Have you ever heard of the "biological pyramid"? There's a reason it's narrower at the top, each level at approximately 1/6 of the one below. Really, considering the amount of fertilizer you get out of the animal as opposed to the feces of the animal that eats it... well,
If you'd like, I can post photographic evidence of the difference between animal waste and blood meal on grass production, or I could tell you that the difference is astonishing. I just did a factorial experiment on exactly that comparison this year.
Most enrichment of the soil comes from decaying vegetable matter.