Horses Eating Thistle

Q: I suspect at least one of my horses is eating thistle. They’re not eating the big mature plant, but rather the immature rosette stage of the plant. Will this harm them (especially due to the prickly parts of the leaves)? I have been trying to eliminate thistle from the pasture and am making headway. However, I’m unable to eliminate the plants. My pastures aren’t ideal due to the drought over the past few years, but I only let my horses graze a couple of hours a day. —Susan, Austin, Texas

A: It’s challenging to answer this question specifically, because I don’t know the exact variety of thistle in your pasture. Different varieties can have different effects. So, here’s some information that I hope will help you.

With regard to the prickly parts of the leaves, check your horses periodically to make sure nothing has gotten lodged in the soft tissues inside the mouth. If you notice any abscesses, soreness, or pus, contact your veterinarian right away.

Some thistle varieties known to be toxic to horses include the following, all in the Asteraceae family:

  • Asters;
  • Broomweed;
  • Cockleburr;
  • Groundsel;
  • Jimmyweed;
  • Snakeroot;
  • Sneezeweed; and
  • Yellow star thistle.

Your efforts to remove the thistle from pastures are well worth your time, but complete eradication is difficult, especially with stressed pasture like you describe. You can contact your county’s extension office for advice on how to further control weeds and manage your pastures. Minimizing the horses’ exposure to these weeds is also a good strategy.

In general, thistle consumption isn’t ideal. In cases of the varieties mentioned above, intoxication can be cumulative and lead to a variety of clinical signs. Should your horse exhibit any unusual behavior, such as uncoordinated movement, involuntary chewing or twitching, lethargy, diarrhea, etc., contact your vet immediately.