Transcript:

[0:03]

Before we get going today, if you can just hit that subscribe button, I’d really appreciate it and show your support for this podcast and this information that we’re putting out. Now, today the topic is: what plants are toxic to your horse and how do you spot them? This is a critical podcast for any horse owner because no matter where you live in the world, there are going to be plants that are toxic to your animals.

[0:30]

And chances are, they may come across them, and they could get sick and in some cases even die — which we’re going to talk about today. And if you think about it, you know, the cycle of not just plants but weeds is a big concern in many of our paddocks and pastures, where in the spring and summer there’s lots of growth of grass. If weeds haven’t taken root there, the grass flourishes — but as horses graze, sometimes with their grazing patterns they may graze an area intensely. That depends a lot on your pasture management, and that could leave a bare area where weeds can take root and take hold.

[1:16]

We typically see that no matter where it is in the world — weeds can start to invade a pasture, ring a paddock, or even pop up elsewhere on your property. And then oftentimes people will go and plant things to make their properties look nicer, their gardens nicer, without knowing these plants could kill your animal if they eat them. So again, a lot to consider today, and a lot that we’re going to talk about.

[1:42]

Really, the challenge with this podcast is that it’s going to be difficult to tell you, “Yeah, this plant, that plant, that plant, and that plant are toxic,” because there are hundreds of plants throughout the world that are toxic to horses. And in the modern world today, we see plants that normally weren’t in, say, North America being imported in — like a lot of ornamentals and things for gardens — that we come to find out are really deadly.

[2:26]

I’m going to give you some tips today on how to identify toxic plants and then what to do with them — a lot of management. How do we manage our horses so they don’t eat them? Usually, a horse will not go and eat something that is toxic to them. Many times, these are bitter-tasting, not palatable; the horse won’t want to eat them and will tend to avoid them. But there are situations — like if a horse is really hungry — where they will. Or you just have that horse, no matter what, that’s looking for trouble anywhere, and they’ll go and possibly eat a toxic plant.

[3:07]

And it’s not just weeds. We’re talking trees, shrubs, and again these ornamentals that are found in gardens throughout the world and are being shipped around the world to be grown and cultivated. We find them not just on our properties — we find them in different gardens, along trails, or in landscapes where horses might graze a little bit if you’re out with them.

[3:37]

So, pretty fun journey today — we’re going to talk about the issues with horses eating these plants, how toxic they are, and how to identify them. Regionally, that is a challenge to cover in an hour, but I’m going to give you some tips and tools to use that you can start today to begin identifying plants. Then we’re going to talk about what parts of the plant are the most dangerous — the ones you really need to look out for. We’ll definitely highlight that, as well as symptoms to look for to see if your horse is experiencing a toxicity event or poisoning. And then we’ll finish out with some management tips on how you can hopefully limit your horse’s exposure or even prevent it.

[4:27]

This is a global problem, and when you go into the scientific literature — I go and do a search and find out what the issues with toxic plants are — article after article comes up discussing which plants are toxic, case studies, and overall studies. For example, here was a review article in Equine Veterinary Education just a couple years ago: “Plants Poisonous to Horses in Europe.” I have a case study here out of Australia, and in North America we’re going to talk about that — definitely a big issue.

[5:10]

Here’s another example — this was a veterinary journal out of Brazil, published recently, talking about toxic plants affecting the nervous system of ruminants and horses in Brazil. I highlight this one because finding statistics on how many horses die per year due to plant toxicity is difficult, and I’ve got a couple of studies that highlight this. In this study specifically, they said yearly it kills up to 38,000 horses per year in Brazil. That is a lot of horses dying from eating these toxic plants.

[5:59]

When I went to go look for statistics in Europe or the United States, that’s more difficult. As this paper out of UC Davis states — and I’ll quote it here — “As difficult as it is to estimate mortality losses from consumption of these plants, it is nearly impossible to determine the sublethal effects of consumption of toxic plants.” Even this paper said it’s very difficult to estimate mortality because, without doing a necropsy, sometimes horse owners won’t want to spend that money. Without a veterinarian able to specifically identify the plant — or the horse being observed eating the plant — these numbers are very difficult to come up with.

[7:04]

The National Academy of Sciences, which is some of the top brains in the United States, estimated that 8.7% of the nutritionally sick animals in the Western United States are sick as a result of eating poisonous plants. Now, just because a horse eats a weed doesn’t mean it’s going to kill them — but the sublethal effects could make them sick, underperform, or suffer other issues like laminitis. It doesn’t always have to be mortality — these plants can cause a lot of issues with horse health, well-being, and overall happiness. I think that’s an important point to make, even while highlighting mortality, because that’s the one we want to avoid the most.

[8:02]

When you look at how many horses die per year from eating toxic plants, a very conservative estimate — again, this was from a paper out of UC Davis titled “Poisonous Weeds: Their Impact on Livestock and Man” dated 1985 — is about 2% annually. That’s conservative. They also gave an even more conservative estimate: if just 1% of all the horses in California died each year from plant toxicity, and back in 1985 there were 570,000 horses in the state, that’s 5,700 horses dying that year. At 2%, that’s over 11,000 horses dying each year in California alone. If you carry that out — even conservatively — to the U.S. population of about 8 million horses, that’s close to 88,000 horses dying every year from toxic plants. Extend that to Canada, Europe, or Australia, and you’re talking tens of thousands to hundreds of thousands of horses globally.

[10:11]

So, why would a horse eat toxic plants? You always have that one horse that says, “Wow, that looks kind of yummy,” and eats it — even if it tastes off. But typically, what we see is a lack of forage. They’re hungry, didn’t get enough feed that day, or it’s late summer or fall going into winter — the pastures are barren, not growing much, and that green-looking plant over there looks tasty. Hungry horses not getting enough nutrients will wander over and eat it. That’s what happens.

[11:02]

In management, we want to make sure we’re feeding our horses as much hay as they can eat per day. That’s where nutrition is changing a lot now in the 2020s. But pasture management is also key — if the pastures aren’t productive and the horses aren’t getting enough nutrition or gut fill, we need to supplement their feed. Other things, like nutrient deficiencies, also play a role. And I’ll say this — we’re going to have a future podcast on this — I don’t care where you live in the world, your hay or your pasture is going to be deficient in something. It will be deficient in some nutrients. We know this after doing analysis after analysis: hay and forages will be deficient in certain minerals or certain vitamins.

[12:06]

And just to give you an example, the big one that maybe you are familiar with is selenium-deficient soils. Throughout the world, there are soils that are deficient in selenium. It’s a very important antioxidant, and forage growing on that deficient soil means it doesn’t have any selenium or very little. Hays grown on those soils mean they don’t have very much selenium — or any — and so when you feed that to the horse, the horse isn’t getting selenium in their diet. And that’s just one nutrient; there are many nutrients they’re missing. So you want to make sure you’re feeding your horse a well-balanced diet — again, management.

[12:44]

The other thing is boredom. The horse is curious. That gets into lack of stimulation, lack of exercise — maybe the area they’re in is very small, they can’t get stretched, and they just start developing those stereotypic behaviors. That’s one thing. It could be a habit they learned when they were young and curious — again, that horse that just finds trouble wherever they go. It could have been something like, “Oh, I remember eating that when I was young,” because they’re curious about the world and what to eat and what not to eat. They may think, “Oh, I remember that and it tasted good.”

[13:27]

One example is red maple leaves — the iconic leaf of the Canadian flag — which horses may seek out. I found this quote in an article: some harmful plants are attractive to horses at certain times of year. For example, wilted red maple leaves are among the plants most toxic to horses, but they can be hard for horses to resist because the leaves’ high sugar content draws the animals. This is an issue in Canada, but also in parts of the United States where there are red maple trees.

[14:09]

Horses can be exposed to red maple leaves in the fall when drying leaves blow into pastures. Ingesting one and a half pounds is toxic in a horse; three pounds is lethal — that’s about half to one and a half kilograms. Not much. So that’s a big challenge. If you have red maple trees on your property and you have horses, you absolutely have to get rid of them. I’m sure some of you have, or know people that have. But then, what do you do about your neighbors? We’ll address that at the end.

[14:53]

So, how much do they need to eat before it’s toxic? For red maple leaves, about three pounds in a horse. Certain plants require less. One big one is the yew tree — an evergreen plant. As little as a few ounces of these leaves can be lethal to a horse within minutes to hours of ingestion. This is a popular ornamental plant shipped around the world. Wherever you live, there’s probably somewhere in your country where a few ounces could kill a horse.

[15:39]

Oleander — these ornamental plants — the leaves can be extremely toxic and potentially fatal to horses. Ingestion of just a few leaves can cause serious cardiac issues. Red maple, just talked about that. Rhododendrons and azaleas — again, a few leaves — are popular ornamentals. Fiddleneck, foxglove, and the list goes on and on. Sometimes small amounts of these can be toxic; other times, the horse has to eat quite a bit. But the yew tree — all parts of the tree are poisonous, and the leaves are really toxic when dried.

[16:36]

Usually, the problem arises when people trim these bushes — because they grow out — and those trimmings can blow into a pasture or paddock. I even saw an example where a neighbor, not knowing anything about horses, trimmed their trees and thought, “Oh, maybe the horse will want to eat this,” and put it over the fence — and they just killed the horse. That has happened.

[17:04]

When I started researching this topic in more depth — and even back when I taught about toxic plants — my advice was always: talk to your local experts. Have them walk your pastures and try to identify any toxic plants in your gardens. This was a couple of decades ago when I started teaching at the university level and going out and talking to horse owners. But now, we actually have more technology that you can use, and you can do the walk yourself and try to identify these plants.

[17:46]

It can be frustrating because there are hundreds and hundreds of plants that could be toxic. One good source is the ASPCA website. They have a list of 277 plant species that are toxic to horses, and a list of 142 plant species that are not toxic to horses. On the ASPCA horse plant list, you can click on each plant and get good descriptions.

[18:37]

It can be frustrating because it’s not like there are just three or four plants I could tell you to look out for — there are hundreds that could be harmful to your horse. When I went to that list of toxic and non-toxic plants, the first one at the top is “Adam and Eve” — also called arum, Lord-and-Ladies, wake robin, starch root, bobbins, cuckoo plant. You can click on it, see a picture — it looks like a big lily — and it’s toxic to dogs and cats as well. The toxic principle is insoluble calcium oxalates. If a horse ate the Adam and Eve plant, clinical signs include oral irritation, pain and swelling of the mouth, tongue, and lips, excessive drooling (and we’ll talk about slobbers later with clovers), vomiting (though horses can’t vomit), and difficulty swallowing.

[19:43]

This is an ornamental plant that originated in Vietnam and Southeast Asia but is also native across most of Europe, as well as Eastern Turkey and the Caucasus. It’s a flowering plant and ornamental. And you might think, “Okay, somebody plants that in their garden — my horse is never going to see it.” But like I said, sometimes people landscape near their properties, around their barn facilities, to make them look really nice, and they’ll plant all these beautiful flowers. If you’re out there with a horse on a loose lead rope and they put their head down, they could grab some of that plant — and for some of these, it doesn’t take much to kill them. So you want to be very cautious with these plants, especially around the property line — or again, your neighbors.

[20:51]

And I’ve got a case study here out of Australia that typifies what happens sometimes with these ornamental plants. This is titled “Suspected Nerium Oleander,” so it’s oleander poisoning in a horse, published recently out of Australia. Oleander is a very toxic plant to horses — again, a beautiful flower that people plant all around the world. The issue is cardiac glycosides, which are very deadly, and sudden death is the most common result of plants related to cardiac glycoside toxicity.

[21:44]

This case study was a seven-year-old Arabian horse. All of a sudden, the owner noticed it wouldn’t eat and looked depressed — the horse went from fine to starting to crash. When the vet performed a physical exam, the horse was in moderate body condition — so it wasn’t starving or eating something out of lack of feed. There were mild signs of depression, an elevated respiration rate of 40 breaths per minute, and an irregular heartbeat at 40 beats per minute — an arrhythmia.

[22:22]

On presentation three hours later, the horse was dehydrated, with 56 breaths per minute, reduced gastrointestinal motility (leading to colic), pale pink and dry mucous membranes, and a capillary refill time of two seconds. The horse was starting to crash and deteriorated quickly. Intensive therapy was started at the vet hospital to try to save its life, but after 20 hours it died.

[22:56]

Further questioning of the owner indicated the horse had exposure to oleander clippings in the 24 hours preceding presentation. This property had clipped the plants back, and the horse ate those clippings. It’s unclear if they were fed directly or simply accessible, but the horse died within roughly 24 hours of showing symptoms. Oleander, also known as Rose Bay, is a shrub or small tree cultivated worldwide in temperate and subtropical areas as an ornamental and landscaping plant. It is very deadly to horses, and it’s just one of hundreds of plants that could cause problems.

[23:56]

So, how do you identify the plants in your area? This is regional — the plants I have here in New Zealand are somewhat similar to North America and Australia, and some to Europe, but our native plants are different from native plants in the Americas. One thing you should always do is constantly walk your paddocks and pastures, often, to look for weeds or plants that don’t belong — those not normally part of your pasture. Note anything unfamiliar, and if you have no idea and have lots of plants around your property, start trying to identify them.

[24:56]

If you live in the United States, I would suggest speaking to your local extension office if available. Ask one of their officials to walk your property with you — they’re trained to identify toxic plants for livestock species. They can point out specific plants and advise on pasture maintenance. If you live in a country with a similar service, use it.

[25:24]

If you’re on your own, embrace technology. I suggest using the iNaturalist app, available on your phone. I tested it before making this presentation — it’s an incredible app, fun to use, and works for more than plants. It can identify animals, insects, and more. You take a picture of the plant, upload it to iNaturalist, and it will identify it. It may give you a few options if there are similar-looking plants, so you might need multiple pictures or to request help from the community.

[26:22]

This morning, I went into my garden, took a picture of a large weed, uploaded it to iNaturalist, and it came back as European black elderberry. Obviously not native to New Zealand, but like many non-natives, it was brought here. This plant also occurs in parts of Australia, New Zealand, and elsewhere. I didn’t know if it was toxic to horses, so I searched online — turns out, yes, it’s quite toxic. Common elderberry, black elderberry, European elder — all parts contain cyanogenic glycosides (the same class of toxin involved in the Australian oleander case) and can be harmful to horses. The bark also contains calcium oxalate crystals.

[28:01]

So here I had a toxic plant right behind me, and I didn’t even know it. That plant will be removed this weekend. This is why it’s important to walk your pastures and property, take pictures of unknown plants, keep notes, and then research them — either online, through the ASPCA toxic plant database, or through local resources. If they are toxic, remove them.

[28:48]

I wanted to give you some ideas of the resources available to you. In Australia, New Zealand, Europe, the UK, and elsewhere, there are websites, pamphlets, and guides on toxic plants for horses. We live in the information age, and much of this is available at your fingertips or on your phone. For example, “Plants Poisonous or Harmful to Horses in the North Central United States” — published by the University of Minnesota Extension — is over 40 pages and covers species like black walnut, chokeberry, different types of clover, maple, oak, and more. It even has a section on plants that cause death.

[30:10]

The ones I highlighted — specific species — were foxglove, rhododendron, oleander, and yew, specifically Japanese yew. Those plants are horrific for horses. Foxglove was introduced from Europe. Rhododendron, including azalea, is native to North America, Europe, and Asia — which covers most of the world. Oleander is native to the Mediterranean and Asian countries but has been exported around the world. Japanese yew was introduced from Asia. These plants are used for landscaping throughout much of the United States and Canada.

[30:49]

There is a resource online that lists the most common toxic plants. It’s not exhaustive — which is why you want to walk your property and identify plants there — but it’s a good start. For the UK, you can go to the MSD Animal Health website for “Poisonous Plants That Can Damage Horse Health.” They list ragwort, foxglove, buttercups, yew, oak, rhododendrons, and more. Europe also has published papers on this. If you’re curious about specific plants, you can visit madbarn.com — there are articles on many different plants, including oxalate toxicity, buttercup, black walnut, and slobbers, which I’ll discuss in a minute.

[31:46]

We’ve talked about why this matters — because toxic plants can kill one to two percent of horses per year. That’s a conservative estimate: one to two horses out of every hundred. We’ve also covered resources — apps like iNaturalist, available worldwide, and other plant ID tools — and the importance of walking your properties, identifying every plant horses can access, and researching toxicity. Hundreds of plants can be harmful.

[32:31]

When you ask what parts of a plant are toxic, it depends. For some of the most deadly ones: yew — all parts are toxic, including leaves, bark, seeds, and berries. Red maple — primarily wilted or dried leaves contain the toxins. Oleander — all parts are extremely toxic due to cardiac glycosides, which can cause heart problems and death. Rhododendron and azalea — all parts, including leaves and flowers, contain toxins. Foxglove — all parts are toxic. Black walnut — the concern is with shavings used as bedding. Fiddleneck — seeds and foliage are toxic. Nightshade — berries and foliage contain solanine, causing gastrointestinal and neurological symptoms. Bracken fern — all parts are toxic. Hemlock — all parts are toxic. Oak — leaves and acorns contain tannins, which can cause kidney damage and gastrointestinal irritation.

[35:03]

From reading that list, I would treat all these plants as deadly and remove them from your property. Now, symptoms — this is where it gets difficult. Many symptoms could also be caused by disease, which is why it’s hard to determine exact mortality from toxic plants. The first signs are changes in behavior — lethargy, depression, not eating, isolating from the herd, reluctance to move. Gastrointestinal signs include colic, abdominal pain, painful diarrhea, or constipation. Neurological signs include wobbliness, muscle tremors, or seizures. These can mimic other conditions like West Nile virus, so the vet has to determine the cause — and without expensive diagnostics, it’s hard to know for sure.

[36:43]

One specific symptom is excessive drooling — known as slobbers. This is often caused by white clover and red clover. These legumes, when stressed (during heat or drought), can host a fungus that produces slaframine, a toxin that stimulates salivary glands and causes drooling. It can also happen if clover is baled in hay. Usually, removing the horse from the pasture resolves the slobbers. It’s not deadly but indicates something is off in the forage and adjustments are needed.

[38:20]

Other possible symptoms include irregular heartbeat, increased heart rate, weakness, collapse, increased respiratory rate, nasal discharge, kidney shutdown, liver damage, and jaundice. The problem is, these signs also look like other illnesses. If you notice any of these, it’s an emergency — call your veterinarian immediately. As in the Arabian horse oleander case, some horses won’t make it. But with supportive care, many can recover and flush the toxins from their system.

[39:27]

Okay, so that’s a lot of information, and really what it boils down to is managing risk. Just keep this in your mind when you think about toxic plants: detection, removal, and prevention. Detection, removal, and prevention — that should be your toxic plant management plan.

[39:54]

Detection — you need to go out and walk your pastures, walk your property lines, and walk around your facilities. If you board your horse at a facility, walk it yourself — don’t just depend on the barn manager. I was a college student managing a barn, and we didn’t look for toxic plants very often, if at all. We were busy feeding horses, managing the property, cleaning up, dragging pastures — and I didn’t even realize I had a toxic plant in my own backyard. We weren’t trained professionals. Some of you may have people like that, so don’t depend on somebody else — do it yourself. That’s not to say there aren’t excellent barn managers out there, but it’s always worth double-checking yourself.

[40:41]

Walk, identify plants, use that app, take pictures. Any plants bordering your pastures or paddocks — if your neighbors have toxic plants, like maple trees or yew, talk to them about removing them. If they do trim those plants, ask them to pick up all clippings, because wind can blow them into your pastures. With red maple trees in fall, if leaves are blowing onto your property, pick them up.

[41:59]

Remove any and all potentially harmful plants, and when you pull them, get the roots so they can’t regrow. If you use an herbicide, make sure it targets the plants you want to kill, and follow the label carefully — especially the grazing restrictions. After spraying, there will be a “keep horses away” timeframe, sometimes until after a good rain or irrigation.

[42:47]

More detection: check your hay. Buy weed-free hay, but always look through your bales for contaminants. Same with bedding — know what it’s made from. Never use black walnut shavings, because black walnut is poisonous to horses. Horses can get laminitis just from standing in it. Research suggests toxins in walnut shavings are absorbed through the skin around the coronary band, increasing hoof blood pressure and inducing acute laminitis.

[44:08]

Prevention — more podcasts are coming on diet and pasture management, because these are the two biggest prevention tools. Make sure your horse is eating a balanced diet and getting all nutrient needs met. If they’re lacking nutrients, they may eat other plants to fill the gap. Free-choice hay is one of the best ways to prevent this, unless you have highly productive pastures.

[44:55]

Pasture management: in late summer or autumn going into winter, when pastures aren’t productive, feed hay. Pull horses off pastures that need rest. Overgrazed pastures let weeds take over — weeds thrive in bare spots. Also, enrichment matters for boredom. Provide toys, both stall and pasture, to keep horses occupied. A bored horse might go looking for trouble (and toxic plants).

[47:26]

Keep them well-fed so they’re less tempted by bitter plants. Detection, removal, and prevention — those are the big three. Final tips: clean feed bins, buckets, and feeders often, because seeds from toxic plants can blow in and accumulate. Despite all efforts, accidents can still happen — if you notice any problems, call your veterinarian immediately.

[48:17]

Use the iNaturalist app, identify plants, make a list, check their toxicity, then go through detection, removal, and prevention. Doing this can save you vet bills and heartache. I know it’s a big topic, but hopefully this gives you tools and ideas to make some changes. If you enjoyed this, please thumbs up, subscribe, and visit madbarn.com for over 500 articles on horse health, nutrition, and toxic plants. Also, stay tuned for the upcoming free Mad Barn Nutrition Course — covering hay, minerals, vitamins, and how to meet your horse’s needs, which ties directly into preventing toxicity. Thanks for listening, and we’ll be back next week with another topic.