well milkweed is usually poisonous for most animals, and the only thing that really eats it are those catepillars. I haven't seen milkweed on any edibles lists. So if you really don't mind doing all that research over milkweed, I would just toss it, why bother ya know?
Ok here is my two cents worth.
I have to disagree with all of you. I believe it is prickly lettuce. I have it in my yard too and I have never seen any of my torts eat it Greeks or DT. However they say other lettuces came from this. Check out these photos and see if this isn’t what you have.
prickly lettuce: (three sites) http://calphotos.berkeley.edu/cgi/img_query?where-taxon=Lactuca+serriola
Is there a horticulturist or a college that teaches horticulture close to you that you could take it too? They may be able to identify it. There seems to be quite a lot that are very close to your picture. (shoot I thought for sure I had it) Have you ever seen them flower?
I emailed a college that has been doing a study and attached some pics. Let's see if they answer. I'm sure they have flowered before but I have so many different weeds growing around I can't remember if they were yellow, purple or whatever but I will pay attention this time. I usually just cut them down and toss them.
I do the same, just cut them down- hate the spikes. But it seems the flowers usually give a much better clue. good luck with the ID hope they answer. Please keep us updated.
OK so I have bragging rights here lol. That is how I ID'd it by those darn spines. I also found in talking to Michael today the torts love it when it is a young weed and it is a good one for them to eat. So there we have it.