I asked a similar question earlier this year.

https://talk.newagtalk.com/forums/thread-view.asp?tid=1106937&posts=5&highlight=pasture%20&highlightmode=1#M10312943

From talking to a couple of people that do drone spraying it could be done by taking areal photo then circling area on map/photo. Then drone would spray each circled spot as if it is a separate job from all the other circled areas. If there are very many spots could take as much or more time to do this verse spraying whole field. So they are wanting same price to spray whole pasture or to do spots (not including chemical). They charge more than airplane so just as well hire plane to do whole thing.  

When they get it so a drone can fly whole field and turn on and off based on where weeds are at, I would consider hiring a drone or buying one.