I have forwarded this question on to our Extension Weed Specialist to accurately identify it. Once he has it ID'd I will help in regard to whether it is poisonous or not to horses.
Any news on this? it's been 15 days....
Sorry our weed specialist has been out for a month. Here is the direct contact info: https://cropandsoil.oregonstate.edu/users/andrew-hulting
Hello. I just found out that the weed specialist has been out of the office. I think this weed is what we call tarweed. Can you tell me are the leaves sticky and when you touch the plant is there a slight odor?
There are several type of tarweed species. Let me know if it is sticky and I can proceed to help you what I can. I have attached a 'stock' photo of tarweed...would you say they are the same plant?
Some species of tarweed, called Fiddleneck are poisonous while other species are not. I will try to help you figure this out. I do know that people are dealing with this same plant other places and have never had it before.
Let me know about the stickiness of the leaves and we can proceed.
It sure looks like your picture... It is however not fiddleneck, as we are familiar with that plant. The stems are hairy and slightly sticky, I will have to go check the leaves....
I’ve included two more photos, one an attempt to do a closeup of the stem and leaves.
Can you send these directly our OSU Extension Weed Specialist? He should be able to help you figure out what it is. https://cropandsoil.oregonstate.edu/users/andrew-hulting