There are strong advocates for using herbs for horses, because they are less invasive, more natural. I say to the most extent I agree with this, but I draw the line at some things. I give chemical wormers to my horses. I would rather they not have worms, but I combine my worming program with a tablespoon of garlic powder in their evening feed. This means I only worm four times a year. There are those who say that garlic powder is not good for horses, but I have not seen any evidence of this with my own horses.
I also give them a tetanus/strangles needle every year. It is not worth the risk.
When her ladyship was around nine months pregnant with the boy, I gave her fennel seeds. These are supposed to assist with milk production. She had it all the way through to when she gave birth and for about six weeks afterwards. The boy grew like a weed. He was very strong born and stayed strong.
When he cut himself badly one time the vet gave us a spray for the wound and a spray to keep the flies away. The chemical fly spray just did not work. I did not want the wee beasties blowing in his wound, so I mixed a few drops of citronella oil into some warm water and sprayed the area around the wound. No flies. Twice a day I did it. I also rub a little of this mixture around the eyes, being very careful not to get it in the eyes. Again, no flies. Her ladyship will wear a fly veil, but the boy felt it was not macho going around looking that stupid.
The willy wagtails were not impressed with the lack of flies and let us know about it as well. The also protested the mare wearing a fly veil.
Using warm water in a spray mix is good, because the cold spray tickles and the horses learn soon to jump away. If the water is warm, they don't really feel it and don't jump so much. Actually with the wound, we couldn't warm the spray and the boy learnt to tolerate it quite well. We had begun to spray him with cold water when he was very little (less than a week old) so that it never bothered him much. The mare would lay back her ears if we sprayed her, and swish the tail, but she put up with it.
If you want to give your horses herbs and natural things, be sensible enough to read as much as you can about it, books, internet articles, etc. Experiment with your horses till you find out what works for them and what doesn't. Have enough sense to worm them, maybe as above, but worms are not nice. Also, as I said the tetanus and strangles shots. They work. A horse near our place had strangles, and gave it to a couple of others, but ours were fine, not even a sniffle.
I think letting them grow their winter wool is also useful, prior to using the rugs. I like mine to go into winter very fat as well. They seem to last the weather better. Then a stable isn't a really necessary thing. Just ask her ladyship.