Ugghh, flies. I live in hot and humid NC and my horse lives out in his own 1 acre pasture 24/7. I also use Shoo-fly boots and a fly mask. However, his new neighbor that he shares a run-in with and he like to play gelding games, and I now find my horse’s fly mask regularly in the other side of the run-in.
The run-in is sturdy and has fans attached to a temp gauge so they turn on over a certain temp. They help to keep flying things away.
I use flyspray that has cypermethrin in it, such as Endure. It’s the next generation pyrethrin. I find it repels flies for maybe hours? “They say” it repels flies for a week, which makes me laugh and wonder how they can legally make that claim.
I aim the distribution of the fly spray based on the time of the season. Right now, we have bee-like biting flies that hone in on his sheath, so I gently spray that and his inner thighs. Gnats hone in on his navel, so I gently spray that as well. Deer flies hone on on his ears. Later in the summer we have green-headed horseflies that aim more for his shoulders and sides and by august we get “the black bombers” - huge black horseflies that land on his topline and especially his butt. So I spray those areas depending on what is out.
Luckily, my very sensitive horse is very happy when I WHACK horseflies to kill them on him - especially the black bombers. He hears them and feels them land and is more than happy to stay still while I aim to WHACK them. After a ride and hosing, I feel like I’ve had a great day if I can kill 4 or more horseflies in a day. It just makes me feel good. Snd he often drops when I start whacking flies on him (we won’t go there). I don’t whack but I squish the horseflies on his sheath. Blood sucking flies freak me out.
Oh, and since you’ve switched to 24/7 turnout, it is good to check for ticks. We have few ticks at our barn because the pastures are well cared for, but if I find them, I find them where the inside of hind legs meet the body (so run my fingers between the sheath and the legs and flank, in the folds where the inside of the front legs meet the body, and occasionally on the forehead. Anywhere were something of the horse meets the ground and the ticks can climb as far up as they can before lodging. This will be especially important if deer frequent your horse’s pasture or the pasture isn’t well maintained.