It's mainly burps. They chew their cud.

Cows produce around 50 kg CH4 per head per year. There's around 1.5 billion cattle. That's around 75 teragrams (million metric tons).

Humans release about 410 teragrams altogether. About 40 percent is agricultural. And another 40 percent is from fossils, namely natural gas.

The top 100 natural gas leaks release over 20 teragrams of methane a year. That's just the leaks. That's 400,000,000 cattle worth of methane just pissed into the wind.