In reply to my other tip, it only lasts 4 a couple of hours, and I also add 1/2 jif lemon, 5 drops tee tree oil, 3 drops lavendar oil and 3 drops eucalyptus oil.
When using citronella oil as when it is exposed to direct sunlight it can cause it to burn it think its called photosensitive but not sure be careful as this has happened to my horse but luckily it wasnt too serious
Another good homeade fly repellent is to get your horse to eat either garlic or onions, I can't remember which one, but I know it's one of them; and the the smell comes out of the horses pores when he sweats, which is everyday in the summer, really. The fly's hate the smell and stay away!
The downside to it is that YOU have to put up with the smell when riding him!
I know that several people do feed garlic powder in their horses feed. Another good one is Apple Cider Vinegar. My friend once took care of a horse that got 1 cup of Apple Cider Vinegar in her feed. I'm not sure if it works, as I myself have never tried it.