Dandruff in your horse's mane is comprised of flakes of skin that have shedded due to dryness. A range of issues can cause or worsen this problem -- some benign and some not so benign -- including stress, hormonal changes, dehydration, swift weather changes and allergies. Always consult a vet to rule out serious issues, but you can also help your horse yourself.
More often than not, dandruff in a horse's mane is caused by a minor issue, but occasionally it can be a result of a more serious problem. Before you start trying to cure it yourself, you should first call in a specialist equine veterinarian. She will be able to tell you if the dandruff has a clear cause -- such as an allergy -- or if your horse has an underlying issue which the dandruff is a symptom of.
Too much and too little bathing can both be a factor in horsey dandruff. Washing your horse's mane more than twice a month could dry out his skin, thus causing this flaky issue. Too little bathing can lead to a build-up of skin cells or can allow a fungal infection to run rife. If you've been keeping your equine pal too clean, give his mane a bath-free break for a few weeks. If you've been neglecting your bathing duties, wash your horse's mane with a medicated shampoo or with a couple of drops of tea tree oil mixed into a regular horse shampoo. Make sure you rinse the shampoo out thoroughly, because leaving residue behind could also trigger a dandruff breakout.
Your horse's dandruff could be a sign that his diet isn't meeting his nutritional needs. Think carefully about what you're feeding him and make sure he's getting a balanced diet that provides enough energy for the amount of exercise he gets. Too little fat in his diet could cause dry skin; supplementing his food with corn oil or flax seeds will help with this. Some horses are sensitive to excess additives in their food, so be sure you're feeding your horse as natural a diet as possible.
Treating the area of dry skin directly can help to reduce dandruff. While you can buy commercial horse moisturizing products from your local equine supply store, many people opt for a simple mineral oil, aloe vera gel or petroleum jelly. Massage your chosen moisturizer into the base of his mane to lubricate the area and minimize flaky skin. Make sure you don't use any harsh products that could exacerbate the problem.