I feed a product called BioMane and it seriously has thickened up my horses manes and tails. (And they grow like crazy!) I haven't found it in feed stores but I order it off the website (
Home page) and I get it within just a couple days. It's a great product. I wouldn't brush very often, and when you do make sure and use a detangler spray and brush that you would use on the horses body (NOT A CURY COMB!) but a soft bristled brush. I takes longer but I promise you won't pull out as much hair because the soft bristles don't cut or pull through knots and tangles. Hope this helps