???
How many cows in a herd? How many humans are you comparing to this imaginary herd? Is it a one to one basis?
You are aware that the large the vat the greater the quantity of methane produced, right? It seems to me that bovine digestion, what with the cud chewing, the four stomachs, the diet being mostly non-animal in nature and the simple tonnage involved might have something to do with the amount of methane produced but, since I have NO clue exactly what you are referring to, I must simply say, huh?