There are hundreds of poisonous plants growing wild in North America. Luckily horses are pretty discerning and don’t tend to bother with strange, bitter plants when their pasture is full of delicious grasses to graze on. They also have size on their side. It takes a lot of leaves to impact a thousand pound animal! However, some of the plants on this list are extremely dangerous because of their potency and the fact that repeated grazing can build up over time, leading to catastrophe.
The ability to recognize the following vegetation by sight will help you protect your horse at home and when you are riding in unfamiliar areas. According to Colorado State University, and their website, Guide to Poisonous Plants, these are the 10 most toxic plants to horses in North America:
1. Bracken fern(Pteridum aquilinum)
Also known as: brake fern, eagle fern
Bracken fern grows in clumps all across the United States in woodlands and moist open areas. Ingestion of bracken fern inhibits the absorption of thiamin (vitamin B1) leading to neurological impairment. Signs of vitamin B1 deficiency may include depression, incoordination and blindness.
Although ingesting a few leaves at one time is relatively harmless for horses, the effects of bracken fern can build up with repeated ingestion, and some horses seem to develop a taste for it. If consumption is discovered before the neurological signs are severe, horses can recover with treatment.
2. Hemlock (Conium maculatum)
Also known as: poison hemlock, spotted hemlock
Hemlock grows wild throughout North America. Its leaves, stems and seeds contain several neurotoxins that affect the nervous system, causing nervousness, tremors and incoordination within 1 – 2 hours. Depression and diminished heart and respiratory rates come next, followed by death. Four to five pounds is a lethal dose for a horse, and there is no treatment. Animals that ingest smaller amounts may recover with supportive care.
3. Tansy ragwort (Senecio spp.)
Also known as: groundsel
About 70 species grow throughout the United States – many of which are common in pastures and along roadsides. Toxicity levels vary, but all ragwort is thought to contain pyrrolizidine alkaloids, which inhibit cell division, especially in the liver. Symptoms include photosensitization (sensitivity to light), diminished appetite and weight loss, progressing to depression, incoordination and jaundice. Consuming between 50 and 150 pounds at once or spread out over time causes cumulative and irreversible liver damage.
4. Johnsongrass/Sudan grass
Johnsongrass grows wild in open, uncultivated areas in Southern climates. Sudan grass is cultivated throughout the United States as a forage crop. The leaves and stems of both grasses contain a cyanide compound that is typically harmless to horses if they ingest healthy adult plants.
The danger comes when the grasses are damaged through wilting, trampling, frost, etc. This can chemically alter the cyanide within the leaves, making them extremely toxic to all species. Both grasses can also accumulate toxic levels of nitrates if overfertilized.
Cyanide poisoning inhibits the body’s ability to absorb oxygen, which leads to death by suffocation. Signs include rapid breathing, tremors, frequent urination and defecation, gasping and convulsions. Veterinary treatment can be effective in cases of mild poisoning.
5. Locoweed (Astragalus spp. or Oxytropis spp.)
Also known as: Crazy weed
Several toxic species of locoweed grow in varied terrains throughout the West and Southwest. They contain swainsonine, which disrupts the function of brain cells when ingested. Horses with “locoism” may bob their heads, stagger, fall over, or display an awkward, high-stepping gait. There is no treatment for advanced locoism, and its effects are irreversible. Horses with less severe poisoning may recover when access to the weed is removed.
6. Oleander(Nerium oleander)
Also known as: Rose laurel, adelfa, rosenlorbeer
Beautiful but deadly oleander prefers hot climates, and is used in landscaping across the southern United States, from California to Florida. It is also grown as a potted plant in northern areas. The entire plant contains the toxins oleandrin and neriin, which cause irregular heart beat. 30 to 40 leaves can be deadly to a horse. Signs include colic, difficulty breathing and tremors. The toxins act fast, but horses can survive if treated quickly with activated charcoal to inhibit toxin absorption and drugs to stabilize the heart.
7. Red maple trees (Acer rubrum)
Red maple trees are native to eastern North America, from Canada to Florida and west to Minnesota and eastern Texas, but have been planted all over the country. As with Johnsongrass and sudan grass, wilted leaves are what pose a threat to horses. The toxins in wilted leaves cause the red blood cells to break down, preventing the blood from carrying oxygen. Organ damage or failure may also occur. As little as a pound or two of leaves can be fatal to a horse.
Signs include lethargy, refusal to eat, dark red-brown or black urine, pale yellowish gums and mucous membranes at first, advancing to dark muddy brown, increased respiratory rate, rapid heart rate, and dehydration. Symptoms may appear within a few hours, or take as long as four or five days after ingestion.
The only treatment is massive doses of IV fluids and possibly blood transfusions. Recovery depends on how many leaves were consumed and how promptly the horse receives veterinary care.
8. Water hemlock (Cicuta spp.)
Water hemlock is one of the most toxic plants in the US and grows throughout the country in marshy areas and along streams or irrigation ditches. All parts of the plant contain a cicutoxin alkaloid that affects the central nervous system, but the toxin is most concentrated in the root. Less than a pound of the leaves and stems can be fatal.
Symptoms such as drooling, dilated pupils, nervousness, trouble breathing, convulsions and damage to cardiac and skeletal muscles can occur within an hour of ingestion. Death from respiratory paralysis follows within two to three hours. If supportive care is started before the convulsions begin, it may offset the worst effects of the seizures, but even horses who survive are likely to have permanent damage to the heart and skeletal muscles.
9. Yellow star thistle/Russian knapweed (Centauria spp.) Barnaby’s thistle
Both plants appear throughout the Western United States, approximately from Missouri to California, and from Mexico northward, almost to Canada. Both plants contain a toxic agent that inhibits the nerves that control chewing. Poisoning is chronic in nature. A chronic dose is 50 to 200 percent of the horse’s body weight consumed over 30 to 90 days. Symptoms of exposure include tense or clenched facial muscles, inability to bite or chew their food effectively, and weight loss. There is no treatment, and nerve damage is permanent. Euthanasia is recommended if the horse is too debilitated to eat.
10. Yew (Taxus spp.)
Western yew and American yew are native to the West Coast and to the Eastern and central United States, respectively. These and other species are commonly planted as ornamentals nationwide. All parts of the yew plant, except for the berries, contain taxine, an alkaloid that causes respiratory and cardiac collapse. The leaves remain toxic even after dried. A single mouthful can be deadly to a horse within minutes.
Sudden death is the most typical sign of yew ingestion. Animals found alive may be trembling and colicky, with difficulty breathing and a slowed heart rate. There is no treatment for yew poisoning.
H/T to Equus Magazine