They are both Thistle. The darker one is Russian Thistle (Tumbleweed). The lighter one in question is another variety of Thistle. I don't know if they are toxic, but in my limited experience, chickens won't eat anything that could be harmful. Hope that helps. Also, if you want to use herbicide, you'd have to mix the Roundup with Savage (a 2-4D) and a solution of water miracle grow prior to pouring in the products. There's a lot of bad press about Roundup, I'm not too sure about all that, but that's how you kill it if you want to.