My good Tammy was able to teach her gelding, Chucho to empty is bladder on command. Everyday, everyday, everyday. She spent a lot of time with him and when he would voluntarily pee she would respond with a verbal cue "go pee-pee" (lol), tap on his back where his kidneys are located, and reward after he was done. The communication between horse and human is awesome between these two. Here's the video!