horse's diet. Perhaps I should have done this earlier in the year but, hey I didn’t think to. I have only covered it here because I was hit on the head by a load of acorns as I walked my dog the other day and that reminded me just how many plants and trees can be poisonous for horses. So, apologies for the lateness, but as the English say – better late than never!It's important to know what plants to look for in each season. Different plants grow and become toxic at different times of the year. For example although red maples are toxic when green during the growing season, the wilted or recently shed red leaves in the fall are most potent in causing anemia and kidney damage in horses. Other poisonous plants are most toxic as young sprouts in the early spring or when toxic seeds are produced late in summer.
Sometimes, what is safe for a person to eat can be toxic to a horse. Cabbage, broccoli, onions, chives, kale, elderberry, pokeweed, and rhubarb can be toxic for herbivores such as horses. So be careful what you toss over the fence to the horses!
The Solanaceae family of plants provides a good example. Though it includes the infamous deadly nightshade, we put several of nightshade's relatives on our dinner plate. Luckily, tomato's fruits and mature potatoes aren't toxic to us. But, the green parts of these and many related plants of this family contain atropine or atropine-like substances, and these are bad news for horses. Atropine will slow the gut. That's the last thing a horse wants because it will cause colic, which can kill a horse. Other toxic members of this family include ground cherry, henbane, and jimsonweed.
Some plants contain saponins, which are detergents, or soaps, and present another danger for horses. Soapy substances can irritate the digestive system and cause diarrhea, cramps, and colic, Plants that contain saponins include pokeweed, bouncing bet, corn cockle, and English ivy.
Horses don't normally get sunburn, but if you notice blistering and cracking on any white areas of your horse, he may have been munching on large amounts of prairie groundsel, rattle box (Crotalaria), and kochia (fireweed). Though members of different plant families, they all kill liver cells. When plant toxins damage the liver, photoactive substances derived from chlorophyll build up in the blood. These substances are exposed to the sun only in the white areas, so that's where you'll notice blistering, cracking, or loss of outer layers of the
skin.
Other photoactive substances, such as wild parsnip or St.-John's Wort, cause the
same skin lesions without first damaging the liver.
Milkweed, dogbane, oleander, yew, lily-of-the-valley, white snakeroot, azalea, and other Rhododendron species and purple foxglove all contain substances that may affect a horse's heart. These plants can hit the heart really hard and cause heart failure. Japanese yew can stop the heart suddenly. One or two leaves of oleander, a plant found mainly in California, Arizona, and Florida, can kill a horse. White snakeroot causes scarring in the horse's heart and may decrease performance and cause severe heart failure.
Horse owners should learn the list of plants, including many popular ornamentals, whcih can cause the biggest problems and even death in horses. A few castor beans or one little potted hydrangea can kill a horse. Larkspur, a plant sometimes found as an ornamental in the Midwest, can cause paralysis and respiratory failure. Wild black cherry, chokecherry, plum, and peach contain cyanide poison in the leaves, and especially in the seeds, which can be rapidly lethal. Sorghum can also be a source of cyanide. Socrates demonstrated the
deadliness of poison hemlock, a plant that is widespread in the Midwest.
If a poisonous plant is consumed by your horse, another animal, or a human, take a sample of the plant, estimate the quantity eaten, note the time of onset of signs, and call a veterinarian or physician immediately.
If you are not sure what certain poisonous plants look like, the University of Illinois even has a virtual tour of its garden at http://www.library.uiuc.edu/vex/vetdocs/toxic.htm
That was just a quick summary but for those of you who want to know more I have cover some of them in more detail below and in the next 2 issues. Today we cover:
• Yew
• Oleander
• Yellow Star Thistle
• Locoweed
• Timber Milk Vetch
• Lupine
• Poison Hemlock
Over the next 2 issues we’ll cover a lot more.
Now you need to know that I have not written the following – I am not a Vet or a plants man, so I have copied this information simply to make it easy for you to read. I would not re-write it because I was afraid of getting something wrong that may prove fatal to your horse, so I make no apologies for copying this.
Yew
The yew is a small evergreen tree found in the Pacific Northwest. All parts of the yew, along with many of its cousins, are extremely toxic and contain taxine,
a cardiac depressant. Symptoms include trembling, a slowing of the heart rate and cardiac failure which can occur in as little as five minutes after ingesting the plant. Some deceased horses were found with yew leaves or twigs still in their mouths. There is usually no time to initiate treatment and there is no known antidote. 
Oleander
Oleanders are a common ornamental shrub in the west and south, often growing along roadways and used as a visual barrier or wind screen. All varieties are extremely toxic. The horse's first signs of oleander poisoning will often be profuse diarrhea which may be bloody. Immediate veterinary intervention and a lot of good luck is required to prevent death, which often occurs within 8 to 24 hours after ingestion. Administration of laxatives to purge the remaining oleander from the horse can sometimes save the horse. 
Yellow Star Thistle
Eating yellow star thistle and Russian knapweed brings on nigropallidal encephalomalacia, commonly called "chewing disease". Typically horses eat the plant over an extended period of time before
symptoms appear. The ingested toxins actually cause a softening of parts of the brain and once symptoms appear, the animal will likely die. 
The horse will not be able to eat or drink, although he may attempt to do so. He may chew food only to spit it out when he cannot swallow, often acting as if something is caught in his throat. (Symptoms resemble the early onset of rabies, so you should consider rabies also as a possibility and not go reaching into the horse's mouth to look for obstructions.)
The horse may chew without having anything in his mouth, move his tongue and lips in unusual fashions, yawn frequently and assume unusual postures such as head hanging or pushing against solid objects.
Because this disease damages the brain and prevents the horse from eating, there is no recovery. Clinical experiments where poisoned horses were fed through stomach tubes have shown that even after extended periods of time, affected horses will not regain the ability to swallow.
Locoweed
Locoweed is a common name for plants which fall into two genera; Astraglus and Oxytropis. While there are hundreds of species of these plants, only about 20 are considered poisonous. These plants range from low growing ground covers to two foot tall clumps of flowers. They can be found throughout the west from Canada to Mexico in semi-arid foothills and plains. Some of these species are very similar to each other, so even experienced botanists can have difficulty differentiating between a poisonous and non-poisonous specimen.
Horses typically avoid locoweed, but once they have sampled it a few times, they can become addicted to it. They have to graze on it for a period of time before symptoms appear and the most obvious symptoms may not appear until well after the horse has stopped eating it. These symptoms include altered gaits, aimless wanderings, sometimes in circles, impaired vision (to the extent they bump into things or fall into arroyos or other depressions) and erratic behavioral changes. They may appear listless or complacent, then wildly overreact to some unexpected event. 
Locoweeds can have such an alkaloid content that one Nevada species has even poisoned the bees which were pollinating the plants. Alkaloid poisoning has a cumulative effect which can be absorbed over long periods of time until symptoms appear and the effects in many cases are irreversible. Horses found eating locoweeds who have then been confined away from the plants prior to showing advanced symptoms, and who have been fed good quality hay and feed, have experienced a slow but successful recovery.
Timber Milk Vetch
Like the Locoweeds, this group of plants falls into the genus Astraglus although the symptoms of poisoning are different and their onset is much more rapid. When excited, the horse will often produce a roaring sound when exhaling. Other symptoms include salivating and staggering. Death is usually sudden from asphyxiation.
Lupine
Lupines involve a large genus of plants, many species which are not poisonous. They typically display bunches of white, blue, purple or pinkish pea-shaped flowers on upright stalks. They are found in a wide variety of landscapes from ocean beaches to high mountain pastures. Like locoweeds, it is difficult for even the experts to distinguish the toxic from the non-toxic, so one should assume that all lupines are harmful if eaten.
While poisonous throughout the year, young lupines and those going to seed are the most toxic. Fortunately the effects are not cumulative so a lethal dose must be consumed over a short period, otherwise if removed from access to lupines and if symptoms are properly cared for, a poisoned horse should recover.
Symptoms include gastrointestinal irritation accompanied
by diarrhea. The horse's gait may change; the horse being reluctant to move and lifting his feet higher than normal when he does, acting nervous and displaying leg twitching. Loss of muscle control, prostrations, convulsions and coma may follow. While the "one time ingester" of lupines may completely recover, continuous consumption can produce toxic hepatitis. This can result from lupines being baled in hay cut from poorly managed fields and being fed to horses over time. Other than separating the horse from the toxic plants and treating the visible symptoms, there is no published treatment for lupine poisoning. 
Poison Hemlock
Poison hemlock, a relatively common range plant, kills a number of horses sheep and cattle each year and is extremely toxic to humans. All parts of the plant, commonly known as deadly hemlock, spotted hemlock, poison parsley, European hemlock, California fern and Nebraska fern, are poisonous. In the spring until the plant flowers, the leaves are especially poisonous.
Poison hemlock is often found along roadsides and creek beds. Humans are poisoned when they confuse poison hemlock roots with wild parsnips, or the seeds as anise seeds. This plant is so deadly that whistles made from the plant's hollow stems have been known to kill children. Fortunately, the toxin is relatively neutral after the plant has cured, so poison hemlock accidentally baled into properly dried hay has not posed a problem. 
Symptoms of poisoning will appear within a few hours after ingestion and may range from sudden, unexpected collapse to nervous trembling, salivation, lack of coordination, especially in the hindquarters, and dilated pupils. The horse's pulse may be weak and he may appear cyanotic (bluish appearance of the mouth and gums), and he may be disoriented or unaware of his surroundings.
In serious cases, death occurs within ten hours of the onset of symptoms due to respiratory paralysis. Less serious poisonings, which still may include periods of coma, can sometimes benefit from early use of stimulants and stomach tubing with mineral oil. If respiratory shutdown can be avoided and the horse can pass any remaining ingested leaves, full recovery is possible without any long term ill effects.
From the author:
Roger Bourdon has written a book on Introduction To Horsebackriding and Horse's Health. (Article Adapted by All About Horses)
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