The Wrong Herbs for Your Herbivore: When Plants Attack

Eliminate these items from the large animal lunch menu

Published: March 25, 2013

Photo by Teri Ann Oursler, DVM
Most hooved animals such as horses, cattle, sheep, and goats are herbivores – plant eaters.  From this terminology, it’s easy to get the idea that herbivores and herbs go together like peas and carrots (as Forest Gump would say).  But if we humans sometimes have an uneasy relationship with our food, think of how much more difficult it would be to exist in a world where the line between “food” and “not-food” was blurry beyond the usual ‘ordering clams at a chain restaurant in a land-locked state might be a bad idea’ worries.

Contrary to popular mythology, animals don’t have an inborn alarm system that sounds whenever their lips touch something that falls into the “non-food” category.  I’ve known some horses, for instance, that simply divide the world into “things that move” and “things I can try to eat.”

Even classification under “things that move” doesn’t always eliminate an item from the equine lunch menu. While sheep are a bit more selective, goats tend to take a very similar approach to diet, and cattle have been known to nosh on dead truck batteries.

All of this means that even herbivores aren’t necessarily great at distinguishing between yummy, nutritious plants and plants that want to destroy the animal in science-fictionally horrific ways.  Translation: Poisonous plants that convince you to install them as part of the landscaping near animal enclosures or that infiltrate your yard and pastures are, in fact, trying to kill your animals.

Here’s a breakdown of some common offenders.  A more complete list can be found  from the folks at the ASPCA Animal Poison Control Center.

Landscaping Plants

Residents of central California know oleander as “the freeway plant.”  Common to freeway medians and public landscaping in areas with “Mediterranean” (translation: hot and dry) climates, this plant with its long, shiny, dark green leaves seems deceptively mundane.  It isn’t.

Oleander kills.  All parts of the plant are toxic, causing severe gastro-intestinal, neurologic, and cardiac effects.  Bit of trivia – it takes two oleander leaves to kill a cow.  Oleander might be attractive, but it is best viewed from a distance, especially if you eat plants for a living.

Azalea (Rhododendron)
Another common landscaping shrub with pretty flowers, azalea is right up there with oleander in the “attractive but deadly” category.  According to the ASPCA website, “rhododendron is typically not very palatable to horses unless it is the only forage available, but sheep and goats may graze readily on the plant.”  Grazing on even a few leaves causes badness ranging from “digestive upset” to coma and death.  Did I mention that this is bad?

Locust trees
The sinister nature of this cute, rather fluffy tree first hit my radar when two patients of mine, unfortunately belonging to a good friend, even more unfortunately consumed large quantities of locust tree (bark, leaves, etc.) during a period of “we’re tired of standing here like good horses” boredom.  The hungrier (or perhaps more bored) of the two horses died, showing signs of severe abdominal distress (colic) and shock.  We managed to pull his buddy through with lots of IV fluids and I learned more than I ever wanted to know about black locust tree toxicity.  This information actually came in handy later that same summer.  Public Service Announcement:  Do not tie your horse to a locust tree.  Do not pen your horse in a pasture or corral containing a locust tree.  Do not feed locust trees to your horse.  If your horse shows signs of colic after eating parts of a locust tree, DO inform your veterinarian immediately.  Do not pass “Go.” Do not collect $200.

Despite living in an area that probably has a eucalyptus population second only to Australia, I’ve had far more questions about eucalyptus toxicity than actual cases that could be attributed to malice on the part of the eucalyptus.  I have seen a couple of horses impale themselves on fallen branches, but that isn’t the tree’s fault.  Horse like to impale themselves.

The short answer is that yes, there are toxic chemicals in eucalyptus, but the animal has to eat a lot in order to experience toxicity.  I’ve known a few goats who’ve spent most of their lives grazing the fallen leaves of the eucalyptus with no discernible ill-effects.

This evergreen may have a happy, Christmas-tree look to it, but it is no jolly St. Nick of a tree.  The yew means business.  Eating the leaves and berries can cause sudden cardiac failure.

An Irish priest once told me that parishioners in Ireland planted yew trees around their cemeteries to prevent the British landowners from grazing their cattle among the graves.  I have no idea whether this story is true, and frankly, it’s far too colorful for me to risk the disappointment of researching it only to have it debunked.  However, true, or not, the clinical moral to the story is the same.  Keep your cattle (and horses, sheep, goats, etc.) away from yew trees.  Also, grazing livestock in the cemetery is just rude.

Another common landscaping staple, boxwood is usually trimmed into pleasing hedge or topiary-style geometric forms.  It’s also bad to eat.  Boxwood ingestion in horses causes colic, diarrhea, respiratory failure, and seizures.  Trust me, these are not considered good conditions from the horse’s standpoint.

Pasture infiltrators

When you have an animal that ideally lives among and dines upon plants, it can be pretty challenging to remove it from potentially sinister plants – particularly when those plants infiltrate the animal’s habitat.

The “pasture walk” is a not-uncommon diagnostic tool for the large animal veterinarian confronted with one or more (especially more) animals displaying weird signs.  This stroll becomes even more necessary when the owner says, “he/she/they seemed fine yesterday.” The pasture walk does not resemble the “silly walk” but it does involve a lot of ground-gazing and periodic sudden stooping.

The problem with any sort of summary of pasture plants is that wild plants, unlike gum, are regional.  This means that my list of common pasture invaders may not match the “most wanted” list for your neck of the woods, so be sure to chat with your veterinarian about your local pasture menaces.

This one gets my top toxin vote not just because of its nifty name (given because the flowers resemble the neck of a violin or fiddle) and pretty color, but also because it has killed several of my patients and caused some expensive veterinary bills for a few more.

Fiddleneck contains a toxin called pyrrolizadine alkaloid (PA).  In my head, I translate this term into “kills the liver.”  Fiddleneck and other PA-containing plants such as tansy ragwort and common groundsel cause liver failure in pigs, chickens, cattle and horses.  Sheep and goats are less sensitive to PA toxicity.

Fiddleneck and its PA-laden friends don’t just live in pastures, where animals might avoid them in favor of yummier grass, but they also grow in hay fields.  Animals are notoriously non-finicky about anything provided to them in the context of a “meal” (sort of like humans and anything provided in a fast-food container), and can easily consume toxic doses of these plants if their hay or pellets are contaminated.

In my experience, some animals will eat anything that their little tongues and teeth can reach.  They don’t seem to care whether the books say that a plant is unpalatable.  They just want to test it for themselves.

Though, because animals don’t generally go out and make a giant PA salad, toxicity usually builds up over a long period of time.  However, because the liver is very forgiving (almost doormat-like), it can take a ton of damage before it starts sending up the warning flags that say ‘all is not well here.’  Unfortunately, the flags sort of go up as the liver ship is sinking, meaning that by the time owners (and veterinarians) see clinical signs in animals suffering from PA-toxicosis, it is often too late.

Signs may include weight loss, diarrhea or constipation, and jaundice, but often also include signs that the brain is being affected by a build-up of by-products normally filtered by the liver.  These signs may include stumbling, head-pressing (the patient basically walks into the nearest object – fencepost, barn, human and presses its head relentlessly against said object), circling, and aimless wandering.

These things are all quite detrimental to the long-term survival of the animal.  PA toxicity in horses tends to end with fewer horses in the pasture.

Lupine (also spelled Lupin; see photo at top of article)
The pretty purple flowers dot the foothills in many parts of the U.S. every spring, bringing joy to most but problems to others. While the bean pods of the lupine can be toxic to humans as well as animals, causing depression of the heart and nervous systems, the more common issue is for grazing livestock. When pregnant animals graze on lupine, the chemicals in the plant can cause life-threatening birth defects in the fetus.  

Alsike clover
This pretty little plant causes damage to the skin, liver, and mucous membranes (mouth, tongue) of horses. Alsike clover causes the skin to be sensitive to light, resulting in lesions that look like severe sunburn.
Systemic symptoms include weight loss, liver disease, nervous system depression, and discolored urine.

Teri Ann Oursler, DVM
Bracken fern
Bracken Fern (photo at right)
If you live in wooded areas, your horses might run into this one.  All parts of the plant are toxic and interfere with thiamine (an essential vitamin) absorption.  The resulting thiamine deficiency causes weight loss and severe neurologic signs.

This one always makes me think of Socrates (“I drank what??”).  But hemlock is a real, modern plant, and ingestion of hemlock by animals produces the same end-effect as Socrates’ bowl of hemlock tea.  Did I mention that the hemlock latte was the ancient Greek equivalent of lethal injection?

Fortunately, hemlock is apparently not all that tasty, so poisonings aren’t very common.

Here’s another plant with lovely historical overtones.  Nightshades contain chemicals that mimic the drug known as atropine.  In small doses, atropine has a lot of beneficial activities.  However, in a nightshade salad, the atropine is overdone, causing excessive salivation, loss of appetite, GI upset, depression, weakness, decreased heart rate, and pupil dilation.  (Historical trivia digression: upscale ladies during the Renaissance used nightshade to make their eyes look bigger – think the cats on black velvet look.)

The list of killer plants could extend way beyond the scope of this article, and we can’t just name them all Audrey II.  Hopefully, though, you’ve got the basic idea.  Not all plant-eating animals should eat all plants.  Know what is in your hay (hint: With hay, as with so much else, you get what you pay for; junk hay often sells for junk prices.), and know the pedigree of any plants in, or near, animal enclosures.  Don’t count on your animals’ “wild instincts” to protect them.  We’ve bred the wild out of most of our domestic species and trained them to believe that almost anything within reach counts as food.



June 8, 2019

Thanks for the humour throughout, great reading.  I am impressed with how my sheep avoid the rhododendron and just nibble on the nightshade - hopefully their instincts will keep them safe in my garden

Chris Crewes
January 19, 2019

Good point about the black locust tree. Walnut trees can also be even more lethal to horses. Bedding made from walnut sawdust or chips can even kill horses in a few hours. Note: Although the black locust tree is toxic to horses, the honey locust tree, with much larger thorns, isn't. In fact the bean pods from honeylocust trees make excellent livestock feed while the thorns will keep out unwanted visitors if used in a hedgerow.

Christy Corp-Minamiji, DVM 
March 25, 2013

Thank you for your feedback and for your question/clarification.  I double-checked, and, according to an entry by the University of Illinois, in some nightshades, the atropine-like compounds are predominate.  However, the chemical causing bradycardia (decreased heart) rate is solanocapsine.  Thanks very much.  I learn something new every day! 

March  25, 2013

Hey, nice article, I just want to clarify that a high dose of atropine would cause a severe tachycardia, not a decreased heart rate as you mentioned. It would also reduce salivation, not increase it. Atropine competes for the muscarinic receptors.

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