Morphine is never given orally to horses under veterinary supervision as it can cause colic so how its supposed to have been found in horse feed beats me. The 'mickey finn' of choice in the racing world is Phenylbutazone which can and is given orally. Morphine can act as an excitant in horses so I presume if you were going to give it to a racehorse it would be to gip it up a bit to make it run faster.