It does not look at all like the Goldenrod I am familiar with in here in NH but I think there may be different varieties of it in different locations. Do you have a local Extension Service you could contact, maybe email the picture to?
I believe that is goldenrod. My "Plants Poisonous to Livestock in the West" says that daily consuption of 1- 1.5 percent of an animal's weight of the green plant for 1-3 weeks will produce signs of poisoning in horses, cattle and sheep. I don't think a stem or two would matter. Probably your horses won't even eat it. The toxin is tremetol.