You can put one shot of cider vinegar into the fodder (independently of the season) and it will not only support the resistence of the horse but also helps to heal chafing points. During the summer it helps to repel annoying flies.
Itchiness and inflammation of the hair follicle
If your horse has an itchiness at the tail and mane or if he suffers from an inflammation of the hair follicle in the area of the saddle cider vinegar can relieve the pain. You can mix it up with some water and just lubricate it on the respective areas.
Additionally on warm and muggy days your horse may like a shower with the cider vinegar-water mix: just put it on the horses coat and it will be a great cooling.