A horse who rubs his tail is telling you he itches, and the broken hairs at the top of the dock are only the visible part. Restraining him from rubbing, or bandaging the tail and leaving the cause alone, gives you a tidier tail on a still-itchy horse. Work through the likely causes in order, from the simple and visible to the ones that need a vet.
Start With the Dock Itself
Part the hair at the dock and look closely, in good light. Dry, flaky skin and dandruff through the tail head are common, and they itch. Wash the dock and the top of the tail with a gentle equine shampoo, rinse thoroughly so no residue is left to irritate, and let it dry before judging the result; repeat washing on a regular rhythm through the rubbing season rather than scrubbing daily, which can dry the skin further.
Check the skin under the tail, around the anus, and between the hind legs for crusts, small scabs, flaking patches, or tiny parasites moving in the coat. Note exactly where any marks are, because the location is a clue: rubbing aimed at the dock and hindquarters tells a different story from a horse who is itchy along his neck, mane and belly line as well.
While you are there, look at what he rubs on. A rough fence rail, a protruding bolt, or a splintered post tears hair and breaks skin; smoothing or padding the favoured rubbing spot protects the tail while you solve the itch, and a clean, safe stable wall is far kinder than wire or rough timber.
Cleanliness Where Dirt Collects
Grime in the folds of a gelding's sheath builds up over time and can make a horse stamp, rub and swish, and mares can collect dirt and smegma between the teats of the udder, which itches in the same way. Cleaning those areas gently, as part of routine care, removes a cause that no amount of tail washing will reach. If your horse resents sheath cleaning strongly, kicks, or the area looks swollen, sore, or has an unusual discharge or smell, stop and ask your vet to do it, possibly with sedation, rather than turning it into a fight.
Seasonal and Parasite Causes a Vet Can Test For
Rubbing that arrives each year in the warmer months, worsens around dawn and dusk, and affects the mane, tail head and belly together can fit sensitivity to biting midges, often called sweet itch. Management is about reducing exposure and protecting the skin: stabling at the worst hours where practical, a well-fitting rug designed to cover the mane, belly and tail, and fans in the stable, since midges are weak fliers. Your vet can advise on products suited to your horse and confirm whether the pattern fits.
Other possibilities sit firmly in vet territory. Pinworms lay their eggs around the anus and the irritation from them can drive hindquarter rubbing, and lice or mites in the coat can do the same. Neither can be confirmed by looking alone, and guessing at treatments wastes weeks while the skin gets worse; a vet can test, examine the skin properly, and advise on the right course for your horse. Book that visit promptly if the skin breaks, hair loss spreads, your horse seems distressed, or a fortnight of careful cleaning and protection changes nothing.
Keep a dated note of each wash, each cleaning, what you found, and how much rubbing you saw that week, with a photograph of the dock taken from the same angle. Patterns across weeks are what separate a seasonal cause from a cleanliness one. Recording those checks in KinsLe's Care Teams, where a helper's observations sit alongside yours in date order, gives your vet a clear history instead of a guess about when the rubbing began.