Is this it? Senecio jacobaea L.? on googling, I see references to this species on googling to horse poisoning in the UK.

Doesn't grow in the southeast USA, according to the distribution map, but is found in the northeast, northwest, and Canada. Here, it's common name is apparently stinking willie in the east and tansy ragwort in the west. It is in the same genus as the assorted plants in these parts that are known as ragworts.

Sounds like all of them have toxicity to some degree or another. (Some are used for cancer treatments.) Apparently livestock tend to avoid grazing it but once dried, the plants lose their unpalatable taste. The greatest risk to livestock, it seems, is when ragworts invade hay fields and get baled in with the hay.