I spray in the spring once the nettles have come through so I can see where they are!!
I use grazon90 in a backpack sprayer. I have 2 1/2 acres split into 2 fields & it takes me about 45 mins each. The horses have to stay off the field for 2 weeks (according to the guy in the shop) in case they decide to eat the wilted plants.
It kills nettles really well, brambles, has a go at docks & also kills these funny plants we have that look like rhubarb but all green & grow massive sticky balls.
Hope this helps? I've sprayed my fields 2 years running & last year only got about 1/3 of the weeds I had the year before.
Grazon is good, but if you have creeping thistle or other persistent weeds, I suggest repeating the application about a week later. Sometimes, you may have to attack some patches a few time before you've nailed it!
I need something to control docks & wretched ragwort- pulling by hand has beaten me- this year I've got to spray. Has anyone tried that new ragwort spray- "Barrier H"- & if so, how was it? Any advice for dockens?