While MTG is good you have to be really careful with it- some horses (like my guy) are violently allergic and it'll cause them to lose hair and rub to the point of making themselves bleed. Do a small spot test before applying it to the whole mane/tail! Also, MTG can cause blistering if exposed to strong UV light (sunlight) so I'd recommend applying it in the evening or when you know the horse is going to be inside for a while.
I've had really good luck with Smart Mane&Tail with two of my horses- might be worth a try, as well as using a good spray-on conditioner like Sunflower Suncoat
I use mtg to promote hair growth.