A combination of these, as you have time and equipment, and money for:
1. Drag the pasture as often as possible, so manure piles are spread out to dry. As an alternative, pick up manure piles. Good in theory, at least.
2. You've tried the feed through fly control. For that to work well, every horse on the farm and any others close by need to be on it.
3. I've had minimal luck with traps on a large scale, but in a small area like a tack room or feed room in a barn they can be effective. Down side is that once they start to fill up with dead flies, they smell horrible.
4. I have an automatic fly spray system in my barn, but more flies immediately move in to replace their killed friends. It does control the barn spiders pretty well, though.
5. If you irrigate pastures, don't overdo it. Flies lay eggs only in the outer moist layer of a manure pile, they don't burrow inside it. So give the outsides of the piles a chance to dry.
6. The pyrethrin based fly sprays don't seem to me to really repel flies from the horse. What does seem to are the ones with natural oils. The down side is cost, and that they don't last all that long. Need to spray the horses at least twice a day, and preferably every 8 hours. Horse Journal, which started as a 4 page newsletter years ago as the Consumer Reports for horse products, likes Ecovet spray. I haven't tried it yet.
Ecovet Fly Spray is Strong | Horse Journal
7. I know several farm owners who swear by fly predators. Butt they work best in and immediately around a barn. I'm not sure of the range.
https://www.smartpakequine.com/pt/f..._campaign=fly predators&utm_term=fly predator
8. Fly sheets, particulary those with belly guards and neck hoods, plus an eared fly mask work pretty well, if the horse will tolerate them. The main problem I've found is that even as thin as they are, the horse gets really hot and sweaty.
9. Some horses seem to be partictularly tasty to flies. I'm not sure why. I have one, a quarter horse, who the flies just love. No matter what I do for him, the flies cover his lower front legs all summer.
10. Speaking of fly sprays, have you ever noticed how much is wasted because the spray pattern of the bottle is so wide that much of it misses the horse? That evidently is by design so that you use it up more quickly and have to buy more. I saw a marketing study years back about how much waste a buyer would tolerate so the spray pattern could be optimized for profit.