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Male horses really don't care where they urinate. I had an old gelding a few years ago who would pee in his stall as soon as I turned the light on to let him out in the morning.
One thing that I tried that did work for me was using kitty litter. You can even use the clumping kind and then it's easier to scoop out. This controls the odor and the horses don't mind it too much.
You can either mix it with the bedding or use it straight on top.
I hope this helps!