Canadian Thistle in Alfalfa/Grass

I have a field that has a good stand of alfalfa and orchard grass, with one corner that has a steadily increasing patch of canadian thistle (thanks to a neighbors unkept burn pile near the fencerow).  Overall, the area of thistle covers about 1/2 acre, enough to be a real pain and lead to some frustrating loss.  Ideally, I would like to be able to find a product to spray this area to eliminate the thistle, but not lose the alfalfa/grass stand that we have.

 

So, was looking for some recommendations on herbicide options and timing of application.  I know herbicides that fit my criteria will be limited, but what have you used with success in the past?  From reading older posts and some university studies, seems like Pursuit may be an option, but control of thistle is not the greatest.  Appreciate any thoughts and recommendations anyone has.

Canadian Thistle in Alfalfa/Grass