Teaching Horses to Urinate on Cue

My cue is to stand with feet apart like a mare preparing to pee, and stretch out my arms

This is how I taught my horses to pee outside. Other people report success with the same approach. On buggy summer days when they stay in the barn, this gives them a mid-day stretch and saves on bedding. Even though Brandy spends days in her screened in shed, she is included in the routine because she does not like to be left out.
1. Every time I saw them spontaneously pee outside, I rewarded them. (I used Click and Treat, but any reward will do.)
2. When they started watching me while they peed (anticipating a reward) I added a cue.

Bronzz often needs to walk a circle or two while he thinks about it

3. Then when they had been in the barn and I knew they hadn't peed in a while, I led them to the toilet area in the back of the paddock and used my cue. This took some patience. Sometimes we needed to walk a few slow circles at the pee area, pause, and think about it. Leading them thru high grass or weeds seems to stimulate peeing. It can also help to have some non-splash material underfoot like shavings or sand. With practice everyone got more efficient about peeing on cue.
Our Sapphire was the only one who never peed on cue. She was so fastidious she could not stand to splash her legs. Using non-splash bedding in her stall was more important to her than any treat.

Now I can send Shiloh out to pee on her own

While she's busy, I scoop the manure out of her stall.

This time she detours to the water tank for a drink

My horses know that it is always ok to pause whatever I have asked them to do, in order to get a drink.

On buggy days, she wastes no time

Trotting back to a clean stall and a treat