Project leader Sebastian Hoppe and his team working for the North Rhine-Westphalia agriculture chamber examined two groups of 48 cows for 120 days - feeding them different menus.
Methane is considered to be around 25 times as damaging to the ozone layer, as carbon dioxide and thus a strong contributor to global warming.
The cows which ate mostly maize, produced on average 340 grams of methane each, per day. Their sisters who were fed mostly grass produced an average of 360 grams each day.
The study was one of the first ever to examine methane production in cows in real-life conditions - most other evidence on their farts and burps has been the result of laboratory tests, the agriculture chamber said.
Further tests are being conducted to see whether tannins in food could affect the bovine production of methane.
But Hoppe warned against drawing too many conclusions from his work, saying the entire production chain must be considered - such as the positive effect of grasslands on the absorption of carbon dioxide, another greenhouse gas.
Federal Environment Office figures from 2011 suggest more than half of Germany's methane emissions come from agriculture, although it is not know how much comes directly from cow digestion.