Been there done that! We did have a loo at the old yard, but it was like the one from trainspotting! Add to that spiders the size of mice in every corner and I soon learned to pee on the bankings in the stable! Only got short for a poo once thankfully.
......skipping out today. Sudden desperate urge to poo- no loo. So pooed on dirty straw and chucked it on the muck heap.
It won't have damaged the muck heap to have a small percentage of human in with the horse, will it? Have visions of a small dead patch in the potato fields of the farmer who takes it. I'm being stupid, aren't it?