Poisonous plants in horse feeds

Foto einer Giftpflanze für Pferde

"All things are poison, and nothing is without poison; the dosage alone makes it so a thing is not a poison." This maxim is from the 16th-century Swiss physician, natural philosopher, and alchemist Theophrastus Aureolus Bombast von Hohenheim, one of the most famous European physicians in history and today known primarily as “Paracelsus”. His teachings and observations are still considered the basis for naturopathy, spagyria, and other areas of holistic medicine. But let’s talk about poisonous plants. What considerations must be heeded in the feeding of horses and their pasture turnout? We take a closer look at these and other “toxic” aspects below.

How does poisoning occur?

Nature has provided us with a variety of different plants, all of which have their place in the circle of life. Every plant has a purpose, even if we humans may not immediately recognise what that is. As the above-mentioned Paracelsus quote tells us, there are poisonous plants which, through their strong effects, can often also be used for medicinal purposes. However, knowledge on their handling and dosage are essential. Plants with strong effects may not be simply eaten like normal feed plants, as even small quantities can elicit a strong systemic effect in the organism, with corresponding symptoms of poisoning occurring, depending on the area of action.

Wild animals have extensive natural instincts, giving them the ability to distinguish plants that are food from those that are poisonous. This ability has been observed not just in those wild animals that learn whilst very young from their mothers what to eat and what to avoid, as is the case with primates. Other wild animals that begin life on their own – tortoises, basilisks and other reptiles, also know instinctively which plants are edible for them, which are not, and even which will poison them. It has been repeatedly observed in various wild animal species that, should they fall ill or have other problems, will often travel great distances in order to find and eat a certain plant – a plant which they normally would avoid. The pharmacological knowledge of wild animals is currently being extensively researched in the relatively new science of zoopharmacognosy.

Poisons in the pasture

Our domesticated animals still possess some of these instincts. However, increasing breeding activity with certain breeds emphasises certain characteristics in animals whilst increasingly de-emphasising others, which are then lost. With animals that are kept in stabled in groups and fed daily by humans, or young animals that can no longer grow up on pastures with their mothers, their natural instincts may recede into the background, as these instincts are no longer needed due to the circumstances of their keeping. In losing their natural instincts, the animals also lose some of their ability to distinguish between edible and toxic plants in nature. This leads to frequent poisonings – especially young, inexperienced animals – when they are allowed to graze at pasture. Such poisonings occur less frequently in older, experienced animals that are accustomed to pasture grazing. As a precautionary measure, it is recommended that all known poisonous plants be removed from in and around the pasture so that they are no longer accessible to the animals.

Poisons in feed

Another way for an animal to ingest poisonous plants is when they are present in hay or other feeds. A healthy pasture should consist of a variety of herbs, grasses, and flowers, not all of which are digestible by the animals. One example is the widely found buttercup. Animals will avoid this plant when grazing, as it contains the toxic substance protoanemonin. Here, the natural instincts of the pastured animals are fully functioning. However, when buttercup is mixed with hay in dried form, it is much more difficult for animals to select around it. When fresh, buttercup (depending on the species) is classified as slightly toxic to toxic. The toxin that it contains, however, is converted into non-toxic anemonin during the drying phase. This process naturally takes some time. Consumption of fresh forage can therefore still cause symptoms of poisoning or indigestion, but in a well-dried state the buttercup is completely non-toxic and can be fed without hesitation.

In other plants, such as the lethally poisonous meadow saffron, the drying process does not remove the substance colchicine. Meadow saffron is indeed a highly toxic plant for horses. They will avoid the plant when it grows in the pasture, but it can make its way into their hay, especially when the hay is cut late in spring. As little as 400 g of dried meadow saffron can be lethal for a horse. The contamination of hay from meadow saffron is therefore highly problematic. In addition, the first signs of poisoning from it are frequently not recognised as such. Colchicine is a cytotoxin and is quickly reabsorbed by the organism. The first signs of poisoning occur in the inner organs with high cell growth, for example in the gastrointestinal tract. It causes colic, bloody diarrhoea, and cardiovascular disturbances. There is currently no antidote for colchicine, which is why colchicine poisoning should be avoided at all costs. In serious cases, only the symptoms can be treated. The administration of fluids containing electrolytes will help to flush out toxins and compensate for a substantial loss of fluids. Preparations made from milk thistle support cell regeneration as well as our most important detoxifying organ: the liver. Preparations made from milk thistle seeds can even be used successfully to treat death cap mushroom poisoning. To prevent meadow saffron poisoning, the spread of the plant should be curbed in hayfields. 

Powerful plant toxins

This explanation of the deadly poisonous meadow saffron brings us to the middle of Hecate's herb garden. In Greek mythology, Hecate is the goddess of magic and religious rites. She was the guardian of thresholds and crossings, as well as the gateways to other worlds. As an oracle, she communicated with spirits and the dead, and see into the future. Many have grown up around her secret garden where numerous toxic and hallucinogenic medicinal plants grow. These include strong poisonous plants such as deadly wolf’s bane (Aconitum); the foxgloves (Digitalis), which are used in heart medications; deadly nightshade (Atropa belladonna), which contains atropine; henbanes (Hyoscyamus); and hemlock (Conium maculatum), which has a smell similar to that of mouse urine and which the Greek philosopher Socrates was sentenced to drink. English yew (Taxus baccata), which is often planted as a hedge or cemetery tree, is the only conifer that is lethally poisonous in all parts of the plant, except for the red berries that enclose the poisonous seed kernel. This may be why, in Nordic mythology, the English yew served as a guardian between this world and the realm of the dead.

This and other powerfully toxic plants may therefore be welcome in Hecate’s mythological, medicinal garden, but not in our horse pastures, and so great care should be taken in the selection of ornamental plants for hedges and gardens that share borders with pastures. Problems repeatedly arise when young animals nibble on poisonous hedges or when poisonous plant parts make their way into the hay. The strong toxins found in the plant kingdom can be merciless. Look, for example at atropine. The genus name of deadly nightshade, “Atropa”, is derived from the name of one of the three Fates of European mythology. Atropos, “the unturning one”, is the Fate that cuts the life threads of all living creatures. But even with our knowledge of strong poisons and our duty to take extreme care, the wisdom of Paracelsus should not be forgotten. Thus the toxin in meadow saffron is also used as an anti-inflammatory and analgesic substance to treat acute gout. Paclitaxel, found in the deadly poisonous English yew, is used in cancer therapy and atropine is highly effective in treating low heart rate in intensive care and emergency medicine.   

Toxic effects of everyday plants around us

Toxic hedge shrubs

Many of our gardens, fields, and pastures are bordered by hedgerows. Popular, low-maintenance evergreen shrub species include thuja, English yew, and boxwood. These, however, are all poisonous if ingested. Thuja (“tree of life”) is a cypress plant and contains the toxic substance thujone. In humans, simply touching it can cause skin irritations. Unfortunately, pasture animals are repeatedly poisoned, even fatally, when they graze on hedges or hedge cuttings left on the ground. All parts of the popular boxwood are poisonous as well. Other typical but toxic hedgerow plants include yellow-flowering broom, which can cause nausea, gastrointestinal complaints and circulatory problems; common laburnum, which can cause symptoms in small children with the ingestion of just 2 or 3 seeds; privet, which is toxic for humans and animals; common laurel, which contains cyanogenic glycosides and causes death by respiratory paralysis, and red honeysuckles, with berries that are enticing for children and animals but which contain saponins and cyanogenic glycosides that cause unpleasant symptoms if ingested. When planting hedgerows and borders, therefore, it is important to select plants that are harmless for the animals. Being well-informed on which trees and shrubs are suitable for planting in pastures is essential. A widespread misconception, for example, is that horse chestnuts were once fed to horses. Horse chestnuts are actually toxic for horses and not suitable for feeding. However, horse chestnut was formerly used in small doses in decoctions to treat equine coughs. Various fruit trees, blackthorn, hawthorn, rose hips, hornbeams, birches, serviceberries, dogwoods, hazelnuts and numerous berry bushes are well suited for planting in pastures and their adjacent hedges. These tend to grow quickly into hedges, often sporting thorns and ensuring dense growth. Grazing animals will happily nibble on these plants, as well as many other non-toxic hedge shrubs, without harm. The plants are also popular with bees.

Poisonous woodland and meadow herbs

Poisonous plants can be found even amongst the everyday herbs that we encounter at the edge of the forest or in our meadows. Some are quite well known, such as the liver-damaging wood ragworts. One of the most well known of these is the yellow-blossomed common ragwort, also called stinking willie. The pyrrolizidine alkaloids they contain, which can cause liver function disturbances, are also found in numerous other plants, for example those in the family Boraginaceae. But, to paraphrase Paracelcus, it’s the dose that makes the poison. So although wood ragworts are to be avoided at all costs, occasional consumption of borage is considered harmless, since here the alkaloid content is so low that no symptoms of poisoning occur from the ingestion of moderate amounts. Borage should not be consumed in larger quantities and is therefore generally not a welcome sight on pastures. And some herbs are harmless to humans, enjoying regular use in cooking and home remedies, but are poisonous or even highly poisonous for certain animal species. One example is ground-ivy, also known by a long list of names. Herb lovers know this creeping plant as a popular spring herb. A delicious addition to salads or soups, it brings variety to the menu and also stimulates metabolism. For horses, however, ground-ivy is highly toxic and can even lead to death if ingested in large quantities. Symptoms of poisoning include dilated pupils, raised pulse, sweating, and shortness of breath.

There is a long list of further plants that can cause mild, moderate, or even severe poisoning. The examples given here are primarily intended to encourage people to learn more about poisonous plants so as to avoid possible poisoning of their animals. Detailed herb books can be helpful, or a tour of the pasture together with an expert.  

Bacteria and poison mushrooms – a glimpse into the microcosm

Another often overlooked aspect of poisoning is the ingestion of fungal toxins or harmful bacteria through spoiled food. This is why feed quality is especially important. Bacteria and fungi are all around us. Most are harmless and belong to our natural biome. For example, the bacteria in our intestinal tract are vital for our existence. However, other bacteria and fungi in our environment can cause diseases or develop harmful degradation products. For example, the degradation products of various moulds, the so-called aflatoxins, and other mycotoxins are considered highly poisonous and there are legal limits for their presence in feeds. Feeds used should be of good quality in any case. A constant ingestion of harmful microorganisms can lead to chronic stress for the animal and thus cause various ailments.

Some fungi contaminate feed grains while they are still in the field. One example of this is ergot (Claviceps purpurea). The ergot fungus attacks the ears of various grains as well as various grasses. Infested plants display black growths on individual grains. The ergot fungus sclerotium contains over 80 different alkaloids, which can be hazardous for humans and animals. One of the best known is lysergic acid diethylamide-25 – LSD – which was first isolated by the Swiss chemist Albert Hofmann. When ingested, these strong hallucinogens cause ergotism (ergot poisoning), also known in the Middle Ages as St. Anthony’s Fire. Symptoms include delusions, headaches, confusion, vomiting, sensory disturbances, and diarrhoea. Acute ergot poisoning can lead to respiratory arrest and even death by cardiac arrest. In addition to grains, the ergot fungus can also infect over 400 different grasses, so contamination is also possible through hay, especially from wet meadows or adjacent grain fields. Corn smut is also a cereal fungus. It is a parasitic smut fungus that affects maize cobs. In the initial stage, it is considered edible for humans, but in the further stages it also develops ergot alkaloids and has toxic effects on humans and animals.  

Detecting poisoning and what to do

Symptoms of poisoning can be greatly varied and often not immediately recognised. A slow, chronic intake of toxins can often lead to gradual symptoms, whilst strong poisons are mostly violent and acute. The symptoms will depend on the toxin ingested, but if you suspect that your animal has poisoning, you should contact your veterinary surgeon immediately. The source of the poisoning should be identified as soon as possible so that appropriate countermeasures can be taken, including prompt elimination of the source to prevent further exposure. Once your vet has diagnosed and treated the acute poisoning, you should take gentle steps to rebuild your animal’s damaged organism. Support can be found in medicinal herbs that stimulate the metabolism and excretory organs, like the liver and kidneys. Milk thistle has hepatoprotective and regenerating properties and is frequently used in treatment. However, dandelion, goldenrod, birch leaves, and common nettle can also help to stimulate metabolism and eliminate toxins. Hawthorn strengthens the cardiovascular system, rose hips supply needed amounts of vitamin C, and red echinacea strengthens the body’s immune system. Naturally, any subsequent therapies following an acute poisoning should be thoroughly discussed with the animal’s veterinary surgeon. Ideally, acute poisoning can be prevented by being fully informed about the poisonous plants all around us.



  • Das Buch der Gifte (Gustav Schenk, Safari Verlag Berlin 1954)
  • Pathologie und Therapie (Nothnagel)
  • https://www.botanikus.de/informatives/giftpflanzen/giftpflanzen-und-tiere/pferde/
  • https://eqwo.net/bilderlexikon-die-100-giftigsten-pflanzen-fuer-pferde/featured-de/news_/
  • Artikel: „Selbstmedikation in Wald und Flur“ auf Ärztezeitung.de 2009: https://www.aerztezeitung.de/Panorama/Selbstmedikation-in-Wald-und-Flur-373706.html
  • https://de.wikipedia.org/wiki/Zoopharmakognosie
  • https://flexikon.doccheck.com/de/Colchicin
  • https://www.botanikus.de/informatives/giftpflanzen/alle-giftpflanzen/gundermann-gundelrebe/

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