They produce it the same way you do: they fart and shit.

If you could figure out a way to sequester bovine farts and shit, you could capture it and power much of Canada.

@zenvelo I edited my question. I want to know the biochemical scientific explanation? How do cows change grass into methane?

Edited I really don’t care just making conversation.

Same way you convert vegetables into methane. Your gastric juices (acids) break the plant down into component elements and produce long chains of CH4, which is methane.

Here ya go.

Https://clear.ucdavis.edu/explainers/how-do-cows-produce-methane

Bacteria living in the digestive tract produce it.