Chemicals work for that season but the docks do come back. Topping twice (at least) each summer works - but if you stop topping, they'll be back.
If they're not too extensive, certain types of sheep will eat them. If they've taken over the pasture, hard grazing is not a good plan, as (I believe) the sheep will suffer from excess oxalis acid.
A pig will rootle them too - although it gets worse before it gets better, as all the seeds in the soil get brought up.