How do you desensitize a horse to being sprayed with a hose or fly spray?

Desensitizing a horse to spray — whether water from a hose or fly spray from a bottle — follows the same progressive exposure principles used for other desensitization work, but there are specific considerations for spray that make the sequence different from object desensitization. Clinton Anderson begins hose desensitization at the horse's feet, not its body. The feet and legs are areas horses are most comfortable having touched and most comfortable having water applied to, so starting there reduces the initial concern and gives the horse an early experience of being comfortable with the water before it is applied anywhere frightening. From the feet, Anderson moves the hose progressively up the legs, to the belly, to the neck, and eventually to the head — always watching for the horse's relaxation before moving further. The spray bottle desensitization for fly spray follows the same bottom-up sequence, with an additional consideration: the sound of the spray pump. Anderson desensitizes the horse to the sound of the pump trigger before ever spraying the horse — pumping the spray bottle near the horse without pointing it at the horse, and allowing the horse to associate the sound with nothing happening before associating it with something being applied. This prevents the horse that spooks at the sound from being accidentally sprayed in the process, which would reinforce the fear. Warwick Schiller adds the observation that horses which are genuinely anxious about being hosed typically have a nervous system issue that extends beyond the spray itself — they are horses that are generally more reactive than confident. For these horses, he recommends ensuring they are in a regulated nervous system state before the hose is introduced, and keeping sessions very short initially — just a few seconds of water on the feet and nothing more — with long breaks between sessions to allow the horse to process and habituate before more is asked.