Catching your horse
The parelli way
http://www.parelli.com/catching-a-horse.html
Get a couple of people to help and section off the field with electric fencing. If the horses are down one end of the field, run a line of fencing across the middle (or as close to them as you can get) then gradually move it in so the horses are corralled into a smaller space. I would then take away the other horses and keep reducing the size of the patch the mare is on, until it's small. Take in feed, put in hay, carrots, anything so she has something to concentrate on.