Diane,
Yes CAnadian thistle is beneficial, of course! Especially to itself! Beyond that, I am sure Canadian thistle has plenty of uses and value in ecosystems. It helps heal land by keeping people and livestock off it, for one thing! I'm sure its tap root is quite good for loosening and building soil. It has its community niches to fill, its services to the system to do. You can either let it do them and deal, or work out what its services are, perform them yourself and make it unnecessary for the system to be healthy. I suggest doing a good thorough niche analysis of the thistle to see what you can find out about it. Start with the Fire Effects Info System
http://www.fs.fed.us/database/feis/index.html and see if it is in their database. POke arounda bit and accumulate all the data you can on it. That will help you learn how to work with or around it.
I do not own goats, so I do not know if they will eat this species. I'll let someone else answer that question, though my guess is that at least one breed will do so under the right conditions.
To eliminate--test soil and correct mineral imbalances, number one. Two: dig pits and see if there are restrictive horizons the thistle is helping to break up. Then work out how to break them up yourself. Three, do a comprehensive site analysis and assessment and work out a comprehensive site preparation plan that will deal with all the issues you face there, including the thistle.
In a project I am involved in we invented a great new technique for hard to kill species. We sheet mulch with cardboard, very dense cardboard coverage, really careful overlaps and covering ALL holes. We pin the cardboard down with metal earth staples. Then we lay either jute mesh or coconut fiber mesh over the cardboard and pin that down. Then we leave it for at least two years! The mesh holds the cardboard down, makes it look decent from a distance, and ALLOWS THE CARDBOARD TO DRY OUT. By doing this the cardboard can cover the ground completely for two or more years without decomposing, and shade out all our problem species. Then we can plant through the cardboard, sheet mulch with wood chips or whatever over it, and the whole thing turns to soil. It has worked well so far with our problem species, crown vetch. It may work for you.
You may need some deep ripping to break up hard pan. You can also try composting the thistle and applying the
compost to the soil. Or you can throw the thistle into a barrel of water, let it rot, then water things with that--this can help spread around the minerals the thistle is pulling up, help it do its job, while also killing the thistle plants.
Enough said for now, I have to go. this post feels incomplete to me, but I hope it helps.
Blessed be!
d