I had a trainer friend years ago who had trained at the track where it saves a lot of time if the horses will urinate "on command", as Heidi suggests, usually for whistling.
She had an appaloosa mare she had raised from birth who would urinate when she said "go pee pee Nikka." From birth, every time she caught the little filly urinating, she gave this command and eventually conditioned her to perform.
I confess I have tried it with sporadic results, tempted to report "piss poor" results.
