Very red maple leaves on branches.

Don’t Let Your Horse Eat Red Maple Leaves

Samantha ShieldsAnimal Health

The red maple (Acer rubrum) is one of the most common and widespread native deciduous trees of eastern and central North America. These fast growing trees are usually planted for their beautiful red leaf color in the fall season, and as a shade tree. However, the fallen leaves could be deadly for horses. Ingestion of wilted or dry leaves by horses can result in red maple poisoning, and if left untreated might be fatal. If associated signs of poisoning are observed or suspected, a veterinarian should be called immediately.

Signs of Red Maple Poisoning

A horse only needs to eat from 1.5 to 3 pounds of leaves to be affected (Lenz T, 2019). Within one to two days after ingestion, a poisoned horse would exhibit these clinical signs :

  • Severe anemia.
  • Depression.
  • Lethargy.
  • Pale to yellow mucous membranes.
  • Red to brown urine.
  • Increased respiratory rate.
  • Fever.
  • Weakness.

Eating the leaves results in methemoglobinemia, which means abnormal levels of methemoglobin are circulating in the bloodstream. A compound in the leaves causes oxidation of iron in hemoglobin that creates the methemoglobin, so it can no longer release oxygen throughout the body. The abnormal hemoglobin results in damage to the red blood cells. The breakdown of red blood cells leads to blood pigments being filtered out through the kidneys and excreted in the animal’s urine. (Wright & Leuty, 2006). A telltale and concerning sign of red maple toxicity is dark red/brown urine, but some of the clinical signs resemble those of other diseases. If horses or other livestock present with these signs of toxicity, or grazed in an area where red maple leaves were on the ground, a veterinarian will be able to rule out other causes. Treatment of red maple poisoning is focused on relief for symptoms and preventing further ingestion of the leaves.

How to Identify Red Maple Trees

Red maple trees are easy to identify by their v-shaped, 3-lobed leaves that turn bright red to yellow during the fall, and their smooth gray bark. Green leaves are not toxic if eaten right off the tree. Once the leaves are damaged, begin to wilt, or fall from the tree they become toxic. The cause of toxicity is still unconfirmed, but many believe it is related to gallic acid (Baisden J, 2013), which also occurs in silver and sugar maple leaves. Further testing and experimentation are required to confirm what actually causes the poisoning.

Biosecurity Measures to Protect Horses

Colts, foals, yearlings and fillys are at the highest risk for poisoning, along with horses kept in dry lots, and those that don’t get enough roughage. All horses are potentially vulnerable though. Biosecurity measures that limit an animal’s exposure to fallen red maple leaves include:

  • Removing trees or cutting down branches to be out of the reach of animals.
  • Diligently removing leaves and fallen branches from pastures.
  • Avoiding red maple trees in or around pastures.
  • Allowing access to plenty of hay and water.
  • Monitoring animals in pastures daily.

When planning a layout for pasturing horses or other animals, it is important to look closely at the vegetation surrounding the area; red maple is not the only plant that is toxic to livestock. Some other common toxic plants are nightshade, hemlock and jimsonweed. Daily biosecurity observation and monitoring for signs of livestock diseases is also useful for early detection of poisoning. Whether the issue is an infectious disease or toxic plants, a veterinary consultation and examination is the next step in providing appropriate treatment as soon as possible.


Lenz T. (2019). Red Maple Poisoning. Retrieved from

Wright B. Leuty T. (2006). Red Maple Leaf Poisoning in Horses. Retrieved from

Baisden J. (May 2013). Isolation and Characterization of a Suspected Phytoalexin from Wilted Red Maple Leaves. Retrieved from

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About the Author

Samantha Shields

Samantha Shields was an undergraduate student at the University of Vermont, studying for a Bachelor of Science in Biological Science, and a minor in Animal Science. She worked with ADBCAP Director Julie M. Smith, DVM, PhD, as an online outreach assistant intern. Samantha assisted with content development for the Healthy Farms Healthy Agriculture website by evaluating, summarizing, and presenting information about protecting animal health. At the University of Vermont, she was an active member of many programs including RALLYTHON, University of Vermont Program Board, Campus Recreation, and the Women’s Club Hockey. Samantha is interested in pursuing the field of epidemiology after graduation. She has a particular interest in the research of different factors that result in diseases, as well as public health emergency planning and response.

About the Editor

Joanna Cummings

Joanna Cummings received a Bachelor of Science in Horticulture from The Pennsylvania State University (PSU), with a specialization in vegetable crop and greenhouse production. At PSU, she was a research technician on no-tillage vegetable crop experiments, and a greenhouse assistant in the All-American Selections Research Gardens. Her career in the agriculture industry includes field research, work with dairy and vegetable farms, and as a greenhouse manager, estate gardener, landscaping and market garden entrepreneur. She transitioned to the science communication field after receiving a master of science degree in environmental communications from Antioch University New England. At Antioch, Joanna was a field botany laboratory teaching assistant, manager of the herbarium, and editor of the department's student newsletter, Notes and Niches. She currently works with Research Professor Julie M. Smith as a communications professional in the University of Vermont Animal and Veterinary Sciences Department. She is the webmaster for the Healthy Farms Healthy Agriculture website.

About the Editor

Dr. Julie Smith

Dr. Julie Smith is a research professor at the University of Vermont. Julie received her B.S. in biological sciences, D.V.M., and Ph.D. in animal nutrition at Cornell University. Since 2002, she has applied her veterinary background to programs in the areas of herd health, calf and heifer management, and agricultural emergency management. She has taught courses in animal welfare, calf biology and management, and ABCs of biosecurity to undergraduates, mostly in the Department of Animal and Veterinary Sciences. As a veterinarian and spouse of a dairy farmer, Julie is well aware of the animal health and well-being concerns of dairy animals. Julie has conducted trainings for Extension educators, livestock producers, and community members on the risks posed by a range of animal diseases, whether they already exist in the United States, exist outside of the United States, or pose a risk to both animal and human health. In all cases, she emphasizes the importance of awareness and prevention. In addition, she has led a number of projects on biosecurity and emergency disease preparedness.