Rocky Bay Equine Newsletter
August 2020


While it’s likely that most every pasture contains plants potentially toxic to horses and other livestock, realistically only a few of them cause serious problems. There are many modifying factors that impact the the health consequences of toxic plants in the environment. For one thing, many of them don’t taste very good and animals with access to good quality forage are unlikely to ingest significant amounts of unpalatable plants. In most cases (with some notable exceptions) it takes more than a few bites for plant toxins to adversely affect a healthy animal. As long as your pasture is in decent shape, you are supplementing forage as needed and your animals are in good health it’s highly unlikely that ingesting a toxic plant here or there will cause any problems. That said, there are some plants that host powerful toxins where even a nibble can be life threatening. Other plants need to be consumed over long periods of time to accumulate enough toxin for serious illness to develop. The toxic agents in many plants have not been specifically identified and there is often no direct treatment or "antidote" for poisonings, we can only help them with supportive care. With some poisonings if the animal doesn’t die it may be permanently disabled. Let’s face it, most of our pastures are stressed and not optimal all year long for horses, so it’s a good idea to walk around and make sure that you aren't un-intentionally propagating toxic plants.

It’s also important to keep in mind that not all toxic exposures come via plants ingested on pasture. I’m sure you already know that even benign plant material like grass clippings can cause colic when given to horses. There is nothing more horrific than inadvertently poisoning your own animals by throwing toxic leaves and stems from landscape pruning into a pasture. You might remember the reports a few years back of poisoning from black walnut husks used as horse bedding. Also keep in mind that many different plants can be caught up in the mix when hay is baled. It’s smart to regularly check your hay bales for extraneous plant material and buy from reliable sources whenever possible. Locoweed, for instance, may not be common on the wet side of our state but much of our hay will be transported from other environments where it is more common. 

Local agricultural extension agencies are good resources for recognizing toxic plants and for managing pastures to minimize their impacts on horses and other livestock.  This link will give you a more complete list of toxic plants and their effects on animals.

Here’s a top ten of “bad apples”, toxic plants most detrimental to horses in our region.

1.  Bracken fern ; common in our area. I’m sure many of you have seen horses consume this plant without obvious adverse consequences but if your horse is particularly fond of foraging on ferns, you should know that the plant contains thiaminase, a compound that interferes with vitamin B1 (aka thiamin). Thiamin is a necessary component for nerve function and B1 deficiencies can result in depression, incoordination and blindness. Be aware that to some animals bracken fern is yummy and too much of a good thing is, well, a bad thing in this case. 

2.  Poison hemlock ; the purple spotted stems give them away. The leaves, stems and seeds all contain potent neurotoxins. As little as four pounds can be lethal for a horse. The good news is that most animals avoid it.

Water hemlock ; is an even more toxic variety in the same plant family. It isn’t easy to differentiate species, both have hollow stems and umbrella shaped flower clusters, so just consider them all potentially poisonous. They grow in marshy areas, near water and in ditches. Fresh or dried, it is considered one of the most toxic plants across the country, all parts of it contain toxin but especially the roots. Most animals avoid it, thankfully, because the plant's neurotoxic components cause a variety of signs ending with convulsion and death by respiratory failure. 

3.  Tansy ragwort ; another common PNW weed we love to hate. There are many species in this plant family and toxicity varies between them. The toxic components in all of them are pyrrolizidine alkaloids which cause liver damage in animals with chronic exposure over time.

4.  Red maple (Acer rubrum); toxic components may also be in related species like silver and sugar maples and also box elder (at less toxic amounts). This is not a native plant in the PNW but ornamental specimens are planted here. Ingestion of young, growing leaves seems to be ok, but eating wilted leaves is extremely toxic. As little as a pound of leaves can be fatal, causing red blood cells to hemolyze (break down) so the blood can no longer carry oxygen, resulting in organ damage. 

5.  Yellow star thistle/Russian knapweed ; a bit more common on the dry side of the state, both plants contain neurotoxic agents that have less immediate impacts than those in hemlock. Poisoning usually occurs with ingestion of fairly large amounts over time. Toxic components affect the animals ability to chew.

6.  Buttercup ; found pretty much everywhere especially in overgrazed pastures. The plants contain ranunculin which is a bitter tasting compound that causes irritation to mucous membranes and the GI tract. You might notice swelling and blistering around the muzzle, mouth or face with increased salivation and in severe cases, colic and bloody diarrhea. Toxicity of the plant varies, the fresh flowers are the most problematic. The good news is that they are also bitter tasting so horses will generally limit consumption unless the pasture has nothing else to offer. Most poisonings will resolve with supportive care.

7.  Yew ; a NW native that is a mixed blessing. Most parts of the plant contain taxine, an alkaloid that is used in cancer therapy (taxol, tamoxifen) but it can also be rapidly deadly when even small amounts are ingested. Sudden death is the most common sign, most often caused by shrub trimmings thrown into a pasture after pruning. 

8. Foxglove, rhododendron, oleander ; these plants are also popular in landscaping, they all contain cardio toxins that interfere with normal heart function. A handful of oleander leaves, 3 to 4 ounces of foxglove or 1-2 pounds of rhododendron may be enough to cause cardiac arrhythmia and death without rapid supportive care. 

9. Ground ivy (aka Creeping Charlie or Creeping Jenny); This is another common invader in conditions of poor pasture management. Fortunately it’s bitter tasting, most livestock avoid it but beware that it can be included in alfalfa hay bales. Signs of toxicity include sweating, salivation and respiratory distress. 

10. Locoweed ; a dry, sandy soil lover, this plant takes ingestion over time before it causes problems. The plant contains an enzyme that interferes with sugar metabolism and brain cellular function. As its name suggests, signs start with odd behavior, exaggerated, high stepping gaits and staggering. The horse may recover in mild cases but the effects in more serious cases are irreversible. 


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Sending all our best wishes for your personal health and wellness as we continue to negotiate the pandemic.Thanks for working with us to minimize spread of covid 19. We are here for you when your animals need us. Take care, stay healthy!