Harmony is quite correct - stock should not be back into the field until you are quite sure that the ragwort is unpalatable.
If you wish to go down the chemical route, the thing to remember with any grassland herbicide is that most manufacturers will say that you require an on-going treatment programme which effectively means you need to spray every year.
Also, the way the weather has been this year, product control has not been as successful as if it was a warm and sunny season.