|MadSci Network: Zoology|
Thank you for your question: What makes a cow produce methane gas in its system?
In short: cows feed mainly on grass. Grass contains a great deal of cellulose. Cellulose generally is quite energy rich but difficult to digest. For breaking down cellulose into digestible molecules special conditions - exclusion of oxygen (an anaerobic environment), and enzymes are needed. The cow does not produce the required enzymes itself but gets help from bacteria living in the stomach. These bacteria break down cellulose into certain sugars but also methane. Methane is a quite general side product of such anaerobic breakdown, occurring also in termites (where other bacteria break down cellulose for them), but also in land fills and rice fields (where other organic products are digested by bacteria).
If you want detailed information on how the breakdown happens in the cow see following WWW-page on digestion in cows found at the University of Alberta. If you are interested in the general problems due to methane production do a search on methane and environment on any search machine.
Besides, there is a different way to understand your question and then the answer to your question would be: If you believe in creation: God has made the cow to produce methane, if you believe in evolution there is no reason at all, just chance.
Try the links in the MadSci Library for more information on Zoology.