Dung

Keep a horse at base long enough and it starts leaving piles behind it. Dung has no recipe and nothing on the site uses it as an ingredient, so the only question is what to do with the piles.

Drop them into a composter. It turns compostable items into fertilizer, and fertilizer raises the ground condition in a planter box, which is what helps crops grow. A group that farms and stables a horse inside the compound gets a slow, free supply this way.

Put the composter right next to where the horse stands, so the piles never travel across the base. Pick them up with E as you walk past, throw them in, and move on. If you do not farm, leave it on the ground. It is a minor item and not worth a slot in a box.