Someone I met at a conference once mailed me a book that she thought proved that animals have nutritional wisdom. She said that this book was proof for the concept of planting a variety of plants thought to have medicinal value in horse pastures so that “when they get sick, they can self-medicate”. Some of the plants she thought should be included were considered toxic by experts in poisonous plants, but she was quite certain that a horse would only eat them ‘as needed’ in the proper amounts and that animals have innate knowledge about medicinal plants that exceeds that of humans. I have poor control over my facial expressions, hence without saying anything I must have communicated my skepticism. I did read the whole book, but I did not find any evidence to support her recommendation. It used the example of chimpanzees that ate plant leaves that attached to intestinal parasites and allowed them to be eliminated. That is a real thing, with good evidence. https://www.newscientist.com/article/mg15120442-500-science-clean-sweep-for-leaf-eating-chimps/ However, I see no logic or reason to suggest that horses are capable of nutritional wisdom. There is much evidence that shows they are nutritionally stupid. If you see your horse eating a strange plant, how do you know if it’s causing or curing some health issue?

A horse is not a chimpanzee

Chimpanzees nurse from their mother for about 5 years and stay close for another few years. Males will stay with the same family group all their lives, while females join another family group when mature. They stay in the same bio-region with the same plants all their lives. As one of the most intelligent animals, their long lives and close family ties give them the tools and social structure for forming a rudimentary culture where knowledge about food can be passed along to the next generation. This allows chimpanzees to develop a unique set of rules about what to eat and what not to eat. Orangutans live fairly solitary lives and young stay near their mother for up to 8 years. Baby orangutans stolen from their mothers for the pet trade require long term education by humans to learn what and how to forage for food before being released back into their native habitats. Successful foraging is not instinctual even in these highly intelligent species. https://www.bbcearth.com/blog/?article=can-captive-animals-ever-truly-return-to-the-wild

Yellow star thistle is palatable to horses. It paralyzes the nerves in the face. Click on the picture to learn more.

Honestly, do you think horses are as intelligent as chimpanzees? How many horses live with their mothers long enough to be taught what to eat? How many horses live in the same pasture they were born in? Even the ones that do stay in the same pasture all their lives may encounter new plants when environmental conditions or overgrazing cause changes in plant populations. Why would you expect them to have the knowledge or instinct to choose plants that will cure them of illness when they also tend to eat things that will make them sick? Yellow star thistle destroys the nerves in the face so horse’s cannot chew. They find it very palatable and continue to eat it as long as they are able. A reasonable extension of the concept of nutritional wisdom would imply that if a horse eats a poisonous plant, it is suicidal.

This faulty logic is selling a lot of herbs for horses. Search online for ‘herbs to plant in horse pasture’ and find many different blends so your horse can ‘self- medicate’. There’s scientific sounding words like: Zoopharmacognosy. Here’s a blog post on that subject by a vet that has good information, and a FB page. You may find advocates of horse self-medication from people with ‘Dr.’ before their name, proving that getting an advanced degree does not ensure that a practitioner has a good grasp of scientific methods or ethics. This is Big Natural business, making money trashing Big Pharma. You can take online courses. It’s all testimonials and no evidence, but if you make people feel good and offer them a magic potion, they buy stuff. When I search on scholar.google.com (my favorite source of information) “self- medication horses’ there is not one entry. Not one scientific reference. Another life lesson: if you want to make money, don’t be a good scientist. To make money, sell people things to absolve them of responsibility and keep the boogieman away.

Range cows have some nutritional wisdom

While no studies have been done on horses, there have been some done on cattle grazing open range where toxic plants are common. If a cow eats something and gets sick right away, they do learn not to eat it in the future. They do learn from their mothers and other herd members what to eat. Calves that were born elsewhere and moved to a range with toxic plants get sick more often than calves that were raised with their mothers on that same range. If a toxic plant does not have an acute mode of action to make a cow sick right away, they don’t learn to avoid it. As many toxic plants create illness only after long term ingestion, range scientists have devised methods to teach cattle not to eat them. They keep cattle in pens for a short time and feed them toxic plants mixed with a substance that makes them feel sick immediately, but has no lasting effect on health. This educates the cows to avoid (be averse to) eating that plant. However, education only lasts if they don’t mingle with uneducated cows. They tend to eat what other cows eat. (How’s that for a life lesson?) Aversions are retained in long term memory and may last indefinitely if averted cattle graze separately.

Chronic poisoning

Groundsel, or senacio causes liver damage over a period of time. For more info, click the picture.

Many plant toxins don’t manifest with obvious symptoms until after months of being eaten. There are many common pasture plants that cause accumulative, irreversible liver damage that do not create noticeable symptoms until the liver is severely impaired. This is far too long for animals to make an association, and unfortunately too long for many horse owners to notice as well. Overgrazing and drought, combined with an erroneous assumption that horses won’t eat stuff that is bad for them, kills horses. Yes, it can take a lot to kill them, but it does. Does anyone notice if they are a little bit sick, or do they just say “gee, he’s gotten so lazy”? Why do universities put up articles warning people about the toxic plants in their area if horses have the wisdom to make safe choices about what to eat?

Some examples of university articles about plant poisoning in horses:

Russian Knapweed and Yellow Star-Thistle Poisoning of Horses, Poisonous Weeds in Horse Pastures, White snakeroot: a toxic plant to horses

Every single time I have walked a pasture and pointed out toxic plants, the owners say “but my horse doesn’t eat them’” Then I point out plants that are partially eaten yet they continue to choose to believe that their horse won’t eat them. Every. Single. Time. Once I pointed out patches of a toxic plant in an overgrazed pasture that causes chronic liver damage. Later the owner of that pasture posted on the internet about one of her horses dying of liver disease, but she continued to deny it could be about the toxic plants that she chose to ignore. Stubborn confirmation bias kills horses. Apparently some people find that attributing the death of a horse to some unknown and uncontrollable reason is easier than admitting that their ignorance and refusal to intervene allowed it to happen.

Sometimes, only one horse in a herd may eat poisonous plants. Why take the chance? Tansy ragwort poisoning in a horse in southern Ontario

Special problems for Metabolic Horses

Weeds may be higher in sugar than grass. Once a client told me that she was worried about her horse going without fresh greens while in confinement for laminitis, so daily she picked a basket full of dandelions “because they are a liver cleanser and she really likes them”. Dandelions are also full of the same kind of inulin-type fructan that researchers use to induce laminitis in clinical studies. It is entirely feasible that the greens she thought were treating her horses’s laminitis were what was causing it.

Appeal to Nature

The ‘appeal to Nature’ is a logical fallacy. https://yourlogicalfallacyis.com/ These are disallowed in structured reasonable debates. They are very common of late for unreasonable debates. For a long time, the appeal to nature was used as an argument in politics and morality. The basic ‘nature’ of women was to be ruled be men. The basic ‘nature’ of men was to be ruled by God. This Appeal to Nature has changed recently to mean that anything that is made by Nature or your deity of choice is intrinsically better than something created or modified by humans. A long but good read titled: The Naturalistic Fallacy is Modern

Buzz words abound suggesting that “natural, organic,” things all made from chemicals are better, safer, and healthier than chemicals invented by humans. As an allergy sufferer that recently tested positive to 40 out of 47 local allergens, the whole ‘natural is healthier and safer’ mantra just doesn’t work for me. Your skin lotion and shampoo full of ‘essential’ oils makes me sick. I’ll stick to ingredients man-made to be hypoallergenic and tested under scientific scrutiny. I’ve also had a college course in Environmental Toxicology, where I learned that some of the most toxic, carcinogenic substances known are made by nature. Aflatoxin, prussic acid, botulism, and cyanide are just a few of the many toxins made by naturally occurring organisms, thus by some peoples definition ‘organic’. The appeal to Nature as being benign and safe just doesn’t make good sense.

Wishful thinking cannot absolve our responsibility to control our horse’s environment.

People are afraid of things they don’t understand. Fearful ignorance is easy to exploit. While the internet is full of testimonials selling products that assume that nutritional wisdom exists in horses, the people I have asked with Ph.D.’s in equine nutrition, and all the equine nutrition textbooks have all told me that horses only have nutritional wisdom about salt and maybe phosphorus. (‘Maybe’ in scientific language means limited data without enough corroboration to be publishable.)

While cafeteria-style supplementation of individual minerals may provide you with a false security that ‘nature knows best’, it will not provide your horse with optimum mineral supplementation. They may curiously taste things, but horses eat what tastes good to them. Suck on a penny to find out why horses won’t eat enough copper. Yes, they sometimes develop perverse tastes to strange things, but it has nothing to do with what is good for them. They are bored, perhaps hungry, but still nutritionally stupid. Yet people choose to believe this without any evidence and will spend money to stop worrying about proper mineral supplementation.

Useful Resources for Learning

This is a really good book on A Guide to Plant Poisoning of Animals in North America by Anthony Knight, a veterinarian at Colorado State University, and Richard Walter, a botanist . In addition to good pictures to ID toxic plants, it includes treatments for affected animals that can save your horses life. If your vet doesn’t have this book, buy her one or get it yourself. CSU also has a searchable website here. If you need more motivation to learn, put ‘sudden death’ into the search box. There is no opportunity for a learning curve with some plants. You just find your horse dead in the field.

There are many university sites around the world dedicated to helping people prevent plant poisoning by grazing animals. Search “poisonous plants <your country, state or province> “. Since university sites with .edu extensions are often more credible, you can add ‘site:edu’ so it filters for those sites.

Diversity is a good thing, but if you are encouraging a diverse, wild type pasture you must accept responsibility to identify and control poisonous plants by some means. That requires walking your pastures monthly and identifying every plant that is there. This may be daunting at first, but it gets easier when you learn to observe different leaf shapes, configurations, and other specific plant parts that are key to identification. Taxonomic plant keys used to be the only way to do this, but frankly, I’d rather take a beating than key out more than a few new plants a day while trying to remember what all the descriptive Latin words mean like ocrea or glabrous. Now there are many apps available for phones and computers that can help you ID plants from good quality close up photographs. Search for ‘poisonous plant ID app’. An alternative to learning how to ID plants is to hire someone who already knows. Plan on paying professional fees of $60-70/ hour. It takes a long time to learn this stuff and develop a good eye for finding different plants among all that green. All crop consultants are knowledgeable about plant ID and have trained eyes. University extension agents should also be able to ID toxic plants in their region.

When you DO know what is in your pasture, you can then be assured that your nutritionally stupid horse will not accidentally poison himself by trying out exotic cuisine.

Thinking critically about nutritional wisdom and horses