Top 10 Poisonous Plants for Horses and Livestock

Do You Know Which Plants Can Kill Horses?

Horses, goats, sheep, and other animals eat plants for food. However, not every plant is safe for even the most robust herbivores. Lots of plants make animals (and people) sick. But some plants can kill.

Horses on a hill. Horses can die if they eat too much toxic plant material

Photo by Lindsay Shaver

Toxic Plants

Not all plants are equally toxic. Some plants are only toxic if a large quantity is eaten by the animal. For example, hemlock is toxic to a horse if only four or five pounds of leaves were eaten by the animal. On the other hand, a single mouthful of yew (Taxus spp.) can kill a horse. Some plants are more toxic than others and their effects on the animal are more noticeable.

Occasionally, the California Supplemental Exam for landscape architects will include a question or two about what kind of plants are safe to use around an equestrian trail. This list will help you understand which plants can cause trouble for our four-legged companions.

These are the most dangerous plants for horses and other livestock.

  1. Water hemlock (Cicuta spp.)
  2. Yellow star thistle/Russian knapweed (Centauria spp.)
  3. Yew (Taxus spp.)
  4. Johnsongrass/Sudan grass (Sorghum spp.)
  5. Locoweed (Astragalus spp. or Oxytropis spp.)
  6. Oleander (Nerium oleander)
  7. Red maple trees (Acer rubrum)
  8. Bracken fern (Pteridum aquilinum)
  9. Hemlock (Conium maculatum)
  10. Tansy ragwort (Senecio spp.)

Some of these plants are wild species and grow in pastures. Other, such as oleander and red maple, are common ornamental species and are widely planted in parts of the state.

Lets look at each species in more detail.

Water hemlock (Cicuta spp.)

Water Hemlock Is Toxic to Horses

Water hemlock flowers. Photo by SCCF Nursery

Water hemlock is a close relative of poison hemlock and can kill animals if only a few pounds are eaten.

Water hemlock is considered one of the most toxic plants in the United States.

Water hemlock is a perennial plant that has six-foot tall stems topped with flowers. All parts of the plant are toxic and have a powerful neurotoxin that causes a variety of maladies including respiratory failure. This is one of the most dangerous plants for humans and animals.

Yellow Star Thistle/Russian Knapweed (Centauria spp.)

Yellow star thistle is an invasive annual weed of Eurasian origin. The spiny foliage and yellow flowers make it easy to spot along roadsides and in fields. It is more common in interior areas of the state and grows well in infertile, disturbed soils. Horses and other animals are cumulatively poisoned by this plant and must eat half or more of their body weight of this particular weed before showing symptoms of poisoning. The plant has a toxic agent that has a neurological effect on the brain that inhibits the nerves and control chewing. There is no treatment for the neurological damage caused by yellow star thistle.

Yew (Taxus spp.)

Yew is a common landscape plant in the northern part of California. There are also a couple of native species that grow in the Coastal Ranges and Sierra Nevada mountains. Yew is used by landscape architects for its great architectural lines, deep green color, and nice texture. However, this good-looking plant has a dark side. The foliage (and to a lesser extent, the berries) are highly toxic to horses and humans. If a horse eats only a few ounces, it could be dead. Yew has toxic compounds in the foliage that contain taxine, an alkaloid that causes respiratory and cardiac collapse. The effects of yew poisoning are immediate and irreversible.

Johnsongrass/Sudan grass (Sorghum spp.)

Johnsongrass is a seductive plant for herbivores. The grass grows up to six feet tall and has a wide blade. Johnsongrass is an invasive weed in California and is commonly found growing in moist spots and drainage ditches in agricultural areas of the state. Cyanide poisoning can happen when the foliage is eaten by animals. Always consult with a veterinarian if you suspect Johnsongrass poisoning.

Locoweed (Astragalus spp. or Oxytropis spp.)

Loco means “crazy” in Spanish. As you might guess, animals start behaving strangely after eating this plant. While it may be desirable to remove locoweed from the area, some species are protected by federal and state law. Poisoning is cumulative and the plant has a toxin that interferes with the body’s ability to process sugars.

Oleander (Nerium oleander)

Nerium oleander is toxic to horses and other herbivores. Photo by Leonora Enking

Nerium oleander is toxic to horses and other herbivores. Photo by Leonora Enking

More horses and other livestock are killed by eating oleander than any other toxic plant. Oleander is a common ornamental plant in California. Do not plant oleander near areas where horses are kept or ridden.

About 30 to 40 leaves can be deadly to a horse.

It does not take much foliage to poison a large mammal. The effects of start in a couple of hours and can last more than one day. With immediate care from a veterinarian, the effects can be reduced.

Red maple trees (Acer rubrum)

Red maple is an exotic shade tree that is widely planted in cooler areas of the state and is appreciated for its brilliant fall color. Fresh foliage is not much of a hazard for horses. However, wilted foliage has chemical compounds that affect the blood system. Horses usually encounter wilted red maple foliage after a storm breaks branches and the leaves begin to wilt. The survival rate depends on how much foliage was eaten by the horse and the amount of supportive care available.

Bracken fern (Pteridum aquilinum)

Bracken fern and its relatives are commonly found in open forests or fields in the mountains of northern California. While most horses will usually find something more appetizing, some animals seem to develop a taste for bracken fern and will seed it out even when other dining options abound. Fern fronds contain a chemical that prevents the animal from absorbing vitamin B1. Fortunately, a horse must eat hundreds of pounds of the foliage before the effects become severe.

Poison Hemlock (Conium maculatum)

Poison hemlock is a perennial weed found in pastures and roadsides throughout the United States. As the name suggests, the plant is quite toxic to humans and animals. Only a few pounds of the foliage and stems can lead to death from respiratory failure. The signs start to appear in just a couple of hours and the survival rate depends on the amount of plant that was eaten by the animal. Children have been killed by the toxic effects of this plant. Usually, kids will use the hollow stems as “pea shooters” and are poisoned when they put the stem in their mouth.

Tansy ragwort (Senecio spp.)

Tansy ragwort is a widespread weed with many species native to California and the western United States. While the toxicity varies from species to species, all members of the genus have toxic alkaloid compounds that cause liver damage if a large amount of the plant is eaten.

What Can Landscape Architects Do To Prevent Poisoning Animals?

Avoid planting these plants near areas where horses or other livestock could come into contact with the foliage. Maintenance personnel can help keep the invasive weeds in check and reduce the chance that horses will eat some of the wild toxic plants.

For a more thorough discussion, please read “10 Most Poisonous Plants for Horses” at  EQUUS magazine’s website.

Read more about toxic plants and planting design safety in “Pass the California Supplemental Exam for Landscape Architects” and “Practice Problems for the California Supplemental Exam”.

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John is a landscape architect who is currently preparing to take the California Supplemental Exam to become licensed in California. He is currently a licensed professional landscape architect in Georgia and Florida. John graduated from California State University, Pomona with a BSLA degree in landscape architecture in 1997 and has extensive practice experience in residential and commercial landscape design.

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