There are other "methods" I have used and it helped but not sure with how tough & numerous the flies are right now...
Golden Malrin is a pelleted, small like coffee ground sized, fly bait you very, very lightly sprinkle on the ground where the horses reside and hang out at...
It attracts and repels, kills the pests and also can help control the fly population...
Make sure once you sprinkle it you not sweep it gone...it works for several days.
Its
not cheap, but I found it helped.. We used it in show barns, breeding farms and I do use it at home in my backyard barn. Can be bought online, in stores...
Get several gallon sized Ziploc storage bags, a shiny penny for each bag....
Fill the bags 1/3 - 1/2 full of water, drop the penny in, seal and hang them from the doorways, around a overhang...just make sure where hung is out of the horses reach.
I know it works...why, not sure but it kept the flies away..
They all helped around my barn...a combination of applied animal fly spray, the dispenser thing, Ziploc bags hanging and Golden Malrin on the ground together should make a huge difference....then you have to convince the horses to go where you made a protected zone for them.

🐴...