Our committee chair, Sarah Jackson, brought to my attention some toxic weeds that we had growing at the Equestrian Center.  She also shared this post with me that she found on Facebook that explains why it is toxic and what signs to look for if you horse does ingest it.
The post was generated by a highly acclaimed large animal veterinary practice located in the Santa Barbara area, Alamo Pintado Equine Medical Center.
I have included the link below — but in case you don’t have Facebook I included the contents of the post as well.

Spring has sprung and so has fiddleneck (also know as tarweed or yellow bur weed.)

Fiddleneck is a flowering plant that is extremely poisonous to your horses.

Dr. Lisa Teske has recently seen the plant at three separate appointments here in our area. Small amounts of fiddleneck, if eaten, may cause serious damage to a horse’s liver and in some cases complete liver failure, which is very difficult to treat.

Symptoms of ingestion of fiddleneck include depression, diarrhea, weight loss, skin changes and discoloration, swelling of the legs, etc.

Contact your veterinarian if you notice any of these symptoms in your horse and please be on guard this season. While the blooms we see all around us certainly are enjoyable, in some cases they may be deadly for horses.

Check out https://csuvth.colostate.edu/poisonous_plants/Plants/Search for more information about the plants that may be dangerous for your four legged friends.