Toxic plants, shrubs and trees

Toxic plants in pasture, roadside or while out riding

Jacob's wort

The most famous and poisonous plant is the Jacob's wort. Fortunately, horses leave this fresh in the field. Only when it is dried and ends up in hay or haylage does the horse eat it. An amount of 5% of body weight of dried Jacob's wort can be lethal. All parts of the plant contain toxins, falling under the Pyrrolizidine Alkaloids. These cannot be broken down in the liver, causing liver cells to break down. This is irreversible and can lead to liver failure.

Poisoning often does not manifest itself until serious liver damage is already present. The first symptoms of illness are languidness, weight loss, diarrhea, decreased appetite and lots of drinking. Jacob's wort eventually affects the heart and central nervous system. Poisoning can be acute when the horse has ingested a lethal dose in a short period of time. But it can also be dormant, when the horse ingests small amounts each time. It can then just take years before symptoms become apparent. Unfortunately, there is no cure for poisoning from this plant.

Horsetail species

Field horsetail or horsetail and swamp horsetail (also called Lidrus) are most common. They grow on calcareous soils. Marsh horsetail grows especially in wet meadows and is most poisonous. The entire plant is poisonous; however, dried in hay it is less toxic than as a fresh plant. The toxic substance equisetin blocks the absorption of Thiamine (vitamin B1). Vitamin B1 plays an important role in carbohydrate metabolism and the breakdown of carbohydrates to obtain energy, especially for the brain and nervous system. It is important for the proper functioning of the nervous system. It promotes the transmission of impulses between cells, especially in the muscles. In addition, vitamin B1 plays a role in the production of hormones, proteins and enzymes.

Vitamin B1 deficiency can cause slow/low heart rate, muscle twitching, ataxia and paralysis. Deficiency can also manifest as abnormalities in skin functions, nerve abnormalities, anemia, poor growth and reduced fitness. Ultimately, vitamin B1 deficiency can lead to brain damage.

Bastard clover

Unlike red and white clover which are reasonably safe to eat, bastard clover is an unwelcome guest in pastures. Bastard clover often grows in acidic, nitrogen-poor soils, and the entire plant is toxic to horses. Even when dried in roughage, the plant remains toxic. The growing period is from May through September and is most toxic in spring and in moist pastures. With high ingestion, the horse may develop red skin rashes (photosensitivity) and swelling. This is seen especially on light or white skin and is easily treatable. However, liver damage can also occur because the body cannot process the toxin properly. Unfortunately, in this case, the damage to the liver is irreversible.

Water hemlock and spotted hemlock

Water hemlock blooms from June through August. It grows mostly on ditch sides and on peat soils. Even with this plant, the entire plant is poisonous, but the roots are the most dangerous. Even a small amount can be deadly. The plant contains neurotoxins and affects the extended medulla in the brain. This is where the respiratory center is located; it becomes paralyzed, resulting in respiratory arrest. After eating water hemlock, within an hour or two the horse will develop symptoms such as cramps, nervous behavior, wide pupils, dilated nostrils and coordination disorders. Eventually, the horse goes into respiratory arrest and dies. Unfortunately, there is no treatment and help often comes too late.

Dogwort

Dogweed grows mainly in wet and shady soils. The plant is toxic both fresh and dried in forage, but in the latter the toxicity does decline. After three months of drying, the dogstraw has lost a lot of its toxicity. When the horse has eaten larger quantities of the dog's trotter it causes a rapid weak pulse, excessive sweating and difficult breathing. It is fatal if the horse ingests large amounts.

Often when dogweed appears in your pasture, it indicates that soil health is not optimal.

Eagle Fern

Eagle fern grows particularly in dry, acidic and nutrient-poor soils in light deciduous forests. It is a plant you will encounter especially with an outdoor ride. The entire plant is poisonous, but the rhizome and young leaves are the most poisonous part of the plant and remain poisonous even dried. It mainly contains the toxin thiaminase (the word says it all); it interferes with the absorption of thiamine, or vitamin B1. The effects are similar to those of the horsetail. In fact, it also causes a vitamin B1 deficiency. If poisoning is diagnosed early, extra vitamin B1 can be given to restore vitamin B1 levels in the body.

Nightshadeaceae

There are many different types of nightshade and many are toxic to horses, including the black nightshade, the bittersweet, even tomatoes and potatoes are included in the nightshade species. These plants contain the toxin solanine, which causes breakdown of red blood cells. Thereby, it affects the central nervous system and the gastrointestinal system. After eating a nightshade-like plant, the horse suffers from gastrointestinal problems, gastroenteritis, diarrhea, loss of appetite and excessive salivation. Because solanine also affects the central nervous system, the horse develops coordination disorders, shakes and suffers from muscle weakness. When solanine affects the heart and respiratory center, the horse may die from it.

Of the black nightshades, the black berries contain the toxin and these are most toxic when still green. In potatoes, solanine is mainly found in the potato fruit and sprouts.

Poisonous plants in the garden or fence

Especially in gardens there are many plants that are very toxic to horses. The list is long, but we only pick out the ones that are commonly used as hedges or fences.

Yew

The most poisonous shrub, often used as a fence is the yew. Just one twig can be deadly to a horse. The yew contains Taxine, which affects the heart and respiratory system. The highest concentration of Taxine is found in the leaves. With yew poisoning, hardly any symptoms occur because it is so deadly. Treatment is virtually impossible.

Boxwood

Boxwood is also very toxic to horses and is commonly used as a hedge. Both the foliage, seeds and bark contain toxins. Even a small amount is lethal to the horse. The toxins affect the central nervous system and gastrointestinal tract. The horse develops severe diarrhea and colic shortly after eating boxwood. Coordination disorder, muscle weakness, paralysis and trembling follows. Intoxication often ends with respiratory arrest.

Bay laurel

The cherry laurel contains the toxic substance cyanogenic glycoside. After eating the plant, prussic acid is formed, it enters the blood fairly quickly. The substance is rendered harmless in the liver, but in larger quantities the liver cannot render it completely harmless and prussic acid poisoning occurs. Prussic acid blocks oxygen transport; with mild prussic acid poisoning, the upper respiratory tract and mucous membranes become irritated. At moderate concentrations, the horse suffers from cramps, accelerated breathing and may become unconscious and even die. At very high concentrations, prussic acid is almost immediately fatal.

Rhododendron

Rhododendron is common in gardens or you may encounter them with an outdoor drive. The leaves contain the toxic substance andromedotoxin. This substance affects the central nervous system and muscle tissue. It causes paralysis of the muscles. It is unknown how much the horse must eat of the plant until symptoms can be observed. Symptoms include excessive salivation, colic, paralysis and eventually the horse dies from respiratory failure.

Foxglove

Foxglove is a very striking plant with pink, purple and sometimes white flowers. It contains three poisons especially in the leaves: digitoxin, gitoxin and gilatin. The plant is highly toxic and even a small amount of this (100 to 200 grams) is lethal. The symptoms are colic, diarrhea and cardiac arrhythmias, where you first hear a reduced, then an accelerated and irregular heartbeat. Eventually, the horse goes into cardiac arrest.

Toxic trees

Maple

Leaves or nuts of various trees can also be toxic to horses. The most well-known is the Maple tree. The seeds of the Maple are very recognizable, typical helicopter seeds. These seeds and seedlings of the Maple tree contain the toxin hypoglycine A. Eating the seeds and seedlings, most common in the spring and fall, causes atypical myopathy. This is a serious, often fatal muscle disease. In this disease, damage to muscle tissue occurs; this involves the release of myoglobin. Symptoms are similar to muscle weakness. Usually the horse dies within three days of the symptoms becoming apparent.

Oak

The oak is also a well-known tree which is poisonous to horses. The acorns and leaves contain the substance tannin. Tannin binds protein and iron; poisoning is seen only after eating larger amounts. If a high dose of tannin is ingested, the intestinal wall can be damaged, edema develops in the gastrointestinal mucosa and the horse develops severe diarrhea. The risk of poisoning is greatest in the fall, when the tree loses its leaves and acorns.

Beech

Beech is also a poisonous tree for horses. In particular, beechnuts can be poisonous, except that the horse must first ingest a large dose of beechnuts. Beech nuts contain the toxic compounds oxalic acid, thiaminase and phagine. These substances can affect red blood cells and cause hemolysis, kidney damage and anemia. In the process, the gastrointestinal tract is affected and the horse develops colic. The substance thiaminase blocks the absorption of vitamin B1.

Chestnut

To a lesser extent, chestnut is toxic. It is unknown how much the horse must ingest until it causes symptoms. In any case, the amount is quite high. The risk of poisoning is greatest in late summer, when the unripe chestnuts can fall from the tree and they contain tannins. The concentration of tannin is lower in chestnuts than in acorns.