......We had 2 acres sprayed with a herbicide last year, and whilst it has knocked the docks back they're not completely cleared......
It does say on a lot of grassland herbicide product cans that an
'on going control programme should be implemented' (or words to that effect).
Basically - 1 spray will not do the job especially if they are large established weeds. Other factors can affect the effectiveness of a product - growth stage of weed when sprayed, temperature fluctuations between night and day, drought, cold winds, water logging, generally any thing that will stress a plant out.
I've got a grass field that we sprayed this year for docks and the spray has done some of them but others have come back so we'll have to come back with another spray next year as its too big an area to hand weed.