Whorled Milkweed. (Asclepias verticillata): VERY TOXIC – this is what killed the 19 mustangs!

Take a really good look…

Whorled Milkweed.  Look familiar?

Whorled Milkweed. Look familiar?

I’m not sure if any of you read about he 19 mustangs in the BLM Prison facility in Canon City, Colorado who fell ill and died…

Well, it turns out that their round bales included Whorled Milkweed (Asclepias verticillata).  This weed is what killed them.

So sad and so fast!

These horses were down almost immediately.

Take another look – and then tell me that you recognize it.

I do.

If you see it in your hay, DO NOT FEED IT.

I don't know why the horses ate it... maybe it masks its poison to smell and taste.  dunno

I don’t know why the horses ate it… maybe it masks its poison to smell and taste. dunno

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This weed pops up on the edges of fields and along roads.  It is very hearty and thrives…

Asclepias verticillata L.

Whorled milkweed, Eastern whorled milkweed

Asclepiadaceae (Milkweed Family)


USDA Native Status: Native to U.S.

Whorled milkweed is a single-stemmed, unbranched perennial, 1-3 ft. tall. The narrow, linear leaves are whorled along the stem. Small, greenish-white flowers occur in flat-topped clusters on the upper part of the stem.

Because of its toxicity to livestock, this plant is considered a weed in range areas.

I’m sure this kind of thing is why hay farmers spray…  Clearly, whomever was buying the hay for these BLM horses wasn’t using a reputable hay distributor.

So sad.

I guess for the locals, this was bittersweet news.  Their water wasn’t contaminated, but their wild horses died.

Another angle...

Another angle…


I found this article…

CANON CITY — Authorities have determined a noxious weed likely caused 19 wild horses to become ill and die at the East Canon Prison Complex where they are cared for by inmates until they find new homes.
According to BLM spokeswoman Lauren Gapinski, the ingestion of whorled milkweed, a highly-toxic plant, is suspected to have caused the deaths based on preliminary lab results issued from the Veterinary Diagnostic Lab at Colorado State University. The weed was in the hay fed to the horses.

The BLM has been working to probe the cause of the illness that killed 11 horses Dec. 3 and another 8 on Dec. 4. Some of the horses died on their own while others had to be euthanized due to their conditions.
This week, nine of the horses also in the same pen that exhibited similar symptoms have recovered or are recovering quickly.




I must admit that when I first saw this report, I had to look up whorled milkweed (Asclepias verticillata) in my copy of A Field Guide to Common Animal Poisons. Milkweed contains cardiac glycosides, the natural substances from which heart medications like digoxin are derived. When ingested in toxic doses, the cardiac glycosides in milkweed stop the heart from beating normally. Very often, poisoned animals are simply found dead, but symptoms like diarrhea, loss of appetite, depression, weakness, unsteadiness when walking, collapse, and seizures may be noted in more mildly affected horses (or other species, for that matter).

MONARCH BUTTERFLY CATERPILLAR eats this weed to protect itself from being eaten.  Smart.  Ol' Mother Nature at work.

MONARCH BUTTERFLY CATERPILLAR eats this weed to protect itself from being eaten. Smart. Ol’ Mother Nature at work.



As a biologist, I am actively involved in protecting species at risk. Here in Arizona, we have recently become aware that the Monarch Butterfly uses our rivers and streams as over-wintering and way-station habitat. Many are trying to help them survive and thrive, and the use of various milkweed species in landscaping and revegetation settings is encouraged. Obviously, it should be eradicated near forage crops, however! Readers of Horse and Man should be encouraged to appreciate milkweed in appropriate settings because of its benefit to Monarchs (as well as several other butterfly and moth species that are at risk), but as noted in the article, be diligent in keeping it out of forage crops and the hay we feed our horses and livestock.

All the best,


Environmental Program Manager (my “daytime job”)

Animal welfare activist (my full-time “passion”)


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foal baling twine

Today is the day of the big auction! They think they have enough money to buy them all! We are now working on transportation money!








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HORSE AND MAN is a blog in growth... if you like this, please pass it around!

2 comments have been posted...

  1. Joan Norton VMD DACVIM

    Great article! There are many toxic plants out there that horse owners may not be aware of. Because they often cause vague clinical signs and are hard to diagnose, the problem goes undiscovered and illnesses and sometimes even deaths go unresolved.
    I invite everyone to take our Online Poisonous Plants course that covers many of the toxic plants out there, what they look like and how they affect our horses!

  2. stephanie

    in washington we have tansy ragwort and it will drop a horse dead in hours too, same goes for rhododendron. both are just as deadly as that milk weed and both are native to washington state.

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