Join Date: Dec 2011
Location: Washington State
Many stylists are against the use of Head and Shoulders shampoo on people because it's very harsh on skin.
What I'd do is this:
First, assess your horse's nutrition - is s/he getting all the nutrients that are needed from your pasture and/or hay? If not, balance out the diet with (preferably) a free-choice loose mineral blend. If your horse is the type that either won't eat the loose minerals on their own or will gorge on them (even after their nutritional needs are met), then I'd go with the same loose mineral blend, but in fixed amounts at each feeding based on weight (go with advice from your vet and/or equine nutritionist on amount). A horse who doesn't have adequate nutrition usually will end up with sub-par skin and/or coat health.
Secondly, I'd look at whether the dandruff is from an allergic reaction or other skin condition. Did this spring up all of a sudden? If so, and it is at all possible, have your vet out to do some testing to rule these out.
Finally, re-assess your bathing and grooming habits. Over-bathing can cause dandruff since the surfactants (cleaning compound(s)) strip the oils from the skin causing it to dry out and flake. Try to encourage and spread the natural oils of the coat with proper grooming, the general consensus that I've seen has been that rubber curries and natural bristle brushes do this the best.
Hope your horse's dandruff clears up!