it rather depend on what you have growing there that you don't want.
pasture/grazon are (as the name suggests) selective weedkiller for pasture which will kill thistles, nettles bramble - there's a whole long list - but not the grass (although as Rosemary says it will still kill some of the nice things. IIRC patsure and grazon are basically the same but pasture is more concentrated for use in a tractor mounted sprayer, whereas grazon is for knapsack application.
if you have bracken then Asulox is the selective weedkiller you want - but it's only available on 'emergency license' (was banned by EU a few year ago) and rather expensive.
the other weedkiller that springs to mind is glyphosate (roundup) which kills more or less everything - grass, bracken, bramble, japanese knotweed, trees, etc, but is supposed to break down on contact with the soil and is considered relatively safe for the environment (still need to keep away from water courses), and also has the advantage that animals avoid sprayed plants so is relatively livestock/pet safe.
I would prefer not to use these chemicals but I have a lot of wilderness to tame and this is the last year I can use them without a cert, so I'm having a bit of a blitz (weather permitting), particularly on the bracken.