The amount of manure on a field in part is based on economics of supply & labor and not necessarily volume. Also, this poop is mixed in with with the other substrate components vs just being spread.
Having spread my fair share of manure onto more than my fair share of fields, I assure you that supply and labor are not a problem. Feedlots around the US are dying to find ways to get rid of their manure. The labor involves driving in circles and is part of the life of farming.