What can I do about them in my horses field? Is there anything I can use to spray them and not kill the grass? How long does the horse have to stay out of the field?
Grazon 90 will kill most things except grass, it will be most effective in the spring when new growth starts. I think they recommend keeping animals off it for 2 weeks - not 100% sure but it says on the bottle.
Cut the docks and nettles and leave them to wilt. Your horse will hoover them up and the rest will get mullered and die over the winter and then come spring you can treat them with Grazon or Agritox which will kill the weeds but not the grass (at the correct dose).