My mare's got a lot of white to keep up with. I haven't had much luck with any of the shampoos in particular, but if I have her stand with a foot in one of those rubber grain pans full of warm sudsy water for 10 or 15 minutes on each leg, that helps a lot to break up mud clumps and get stains out. For the tail I just lathered it up really good, and let it soak wrapped up in a trash bag for 15 minutes. I also treat her feather and tail once or twice a month with mineral oil. It works a lot like cowboy magic does when you're grooming, so it's great for getting out knots and much less expensive. Leave it in and it'll condition the hair and make it more resistant to stains in the first place.