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Plants Poisonous To Horses

The traditional agricultural practices both allowed and encouraged a greater variety of pasture species to exist compared to today. Before the 1940's, a paddock would probably have contained well over 100 different varieties of herbage, compared with todays adverage of prehaps 10 species in the modern pasture.
As a number of different species of plants declined in the countryside (mainly due to the use of nitrogenous fertilizers), the general knowledge about the plants affect on animals declined aswell. Many toxic plants disappeared along with the beneficial species, therefore it became less important to identify them as they became less of a threat to livestock.
The trend away from the extensive use of chemicals together with the employment of more traditional methods of farming means that species diversity may increase. Along with the nutural benefit this will bring, there is the risk that many of the potentially poisonous species of wild plants will return, together with those which, although not outright poisonous in themselves, may be toxic to some degree. It is therefore becoming more important to be able to identfy a wider range of potentually dangerous herbage in order to access the risk for your grazing livestock.

We have searched the internet and various great books, to bring this page together, below you will find 119 potentially poisonous plants of varying degrees. You will find out information about each of these weeds and other useful information.

Chemical analysis of plants can determine their actual or supposed action on the chemistry of the body. For practical evaluation, however, there are also other factors which have to be taken into consideration. i.e. - not all parts of some poisonous weeds are toxic at certain times of the year, or even at certain times of the day. An example of this is the opium poppy. At nine am is it reputed to contain a whopping 4 times more morphine in it than it does at midday.
Also small amounts of some toxic species can be quite beneficial for the horse if ingested as part of a holistic nutritional profile, which means that the plant may be regarded as poisonous in some circumstances, but not in others.
The time taken from when the poisonous plant is first eaten to when the first appearance of the symptoms appears may vary greatly. i.e. the effects of eating yew leaves and the cowbane roots show up much more quickly than those produced by eating bracken or ragwort. The initial effects of some weeds vary greatly depending on the species of the animal eating it. Deers are able to eat rhododendron leaves, which if eaten by goats or cattle will most likely cause death. There is also a variation within certain species. i.e. some rabbit breeds are adversly affected by deadly nightshade which causes death, while others seem ammune.

Traditionally plants were classified according to their poisonous ratings. There are many of these including - alkaloids, glycosides, saponins, nitrates, minerals and phenols to name a few. These are either synthesised by a chemical process within the plant itself, or concerntrated by the plant from the soil.

Alkaloids : are present in many plant species and are the dominant poisonous principle in high-rish toxic plants like yew and hemlock. As well as being poisonous, alkaloids can also be used for medical purposes. They can be depressives, stimulants, narcotics or painkillers. Horses will usually avoid alkaloid-containing plants as they have a characteristically bitter taste, although even after poisoning, some horses seemed addicted to the taste. Treatment involves drugs to counteract the effect of the alkaloid on the Central Nervous System. In those that do survuve the treatment, seldom ever recover 100 percent.

Glycosides : are a large group of organic substances, many of which aren't poisonous. Those that are poisonous can be divided into 4 maing categories which determine their toxicity. Although some poisonous glycosides don't fit into these groups. The groups are as follows.
Cyanogenic.... Cyanogenic plants arent toxic as such, but after they have been broken down by the digestives system they become poisonous, releasing the poisonous chemical hydrocyanic acid (cyanide). Cyanogenic plants include plants from families like Rosacaea, Leguminosoa and Gramineae. The flax plant is cyanogenic, and the seeds must be boiled before feeding to destroy its poisonous principle.
Goitrogenic.... The poisonous plants contained in the category mainly come from the Cruciferae family - the cabbage and turnips being examples. Plant goitrogenic inhibit the uptake of iodine by the body, and poisoning is indicated by an increase in the size of the thyroid gland, which is clinically recognised as a goitre. While goitres arising from iodine deficiencies may be treated by giving iodine, this treatment has no effect after poisoning by plant goitrogens as the causative mechanism is different.
Cardiac.... Cardiac glycosides have a direct action on the heart muscle, increasing its contractions whilest slowing the rate. Other affects are gastro-enteritis and diarhoea. If cardiac glycosides poisoning occurs, surgical elimination of the plant from the system is the only option for saving the animal. If the substance is ingested in lethal quantities and left undisturbed death will occur within 12 - 24 hours. Foxglove, Lily of the Valley and Hellebore spp. are sources of Cardiac glycosides.
Saponic.... Saponic glycosides, or saponins are distinguishable by their ability to form a lather, (sapo means soap) and to emulsify oils. They are widely distributed and are found in forage legumes like lucerne, clover and many other plants. Saponins are more toxic to animals when injected than when eaten, although ingestion of large quantities can cause diarrhoea.

Nitrates/Nitrites : are not very toxic in themselves, but are converted by bacteria in the alimentary tracts into nitrites, which are much more toxic. Nitrates are absorbed from the soil by plants and the rate of absorption can be increased by several factors - shade, the use of herbicides and especially the use of nitrogenous fertilizers being a few factors. Foods such as beet, turnips, mangles, rape, swedes and kale are more likely to accumulate high levels of nitrates than other plants, though accumulation in other plants can be significant. Nitrates pass form the gastro-intestinal tract into the bloodstream, where they combine with elements in the blood to form a substancecalls methaemoglobin, which limits oxygen transportation. The symptoms are those of oxygen deficiency - weakness, rapid pulse rate, fall in blood pressure etc. Death can occur within hours of the initial poisoning of eating nitrate-rich foods, though more commonly it takes days for the symptoms to appear. Ingestion of high levels of nitrogen have also been linked to infertility, abortion and vitamin imbalences.

Oxates : exist in many species of plants, but are more highly concerntrated in certain types. Large variations exist in the amound of oxalates present in different species of plants at any one time, depending on soil conditions and climate. Also the more mature the plant comes the more concerntrated the toxic becomes, especially in the leaves. The effect depends on the amount ingested over a given time and on the nutritional status of the animal, the amount of calcium in the diet, etc. The symptoms of poisoning are rapid and laboured breathing, staggering, recumbency and depression. Horses can become adapted over a period of time to higher levels of oxalates in their diet, the greatest risk being when large amounts of oxalate-rich plants are ingested quickly. This causes Hypocalcemia to be produced, which is caused by a combination of oxalates and calcium in the blood. A post-mortem of affected animals show that various tissues and organs - notably the kidneys, contain deposits of calcium oxalate crystals.

Photosensitive Agents: cause unpigmented or partially pigmented areas of the skin such as the muzzle, to become hypersensitive to ultraviolet rays from the sun. This will cause cell damage. These agents - the furocoumarins - are contained in several different species of plants, the St John's Wort being one. They do not need to be ingested to produce this hypersensitive reaction, as skin contact can trigger the reaction. Primary photosensitivity by ingestion will occur when furocoumarins are transported byt he body unchanged to the skins surface. Second photosensitivity can take place after the plant has been broken down by digestion, anbd the likelihood of this will be increased if the alimentary capacity of the liver has been compromised. This means that sensitivity to ultraviolet rays can be increased by a secondary photosensitivity if liver damage has occured through disease, hepatoxic drugs, chemicals, etc.

Danger Ratings.
The weeds have been divided into 2 basic groups, which are represented by the symbol that appears alongside each weed. Qualified advice should be taken if there is even the slightest doubt about the identity of plants that are suspected of being poisonous.

HIGH RISK!!!! Do not allow access.

May be appropriate for ingestion on a regular basis depending on the overall nutritional profile, together with the health status of the horse, but can be hazardous in some circumstances. Still it is best to be rid of these weeds, stopping any accidental poisoning to occur. Better to be safe than sorry.

Please click on the letter links below to view the various pages of weeds.

{A / B}
Alder Buckthorn
Autumn Crocus Avacado
Black Bryony
Black Cherry
Black Locust
Black Nightshade
Black Oak
Black Walnut
Bounching Bet
Buckeye and Horsechestnut
Bulb-Bearing Plants

{C / D / E / F}
Castor Oil Plant
Common Groundsel
Common Milkweed
Common Pokeweed
Common Vertch
Cuckoo Pint
Cutleafed Coneflower
Deadly Nightshade
Death Camas
Death Cap
Dutchman's Breeches
English Ivy

{G / H/ I / J / K}
Green Cestrum
Green False Hellebore
Ground Ivy
Hemlock Water-Dropwort
Hemp Nettle
Herb Paris
Horse Radish
Hound's Tongue
Kentucky Coffee-Tree
Kidney Vetch

{L / M}

Lily Of The Valley
Marsh Marigold
Meadow Saffron
Mexican Poppy
Monks Hood

Murray Phasey Bean


{O / P / R}
Patersons Curse
Poison Hemlock
Rattle Box
Rosary Pea

{S / T / W / Y}
Sensitive Fern
Spreading Dogsbane
Star Of Bethlehem
St Johns Wort
Sweat Pea
Tall Fescue
Tall Laskspur
Thorn Apple
white Bryony
White Snakeroot
Woody Nightshade
Yellow Star Thistle

Common Burdock
Giant Hogwort
Stinging Nettle
Poison Ivy

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