The problem is not exactly cows, rather industrial ag.
Cows have been defecating for a long time (just a guess) the difference is concentration and "disposal".
Instead of broadcast over a large area over time, the present system concentrates an enormous volume; even where the matter is spread for soil enrichment, part of that benefit is lost because of the degradation/elimination of recyclers.
It is a breakdown of what I guess you could call the Fecal Cycle :p

https://www.amazon.ca/Origin-Feces-E.../dp/177041116X