Try a 1300' roll of 'Revenge' Sticky Fly Tape in addition to feeding garlic. Hang one spool up on one side of the barn, string it down to the desired length, thread it through a small pulley, pull it back down the other side of the barn and attach it to a 'take-up' reel, (empty plastic wire spools from Lowes work great for take-up reels) Fly spray and misting devices only repel flies, but with the sticky tape overhead, garlic-hating flies will buzz the poop, circle your horse and fly right up onto the tape... never to breed, or bite you, your horse or your dog ever again! When the tape is full, just crank those nasty flies onto the take-up reel and leave new tape in its wake! IFA sells them for $28-$32 a roll, but 'Farmer Boy AG' @
www.farmerboyag.com/fly-insect-control/99050744.asp has them for only $17. Very effective and depending on the size of your barn, one roll can last months.