Cattle will eat Canadian thistle. Even cattle that have not been trained for it. I have been discovering this by accident as I concentrate the herd in early June to graze the grass that has gone reproductive. They will take the blossom off, plus a few leaves below the top as they get more restricted in what is available. I have not seen thistle feed analyses but I suspect it is pretty good stuff as the plant is taprooted, thus pulling nutrients up from deep in the profile. I would prefer alfalfa or sweet clover for that task, of course, but you work with what you have.
The thistle come in wherever the cattle have congregated, especially under wet and muddy conditions. It is a first colonizer in earth's ongoing battle to heal itself. But I notice that pasture slowly takes the area back, the sward pulls the compaction out as the thistles are controlled. This is different from what happens in the cropping areas, where compaction from early wetness, excessive wetness in our case this year, must be broken up with the tillage equipment, often making it into hard chunks which will not allow the seeds to sprout without a regular rain, and sometimes not even then. Another winter seems to be the only cure. Much of the problem here is that the cropping is annual. Pastures are perennial plants largely. I think the earth regards annuals as nothing more than emergency or interim plants, and that is why it is difficult indeed to maintain and improve soil health using annuals alone.