If I remember my highschool chemistry classes, the difference is;

this is the combustible reaction of cow farts:

CH4(g)+2O2(g)⟶CO2(g)+2H2O(l)

And this is the combustible reaction of human farts. And that's vegan farts, and regular people farts, they're the same.

2H2(g)+O2(g)⟶2H2O(l)

See the difference?

That's why cow farts are a very bad greenhouse gas. It's simple highschool chemistry that anyone that was paying attention would know.