Field Guide to Toxic Plants by Sandra M. Burger (Breakthrough Publications, 1996)
• Rutgers University Equine Science Center Website — http://www.esc.rutgers.edu/ask_expert/Poisonous_Plants.htm
• ASPCA Animal Poison Control Center Website for Toxic Plants & Horses — http://www.aspca.org/site/PageServer?pagename=pro_apcc_horsetoxicplants
• Cornell University Poisonous Plants Informational Database — http://www.ansci.cornell.edu/plants/alphalist.html