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Erm... horses are herbivores. Did this idiot not know this? And you do not feed a working police horse, ever. If the rider says you can give it a pat, fair enough. But you do not feed. Horses have pretty damn delicate insides, they do not need random idiots feeding them crap.
He should have stopped when asked. He became aggressive and abusive when he was asked not to feed the horse. I have the same problem with arseholes throwing stuff into the field for my horse. Burgers, kebabs all sorts of shit. This despite having a sign up saying Please Do Not Feed The Horses. Horses do not eat any of this shit. They can easily get colic and cannot vomit.