Subsequent spraying with Roundup when it is in the flowering growth stage (as you have already done) will decrease Canada thistle populations but not as well as a herbicide like Curtail - it has a lot more activity on Canada thistle than Roundup. Also, Roundup will severely injure grasses and many other plants because it is non-selective and will kill or injure most plants it contacts ... especially grasses. The perennial grasses that are left in your pasture - as well as new plants after seeding - will continue the battle against Canada thistle and eventually gain the upper hand. But one must effectively decrease Canada thistle for seeding to be effective - it is very expensive and you want to make sure you give this effort every opportunity to succeed.

So, with this in mind, switch to using only Curtail and apply it when Canada thistle is in the bolting to flowering growth stages (e.g. next spring ... if necessary). Another similar herbicide that is more effective on Canada thistle than Curtail, is Milestone. When you make your next herbicide purchase, consider switching to it.

http://www.ext.colostate.edu/pubs/natres/03108.html