Mushrooms
A major growth medium for mushrooms is often sterilized horse manure, as part of a compost. In the industry, "synthetic compost" is any formulation that doesn't include horse manure.
When my grandfather was farming, every farm had a manure spreader on it. I had a toy one.
Of course, to be fair, that was back in the good old days when cows weren't harboring nasty e-coli, and you could eat a raw egg without worrying about salmonella.