Just going to say, 20,000 head of cattle is about 4 dairy farms here in Wisconsin. And we have over 50 more the same size or bigger. And there are a thousand smaller ones.
Of course there were herds of over a million buffalo in the US before the 1850s. And the deer population is in the millions currently.
But if you really want to find some CO2 check out leaves and grass decaying in meadows and forests. Life produces CO2 and so does death.